Zonal oil production is an important approach to address interlayer and planar conflicts in high water-cut oil fields, fully exploit mid-low permeability reservoirs, reduce ineffective water circulation and tap remaining oil. Due to differences in development concept, management mode, single well economy, and types of production wells, the technical series of zonal oil production developed in China and other countries are different. In other countries, water-controlling equipment is usually applied since the well completion stage, and the zonal oil production technology is represented by the inflow control technique and intelligent completion technique, and mainly applied to horizontal wells and high-output vertical wells. In China, oil fields generally adopt commingled oil production at the early stage of development. Due to complex interlayer and planar conflicts, the production profile is uneven. After zonal (selective) water injection is applied, interlayer conflicts are to some extent suppressed. However, due to heterogeneity of oil reservoirs and permeability differences between oil layers, injected water advances fast in highly-permeable oil layers, resulting in premature water breakthrough in oil wells and water cut rise of produced liquid, while low-permeability reservoirs are low in producing degree^[1]. After an oilfield enters the high water-cut stage, differences in formation pressure and injectivity between layers grow year by year, the oil and water distribution becomes more and more complicated, and interlayer interference gets stronger. To control water cut of oil wells, reduce interlayer conflicts, fully exploit production capacity of medium-low-permeability reservoirs and achieve balanced recovery of all oil layers, many types of zonal production techniques have been developed and applied in some oilfields in China^[2,3,4,5,6]. For example, in Daqing oilfield, with the progress of oilfield development, the conflicts faced have changed constantly, correspondingly, the functions and implementation methods of zonal oil production have been evolving too, from “zonal oil production (allocation) of flowing wells”, to “water blocking by mechanical means”, then to “hydraulically adjustment zonal oil production” and at last to the current “intelligent zonal oil production, and the supporting zonal allocation tools, packers, and adjustment techniques also have changed correspondingly.

After briefly introducing the development of zonal oil production in foreign countries, this paper presents the development history of zonal oil production in China taking the application of the technology in Daqing oilfield as an example, and points out the development direction of zonal oil production in light of production requirements of oilfields and problems of current zonal oil production techniques.

Different from China, vertical wells in foreign oilfields are generally produced layer-by-layer from bottom to top. The main purpose of zonal production is to seal the bottom water, and bridge plug sealing is generally used^[7,8], including bridge plug set under hydraulic control and bridge plug set under cable control. The bridge plug set under hydraulic control is carried to the designed depth by pipe string, then set under pressure applied through tubing, and released by turning the tubing clockwise or shearing the pin under pressure. At last, the delivery pipe is tripped out of well and the plugging is done. The bridge plug set under cable-control is carried by the cable. After it is lowered to the target depth, an electric signal is sent from ground to ignite the charge in the setting mechanism to provide energy to push the bridge plug to set. After setting, more than 5m of sand or cement shall be laid above the bridge plug to ensure the sealing effect.

With the improvement of drilling technology, horizontal wells have gradually become the main type of development wells in foreign oil fields. During the development of edge- bottom water reservoirs, in order to improve oilfield recovery, prolong the life of oil wells, and prevent premature breakthrough of water and gas, the production pressure difference control tool is often installed in each section of the horizontal well at the completion stage according to logging data to regulate reservoir production profile to keep the liquid production rate consistent in all sections of the horizontal well, make the horizontal well bottom water advance evenly, extend the water free oil recovery period of the oil well, and thus achieve the purpose of balanced production and improving overall development effect^[9].

The early inflow control devices (ICD) were actually nozzles with fixed diameters. By adjusting the number of ICDs, the flowing pressures of different liquid production sections were adjusted to achieve balanced production^[10]. However, the ICD cannot be automatically adjusted according to changes in reservoir conditions, and the inaccurate completion test data would result in a mismatch between the reservoir and the set parameters of ICD adversely affecting the balanced production.

The automatic inflow control device (AICD) is a new generation of inflow control device developed in recent years. With a specific structure or flow channel design, the AICD shows different inflow control characteristics for water, oil, and gas. At present, there are mainly three types of AICDs^[11]: floating disc type^[12], clip type^[13] and flow channel control type^[14]. They can realize oil-water ratio detection and automatic adjustment of inflow resistance according to different physical characteristics of oil and water (such as viscosity). Properly used, they allow us to control water and gas and increase oil production. Tendeka's FloSure automatic inflow control device working according to Bernoulli's principle^[12] is a typical floating disc AICD (Fig. 1). When the oil with higher viscosity flows through the valve, the pressure drop generated by the gap fluid increases, while the pressure in the pressure holding chamber under the disk remains basically unchanged, so the floating disk moves down, the valve opening increases, and the overall pressure drop of the valve decreases. When the water or air with lower viscosity flows through the valve, the pressure drop generated by the gap fluid decreases, consequently, the floating disc moves up, the valve opening decreases, and the overall pressure drop of the valve increases. In this way, AICD can realize automatic detection of oil, water, and gas ratio and automatic adjustment of valve opening, so as to realize the zonal production that integrates water control, gas control and oil production increase. In actual production, when the area of ​​the floating disc is fixed, the inlet size of AICD can be adjusted to meet the production requirements of different displacements. Multiple valves can also be combined in parallel to meet larger displacement requirement. At present, FloSure AICDs in use in the world exceed 42 000, and are widely used in North Sea, Canada, Middle East, Southeast Asia and other regions^[15].

Compared with ICD-based balanced production technology, AICD-based balanced production technology has some unique advantages. It can automatically identify fluid viscosity and limit low-viscosity water and gas without manual intervention. The valve plate of the inflow control device, made of cemented carbide and high-strength stainless steel, has good erosion resistance and can maintain long-term effectiveness. However, there are some prerequisites for the successful application of AICD technology: (1) AICD is only suitable for reservoirs with sections differing in water saturation and significant viscosity difference between oil and water. (2) It requires a start-up displacement of the single-stage fluid production, and the well to have stable flow field. (3) The additional production pressure difference of AICD in the liquid flow channel will increase the energy consumption of surface lifting.

Smart well technology is a complete solution for zonal oil production. With this technology, the oil well is zonally-separated by cable-thru packers, temperature, pressure, flow sensors and hydraulic control sliding sleeve are installed at each layer. The sensor signals are transmitted to the surface through cables to the downhole dynamic monitoring system. The sliding sleeve is connected to the hydraulic control equipment on the ground through hydraulic pipeline. Through hydraulic control pipelines and cables, it is convenient to realize zonal oil production and dynamic monitoring of downhole parameters. The smart well technology was originally designed to realize remote control of complex wells in deep water areas, but it has been applied widely to production wells in shallow water areas and high-production oil wells on land soon. The initial focus of the research was to control the downhole sliding sleeve switch through equipment installed on the ground to achieve selective production of multiple oil layers downhole. With advancement, the smart well technology can use dynamic monitoring data provided by the temperature, pressure and flow sensors in the wellbore to continuously optimize the production plan, effectively control the coning of water and gas, and realize refined management of the oil well production process, and finally achieve the goal of increasing the output of oil wells.

The typical structure of smart well is shown in Fig. 2, which is mainly composed of downhole dynamic monitoring and control system, downhole data transmission system and surface reservoir development program optimization software system^[16]. The downhole dynamic monitoring and control system includes a downhole monitoring system made up of downhole temperature, pressure and flow sensors and a downhole flow adjustment sliding sleeve, and is used for downhole data collection and flow adjustment. Downhole information transmission and sliding sleeve control are realized by a composite pipeline composed of cables and hydraulic pipeline. The composite pipeline is laid outside the tubing and passes through the wellhead to connect to the surface control system. The two-way data transmission between the surface and downhole is completed through the cable. The hydraulic pipeline is used to control the opening of sliding sleeve. The surface reservoir development plan optimization software system acts as the surface control center. According to the real-time monitoring data of downhole parameters and the geological characteristics of the reservoir, the development parameters are optimized and the optimal control parameters are selected, and then the zonal flow rate is adjusted by the hydraulic control slip sleeve in real-time. The smart well technology has unique advantages in optimizing production parameters, improving reservoir management, reducing operational risks, enhancing ultimate recovery and saving workover time.

At present, the typical smart well systems abroad include Halliburton's SmartWell system and SCRAMS system, Baker Hughes's InForce system, etc., which have been applied to some high-production wells all over the world. Nevertheless, the smart wellsystem is complex and difficult to install, and high in cost, and thus only suitable for high-production wells.

According to the characteristics and applicability, the development history of zonal oil production techniques in China can be divided into four stages, namely the zonal production allocation of flowing wells, water blocking by mechanical means, adjustment zonal production allocation, and intelligent zonal oil production.

The zonal oil production technology at this stage is mainly used in the early stage of oilfield development, when the formation energy is relatively sufficient, there is no pumping system in the pipe string, and the pipe string structure is integral.

2.1.1. Concentric zonal production (allocation) technique

In the 1960s, in order to control rise of water cut, Daqing Oilfield successfully developed a zonal oil production process with oil well packers and 625 oil well allocators as the main tools^[2] (Fig. 3). The working cylinder of this allocator is connected to the pipe string and is run into the well together with the packer. A movable plug containing a fixed oil nozzle is installed in the working cylinder of the allocator. The oil production of the corresponding interval is adjusted by replacing the oil nozzle. Because the allocator is of concentric structure, in order to realize the setting and fishing of the plug, the outer diameter of the plug must increase from the bottom to the top, and the pipe string can be connected to a maximum of 5 working cylinders; the plug can be dropped or taken out of the well. When run in, it can only be delivered level by level from bottom to top; when salvaging, it can only be fished level by level from top to bottom.

This process has been used in Daqing Oilfield for more than 2000 well times. After application, the oil wells witnessed increase of oil production and decrease of water cut. This process played an important role in the initial zonal production. However, the concentric zonal production technology has disadvantages such as complicated on-site construction, inconvenient fishing and adjustment, and limited allocation stages.

2.1.2. Eccentric zonal oil production allocation technique

In the 1970s, during the water flooding development of oil reservoirs, the injected water often advanced quickly along high-permeability layers, resulting in premature water breakthrough from multiple layers of the oil well. Accordingly, the zonal oil production gradually changed from a production allocation process to a water plugging process. For this reason, a multi-function eccentric production and water plugging system was developed to solve the problem of plugging multi- layers arbitrarily.

The core of the multi-function eccentric production water plugging system is the type 635 eccentric production device^[2] (Fig. 4a), which is mainly composed of a working cylinder and a plug. The plug is installed in the eccentric hole of the working cylinder of the allocator, and doesn’t occupy the central channel of the tubing. The fishing device can set and fish plug at any layer (Fig. 4b), breaking the limit on the number of water plugging stages. This system enables adjustment of nozzles at multi-stages and any layer, and tripping of pipe string without well killing etc. The system has a 46 mm central channel remained, allowing support test instrument to do zonal test in the production stage without fishing the plug, so the test is simple in procedure can get accurate data.

To sum up, the "production allocation" in the natural flow stage is essentially "production allocation on the ground and implementation downhole", that is, the production of each layer is determined on the ground, and specific nozzle is put into the corresponding production layer, which cannot be adjusted downhole after construction. After the oilfield enters the stage of full pumping, the testing and adjustment channel is occupied by pump, and the above-mentioned zonal production technology is used less and less.

In the 1980s, the production of Daqing oilfield entered pumping stage from natural flow stage. The existence of lifting pipe string made zonal oil production on-site construction and downhole adjustment more complicated. Under this background, the zonal oil production technology transformed into zonal water plugging to a certain extent during pumping production^[2].

2.2.1. Mechanical water blocking techniques

Mechanical water plugging mainly adopts slip-type, drillable and other mechanical water shutoff and release-sub-involved pipe string structures. The water plugging pipe string and the oil pumping production pipe string are separated. The upper part is the production pipe string, and the lower part is the water blocking pipe string. When the pump is checked, the water plugging pipe string does not move, so the lifespan and validity period of the water plugging pipe string are longer than the integral pipe string. The slip-type water plugging pipe string is suspended by a two-way slip packer. The structure of the pipe string includes a releasing sub, a slip packer, an eccentric allocator (i.e. a liquid generator) and a plug (Fig. 5). The slip-type water plugging pipe string adopts hydraulic setting, releasing under pressure, and lifting or fishing with special tools to remove blocking. The drillable water plugging pipe string is generally composed of a drillable packer and an inserted sealing section (Fig. 6). The packer is lowered to a predetermined position step by step, set by hydraulic pressure, the tubing is rotated clockwise to release the packer, and then the sealing section is inserted to complete the sealing of the target layer. When the packer fails or the production stratum is adjusted, the workover rotary table, drill pipe and special drilling and milling tools are used to remove the sealing section, then the packer is grinded, and the extended working cylinder and accessories under the packer are removed. This kind of pipe string can work stably for a long time in under high temperature and high pressure environment, but the operation is complicated, so it is generally used for long-term plugging and is suitable for normal cased wells or wells with casing deformation.

The water blocking device commonly used in the stage of water blocking of pumping well is the sliding sleeve type water blocking device (Fig. 7). Its main components are the switchable water blocking device, supporting shifter, test seal section and blowout preventer. This technology uses the shifter to change the working state of the hollow sliding sleeve to realize the water blocking or oil production of the corresponding layer, and the shifter is inserted once to open or close one water blocking device.

2.2.2. Annular water blocking techniques

In the 1990s, Daqing oilfield entered high water cut stage. In view of problems such as rapid water breakthrough in multiple layers, increase of plugging layers, and difficulty in selecting water plugging layer, the well logging and plugging technology (also known as pumping well sliding sleeve water blocking technology) and suspended zonal mechanical water-blocking technology that integrate zonal adjustment and water blocking were developed for high water-cut pumping wells.

The pipe string structures of the logging and plugging technology for high water cut pumping well and the suspended mechanical water plugging technology are shown in Figs. 8 and 9, respectively, both of them are release-sub-involved strings. There is a corresponding sliding sleeve switch in the release-sub-involved pipe string. When layer adjustment is required, an electric switch tester or shift switch is run in from the annulus between tubing and casing to turn on or off the sliding sleeve switch and thus adjust the production state. The above two technologies have similarities in pipe string structure and construction process, and have achieved good application effects in water plugging. Both of them are suitable for pumping wells with D140 mm casing and pump outer diameter of less than 90 mm.

Due to the high winding rate of steel wire or cable annulus operation^[17], the above two technologies are prone to failures such as obstruction, jamming or steel wire and cable winding tubing during operation in tubing and casing annulus. In addition, the tool can be run in annulus without disrupting the well pumping, but the extension and swing of the tubing string during the pumping process could cause the downhole tool to entangle, so the pump usually needs to be stopped during the water plugging operation. Although the instrument can be run into the annulus without tripping the pipe string, the operation still has a certain impact on the production of the oil well.

In the 21st century, Daqing oilfield entered ultra-high water cut stage. Complex distribution of oil and water, highly dispersed remaining oil, and decrease in water cut difference between layers all added difficulty to the selection of wells and layers for water plugging. As the fluid production and water cut of all sections in a well constantly change during the development process, the plugging sections need to be adjusted accordingly. The water detection and plugging technology developed in the medium and high water cut period requires moving the pipe string during the operation, and the operation process complicated and low in adaptability couldn't meet the development needs of high water cut and ultra-high water cut stage anymore. To solve these problems, a series of water plugging/ allocating technology that can adjust production layers hydraulically and through annulus cable controlled zonal oil production technology have been developed and applied. According to the difference of the types of downhole water blocking devices, the hydraulic layer- adjustment water blocking/ allocation technology can be divided into hydraulic layer-adjustment water blocking technology and pressure wave controlled zonal production technology. The downhole actuator of the former is a mechanical water blocking device, and the downhole actuator of the latter is an electronically controlled switch device. The pressure wave controlled zonal production technology is commonly known as the piezoelectric switch zonal production technology. The technology is essentially to send the adjustment command to the downhole allocator through pressure changes. In order to distinguish from the piezoelectric effect and avoid misunderstandings, in this article this technology is called the pressure wave controlled zonal production technology.

At this stage, in addition to the above-mentioned zonal production technologies, a separate production pump zonal oil production technology was specifically developed for wells with two layers in commingled production but of large pressure difference^[18,19], and a dual-tube cable-control connection zonal production technology was developed to overcome the difficulty in tool-running in the annulus^[20].

2.3.1. Hydraulically layer adjustment water blocking technique

The downhole mechanical water blocking device uses the pressure difference between the wellhead and the bottom of the well to drive the down sleeve to switch (Fig. 10), and the ground pump truck is used to apply pressure to the annulus between tubing and casing to control the action of the downhole mechanical switch to plug water. All layers are adjusted synchronously according to the mechanical design. Each time the pressure is increased, the state of all the water shutoff devices in the well can be adjusted to obtain a new water blocking scheme. Each adjustment takes 2 to 4 hours and the driving pressure difference is 10 to 15 Mpa^[21,22]. According to the different structure of the sliding sleeve switch, some mechanical water blocking devices can only be set in a single-layer open state^[23], and some can switch on and off at multi-layers arbitrarily^[24].

This operation can realize the adjustment of production layers without moving the pipe string, greatly simplifying the operation process, and had good effect in the preliminary pilot test. However, limited by the tool structure, this technology is only suitable for oil wells with no more than 4 sections. Considering the success rate of the operation, generally this technique is mostly used to for wells with 3 sections, so it cannot meet the development needs of refined division of development layers. In addition, after many adjustments, the state of sliding sleeve switch could be in confusion, limiting its scale promotion and application.

2.3.2. Pressure-wave controlled zonal oil production

In the pressure-wave controlled zonal production technology, the control instructions are encoded and transmitted in the form of pressure wave to the downhole electronic control valve through applying pressure at the wellhead. The electronic control valve decodes the pressure wave signal, and the motor drives the valve to complete water blocking operation^[25,26]. The pipe string structure of the system is shown in Fig. 11. The pressure wave signals are generally 2 to 5 MPa in pressure difference, and are coded by the duration of high and low pressures. In addition to adjusting the open and close states, some electronically controlled valves can also have their opening adjusted remotely to achieve zonal production. In addition, this technology can also achieve long-term downhole zonal pressure testing^[26], which has a certain significance for understanding the dynamic changes of the reservoir, but data playback can only be achieved after the pipe string is pulled out, and isn’t timely.

Compared with the downhole mechanical water blocking device, the number of layers is not limited when using the pressure wave control zonal production technology, but the working life of the downhole electronic control valve is limited by the battery life. The short working life of the device makes it unable to reach the goal of long-term effective adjustment. But to tap the potential of the replacement layer requires a dynamic adjustment process, which requires the water plugging process able to be adjusted in long-term. In addition, the technology cannot measure the production and water cut of each layer, and making it difficult to manage the oil-producing layers dynamically, moreover it cannot provide basis for fine geological analysis and tapping remaining oil in time.

2.3.3. Through-annulus wireline-controlled zonal production

The system of through-annulus cable-controlled zonal oil production is mainly composed of surface control system, downhole testing instrument and downhole distribution pipe string (Fig. 12). The test and adjustment instrument is run in from the annulus between tubing and casing and docked with the adjustable allocator. While the real-time zonal flow is directly read on the ground, instructions can be sent from the ground to adjust the opening of the downhole adjustable allocator to achieve zonal production. The pressure gauge is run in through the annulus between tubing and casing, and sit into the allocator with the help of the positioning arm accurately, and then the delivery device is separated from the pressure gauge. At this time, the double cup structure is used to seal the single layer to realize the sealing pressure measurement. After the test is completed, the pressure gauge is fished out with the fishing device. In addition, the technology is also equipped with a water cut tester that can be run in from the annulus between tubing and casing, which can measure water cut in real-time. The annulus cable-controlled zonal oil production technology enables long-term dynamic adjustment of liquid production at any section downhole. It is suitable for pumping wells with intact casing and annulus test conditions, and meets the adjustment needs of multi-layer wells with high and constantly changing water cut. But as the test instrument needs to be run in through the annulus between tubing and casing, it is limited in application scope, and has risks in instrument running and fishing etc. Moreover, zonal measurement and adjustment of pressure and water cut need to run in different instruments separately and take long time to complete.

2.3.4. Water blocking in horizontal wells

In order to increase the contact area between the wellbore and the oil and gas reservoir and achieve the purpose of increasing production, more and more horizontal wells are drilled, and play more and more important role in oilfield development. However, after multi-stage fracturing, the fractures may connect to high water-cut layers. After the well is put into production, water will advance along the high-permeability channel, resulting in an increasing rise of water cut and complex oil-water distribution^[27,28], and water blocking is needed. Horizontal well water control methods mainly include mechanical and chemical methods. Mechanical methods can also be divided into pure mechanical plugging technology^[29,30] and hydraulic layer-adjustment water plugging technology. The latter with no need to move the pipe string has been widely used in oil fields. It has solved the problems of mechanical sliding sleeve jamming, unable to realize conventional steel wire operation, and high operation risk and high cost of coiled tubing operation, and has been effective in controlling water cut and increasing oil production^{[31,32,33,34]}.

Mature oilfields in China have entered the middle and late stages of development. To effectively control rise of water cut and further tap the remaining oil potential, it is necessary to improve the separated layer control level of oil production wells in high water cut oil fields. Under this background, electrical control zonal oil production technologies such as cable control and vibration wave control technologies have emerged to realize online real-time monitoring of downhole zonal production, water cut, pressure and other characteristics. The large amount of monitoring data collected continuously by these technologies provides the basis for analysis and optimization of reservoir performance. Since the high water-cut layers are usually the main producing zones, full opening of downhole nozzle would aggravate the development imbalance of the layers, while direct plugging would cause reduction of single well production and the profit of the well. The above- mentioned electronically controlled zonal oil production allow not only arbitrary adjustment of downhole nozzles, but also real-time adjustment according to the development plan, truly turning "water blocking" into "water control". Zonal oil production has really entered the "intelligent" development stage. However, due to the problems of small application scope, short service time, and high cost, it is still in the exploratory stage.

2.4.1. Pre-installed cable zonal oil production technology

The pre-installed cable zonal oil production technology can adopt a monolithic string or a release-sub-involved string structure. Fig. 13 shows the monolithic pipe string structure. Its oil production pipe string and the zonal production pipe string are integrated. Therefore, it only needs to run the pipe string once. After the cable-crossing packer is set by increasing pressure, the pump can be lowered to start production. The cable is laid on the outer wall of the tubing from the wellhead to the bottom of the well and connects to the allocator of each layer. The wellhead control commands are transmitted to the allocator of each layer downhole through the cable to realize zonal production. The cable supplies power to the allocator of each layer in the well, and is also the carrier of two-way communication. The flow rate, pressure in front of the nozzle, pressure behind the nozzle, temperature and other data of each layer in the well can be transmitted to the ground in real time, and through ground remote transmission equipment (such as GPRS) they can be sent to the production control center, and the production control center can then adjust the downhole nozzle according to the production dynamics. This technology realizes the real-time monitoring of zonal production allocation and downhole state parameters, and is a new generation of electronically controlled zonal oil production technology. However, with monolithic pipe string and cable running through the whole well, it can only be used with rod pumps with smaller displacement, limiting its application range. When the pump or downhole allocator fails, the entire pipe string needs to be lifted out.

The release-sub-involved pipe string is divided into wireless docking cable controlled zonal oil production string and wet docking cable controlled zonal oil production string. Fig. 14 shows the structure of wireless docking cable-controlled zonal oil production string. The zonal allocator is placed downhole by the release-sub-involved string, and a relay unit is installed on the top of the release-sub-involved string. The relay unit communicates with each zonal allocator through cable, and it wirelessly communicates with the downhole tool through the annulus, to realize two-way real-time communication between the downhole and the ground. According to the conditions of the production well, the downhole tool can be set through the annulus for a short time, or fixed outside the production string and run in or taken out together with the production string, without lifting the zonal production string, when the pump is checked. Wet docking zonal oil production technology uses wet docking joints to physically connect the production pipe string and the release-sub-involved pipe string to meet the power supply and communication requirements of the downhole allocator and realize zonal oil production. The long-term reliability of this technology is affected by the wet docking joint, so its service life needs further test. The release-sub-involved pipe string structure eliminates the need to lift and lower the zonal production pipe string when inspecting the pump, so it is simpler in operation process and can be used in a wider scope.

2.4.2. Vibration wave controlled zonal oil production

Vibration wave controlled zonal oil production technology is a new type of zonal production technology developed in recent years. The pipe string is a release-sub structure, mainly composed of suspended packers, zonal packers, downhole zonal production allocators and plugs (Fig. 15). The core of vibration wave controlled zonal production is the vibration wave two-way communication technology^[35,36,37]. The ground control system installed at the wellhead and the downhole zonal allocator are both equipped with a vibration signal generator and a micro-vibration acceleration sensor. The former is responsible for generating vibration signals, which are transmitted down or up along the casing, the latter receives the vibration signals and decodes them to realize instruction implementation or data reading.

When the ground needs to monitor downhole data or adjust downhole parameters, the surface software control center wakes up the downhole zonal allocator through the ground vibration signal generator, establishes a one-to-one communication link, and verifies each frame of data during the trans-mission process to ensure that the data is correct. The downhole zonal production allocator adjusts the opening of the nozzle according to ground instruction to realize zonal production, or upload data such as downhole flow, pressure, valve opening, temperature, etc., to realize downhole state parameter monitoring.

This technology has the characteristics of simple operation process and low risk. It is suitable for pumping wells, electric pump wells, screw pump wells, and is applicable in vertical wells, inclined wells, horizontal wells and other well types. It is similar with pressure wave control zonal production technology in pipe string structure and control method, but it can realize two-way communication, and only a small vibration signal generator is required to transmit commands, making its construction easier; besides its signal transmission is faster and less time-consuming. Since the power supply on the ground is not limited, its downward transmission of the vibration wave signal is relatively stable, but the signal transmission upward still faces technical difficulties: (1) The output of the downhole vibration signal generator is limited due to the low power of the downhole battery. (2) The signal upload is greatly affected by the location of the vibration signal generator and the structure of the pipe string. (3) When the signal reaches the ground, it would be interfered by ground noise. Under the influence of these factors, the current upload distance can only reach about 1000 m, which cannot meet the needs of deep wells and massive data transmission.

The water plugging methods used in oil wells are divided into two categories: mechanical water plugging and chemical water plugging. The concept of mechanical water plugging refers to the use of packers and supporting control tools to seal oil layers with high water-cut to relieve the interference between oil layers or adjust the water absorption profile of the water injection well, and change the direction of the injected water on the plane to improve the water flooding efficiency, increase oil production, and reduce water output^[38]. In short, mechanical water plugging refers to the use of physical methods in the wellbore to discover and block high water-cut layers. The concept appeared early, mainly means using pure mechanical tools for water blocking. Therefore, mechanical water plugging can also be understood as mechanical water blocking. With the advancement of science and technology, electronically controlled water plugging technology has emerged, which can not only realize water blocking, but also adjust the opening of nozzles to some extent, that is, zonal production allocation. Although this type of technology is still called “mechanical water plugging technology”, its technical connotation has actually exceeded the scope of mechanical water blocking, and the realization methods are beyond mechanical means too, especially after the emergence of intelligent zonal production technology, the opening of downhole nozzles can be adjusted arbitrarily and downhole conditions can be monitored in real-time. Mechanical water plugging has played an important role in the development process of oilfields. For a certain period of time, “mechanical water plugging” even became synonym of “zonal oil production”. However, mechanical water plugging is actually a development stage of zonal oil production. With the continuous development and advancement of technology, the technology beyond the connotation of mechanical water plugging should be attributed to the concept of “zonal oil production technology”.

The development of zonal oil production technology has met the production needs of various stages of oilfield development, and to a certain extent solved the stage production contradictions. In view of the problems of low control level by layer of production wells in high water-cut oilfields, difficulty in finding and plugging water layers, high operation costs, and lack of effective long-term monitoring methods for downhole parameters, the future zonal oil production technology should continue to develop in the intelligent direction, which is specifically reflected in the improvement of technical level and adaptability, construction of a comprehensive platform for zonal oil production technology and management, and integration of injection and production.

At this stage, Daqing oilfield has prominent contradictions between layers and complex distribution of oil and water, to judge high water-bearing zones by static data analysis is low in accuracy, and cannot cope with the quick dynamic changes of production status. Therefore, the real-time monitoring level of zonal oil production technology should be improved to realize long-term monitoring of downhole zonal flow, pressure, water cut and other parameters, to get a better understanding on the reservoir, and provide accurate basis for fine reservoir analysis and remaining oil tapping. Meanwhile, when the production plan needs to be adjusted to cope with the dynamic changes of the reservoir, the zonal oil production technology should have real-time adjustment capability. At present, zonal production technology still faces technical difficulties in downhole sensing, two-way communication, downhole tool life cycle, and horizontal well adaptability.

3.1.1. Downhole sensing techniques

Compared with the downhole environment and working medium of zonal water injection, the downhole working environment of oil wells is very harsh, the well fluid may be a mixture of two or more in oil, water, gas, and sand; and the flow state fluctuates greatly under the influence of pumping state. This harsh working environment could cause contamination, corrosion, and even structural damage to the sensitive components of sensors, directly impairing the accuracy, stability and service life of the sensors. At present, except the mature temperature and pressure sensing technology, flow rate and water cut sensors are mostly imported from the fields of water injection and well logging, which have not been verified in accuracy, stability and reliability for a long time. Therefore, key sensing technologies applicable to oil wells should be developed, with the focuses on improving the adaptability of downhole sensors to the unstable flow of oil wells and the ability of the sensing components to maintain stable performance in the long-term.

3.1.2. Downhole communication techniques

The production string in the artificial lift well has the characteristics of continuous casing, discontinuous tubing, discontinuous liquid, complex liquid composition, and noise interference from pump valves, as shown in Fig. 16. Since traditional wireless communication technology cannot be applied in this kind of well, downhole communication has always been a difficult point in realizing zonal oil production.

At present, three methods are commonly used to realize downhole communication, namely cable carrier, vibration wave and pressure wave. Cable carrier technology has problems such as high cost, complex on-site construction technology, and serious grid interference. Moreover, the long-term reliability of cable connection in this technology still needs to be verified. The vibration wave communication technology from underground to the ground has disadvantages such as short distance and limited data amount. Pressure wave signal transmission is affected by reservoir water absorption characteristics and water injection volume, slow in data upload speed, not guaranteed in success rate, and limited in data volume due to the battery capacity.

Taking into account the well structure and the large number of measurement and adjustment and large amounts of monitoring data, the reliability and convenience of cable connection should be researched, and the downhole relay communication technology should be developed. The transmission link should be divided, and the release-sub-involved pipe string should be controlled by cable to connect zonal allocators. A communication repeater may be set up on the top of the release string, through which the communication connection between the release string and the production pipe string is realized in a wired wet docking or wireless manner, and then it is connected with the ground controller in a wired or wireless manner. For oil wells with limited data needed transmission, the vibration wave relay communication technology should be developed, high-efficiency signal generators, signal coding and data compression technology should be developed, so under the condition of limited battery capacity, the service time of downhole instruments can be extended as much as possible.

3.1.3. Full life cycle service capability

Improving the service capability of downhole zonal production allocation tools and avoiding them from becoming a short plank of production allocation pipe string mainly includes two aspects of work: (1) Improving the integrity and reliability of the technology mainly includes improving the long-term stability and reliability of the measurement and adjustment components and providing sufficient power. Downhole zonal production is mainly realized by driving nozzle with motor, and the adjustment is easily affected by factors such as downhole pressure changes, flow channel scaling, sand jam, etc. The mechanical structure and material of zonal production device should be optimized to meet long-term measurement and adjustment needs. Electricity is the basis for real-time monitoring and adjustment of downhole allocators. Except for the pre-installed cable zonal production technology, all downhole allocators of other technologies are powered by batteries. However, the high-energy lithium batteries currently in use can only meet the requirements of limited measurement and adjustment frequency and data transmission, and are not suitable for frequent measurement and adjustment and large amount of data transmission. In order to break the downhole power supply bottleneck, it is necessary to develop large-capacity batteries and downhole power generation technology to provide sufficient power for the downhole zonal production tool, so that the zonal production tool has the ability to serve in a whole production cycle, and thus has production application value. (2) Simplify the one-time operation process, reduce or avoid later operations. The zonal oil production process requires regular testing and adjustment of the production status, and a testing team is required for every time of testing and adjustment under the current technical conditions. The future zonal oil production process should be simple, fast, and highly adaptable to the environment, so as to reduce the downtime of oil wells and improve production efficiency; subsequent testing operations should be completed by one person with portable tools, or even by remote control in the office.

3.1.4. Applicability to horizontal wells

With the maturity and popularization of horizontal well development technology, horizontal wells take a more and more important position in oilfield development. The horizontal well pressure wave control segmented water control technology applied in the early stage was effective at the initial stage, but it also had shortcomings such as short battery life, susceptibility to impact of abnormal formation pressure, and limited valve opening. Currently, there is no mature zonal oil production technology suitable for casing horizontal wells, so related research and development are required. If connection reliability and service life are improved, the cable-controlled zonal oil production technology for horizontal well has a good development prospect. It can provide plenty downhole monitoring data and realize real-time adjustment of downhole development status, making it one of the ideal technical means to realize intelligent reservoir development. For horizontal wells with stable production conditions, good segmentation, and small data volume needed for transmission, the vibration wave control zonal oil production technology has better adaptability, simple operation and low cost. In addition, horizontal well mechanical water search and chemical water plugging should be developed based on intelligent zonal production technology to overcomes the problem of horizontal well testing instrument delivery^[39] and realize rapid horizontal well fluid production profile testing under the premise of reducing disruption to production.

The AICD-based horizontal well balanced production technology has the advantages of simple tool, long life, convenient construction, self-adaptive adjustment, and low overall cost. It is an ideal balanced production technology for high-production horizontal wells with low requirements on real-time monitoring and adjustment. In the future, the application of AICD in the completion stage should be promoted to carry out preventive water control; when AICD is applied as a later water control method, the geological conditions of the reservoir should be analyzed carefully, so that the AICD can control the water cut effectively.

Combining technologies such as downhole monitoring, surface control, and remote transmission for zonal oil production and daily production management and operation items of oil wells, a comprehensive platform for zonal oil production technology and management can be built.

For oil wells that have realized digitization and remote control of the pumping unit, the digital artificial lift system can be integrated with the zonal oil production system, and the downhole monitoring data can be fully utilized for functional expansion such as real-time monitoring of the fluid level to realize the automatic control during the whole process of the pumping unit; at the same time, the combination of the two can build a more comprehensive platform for zonal oil production technology and management and realize integrated management of ground and downhole. For most of the oil wells with daily production management operations completed manually, the daily management can be combined with the zonal oil production system, for example, downhole zonal flow rate and zonal water cut monitored can replace the required test of oil volume and water content every 10d (different in different oilfields).

On the basis of making full use of the monitoring data of zonal oil production and appropriately expanding the functions of the surface control system, the combination of intelligent zonal oil production technology and daily oil well management can realize the automation and intelligence of oil well production, reduce manual operations, and lower the overall production cost of the oil well platform.

During the development process, the relationship between injection wells and effective production wells is complex on the plane and intervals, where multiple sets of well patterns are intersected, the connection state of geological analysis is not in good agreement with the actual connection state, and the layer system is not well sealed. Many oil production wells are affected by the phenomena such as production without injection and intertwined oil layers, which leads to uneven production of oil layers and poor development effects. At present, affected by the overall development concept of the oilfield, technological maturity and adaptability, investment cost and other factors, the water plugging and production allocation technology are implemented in individual wells in small scale; in addition, the zonal water injection and zonal oil production essentially belong to the same system, but they are often implemented separately, failing to reach synergistic effect.

In the future, the collaborative application of zonal oil production and zonal water injection should be developed, and the integrated zonal production and injection scheme should be designed to strengthen the corresponding analysis of the downhole sections between the production end and the injection end, that is, the continuous, long-term, and abundant downhole monitoring data of multiple layers in the injection end and the production end of the same block should be used to carry out big data-driven fine geological modeling, to obtain reservoir fluid saturation and pressure field evolution models under the constraints of zonal injection-production real-time data, and deepen the understanding of reservoir heterogeneity and flow pattern, and reduce the uncertainty of remaining oil distribution prediction. Finally, real-time adjustment technology is used to adjust the parameters of the injection end and the production end to realize the transformation of development adjustment from "delayed control" to "real-time optimization", to improve well pattern control and adjustment level, control natural decline and rise of water cut, and increase producing degree and recovery factor. Furthermore, on the basis of wellbore injection and production automation and massive data processing, artificial intelligence is used to develop a reservoir analysis and optimization system to realize "smart reservoir management" in the true sense.

The zonal oil production technology can effectively control and dynamically adjust production rates of different layers, to alleviate the interlayer and planar conflicts, expand the swept volume, reduce ineffective and inefficient water circulation, tap potential of remaining oil, and ultimately improve the recovery factor and the overall development performance and profit of the oilfield. Zonal oil production technologies abroad are represented by the inflow control technology and intelligent completion technology, while zonal oil production technologies in China have gone through a development process from zonal production allocation of flowing wells to water blocking of pumping wells, layer adjustment and production allocation and finally to intelligent zonal oil production. From the technical point of view, zonal oil production is the last step of various water control measures, and zonal oil production and zonal water injection play equally important roles in improving waterflooding recovery. However, so far, oil wells with water blocking measures account for a very low proportion of the total number of pumping wells, and oil wells adopting the zonal oil production technology are even rarer. In the future, the zonal oil production technology should be directed at improving the technical level and applicability, building a comprehensive technical and management platform for zonal oil production, and integrating injection and production. Moreover, the promotion and large-scale application of such technologies should be strengthened. Zonal production can be combined with refined zonal injection, to form a new development model of tapping the potential of well groups and blocks as a whole. This model will provide effective technical support to the sustained stable production of high water-cut mature oilfields.