Introduction
Shale oil with rich resources is the most promising, strategic and realistic alternative to oil in China[1]. Statistics show that China's recoverable shale oil resources amount to 55×108 t, accounting for 9.7% of the recoverable resources worldwide[2]. Shale oil mining areas of PetroChina Company Limited (hereinafter referred to as PetroChina) are mainly distributed in the Songliao, Ordos, Bohai Bay, Sichuan, and Junggar basins[3]. Since 2010, PetroChina has greatly promoted the shale oil revolution by strengthening exploration deployment and making more risk investment. Shale oil exploration and development pilot tests have been carried out in basins such as Ordos, Junggar, Bohai Bay, Songliao and Santanghu Lake successively, with major progress achieved. With proven geological reserves of 6.24×108 t, controlled and predicted reserves of 24.60×108 t, and 3P reserves of 30.84×108 t discovered, shale oil has become a realistic replacement for crude oil in China[4,5].
Reservoir stimulation technology is crucial for shale oil development. Over 10 years of research and development, introduction, learning and innovation, the main stimulation technology series of "long horizontal well completion + multi-cluster perforation + slick-water carrying sand + staged fracturing" has been developed based on the U.S. experiences in shale oil development. By the end of 2019, PetroChina had built up production capacity of 350×104 t in 7 major shale oil basins, including Ordos and Songliao, etc. However, quite different from North American marine shale oil reservoirs in quality and engineering difficulties, continental shale oil reservoirs in China have low pressure coefficient, high viscosity and low mobility of crude oil, few natural fractures, and large stress difference between maximum and minimum principle stresses in general[6,7,8,9,10,11,12]. Shale oil development in China still faces problems such as fast production decline, low recovery and high cost, making profitable development quite challenging.
By reviewing the development history of shale oil stimulation technology in China, we have systematically summarized the main progress in shale oil stimulation technology, comprehensively analyzed the challenges faced by the technology, and proposed the development direction of the technology in this work.
1. Main progress in shale oil reservoir stimulation technology
Since 2011, PetroChina has been learning from successful experiences of shale oil development in North America, and making explorations and innovations. Shale oil reservoir stimulation technology of CNPC has experienced three stages of development: vertical well conventional fracturing, horizontal well volume fracturing and fracture controlled fracturing. Important progresses have been made in five aspects: shale oil reservoir stimulation mechanism, fracture controlled fracturing technology, geological-engineering integrated reservoir stimulation design platform, low-cost fracturing materials and large platform three-dimensional development mode.
1.1. Stimulation mechanism of shale oil reservoir
Research has been made focusing on the fracture expansion law of shale oil reservoir with beddings, the mechanism of shale oil imbibition and replacement, and mechanism of initiation and expansion of multi-cluster fractures[13,14,15]. For China’s continental shale oil reservoir with beddings, whether hydraulic fractures can cross the beddings and connect the upper and lower reservoir bodies in the fracturing process affects the placement of horizontal well boxes and the reservoir stimulation technology selected. Therefore, the fracture expansion law of shale oil reservoir with beddings has been investigated by large-scale physical simulation experiments and numerical simulations. The experimental results showed that the fractures in shale oil reservoirs with beddings presented "brick wall structure" or "fishbone structure", with fracture height extension restricted. The numerical simulation results also showed that the inhibition of fracture height by the bedding surface increased as the bedding density increased or the cementation strength of the bedding surface decreased. Under the guidance of the experimental results, an inverse composite stimulation mode suitable for shale oil reservoir with beddings has been established: at the beginning of the stimulation, frozen glue is used to break rocks and penetrate bedding joints by rapidly increasing the discharge volume, then slick-water slug carrying sand is injected to activate and support the bedding joints, and finally frozen glue carrying sand is injected into fractures to sustain moderate conductivity. The whole process physical experiment method of "high speed centrifugation-pressurized imbibition-core displacement" was developed, and quantitatively evaluated the recovery enhancement by imbibition. Three main mechanisms of imbibition replacement have been identified: reduction of interfacial tension, wettability change, and viscosity reduction through emulsification and solubilization. The experimental results show that imbibition can enhance recovery rate by more than 4%, laying a theoretical foundation for the development of new fluids to improve efficiency and reduce costs. The large physical experimental method modeling perforation and fracturing has been set up to test special perforation gun and perforation ammunition formulation and research initiation and expansion of multi-cluster fractures in open hole, spiral and planar perforation conditions. The research shows that each fracture cluster eventually leads to single-hole expansion, and by reducing the perforation length and increasing the number of perforation clusters, the fracture pressure and fracture complexity near wellbore can be reduced. These research results provide a theoretical basis for the optimization of multi-cluster perforation mode inside fracturing stage. Under guidance of these experimental results, perforation clusters in each fracturing stage in shale reservoirs have increased from 2-3 to 5-14.
1.2. Fracture controlled fracturing
To overcome the problem that the shale oil reservoir is difficult to "break up", on the basis of unconventional oil and gas volumetric stimulation technology[16,17,18,19,20], Lei et al. proposed the design optimization and supporting technical method system for“fracture-controlled reserves” based stimulation featuring dense fractures[21,22,23]. In this technology, the length, spacing, and height of fractures are optimized according to reservoir properties, stress, and well-controlled reserves so as to make the ratio of the fracture-controlled production” (the amount of oil and gas recovered from the fractures in the target time) to the well-controlled recoverable reserves (the oil and gas reserves in the reservoir unit where the well is located) approach 1, to realize effective control and recovery of the subsurface reserves. The core of fracture-controlled fracturing technology is to make clear four relationships: (1) rock properties and fracture expansion mechanisms; (2) horizontal section length and fracture density; (3) coupling of fluid flow in reservoir and fracture; and (4) match of artificial fractures with well spacing in well pattern. The key points of fracture-controlled fracturing technology include: start fracturing early, select sweet spots and layers vertically, and determine vertical well spacing according to simulated fracture height expansion; determine planar well spacing according to simulated artificial fracture length and production dynamics-based history matching, to achieve one-time well deployment; based on the study on the spatial and temporal evolution of three-dimensional stress field, adopt staggering arrangement of fractures and optimized fracture parameters to control oil drainage area and recoverable reserves and realize one-time fracture placement; control single well cost and production decline by optimizing fracturing scale to achieve one-time stimulation.
The fracture-controlled fracturing technology has been proved very effective in shale oil development of PetroChina[24,25]. From 2016 to 2019, 780 shale oil horizontal wells were treated by this technology, with the average number of stages in a single well increasing from 9.8 to 18.9, and the average added sand volume and fluid volume in a single well increasing by 4.1 and 3.1 times respectively. After the fracture-controlled fracturing was employed in the Tiaohu Formation shale oil reservoir of Tuha Oilfield, the horizontal well spacing gradually reduced from 400 m to 100 m, the fracture spacing from 30-40 m to 8-15 m, and the number of clusters in each fracturing stage increased from 3-5 to 6-10. Among which, 73 wells had daily oil production per well increasing from 13.5 t/d to 17.0 t/d. Compared with neighboring wells in the same block, these wells had an increase of average single well production of 25.9%, effectiveness rate increasing from 11.6% to 80.0%, degree of fracture-controlled reserves increasing from 42.1% to 85.2%, and composite decline rate decreasing to 20%. As a result, the predicted recovery rate of the block increased from 2.5% to 10.2%. In the Longdong shale oil development demonstration area of Changqing Oilfield, the fracture-controlled fracturing technology has also been applied to 58 wells, with well spacing reducing from 600-1000 m to 200-400 m, the number of clusters in each stage increasing from 2-3 to 5-14, and cluster spacing reducing from 22-30 m to 5-12 m, the degree of fracture-controlled reserves increasing from 50%-60% to more than 90% according to microseismic monitoring, single well production increasing from 10-12 t/d to more than 18 t/d, and the first-year decline rate dropping from 40%-45% to less than 35%, reversing the passive production capacity building situation. With the support of this technology, the demonstration area has reached a daily oil production of more than 1000 t and an annual production capacity of 50×104 t.
1.3. Integrated geological-engineering reservoir stimulation design platform
Under complex geological conditions of unconventional oil and gas reservoirs, only the geological-engineering integration organization and research platform can help gradually overcome the development problems and get better effect of reservoir stimulation. The geological-engineering integrated reservoir stimulation mode has been established by building the following "4 platforms" for geological-engineering integrated research: (1) integrated evaluation platform for geological evaluation, sweet spot evaluation, mechanical evaluation and well completion quality evaluation; (2) integrated design platform for establishing geological model, reservoir model, fracture model and economic model; (3) integrated analysis platform for post-fracturing tracking, measure evaluation, effect evaluation, and model correction; (4) integrated sharing platform for sharing experimental results, optimized plans and operation design.
The Fracturing and Acidizing Technology Center of Research Institute of Petroleum Exploration & Development, PetroChina has developed FrSmart integrated geo-engineering fracturing design software[2, 21, 26], which with fracturing optimization and design as core, integrates geological description, completion design, hydraulic fracture simulation, post-fracturing production capacity simulation, economic evaluation, and real-time fracture monitoring, etc., and includes 7 key modules. The software has the following functions: (1) the geological modeling module can establish 3D geomechanical models of individual wells or the whole area by importing the structural, attribute and geomechanical models; (2) the pre-fracturing analysis module can select well sections for fracturing by comprehensively evaluating reservoir quality and completion quality; (3) the fracture simulation module and fracturing capacity simulation module can optimize parameters such as artificial fracture spacing, fracture length and fracturing scale; (4) the economic evaluation module can estimate the payback period and internal rate of return of different schemes based on the net present value model, so as to select the scheme with the best economic benefit; (5) the real-time decision, database and data analysis module, which integrates big data, on-site and remote decision functions, can ensure the accuracy of input parameters and rationality of fracturing design.
The integrated evaluation, design and analysis can increase the drilling ratio of high-quality sweet spot and improve the fracturing effect of shale oil reservoir. During the pilot test phase of Jimsar shale oil reservoir in Xinjiang Oilfield, 10 horizontal wells had a daily initial production of 5.9-40.8 t, failing to reach the 40 t/d, and 1.5×104 t of production capacity per well cumulatively in two years designed by the scheme. Through integrated geological-engineering study, we found that the drilling ratio of high-quality reservoirs was the geological guarantee of post-fracturing production[27]. In the test development stage, horizontal well trajectory fine control technologies such as rotating geosteering and thin sand layer limited test were used. Consequently, Wells JHW023 and JHW025 reached drilling ratio of oil layer as 100%, and drilling ratios of high-quality sweet spots of 96% and 92% respectively, marking a significant increase from less than 70% of wells drilled earlier. After treated by high-intensity fracture-controlled fracturing technology, these two wells had the highest daily oil production of 88.3 t and 108.3 t respectively, and an average cumulative production of 1.0×104 t in 240 d per well. In addition, by adopting 3D seismic technology in Loess mountainous areas, fine log evaluation for shale oil reservoir, horizontal well trajectory steering technology, etc., Changqing Oilfield can effectively predict shale oil sweet spots. By using geosteering, 55 horizontal wells targeting the Triassic Chang 7 Member shale oil had an increase of oil layer drilling ratio of 15%.
1.4. Low-cost materials
1.4.1. Low-cost sealing tools for fracturing
Sealing tools have become the key ones for staged fracturing of horizontal wells in shale oil reservoir, and bridge plugs are the main sealing tools for fracturing of shale oil reservoir, accounting for over 95% of the sealing tools. Bridge plug staged fracturing has the advantages of unlimited number of fracturing stages, large fracturing scale and discharge, and smooth wellbore after drilling and milling. The bridge plugs initially used by PetroChina in horizontal well fracturing were quick-drilling composite bridge plugs imported from foreign countries at high prices. After years of research, we have developed dissolvable bridge plugs and dissolvable ball seats, and these products have been produced and applied on large scale in China, providing new means for improving sealing and fracturing efficiency. The dissolvable bridge plugs with a variety of sizes and controllable dissolution time from 7 to 14 d, have promoted the transformation of domestic unconventional oil and gas development mode, forced the price of foreign products to drop from 197 000 RMB to 35 000 RMB, and improved the core competitiveness of staged fracturing technology of PetroChina. The dissolvable ball seats developed have a pressure-bearing capacity of 70 MPa and can be fully dissolved within 7 d[28]. They have been applied to more than 3600 stages of 189 wells in Changqing Oilfield, setting a record of fracturing 43 stages and removing 24 ball seats in 28 h in a single well.
In order to realize multi-cluster fracturing in one stage, a modular clustering shotgun has been developed. Typical clustering shotguns relied on wires for inter-cluster conduction and selection control, and are complex in on-site assembly, prone to error and inefficient. By using the newly developed modular perforation tool, in combination with key technologies such as optimization of the supporting string, wellbore throughput improvement, ignition stability design, and pumping parameter, the 20 m lubricator allows 15-20 clusters of perforations to be shot (tubing string length of 16.4-17.6 m) in one trip. 12-cluster perforation in a single stage has been successfully tested, greatly improving the efficiency and quality of assembly and significantly reducing the labor intensity in field.
1.4.2. Low-cost stimulation materials
Compared with conventional reservoir fracturing, large-scale volume fracturing in shale oil reservoir uses larger amount of fracturing fluid and proppant, therefore fracturing fluid and proppant are focus of cost reduction. In recent years, fracturing fluid has been developing toward directions of slick-water with adjustable viscosity, recoverable slick-water, and increase of slick-water proportion; while in terms of proppant, quartz sand is replacing ceramsite.
The slick-water system with adjustable viscosity developed by Research Institute of Petroleum Exploration & Development, PetroChina has a mass fraction of 0.01%- 0.10% and an adjustable viscosity of 2-30 mPa•s. It can be converted between slick-water and sand-carrying liquid freely, with a resistance-reducing rate of 80% before the viscosity change and 70% after the viscosity change. The recoverable slick-water systems of EM30 and EM50 have been widely used in Changqing Oilfield, and have a resistance-reducing rate of 70%-80% at the mass fraction of 0.03%-0.08%. In recent years, the proportion of slick-water used in shale oil reservoir fracturing in China has also increased year by year and now stands at about 70%. The proportion of slick-water used in fracturing of the Tiaohu Formation shale oil reservoir of Tuha Oilfield has risen from 36.5% to 82.8%, and the proportion of slick-water used in fracturing of Jimsar shale oil reservoir in Xinjiang Oilfield has increased to 50%-60%.
Recently, the research and application of proppant has been focusing on replacing ceramsite with quartz sand to significantly reduce the cost of fracturing materials. Experiments were conducted to demonstrate the feasibility of replacing ceramsite with quartz sand, the method to evaluate the conductivity capacity considering stress state, placement concentration and production regime has been established. It has been made clear that the effective force on proppant can reduce by 50%-60% in multi-stage and multi-cluster fracturing mode in horizontal shale oil wells, so quartz sand can satisfy conductivity requirement of shallow shale oil reservoirs at depths of 3500 m or less. In addition, results of parallel plate physical model experiments and numerical simulation show that small particle size quartz sand can be transported over long distances, and the support profile can be further optimized by multi-level combination and multi-layer placement of proppant. In quartz sand tests in several oil fields of PetroChina, the application ratio of quartz sand increased from 47.9% in 2014 to 71.5% in 2020. Quartz sand has been used in shale oil reservoir fracturing in Changqing Oilfield. In Xinjiang Oilfield, quartz sand has been used to replace ceramsite in Mahu shale oil reservoirs at depth of 3500 m or less, saving proppant cost up to 590 million RMB from March to August 2020.
1.5. Large well pad three-dimensional development mode
In response to low oil prices, a new theory of high-efficiency field-wide horizontal well development in three dimensions has emerged in North America. It adopts multi-layer superimposed three-dimensional development mode to achieve the recovery of all resources in vertical direction and ensure efficient development of unconventional oil and gas, with wells arrangement and completion done in one time. Since 2010, this development model has been adopted in Permian basin, where each well pad has more than 20 wells; wells in the Wolfcamp Formation and Spraberry Formation have horizontal sections 1500-3000 m long and 1500-2000 m long respectively; the well spacing was 85-150 m within the same layer and 85 m between layers; the single well cost reduced by 15%-30% and production increased by 15%-25%[29].
With the idea of "multi-layer, three-dimension, large well cluster and factory operation", PetroChina explored a new development mode of large platform multi-layer well placement and three-dimensional fracturing in Hua H60 of Changqing Oilfield and Ma131 of Xinjiang Oilfield, etc. On Hua H60 well pad of Changqing Oilfield, 22 wells targetting 3 small layers were drilled, with horizontal section of 1507 m long and 20.8 stages and 121 clusters fractured per well on average. After completed, the well pad had a daily oil production of 360 t and annual oil production of 11×104 t. On Ma 131 well pad of Xinjiang Oilfield, 12 wells were drilled in three dimensions, and had 1132 fractures created in 331 fracturing stages. The wells had an average production 15 t/d higher than neighboring wells in the same period, and a rise of estimated recovery rate from 10% to 17%[30].
2. Challenges faced by shale oil reservoir stimulation technology
Overall, PetroChina shale oil development is still in the early stage of production capacity building, although Changqing, Xinjiang and Tuha Oilfields have made faster progress, it is quite difficult to achieve economic development of shale oil at current oil prices. For the overall development scheme of 300×104 t of shale oil in Changqing Oilfield with EUR (Estimated Ultimate Recovery of a single well) of 2.4×104 t, the average single well cost will be 2524 RMB/t, or about $50/bbl ($315/m3). Drilling and fracturing cost of Jimsar shale oil, Xinjiang Oilfield in 2019 was 61 million RMB, with EUR of 2.6×104 t, the internal rate of return calculated was -6.2% with progressive oil prices. The authors believe there are challenges in the following four aspects.
2.1. Challenges to three-dimensional stimulation technology due to multi-layers superimposition of shale oil vertically
Unlike the widely stable distributed marine shale oil reservoirs in North American basins, continental shale oil reservoirs in China are characterized by strong planar heterogeneity and vertical superimposition of multi-layers. The previously adopted single-layer well deployment has problems of low producing degree and high costs, etc. The development model with three-dimensional fracturing and horizontal well has made remarkable achievements in North America, and is an idea we can draw on. But in-depth research is still necessary in fine reservoir description, sweet spot identification, well deployment on platform, well spacing optimization and three-dimensional fracturing plan design. The following issues need to be addressed: (1) sweet spot identification and horizontal well trajectory design; (2) the best well density for maximizing benefits; (3) the best staggering well spacing for three-dimensional well deployment; (4) whether multiple wells should be arranged in a superposed pattern or in a staggering pattern; (5) whether hydraulic fractures overlap vertically; (6) whether the best fracturing sequence is top-down or bottom-up; (7) how to prevent fractures in combination wells from "colliding".
2.2. Challenges to the optimization of fracturing schemes due to complex fluid flow pattern in shale oil reservoirs
Shale oil reservoirs have permeability at the nano- darcy level with complex and variable fluid flow pattern, the relationships between EUR and benefit related parameters such as well spacing, fracture spacing and fracturing scale still need optimization. For example, for well spacing optimization, the well spacing in shale oil reservoir of the Tiaohu Formation of Tuha Oilfield was gradually reduced from the 400 m to 100 m in field test, and no inter-well fractures have been found. But more inter-well fractures were found in the shale oil reservoirs of Changqing Oilfield at well spacing of 200 m. Therefore, the fracture-controlled reserve volume and fluid flow pattern in different shale oil reservoirs need to be studied further to optimize the fracturing parameters pertinently and make the fracturing scheme design more scientific. Meanwhile, due to complex morphology of artificial fractures, the evaluation of fracture length, height and width is not only important for understanding fractures and guiding fracturing scheme design, but also crucial for guiding the whole life cycle production of oil and gas wells. It is urgent to develop fracture diagnostic technology that can identify and quantitatively characterize the scale of complex fracture networks.
2.3. Challenges to the recovery of remaining reserves between wells and between fractures due to large well spacing and cluster spacing in early development
In the early stage of shale oil development, the well spacing was basically 400 m due to mismatch of horizontal well technology. Later field coring and production dynamic analysis show that with the current engineering technology, the artificial fractures are less than 100 m long, leaving a large amount of undeveloped reserves between wells. After the secondary infilling of well pattern, due to reservoir heterogeneity, the fracturing of the infilled wells would be interfered by the stress field of the neighboring wells in the early stage, and the frac-hit is likely to occur during the fracturing of the combination wells. Therefore, frac-hit reducing measures need to be researched further. Meanwhile, with large cluster spacing and low pumping rate, the horizontal wells at the early stage of development were not completely stimulated, leaving unrecovered reserves between fractures, therefore refracturing becomes a must choice during the whole life cycle of shale oil horizontal wells. Refracturing faces five main challenges: (1) challenge in characterizing remaining oil distribution, and reconstructing flow field; (2) challenge in reconstructing dynamic stress field due to the complex stress field change pattern before refracturing; (3) challenge in reconstructing the horizontal wellbore due to lack of effective sealing tools in fractured wellbore; (4) challenge in determining the time point of refracturing due to lack of technology to evaluate the refracturing timing; (5) challenge in evaluating refracturing effect, due to lack of re-evaluation method.
2.4. Low cost of horizontal well staged multiple- cluster large-scale fracturing as the main shale oil reservoir stimulation technology
The increase in the number of fracturing stages and sand adding intensity, as well as higher drilling rate, would lead to increase of the proportion of fracturing costs in drilling and completion costs. Therefore, to cope with the low oil price effectively, it is necessary to reduce fracturing costs and increase production capacity of shale oil well[31]. Thanks to well deployment from well-pad and factory-like operations, fracturing time efficiency has increased from 4.2 to 10 stages per day, but in China, the average number of shale oil wells per well-pad is smaller, and the average fracturing time efficiency remains low (2-5 stages/d) due to various factors, showing a large gap with large well-pad factory-like operations in North America. It is difficult to increase the proportion of slick-water in some shale oil blocks. Slick-water takes up 61.5% on the average and 85.5% at most of the fracturing fluid used in China. Although the work of replacing ceramsite with quartz sand is pushing forward in an orderly manner, the proportion of transport cost is high, making it harder to reduce the costs of fracturing materials.
3. Technical development direction of shale oil reservoir stimulation technology
Continental shale oil is an inevitable choice as China's continental oil and gas exploration shifts focus from "out-of-source" reservoir to "in-source" reservoir. Medium-to-high mature shale oil is the realistic target for our oil exploration and development in the near future, and reservoir stimulation will definitely play a more important role in the effective exploration and development of shale oil. In light of shale oil reservoirs and technical requirements on stimulation technology of PetroChina, by reviewing the main progress and challenges of shale oil reservoir stimulation technology, we think that PetroChina should learn from the successful experience of shale oil development in North America, and focus the development of shale oil reservoir stimulation technology in the following aspects:
3.1. Strengthen the integrated geology-engineering research to utilize reservoir stimulation technology
3.1.1. Deepen basic theoretical study on reservoir stimulation
It is necessary to further strengthen basic studies on ground stress field and well network arrangement, rock mechanics and fracture expansion, proppant transport and fracture conductivity in shale oil reservoirs, identify geological sweet spots and engineering sweet spots, and reveal the mechanism of formation fluid-solid coupling, to improve the pertinence of shale oil reservoir stimulation. The identification and prediction technology of sweet spots need to be improved by using three-dimensional geological modeling technology, and conducting quantitative evaluation of spatial distribution of key parameters, fracability, and recoverability; the dynamic ground stress field needs to be described to get real-time ground stress distribution by pre-fracturing ground stress field modeling, measuring ground stress while drilling, predicting spatial and temporal evolution of three-dimensional ground stress field; the fracture initiation law and its main control factors need to be revealed by understanding rock mechanics and fracture expansion law, developing technologies of high temperature rock mechanics, large physical modeling and three-dimensional characterization of bedding and weak surface fracture expansion; it is necessary to develop a large physical model of sand transport and study the proppant transport and settlement characteristics under the slick-water reverse-order sand transport stimulation mode, to guide the optimization of fracturing fluid types and parameters.
3.1.2. Establishment of field laboratory for hydraulic fracture study
By drawing fully on the results of field tests in China and abroad[32-35] (Table 1), and focusing on the enhancement of single-well production and reserve producing degree, integrated field test laboratory needs to be built in key shale oil areas to research the optimization of fracturing technology, materials, parameters and scale, test fracturing technologies designed for increasing production, reducing costs and improving efficiency, and establish prolific well templates to guide efficient shale oil and gas development. In the field laboratory, a variety of monitoring and evaluation technologies such as DFIT (diagnostic fracture injection test), full-diameter coring, tracer, micro-seismic, DTS/DAS/DSS (distributed Raman temperature sensing system/distributed acoustic sensing system/distributed strain sensing system), and CSEM (controlled source electromagnetic method) can be used to deepen the understandings on scientific issues such as fracture morphology to guide the selection of well spacing, fracture spacing, fracturing parameters and refracturing measures.
Table 1 Monitoring methods and results used by shale oil and gas field laboratory test in North American.
| Project name | Monitoring methods | Understandings |
|---|---|---|
| HFTS | DFIT for 4 wells, wall coring, core analysis, proppant tracer for 3 wells, fracturing fluid tracer for 8 wells, micro-seismic, inclinometer, bottom hole pressure gauge, coring of large deviation well at 182 m | (1) The fracture complexity beyond that of current numerical simulation modeling. (2) Tensile fractures and shear fractures coexist; the support fractures are only about 10 m high. (3) There is a large amount of unbroken small-grain quartz sand on the fracture surface of the core. Large fractures 0.5 to 1.0 cm wide filled with quartz sand are commonly present in the cores. (4) Energy spectrum logging analysis indicates that more than 90% of the clusters are stimulated |
| MSEEL | Well wall coring, core analysis, micro-seismic monitoring, DTS/DAS, production logging, vertical well coring at 34 m | (1) Cores taken from vertical wells have natural fractures, induced fractures, and horizontal bedding; (2) A large amount of slow slips occur during the fracturing process; (3) DTS shows that the temperature of the fractured stage would rapidly recover to the reservoir temperature |
| Conoco-Phillips Eagle Ford pilot test | Dual well micro-seismic, DTS/DAS, bottom hole pressure gauge, tracer, radioactive proppant, coring from Well S2 at 61 m, coring from Well S3 at 110 m | (1) There are a large number of hydraulic fractures, far exceeding the number of perforation clusters; the fracture complexity is related to the cluster spacing; beddings and weak surfaces, etc. affect the fracture complexity. (2) DTS/DAS show that all clusters contain fluid, but not uniform in volume. (3) Density of fractures in core is not directly related to micro-seismic events. (4) DAS shows that some of the fractures are 457 m long, and the fractures extend upward to the Buda Formation and downward to the Austin Formation. |
3.1.3. Improve integrated geological-engineering reservoir stimulation software platform
The FrSmart integrated geo-engineering fracturing optimization design software platform developed by Research Institute of Petroleum Exploration & Development, PetroChina has taken shape. We should draw on the most advanced international achievements and the advantages of global mainstream commercial software, establish international standards, and realize seamless integration between modules, and the trend of digitalization and intelligence. Given the complex geological characteristics of oil and gas reservoirs in China, we should conduct top-level design, set functional modules, and continuously develop and improve the FrSmart integrated geo-engineering fracturing optimization design software platform by making use of technological innovation outcomes to make its functions more practical, models more advanced, performance more complete, and gradually make it an IT-based intelligent platform.
3.2. Deepen the study on fracture-controlled fracturing technology to improve producing degree of shale oil reserves
As mentioned earlier, the promotion of fracture-controlled fracturing technology in PetroChina shale oil development has achieved good results. Unconventional fracturing operations in North America showed parameters turning better and then worse, indicating there are optimal values for fracture spacing and operation scale[36,37,38]. Therefore, it is necessary to further deepen the studies on four major relationships, namely rock properties and fracture expansion mechanism, horizontal section length and fracture density, reservoir fluid flow and fracture flow coupling, and match of artificial fracture with well spacing in well pattern. In a payback period of 3-5 a, the optimal well spacing, fracture spacing, economic conductivity capacity, and fracturing scale of shale oil horizontal wells in different regions and reservoir conditions need to be worked out to maximize the fracture control and recovery of reserves.
3.3. Promote horizontal well three-dimensional development mode to realize the production of multiple layers longitudinally
Going through exploration of infilling wells, three-dimensional layer-by-layer development, and three-dimensional overall development, North American unconventional oil and gas development has entered completely in the stage of horizontal well three-dimensional develop-ment. In this stage, thanks to the dramatic increase in drilling speed, horizontal wells in superimposed and staggering pattern can be deployed to tap oil in multiple layers to increase the reserves-producing degree in vertical profile and improve quality and efficiency. In contrast, the "multi-layer, three-dimensional and factory-operation" mode of shale oil development has just started in China. In line with the multi-layer superimposed shale oil reservoirs in China, we need to learn from the North American experience, and research well deployment for multi-layers on well-pad, horizontal wellbore trajectory, optimal well pattern density, three-dimensional fracturing mode and fracture morphology, etc., to make unconventional resource development profitable. We need to strengthen research on shale oil geomechanics, shorten the iterative cycle of field sweet spot identification, improve the accuracy of continental shale oil sweet spot identification, and sort out the best target layer for horizontal well; optimize the well pattern in three-dimensional well deployment, the inter-layer and intra-layer well spacing; and optimize the fracturing mode and parameters to maximize producing reserves on profile vertically.
3.4. Research refracturing technology for shale oil horizontal wells to efficiently develop the remaining reserves between fractures
In light of the five challenges in refracturing of shale oil horizontal wells, we should research refracturing technology of horizontal wells, and develop a series of supporting technologies such as "remaining oil description, stress field analysis, evaluation of previous fracturing, selection of refracturing timing, scale design, casing re-engineering, and post-fracturing tracking" to match flow field, stress field and fracture system again to improve the stimulation effect. For casing re-engineering technology, North America has developed mature expandable tail pipe and tail pipe cementing technology, in which expandable tail pipe can improve the pressure-bearing level of overall casing column. The casing of 139.7 mm (5.5 in) after repaired will have a diameter of more than 106 mm, and internal pressure resistance of 70 MPa and external pressure resistance of 35 MPa[39]. Tail pipe cementing involves running 88.9 mm (3.5 in), 101.6 mm (4.0 in), or 114.3 mm (4.5 in) tail pipe inside 114.3 mm (4.5 in) or 139.7 mm (5.5 in) casing, and is applicable under the condition of bottomhole temperatures of 100-180 °C and horizontal section lengths of 760-2100 m[40,41]. Halliburton statistics show that this technology has been successfully applied to more than 112 wells in Haynesville since 2016, increasing the average EUR by about 150%; ConocoPhillips refractures about 100 horizontal wells per year, with mainly the refracturing technology of tail pipe cementing[42,43]. China still mainly adopts the dynamic multi-stage temporary plugging and diverting and double-sealing single-stick technology for refracturing. For dynamic multi-stage temporary plugging and diverting technology, it is difficult to initiate new fractures and predict the stimulation effect due to the influence of the original perforations and depleted fractures. The operation of double-sealing single-stick technology has poor efficiency. Therefore, the refracturing technology of expandable tail pipe and tail pipe cementing need to be developed.
3.5. Develop low-cost stimulation supporting technology to help reduce the cost and increase the efficiency of development
3.5.1. Develop shale oil horizontal well staged stimulation tools
Key tools and equipment are crucial to productivity and are also the major points for cost reduction and efficiency improvement. We need to strengthen the research and development of dissolvable bridge plugs and multi- cluster perforation tools, improve rubber sleeve-free all-metal dissolvable bridge plugs, modify setting mode, and develop a series of all-metal dissolvable bridge plugs for complex well conditions with different casing sizes, steel grades, and different temperatures etc., to realize no drilling, milling or flushing of bridge plugs after fracturing, thus improving operational efficiency. In terms of multi-cluster perforation tool, North America's modular perforation and high-efficient setting tool has a 99% success rate and a maximum of 24 clusters in a stage, compared with 14 clusters in China. Therefore, it is urgent for us to research high-efficient long horizontal section clustering perforation tools to achieve safe perforation in a single stage with 20 clusters or more.
3.5.2. Continuously reduce costs of fracturing materials
The proportion of low-cost slick-water should be increased. The proportion of slick-water used in shale oil reservoir has reached 100% in North America, while that in China remains relatively low at 70%. We should strengthen fracturing fluid recycling, use oilfield waste water, re-injection water, water separated from flowback liquid or shallow desert water to prepare fracturing fluid directly, to significantly reduce the environment risk incurred by fracturing fluid, and provide technical guarantee for "no landing" of fracturing fluid. In light of the characteristics of China’s shale oil reservoirs and stress loading conditions, we need to put more efforts into the field test and promotion of low-cost quartz sand proppant, increase the proportion of quartz sand instead of ceramsite, accelerate the evaluation of localization and economics of quartz sand source and set up quartz sand industrial bases, so as to effectively control fracturing proppant costs.
3.5.3. Develop information technology
As information technology develops fast, the reservoir stimulation system involves many aspects, such as reservoir information, oil casing parameters, perforation degree, sealing tool performance, stimulation plan, surface wellhead conditions, fracturing equipment status and instrumentation monitoring. The Internet of Things (IoT) can be used to collect, exchange, integrate, direct the information in above aspects and endow it artificial intelligence, and ultimately make reservoir stimulation artificially intelligent. Information technology can improve the level of factory-like operations and help reduce costs and improve efficiency in the whole cycle.
International oil companies commonly combine "unconventional reservoir" and "big data" to establish big data bases including operation, production, well construction, reservoir engineering, and geoscience to help shorten drilling cycles, optimize completion designs, and reduce unconventional oil and gas development costs[44].
PetroChina is strengthening the information construction, and based on the collaborative research environment of the Dream Cloud platform, a remote decision-making center for fracturing and acidification can be built, big data technology should be developed to offer online support to the whole process including fracturing optimization design, operation monitoring and remote diagnosis, and post-fracturing evaluation, to improve the quality and efficiency in the whole process.
4. Conclusions
After more than 10 years of development, shale oil reservoir stimulation technology of PetroChina has made significant progress in five aspects: reservoir stimulation mechanism, fracture-controlled fracturing technology, integrated geological-engineering reservoir stimulation design platform, low-cost material technology and large well-pad three-dimensional development mode, effectively supporting the construction of shale oil production capacity in oil fields such as Changqing and Xinjiang. However, overall, the shale oil development in China is still in the initial stage. As oil price lingers at medium to low levels long, it is more challenging to achieve economic development, the requirements and technical difficulty of reservoir stimulation technology are getting higher. We should strengthen the integrated geo-engineering research, deepen the fracture-controlled fracturing technology, promote horizontal well three-dimensional development mode, research refracturing technology of shale oil horizontal wells, develop low-cost stimulation supporting technology, explore shale oil stimulation technology systems suitable for different blocks and different reservoir characteristics, and strive to realize large-scale profitable shale oil development. We should make shale oil a new strategic growth point to guarantee national energy security.
Reference
Progress, challenges and key issues of unconventional oil and gas development of CNPC
Achievements and future work of oil and gas production engineering of CNPC
Geological theory and exploration & development practice of hydrocarbon accumulation inside continental source kitchens
Development potential and technical strategy of continental shale oil in China
Theory, technology and prospects of conventional and unconventional natural gas
Geological features and exploration potential of Permian Tiaohu Formation tight oil, Santanghu Basin, NW China
DOI:10.1016/S1876-3804(14)60067-7 URL [Cited within: 1]
The types, potentials and prospects of continental shale oil in China
Favorable formation conditions and enrichment characteristics of lacustrine facies shale oil in faulted lake basin: A case study of Member 2 of Kongdian Formation in Cangdong Sag, Bohai Bay Basin
Features of deepwater lithofacies, Yanchang Formation in Ordos Basin and its petroleum significance
Characteristics of lacustrine shale porosity evolution, Triassic Chang 7 Member, Ordos Basin
DOI:10.1016/S1876-3804(15)30003-3 URL [Cited within: 1]
Geologic characteristics and exploration potential of tight oil and shale oil in Lucaogou Formation in Jimsar Sag
Formation conditions and exploration potential of tight oil in the Permian saline lacustrine dolomitic rock, Junggar Basin, NW China
On the determination method of rupture mechanism in acoustic emission used in hydraulic fracturing fracture propagation
Solution for counter-current imbibition of 1D immiscible two-phase flow in tight oil reservoir
DOI:10.1007/s13202-016-0248-4 URL [Cited within: 1]
Polyaxial test research on in-plane perforation for horizontal wells
Volume fracturing technology of unconventional reservoirs: Connotation, optimization design and implementation
The core theories and key optimization designs of volume stimulation technology for unconventional reservoirs
The current situation of stimulated reservoir volume for shale in U. S. and its inspiration to China
The revolution of reservoir stimulation: An introduction of volume fracturing
Progress and development of volume stimulation techniques
The “fracture- controlled reserves” based stimulation technology for unconventional oil and gas reservoirs
A novel approach of tight oil reservoirs stimulation based on fracture controlling optimization and design
Test and application of subdivision fracture control fracturing for tight oil horizontal wells in Ordos Basin
Study and application of fracture control volume fracturing in Santanghu tight oil
Technological progress and prospects of reservoir stimulation
Development practices of geology-engineering integration on upper sweet spots of Lucaogou Formation shale oil in Jimsar sag, Junggar Basin
Design and application of soluble bridge plugs for plugging and perforating operation
Theoretical understandings, key technologies and practices of tight conglomerate oilfield efficient development: A case study of the Mahu oilfield, Junggar Basin, NW China
Refracturing horizontal shale wells with solid-steel expandable liners
