Since the “National 13^th Five-Year Plan of China”, 70% of the newly proven oil reserves and 90% of newly proven natural gas reserves of China are mainly low-grade unconventional resources ^[1], which normally refer to shale oil and gas, tight oil and gas, coal seam gas, hydrate, heavy oil, oil sand and so on. It is estimated that the amount of the recoverable unconventional reserves of China are up to 55×10⁸ t, which include 20×10⁸ t tight oil and 35×10⁸ t shale gas ^[2]. Enhancing the effective use and efficient development of these resources has become one of the most important guarantees for the development of the oil and gas industry of China. Horizontal well multi-stage fracturing technology can effectively expand the drainage area, improve the rate of oil recovery and final cumulative production. Therefore, it plays a central role in the exploitation and development of shale oil and gas resources.

Considering the characteristics of shale oil and gas resources at home and abroad, this paper sorted out 6 new developments in horizontal well fracturing technology in North America and 6 new technologies in improving efficiency of horizontal well fracturing in China. According to the demand and future development direction of horizontal well fracturing technology in the "National 14^th Five-Year Plan of China" regard to shale oil and gas, 7 development proposals have been put forward, hoping to provide some technical references for the effective development of shale oil and gas resources in the future.

Since the application of horizontal well staged fracturing technology in North America in 2002, horizontal well multi-stage fracturing technology has experienced a rapid development ^[3]. By the time of 2009, the total number of completed gas well in the Barnett shale reservoir in the Fort Worth basin had reached 13 740, of which 3694 were new wells, 3531 were horizontal wells, and over 95% of these horizontal wells were new-drilled, which had a significant contribution to NA’s shale oil and gas revolution ^[3]. According to the EIA’s (Energy Information Administration) report, the number of fractured horizontal wells in North America exceeded 153×10⁴ in 2020 ^[4]. The horizontal well multi-stage fracturing technology is ripe for making new progresses in improving the producing degree, fracturing parameter optimization, increasing production and tapping remaining reserves, reducing costs, and diagnosing and evaluating cracks.

In order to cope with the problems such as low producing degree of single-layer development in stacked multi-layer shale reservoirs and poor economic efficiency under low oil price, a new concept ^[5,6] of three-dimensional development of whole reservoir by horizontal wells called “stacked pay pad development” or “tank development” was emerged in North America. It mainly evaluates the physical properties, geological mechanics, and stress profile of multi-layer reservoirs, and optimizes the operation parameters with the three-dimensional morphological analysis and dynamic simulation results of artificial fractures, and makes full use of permeable channels between fractures, stages, wells and layers, improving the complexity and control volume of the fracture system, achieving a one-time three-dimensional development of oil and gas reservoirs ^[7]. Three key technologies are used to reach this goal: (1) Low-cost high-speed drilling technology. Currently, the average drilling rate in North America is between 1000-1600 m/d, and the maximum drilling footage of a bit in one trip can reach 5500 m. The drilling process only takes 21%-34% ^[8] of total construction expenditure comparing to 60%-80% in the early days. (2) Optimized design of 3D well staggered layout. The vertical horizontal well spacing is optimized according to sweet spot, favorable sections and simulated heights of the artificial fractures, where the fracturing parameters and lateral spacing is optimized and determined in terms of achieving a successful one-time well deployment by simulating the length of artificial fracture, optimizing fracturing factors after fitting the production history and confirming the horizontal well spacing. (3) Factory fracturing operation is carried out based on research results of time and space evolution of the three-dimensional stress field, implements staggered fractures layout and adopts multi-layer 3D zipper fracturing to achieve a vertically "full coverage" of oil and gas reserves. For example, in 2014, Carrizo Inc. carried out a 3D fracturing test in 2 layers (Layer A and Layer B) of Niobrara shale oil reservoir. Wells in Layer A (with better geologic conditions) had an average production rate of 180 m³ (1135 bbl) per day after fracturing, while wells in Layer B (with poorer conditions) had an average production rate of 168 m³ (1057 bbl) per day after fracturing ^[9]. The better and poorer reservoirs had similar development results through 3D fracturing ^[9].

“Frac hits” and “frac bashing” often occur during shale oil and gas well fracturing process due to stress interference ^[10]. Study shows that integrated research including fine reservoir description, geomechanical characteristic evaluation and fracturing operation process optimization is a new way to solve the problems. The interference and connection condition of artificial fractures in horizontal wells can be figured out through dynamic reservoir production analysis, fracture monitoring and diagnostics, and production history data matching, then, the well pattern and spacing can be optimized accordingly. With promotion of the small spacing and dense well pattern in North America, well spacing has reduced from 400 m to 200 m in general, and 76 m at minimum ^[11]. By narrowing the well spacing from 300 m to 100 m in the Eagle Ford block, Carrizo Inc. expected a 64% increase in single-well accumulated production and a 580 million rise in NPV ^[12].

With the increase of horizontal well fracturing stages, optimization of fracture spacing has become one of the most important contents of fracturing parameter study. Evaluation results of fracture spacing in horizontal well and dynamic production after fracturing by reservoir productivity coefficient method and fuzzy mathematics have confirmed that there is a strong correlation between the density of artificial fractures and the controlled geological reserves, recoverable reserves and economic benefits of a horizontal well ^[13]. Analysis of post-frac production data of over 3000 shale oil horizontal wells in American Permian basin with statistics and fuzzy mathematical methods showed that daily production rate of a single well would increase by 132% by reducing the cluster spacing of fractures from 23 m to just 6 m. But the increase of daily production and EUR (estimated ultimate recovery) slowed down when reducing the cluster spacing further to 3 m, and the cost of investment increased by over 60% ^[14]. The statistics of production of American Eagle Ford shale oil reservoir also showed that NPV (net present value) and IRR (internal rate of return) of the reservoir increased by 1748% and 214% when the cluster spacing narrowed down from 18 m to 9 m, but the increases of daily production and EUR were slower, and the operation costs was increased dramatically as well. Clearly, the cluster spacing is the key factor influencing the production rate of horizontal wells and narrowing fracture spacing within a reasonable range can effectively boost the production rate and economic profit.

To deal with problems such as insufficient reservoir fracturing and low production in early horizontal wells, nonlinear flow and heterogeneous geological models, history matching of initial fracturing production performance, and reservoir numerical simulation are currently used to figure out the distribution of residual oil after the initial fracturing. Meanwhile, the in-situ stress field of horizontal well, the fracture generated conductivity and penetration rate of re-frac can be evaluated. In addition, a series of technologies such as wellbore reconstruction and small casing secondary cementing have been developed for re-fracturing ^[15]. Wellbore reconstruction mainly benefits from the expandable tube technology, which can be used in wellbores with casings from 11.43-13.97 cm. The reconstructed casings have an 8.28-10.69 cm inner diameter and can resist internal pressure up to 89 MPa and isolate 10-2000 m long wellbore sections. Up till now, over 1100 wells in North America have been treated by expandable tube refracturing and expected a 100% to 150% increase in single well EUR ^[16].

Fracturing fluid and proppant material costs have taken over 35% of total cost due to the increase scale of shale reservoir fracturing ^[17]. Developing low-cost materials has become the key factor to the economical fracturing and profitable development of shale oil and gas reservoirs. Under the research and development of fracturing fluid, through the study of reaction mechanisms between shale and water and resistance reduction mechanisms of liquid, the fracturing fluids with polyacrylamide as main functional group have been upgraded to form a series of slickwater systems ^[18]. The application rate of slickwater in North America had risen from 51.8% in 2013 to 95.0% in 2019 (Fig. 1). Slickwater makes up over 95.0% of the fracturing fluid in horizontal well fracturing in shale oil in Permian basin and Haynesville shale gas ^[19].

In terms of proppant, big data statistics, lab- scale conductivity test, artificial fracture modeling and comparison of well groups treated with different types of proppants have been used to evaluate the matching rela-tionship between different types and sizes of proppants with reservoir flow requirements. The estimated fracture permeability and dimensionless conductivity are determined by using production history matching, DFIT (Diagnostic Fracture Injection Testing), and thousands of conductivity test results. Finally, a “just good enough” concept ^[20] was proposed for the conductivity modification of shale oil and gas resources, which has promoted quartz sand to become the major proppant and increase its application scale. The quartz sand used in fracturing in the U.S. in 2019 was 8030×10⁴ t (Fig. 2), which was over 10 times of 2010, and 92% of it was used in fracturing of shale oil and gas wells ^[21]. In the meantime, oil companies in North America took a local-sourced-sand strategy, looking for quartz sand mines that close to the basin blocks to lower the transportation and the proppant cost. Taking American Permian basin for an example, by building a local sand plant, the cost of quartz sand in the oilfield has dropped from 180 USD/t to 50 USD/t, 22×10⁸ USD was saved in 2018. The lowering of proppant cost has already become an effective way to reduce the cost of shale oil to 30 USD per barrel and still achieve a profitable development ^[22].

Although microseismic, distributed fiber, and gradio-graph are commonly used in post-frac evaluation, the accuracy of quantitative description in artificial fracture locations by microseismic technology is affected by reservoir parameters, received signals, and signal-to-noise ratio, etc. Optical fiber monitoring can only reflect the fracturing effect of wellbore perforations and near-wellbore zone. Recently, a tracing technology called “surface-based, controlled-source electro-magnetics” has been developed by a North American oil company ^[23] to quantitively describe artificial fractures more accurately. The injection of fracturing fluid into the formation will cause variations in rock resistivity, fluid conductivity and magnetic field of the reservoir. This technology monitors the magnetic field changes at surface during the injection period to evaluate the shape of artificial fractures and provide information of the dynamic distribution of fracturing fluid during fracturing process in single or multiple wells, establishing a new assessment approach for scientifically optimizing fracturing plan.

Multi-stage horizontal well fracturing technology in North America has brought significant improvement in fracturing parameter optimization and post-frac analysis. At present, one well-pad (with an area of 2.5 km²) can have up to 51 horizontal wells, with horizontal well spacing at only 150-200 m, horizontal length at 5888 m and average artificial fracture spacing for only 6 m ^[12]. For example, Well Purple Hayes1H, a horizontal well with 8244 m in depth and 5652 m in lateral length, took 23.5 d to complete a 124-stage fracturing at 5 clusters per stage^[24]. The multi-stage fracturing technique has directly promoted shale oil and gas production in North America. In 2020, the U.S. had a shale oil production at 3.94×10⁸ t, accounting for 65.1% of the total oil production; and a shale gas production at 6320×10⁸ m³, accounting for 66.0% of the total gas production of the year. The multi-stage horizontal well fracturing technology has become an important approach of improving shale oil and gas production ^[25].

In 2006, CNPC launched a major research project in reservoir stimulation through horizontal wells. Through importation, digestion, absorption and innovation, a main reservoir stimulation technical system of "long horizontal well completion + multi-cluster perforation + slickwater carrying sand + staged fracturing" has been established ^[26]. Sinopec started a technical research program on shale gas horizontal well drilling, completion, and staged fracturing in 2009 as well, and a set of horizontal well fracturing technologies suitable for mid-to-deep shale gas reservoirs up to 3500 m deep have been carried out and put into industrial application ^[27].

Compared to marine reservoirs in North America, China’s shale oil reservoirs often have more complex lithologies, lower pressure coefficients, higher crude oil viscosity, lower fracture development degrees, stronger heterogeneity, and larger difference between two horizontal in-situ stress. Drawing the experience from horizontal well multi-stage fracturing in North America, volumetric fracturing depletion development has become an important means in shale oil and gas reservoir stimulation, but we are still facing issues such as rapid production decline after fracturing (the decline rate of greater than 50% in the first year), low recovery rate (less than 20% of shale oil reservoir, less than 30% of shale gas reservoir), and low ultimate production. Therefore, it is necessary to carry out a series of basic technical research in terms of fracture initiation and extension, stress field distribution and energy supplement.

In the simulation of multiple complex fractures extension, in light of strong heterogeneity of shale oil and gas reservoirs in China, three-dimensional hydraulic fracture propagation experiments under stratified stress loading have been carried out on large rock samples (762 mm× 762 mm ×914 mm) with natural fractures and beddings under lab conditions in physical simulation equipment, the propagation model of complex fractures including natural fractures and horizontal beddings has been established, and the optimization chart of hydraulic fracture parameters for reservoirs with natural fractures and beddings was drawn to provide a foundation for the optimization of fracturing parameters of complex shale reservoirs ^[28]. In the optimization of horizontal well fracturing design, the geo-engineering-integration fracturing software FrSmart has been developed ^[29], which establishes a fluid-solid coupling non-planar three-dimensional fracture propagation model based on the boundary element and finite volume method and adopts adaptive time step and parallel computing algorithms to improve calculation efficiency in fracture simulation. Under the same condition, this software is over 30% more efficiency than the plane 3D model of commonly used fracturing software and is expected to accomplish complete independence in domestic software development.

In recent years, in line with the special conditions like low pressure coefficients (0.7-1.0) and limited natural energy of Ordos Basin shale oil reservoirs, a fracture-reservoir simulation method coupling stress, pressure and seepage field has been established, and the spontaneous imbibition displacement and surfactant migration and diffusion models have been introduced to deepen the study on artificial fracture propagation pattern and the mechanism of liquid energy supplement. A stimulation model integrating fracturing and energy replenishment by liquid injection is able to realize artificial fracture expansion, crude oil imbibition displacement and liquid energy replenishment (with mainly water, and CO₂ in some reservoirs) has been worked out, which can make the formation energy increase by 10% to 30% and the average daily oil production per well after fracturing increase by 78.3% ^[30].

For reservoirs with large differences between two horizontal in-situ stress (greater than 10 MPa), poor brittleness (brittleness index less than 50%), and poorly developed natural fractures, fracture-controlled fracturing technology with dense cutting as the main feature has been worked out ^[31]. In this technology, by examining four relationships, namely that between "rock properties and fracture propagation, horizontal section length and fracture density, reservoir fluid flow and fracture flow coupling, matching between artificial fractures with well pattern and spacing", parameters such as length, spacing and height of artificial fractures are optimized according to reservoir physical properties, stress, and well-controlled reserves. This technique has become a core technology for the stimulation of unconventional and low- permeability oil and gas reservoirs, boosting the economic development of unconventional resources. 283 wells in multiple blocks of Changqing, Xinjiang and Southwest oilfields have been treated by this technology, the average daily output per well is 1.8 to 2.6 times more than that of the control wells.

By using a motor driven fracturing pump, electric drive fracturing equipment changes the traditional diesel engine drive to direct motor drive. Moreover, high-voltage substation system, frequency conversion multi-phase vector control, programmable logic controller, and remote operation are integrated to realize intelligent control of the electric fracturing pump ^[32]. The 5000 to 7000-series of electric drive fracturing equipment have been developed up till now, of which the 7000-series electric fracturing vehicle has the power output up to 5520 kW, a voltage of 6.6 kV, and a maximum pumping pressure of 138 MPa, the maximum pumping rate at 2.03 m³/min, resulting a 30% reduction in procurement cost, 25% reduction in energy consumption, 31% less usage in field land occupation, 28% less staff needed and operation noised drops from 110 dB to 90 dB showing advantages in "cost reduction, environmental friendly, high efficiency and localization". It has been widely used in shale gas reservoirs in Fuling, Changning and Weiyuan areas in Sichuan of SW China, and shale oil reservoirs in Jimsar area in Xinjiang of NW China and Gulong area in Daqing of NE China. In 2020, electric drive fracturing vehicles were used in nearly 6000 fracturing sections in the above-mentioned areas saving an average cost of 5×10⁴ yuan per section.

As horizontal well extends up to 5000 m long in some deep shale reservoirs drilling and grinding traditional drillable bridge plugs after fracturing are more and more difficult, so the soluble packers for staged fracturing are becoming an inevitable trend. Based on the basic electrochemical corrosion principle of magnesium-aluminum alloy in high-salinity liquid environment, high-strength soluble magnesium alloy materials have been developed, dissolution mechanism, dissolution rate and influencing factors of the materials have been studied, and a series of fracturing tools such as soluble bridge plugs, time-delay soluble toe slide sleeves, soluble ball seats have been developed ^[33]. Among them, 10.16-13.97 cm multi-standard rubber cartridge and all-metal-series soluble bridge plugs have a controllable dissolution time of 7-14 d, with temperature resistance of 177 °C and pressure tolerance up to 70 MPa. Soluble tools like soluble bridge plugs have make up over 80% of staged fracturing tools used in shale oil and gas reservoirs in China, becoming mainstream tools for horizontal well multi-stage fracturing.

The cost composition of typical shale oil and gas horizontal well fracturing in Sichuan, Xinjiang and Ordos basins shows that fracturing materials take over 30% of the total cost of the fracturing process ^[34]. Thus, research on new fracturing materials and cost reduction around fracturing fluid and proppant has been carried out. In term of fracturing fluid, the evaluation experiment of lattice expansion of clay minerals in shale when contacting water has been conducted. The results showed that shale has much lower expansion rate than mudstone when contacting water, but water is conducive to the desorption of shale gas and formation of complex fractures to some extent, accordingly, a "low-concentration, low-damage, and reusable" slickwater fracturing fluid system has been formulated. With concentrations of drag-reducing surfactant of 0.05%-0.30%, comprehensive resistance reducing rate of 71%-77%, and recycle rate greater than 95%, this fracturing fluid system has been used in over 90% operations across several domestic shale reservoirs 50-80 yuan/m³ of fracturing cost can be saved by using this system ^[35]. In the aspect of proppant selection, drawing the experience from the North America’s replacing ceramsite with quartz sand and building sand plants near oilfields, the parameters of quartz sand were systematically evaluated to find out the effects on average diameter, closure stress, breaking rate of quartz sand on fracture conductivity. In the meantime, the dynamic performance of over 350 horizontal wells in Mahu depression tight oil in Junggar Basin, Triassic Chang 7 shale oil in Ordos Basin, and shale gas reservoirs in Sichuan Basin were used to analyze the stress condition on proppant in multi-stage fracturing horizontal wells. Under the same reservoir condition, the effective force on proppant in a multi-stage fracturing horizontal well is only 50%-60% of that in a vertical well. Therefore, the evaluation method for force on conventional proppant has been changed, providing basis for replacement of ceramsite with quartz sand. In order to further reduce costs, localization of quartz sand plant has been adopted. Quartz sand plants with a total annual production capacity of 200×10⁴ t have been built in the Junggar and Ordos basins of NW China, making the quartz sand cost reduced by 230-260 yuan/t, which is a 20%-30% decrease. Subsequently, the proportion of quartz sand used in fracturing went up from 47.9% in 2014 to 71.5% in 2020 ^[34].

In accordance with the new development concept of "multi-layer, three-dimension, large well cluster, and factory-frac", the 3D trajectory control of horizontal well, layout of horizontal wells for multi-layers from well-pad, longitudinal multi-layer stress field analysis, and factory-like fracturing mode have been studied, promoting a new model of multi-layer well layout and 3D fracturing on large well-pad. Considering the limited wellsite area in the Loess Plateau of the Ordos basin, according to the three-dimensional geological model of the Triassic Chang 6 and Chang 7 tight reservoirs, 22 horizontal wells for 3 sets of small layers were deployed on the well-pad H60. With an average horizontal section length of 1500 m, single layer well spacing of 300 m, these horizontal wells can produce 390×10⁴ t of geological reserves at once. This layout of wells can save 0.93 km² of land for every 100×10⁴ t production capacity and enhance fracturing time efficiency by 30% ^[36]. In addition, in the Ma-131 well block of Xinjiang Oilfield and Gulong shale oil reservoir of Daqing Oilfield, pilot tests have also been carried out. This new fracturing model is expected to become a new way for future shale oil economic development.

In summary, the ongoing exploration and development of shale oil and gas in China has brought about a substantial increase in horizontal well multi-stage fracturing operations. The annual fractured horizontal wells of CNPC have increased from 550 in 2016 to 1901 in 2020. The fracturing operations have a maximum well depth of 8008 m, a maximum horizontal section length of 4466 m, an average stage length from 800 m increasing to 1300 m, average number of clusters increasing from 2-3 to 6-12, and the cluster spacing reducing from 20-30 m to 10-20 m (4 m at minimum). The daily oil and gas production after fracturing has increased by over 20%, fueling the continuous production increase of shale oil and gas ^[37]. For example, 58 wells in Longdong shale oil development demonstration area of Changqing Oilfield, SW China, have been fractured, with fracturing clusters per stage increasing from 2-3 to 5-12, cluster spacing decreasing from 22-30 m to 5-12 m, fracture-controlled reservoir area from micro-seismic monitoring increasing from 50%-60% to over 90%, single well production increasing from 10-12 t/d to more than 18 t/d, and the first year production decline rate dropping from 40%-45% to less than 35%, achieving annual oil production capacity of 50×10⁴ t ^[38].

To deepen the work model of integrated geo-engineering platform continuously, aim at whole life cycle management of shale oil and gas horizontal wells, and achieve the maximum control of stimulation volume and economic recovery of sweet spots, geology, reservoir, engineering, and management, full life cycle management should be applied to each well and 4 integrated platforms need to be built: (1) Integrated evaluation platform including evaluations of geological conditions, sweet spots, mechanical properties and well completion quality. (2) Integrated model design platform dealing with geological, reservoir, fracture and economic models design. (3) Integrated analysis platform covering post-frac tracking, approach judgement, performance evaluation and model modification. (4) Integrated sharing platform for sharing experimental results, optimization schemes and operation designs. Through these integrated optimization research, the integrated management platform was built, intelligent management can be implemented to realize automation, intelligence and digitization to build modern production bases. In this way, the producing degree of reserves can be maximized, operation efficiency further improved, and land and water can also be saved to improve operation quality and efficiency and lower fracturing cost and guarantee the economic development of shale oil and gas.

In terms of basic theoretical research on fracture propagation and stress field simulation, the research on distribution of in-situ stress field, rock mechanical properties and fracture propagation laws should be further strengthened. Mechanical rock properties at high temperature, physical simulation of artificial fracture propagation in shale with large-scale beddings, and three-dimensional characterization of fracture distribution need to be studied to reveal the initiation law and control factors of fractures in complex reservoirs and laws of artificial fracture propagation in rock layers under different geological conditions ^[39,40]. For example, the Gulong shale oil reservoir with abundant lamellation and laminae, the effect of bedding on artificial fracture shape needs to be examined to find out proper fracturing parameters and main factors affecting fracture shape ^[41]. In the design optimization of horizontal well fracturing, fracture-controlled technology needs to be further improved; according to the investment payback period of 3-5 years, the fracture conductivity, cluster spacing, number of fractures, and fracturing scale are optimized to work out optimization method and design chart of staged fracturing parameters for horizontal wells in different regions and different reservoir conditions.

As horizontal well multi-stage fracturing goes towards a safe, green, efficient and intelligent direction, high- power electric fracturing equipment can not only meet the above requirements, but also has the advantage of performing a long time, high-pressure, large-displacement continuous operation. Therefore, the high-power intelligent electric drive fracturing pump, domestic variable frequency drive technology, high-voltage power supply and distribution system, fracturing fluid automatic mixing and transmission equipment, multi-platform power efficient dispatch and power quality adjustment system and fracturing unit intelligent operation system should be further invested, the technical level of 7000- series electric drive fracturing equipment should be improved, and electric drive fracturing equipment series above 7000-series should be developed.

For ultra-long horizontal section (over 4000 m) and wells with special geological conditions such as large depth (vertical depth over 3500 m), high temperature (more than 150 °C), and high pressure (more than 100 MPa), the rubber-free all-metal dissolvable bridge plug series for different sizes of casings (10.16-13.97 cm) and steel grades and better seating modes need to be developed to realize no-drilling and no-grinding of bridge plug and no-flushing after fracturing. High-efficiency long horizontal-section cluster perforating tool needs to be developed to achieve a safe perforation of more than 20 clusters in a single stage each time. Support equipment for over 8000 m continuous-tubing fracturing needs to be developed to ensure that the injection head has lifting force of greater than 70 t and downward thrust of greater than 30 t, in order to meet the fracturing requirements of ultra-long and ultra-deep horizontal wells, improving the special operation capabilities in deep wells.

According to statistics, at the end of the "14th Five-Year Plan" period, 20% of the horizontal wells will face the needs of re-fracturing, which is an effective way to settle rapid production decline and premature water breakthrough. But some old oil reservoirs have complex residual oil distribution, in-situ stress state, and complex oil-water relationship, making it difficult to produce the remaining oil by horizontal well re-fracturing. To solve this problem, the horizontal well flexible sidetracking and staged hydraulic jet fracturing technology has been tested. Currently, sidetracking can be carried out from 13.97 cm casing and the drilling direction can be controlled. The sidetracked horizontal section can reach an inner diameter of 11.68 cm and length of 50-80 m and has a curvature radius of 2-4 m ^[42]. Sidetracking has been implemented in 15 wells in Changqing and Jilin oilfields, resulting an average daily oil production increase of 2 t per well ^[43]. Improving this technology is of great value for tapping and enhancing producing degree of residual oil in old areas.

As shale oil and gas resource development goes on, the number of horizontal wells is increasing year after year, and the length of the horizontal section is gradually increasing as well. The workover and supporting operation technologies for horizontal well with long horizontal section (greater than 1500 m) need to be researched to meet the requirement of shale oil and gas development. In light of the current technical capabilities, researches in the following 4 aspects are still needed: (1) Horizontal well boosted fishing technology and design optimization of horizontal well fishing string need to be researched to increase the pulling force of downhole fishing booster. (2) Milling technology in horizontal well needs to be improved, and pipe string mechanics software and tools such as ball centralizing milling tool, horizontal well torque universal joint and horizontal well reversing tool need to be developed ^[44]. (3) Multifunctional horizontal well tractor needs to be researched to realize precise positioning and real-time control of the dragged tool, and online data transmission etc., so that the tractor can be used in casing inspection, perforation, milling and other horizontal section operations. (4) Coiled tubing made of composite material, intelligent coiled tubing with optical fiber sensors, and supporting operation machines with capacity to handle more than 2000 m long coiled tubing need to be developed.

Complex reservoir stimulation needs reservoir information, oil casing-sealing tool parameters, perforation degree, stimulation plan indicators, ground wellhead condition, fracturing equipment status and so on. With the rapid development of information technology, the next development direction and goal of reservoir stimulation is to collect, exchange, integrate, command and use all the above information intelligently through the internet of things and big data to realize intelligent reservoir stimulation. This is mainly divided into four steps: (1) Establish a remote decision-making center for fracturing to realize the sharing of reservoir fracturing data and remote decision-making for on-site operation. (2) Save the annual horizontal well fracturing data into database to improve the decision-making ability. (3) Gradually realize the internet of things in each link of fracturing operation to track on-site operation in real time and improve response decision-making capability. (4) Build an artificial intelligence reservoir stimulation decision-making system to give fracturing design schemes quickly by artificial intelligence methods, greatly improving design pertinence and effectiveness, forming a CNPC remote decision support center and fracturing big data platform.

Horizontal well multi-stage fracturing technology is an important means to achieve development of shale oil and gas resources by “fewer wells at higher production” in North America. While learning from North America, after years of development, a series of technologies that meet the needs in multi-stage horizontal well fracturing of shale oil and gas reservoirs has been developed in China and have effectively supported the exploration and development of large shale and tight oil and gas fields in Sichuan, Ordos, Junggar, and Songliao basins. As the oil and gas exploration and development in China is still going on, the geological conditions of unconventional resources such as shale oil and gas will become more complex, and the resources are heading to a lower grade as well. The horizontal well multi-stage fracturing technology, geological-engineering integration, stimulation mechanism and optimization design, high- power electric drive fracturing equipment, long-interval horizontal well fracturing tools, workover supporting equipment and intelligent fracturing technology need to be researched continuously providing the essential technical support to accomplish the stable and long-term production goal of 2×10⁸ t oil and 3000×10⁸ m³ gas a year in China.