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Discussion on effective development techniques for continental tight oil in China
PETROLEUM EXPLORATION AND DEVELOPMENT Volume 41, Issue 2, April 2014 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2014, 41(2): 217–224.
RESEARCH PAPER
Discussion on effective development techniques for continental tight oil in China DU Jinhu1,*, LIU He2,3, MA Desheng2,3, FU Jinhua4, WANG Yuhua5, ZHOU Tiyao2,3 1. PetroChina Exploration & Production Company, Beijing 100007, China; 2. PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China; 3. State Key Laboratory of Enhanced Oil Recovery, Beijing 100083, China; 4. PetroChina Changqing Oilfield Company, Xi’an 710018, China; 5. Daqing Oilfield Co. Ltd., Daqing 163453, China
Abstract: Based on the main geological features and technical breakthroughs made in tight oil exploration, the major challenges facing tight oil development are analyzed, and the key technical trend for tight oil development is discussed in this paper. Mainly found in continental deposits, tight oil reservoirs in China feature small area, poor physical properties, big differences in geological characteristics between different basins, but low porosity, low permeability and pressure in general. In contrast to marine tight oil, tight oil in continental deposits faces such challenges as low production and recovery, and poor economics. Through nearly three years of research and pilot test, an integrated development mode with repeated fracturing of horizontal wells as the principal technique has been proposed, which includes integrated design, platform long horizontal well drilling, massive volume fracturing, re-fracturing stimulation, controlled production, factory-like operation, concentrated surface construction etc. It is recommended that study be strengthened on basic tight oil development theory, practical development technologies, and economic evaluation of tight oil development over the whole life cycle. Key words: continental tight oil; geological characteristics; effective development; key techniques; repeated fracturing of horizontal wells
Introduction Borrowing unconventional technologies used in shale gas development, such as sweet spot identification, long horizontal well drilling, volume fracturing and factory-like operation, etc, tight oil development is booming in the U.S. Tight oil production jumped from 30 million tons in 2011 to 96.9 million tons in 2012 [1−2]. Yanchang Formation in the Ordos Basin, Lucaogou Formation in the Junggar Basin, the Cretaceous in the Songliao Basin, the Jurassic in the Sichuan Basin, the Paleogene in the Qaidam Basin, etc in China have abundant tight oil resources, with the geologic resources of about 8−10 billion tons [1, 3−6]. Based on the main geological features of tight oil reservoirs and recent achievements in the tight oil exploration, the major challenges in efficient development of tight oil are analyzed and the key techniques are discussed in this paper, which will be of valuable guidance for tight oil development in China.
1 Progress in tight oil exploration and development The geological evaluation methods for tight oil have taken
shape in China after nearly three years of research, and systematic understandings have been achieved in reservoir types, relationship between sources and reservoirs, main factors controlling sweet spots, and accumulation types, etc. At the same time, major breakthroughs have been made in tight oil exploration: preliminarily control and predicted reserves are approaching billions of tons in the Ordos, Junggar, and Songliao Basin, confirmed control reserves exceed 600 million tons. Prospective areas of 100 million tons oil reserves have also been found in Santanghu Basin and the Huabei, Liaohe, and Dagang oilfields. Major progresses have been made in research on exploration and development techniques for tight oil, including four sets of key technologies: (1) The seismic prediction technique for tight oil reservoirs has been established based on the multi-parameter petrophysical chart; tight oil sweet spot selection method has been created based on comprehensive evaluation of geology, engineering and economy, which laid foundation for target selection in tight oil exploration and deployment of wellpads for cluster horizontal well drilling. (2) A system of evaluation methods named “Seven Proper-
Received date: 23 Jan. 2014; Revised date: 10 Feb. 2014. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the Accumulation Conditions and Key Supporting Techniques Research for PetroChina Tight Oil Reservoirs (101013kt1009). Copyright © 2014, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
DU Jinhu et al. / Petroleum Exploration and Development, 2014, 41(2): 217–224
ties Relationship” have been created based on well logging techniques to offer references for selection of drilling formation and well trajectory and fracturing design for horizontal wells. (3) A series of techniques of fast drilling and drilling fluid systems for long horizontal wells have been established to support the safe and fast drilling and high formation encounter ratio of horizontal wells. (4) A number of multi-stage fracturing techniques for horizontal wells have also been developed, including sand jet fracturing, hydraulic bridge plug pumping fracturing, and open-hole packer sleeve fracturing etc. Innovations have been made in fracturing design optimization and development of fracturing liquid system, realizing large scale volume fracturing with thousands of cubic meters of sand, and ten thousands of cubic meters of fracturing fluid. Tight oil development has obtained some accomplishments. All horizontal wells tested in the West 233 demonstration pilot block of Ordos Basin produced over 100 cubic meters of oil per day after volume fracturing. Another pilot block, An-83 has been built with production capacity of 300 thousand tons per year, where average production of horizontal wells is about 8 times of vertical wells; 3 tight oil pilots with a production capacity of 139 thousand tons in Songliao Basin also have been set up, and over 10 thousand cubic meters of oil has been produced from horizontal wells cumulatively; an oil production capacity of 65 thousand tons in Jilin Oilfield has been established and the daily production of horizontal wells after multi-stage fracturing reaches 26−53 tons, which is 7 times that of vertical wells; in the Jimusar sag, the production of horizontal wells was 71 cubic meters per well per day, which is 7 times the production of vertical wells in the surrounding areas.
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Geological characteristics Mainly found in continental deposits, tight oil reservoirs in Table 1
Basin
Ordos
Horizon
Ch7
Source- reservoir relationship Inside source
SonFuyu Below source gliao Gaotaizi Inside source JungLuInside souce gar caogou SiDaanzhai Inside source chuan Bohai Sha-3 Inside source Bay QaiLower Inside source dam Miocene Santanghu
Tiaohu
Inside souce
China span a wide range of horizons, and are quite different in reservoir types, physical properties and rock properties in different basins (Table 1). Main geological characteristics are as follows. (1) Continental tight oil reservoirs in China are smaller in area than marine ones abroad. Limited by the sedimentary characteristics of lake basins, the tight oil source beds in China cover several hundred to tens of thousands of square kilometers, in contrast, the Bakken Shale Formation in the Williston basin, Northern America covers an area of 7×104 km2. But the China tight oil source beds have a bigger thickness of more than 30 m in general, and medium-high total organic carbon content, which provided solid material foundation for the forming of tight oil reservoirs. (2) The tight oil reservoirs in China are diverse in types, poor in physical properties and strong in heterogeneity. The tight reservoirs in China include a variety of rocks, such as tight sandstone, conglomerate, limestone, dolomite, tuff and transitional rocks etc. Tight oil reservoirs and sources are in a number of combination modes: reservoir-source in one, reservoir above source, and reservoir below source. Reservoirs in Ch7 member of Ordos Basin are thick (about 10 to 30 m) and rich in micro-fractures. Fuyu layer of Songliao Basin features thin single layers of 3−5 m, multi-layer stacking, and fast change laterally. In contrast, tight oil reservoirs in North America are simple in lithology and better in physical properties, and mainly sandstone and carbonate ones. (3) The porosity and permeability of tight oil reservoirs in China are very low. The tight oil reservoirs in China is generally less than 10% in porosity, and less than 0.1×10−3 μm2 in permeability, while porosity of North American tight oil reservoirs is about 10%−13%. Reservoir pore throat radius in China is of micro-nano scale. For example, the throat radius of CH7 tight oil reservoir mainly distributes from 0.10 to 0.75 μm (Fig. 1).
Typical genetic types of tight oil reservoirs in China’s continental basins Main controlling factors of Tight oil accu“sweet spots” mulation patterns Total thickness/m TOC/% Facies control Fracture Gravity Tight sandThick sandstone 10−30 5−20 Diagenetic Structural flow stone porous Tight sand- Sedimentary Thin sandstone 30−90 3−9 Delta Structural stone -Diagenetic porous Saline Dolomitic Thick dolomitic 50−150 2−15 Diagenetic Dissolution lacustrine sandstone sandstone porous Marl and lime＜2 Lacustrine Limestone Diagenetic Structural stone fracture Marl and lime＞200 1.6 Lacustrine Limestone stone fracture Gravity Tight sandThin sandstone 0.4−1.1 flow stone porous
Source rock Type High abundance shale High abundance shale High abundance shale Low abundance shale High abundance shale Low abundance shale High abundance sedimentary tuff
150
8.1
Sedimentary facies
Reservoir lithology
Lacustrine
Limestone
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Thick tuff porous
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(7) Reservoir buried depth differs widely in different basins. For example, Yanchang Formation in Ordos and Fuyu Formation in Songliao Basin are buried at around 2 000 m deep, while the depth of Lucaogou Formation in Jimusaer is about 3 000 m, and Sha3 member tight oil reservoir in Shulu is about 3 500−4 000 m deep.
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Fig. 1 Pore throat radius distribution results obtained by constant rate mercury injection
(4) Affected by deposition mode, oil saturation differs quite a bit between reservoirs inside and outside sources. Tight oil reservoirs in source beds are generally higher in oil saturation. Oil saturation in CH7 formation is about 65%−85% (Fig. 2), Lucaogou Formation in Jimusaer Basin has an oil saturation of 70%−95%; in contrast, Fuyu Formation, a typical tight oil reservoir below source bed has an oil saturation of less than 50%, besides, wells there generally produce some water. (5) Tight oil reservoirs in different basins of China are quite different in oil density and GOR, and normal or low in pressure, but there are a few high pressure reservoirs. CH7 tight oil reservoir has an oil density of 0.70 to 0.85 g/cm3, GOR of 100 to 200 m3/m3, and pressure coefficient of 0.60−0.80; in comparison, Qingshankou tight oil reservoir in Songliao Basin has an oil density of 0.85 g/cm3, GOR about 40 m3/m3, and pressure coefficient of above 1.20; Lucaogou Formation tight oil reservoir in Jimusaer has an oil density of 0.80 to 0.90 g/cm3, and pressure in normal range. The tight oil reservoirs in the U.S., have an oil density of 0.80 to 0.82 g/cm3, GOR of about 90−250 m3/m3, pressure coefficient of 1.35 to 1.80, representing abnormal high pressure reservoirs. (6) Rock brittleness and stress difference are quite different in different basins. CH7 tight oil layer has a rock brittleness of about 40%, and a fairly small stress difference of about 4 to 7 MPa; Lucaogou tight oil layer has higher rock brittleness, and stress difference of less than 6 MPa in general; Fuyu tight oil layer in Songliao Basin has strong plasticity, and stress difference of around 10 MPa. High brittleness is favorable for the creation of reticular fractures, and the principal stress difference is one of the key factors controlling fracture direction [7−8].
Fig. 2
Porosity vs. oil saturation of Ch7 member
Challenges facing tight oil development
Featuring low porosity, low permeability and low pressure, continental tight oil reservoirs in China are facing problems like fast production decline, difficult energy compensation and poor producing degree in development. (1) The pore radius in tight oil reservoirs is of micro-nano scale, posing challenges to improving producing degree of reserves. Fig. 3 shows that about 30% to 50% of movable oil is stored in the submicron pores with radius of about 0.1−1 μm. The oil production after fracturing generally can not be very high because the reservoir properties are very poor, and the pore connectivity improvement by conventional fracturing is limited. For example, Ch7 reservoir in Changqing Oilfield, wells obtained industrial oil production after conventional fracturing is only about 40% of the total fractured wells, making it hard to produce the reserves. For the Fuyu reservoir in Daqing Oilfield, the fluid supply radius of conventional fracturing well is only 90 m, the control reserves of a vertical well is only 1.9×104 t, and the control reserves of conventional horizontal wells with a 600 m long horizontal section, is only about (7−9)×104 t. (2) The tight oil reservoirs are very low in pressure and permeability, making it very difficult to enhance single well production. Due to low formation pressure and low reserves controlled by individual wells, the natural energy is very limited, so tight oil wells universally suffer rapid production decline after the initial short-term high oil production. Fluids in tight oil reservoirs flow slowly due to low transmissibility of formation pressure, as a result, individual wells features long term low production, and low cumulative production. For example, Ch7 reservoir in Changqing Oilfield, oil production was about 5.8 t/d in formation testing phase, but only 0.6−0.9 t/d in production test phase. (3) It is hard to maintain reservoir pressure, posing challenges to enhance oil recovery. The recovery factor of tight oil
Fig. 3 Distribution of movable oil in Ch7 tight oil reservoir of Changqing Oilfield
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reservoirs is generally less than 10% by natural depletion, while wells may suffer quick water breakthrough after massive fracturing, making oil in matrix hard to be produced. (4) High project investment makes it difficult to improve the economic benefits. For instance, the Fuyu reservoir in Daqing Oilfield, the total cost of drilling a horizontal well and volume fracturing is about (3 000−5 000)×104 RMB, but the cumulative production of individual wells predicted is only 1.48×104 t, so the total cost per well must be reduced to less than 3 000×104 RMB to obtain economic benefit. Big buried depth of tight oil reservoirs in the Junggar, Bohai Bay, Qaidam basins etc, rough terrain and lack of water resources in Changqing oilfield, and low oil saturation in Songliao Basin make it difficult to reduce tight oil development cost in these basins. Therefore, only by establishing specific low cost development mode for each basin according to the tight oil reservoir features, can the tight oil in each basin be developed effectively and economically.
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Effective development technologies
Huge in resource scale, tight oil exploration and development are necessary. However, facing low production and low benefit, the biggest problem is what development approach and technologies should be taken to maximize the production rate, recovery factor and comprehensive benefit. According to the present status of tight oil development technology at home and abroad [9−13], especially the technical research and practice in recent years, this paper puts forward some thoughts on effective development of tight oil. 4.1
Guiding ideology
Shaking off the bound of old concepts and ideas in conventional development, the tight oil development should center around economic benefit, and adhere to the principle of “Five integration”, i.e. “the integration of exploration and development, geology and engineering, economy and technology, surface and subsurface, and research and production”. With “massive re-fracturing and production” as the leading development technology, “factory-like” operation, “centralized” surface facility construction and refined management organization model should be adopted to improve single well production, reduce cost and enhance economic benefit of thigh oil development. 4.2
Connotation of development technology
“Massive re-fracturing and production” for horizontal wells mainly has three meanings: first, improvement flow condition and formation pressure by long horizontal section, multistage multi-cluster and high fluid volume fracturing, increasing single well production rate and cumulative recovery in “primary production”; Second, application of re-fracturing technology to increase the single crack length, or changing fracture direction to form new fracture network, to develop the remaining oil zone and increase formation pressure in “secondary recovery”; third, using integrated technology, includ-
ing integrated design, wellpad long horizontal well drilling, massive volume fracturing and re-fracturing, “factory-like” operation and centralized surface facility construction etc. 4.3
Technology mechanism
Because of low porosity and poor connectivity and low formation pressure, when produced by natural depletion, tight oil reservoir will see fast pressure drop and fast driving force reduction, in turn rapid decline of single well production and low oil recovery efficiency. Therefore, improving reservoir connectivity, complementing formation energy, and creating effective flow driving system are at the heart of tight oil development. Massive re-fracturing and production technology has a scientific theoretical base: during the fracturing process, fracturing fluid is forced into the pores because of fracturing pressure is greater than formation pressure, and balanced before blowout; during the producing process, under the pressure differential, oil in the pores flowing through the fractures into the wellbore, forming industrial oil production. When oil production becomes uneconomic, re-fracturing technology is used to increase the single fracture length, or changing fracture direction to form new fracture network, the formation energy will be re-supplied, and the next new production stage begins. 4.4 4.4.1
Key technologies Integrated design
Integrated design combines the optimization of exploration, development, engineering, and management etc. By extending exploration to the following operation, and early stepping-in of surface engineering design optimization and adjustment, an integrated information processing platform should be set up to meet the evaluation needs of the whole production process (fig. 4). Following the principle of reverse thinking and forward implementation, plan of all aspects will be optimized timely as according to the changes in reservoir parameters in real time. Integrated design mainly includes the following two connotations: first ‘reverse thinking’, based on the surface environment, and the optimization design of surface construction and system management, as well as tight oil resource evaluation results, the position, scale and number of drilling wellpads are optimized, then the well type and well trajectory are optimized according to platform position and reservoir stress direction; finally the horizontal well direction, fracturing scale and well spacing are optimized according to stress direction and fracture characteristics, to complete the well pattern optimization. Second, ‘forward implementation’, as optimized plans of all aspects are completed, well drilling sequence will be optimized according to the geological understandings. After new well data acquisition, geological model, well pattern and surface construction plans will be adjusted immediately until the completion of the overall plan. The goal of integrated design is to maximize the benefits and reserves producing degree.
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Fig. 4 Schematic diagram of interactive information integration processing platform
4.4.2
Long horizontal well drilling from wellpad
Reserves controlled by single well will be increased greatly with long horizontal wells, and adding more wells drilled from one wellpad can increase reserves controlled by the wellpad. One advantage of wellpad is to reduce the surface area significantly, and provide prerequisite conditions for the “factory-like” operation and centralized surface construction. Length of horizontal section, well orientation and well space are the key contents of wellpad horizontal well design. In different basins, tight oil reservoir conditions are different, so the design parameters are not the same. For example, tight oil reservoirs in Changqing oilfield, horizontal section length of horizontal wells is about 1 500 m. Reasonable well spacing calculated by crack monitoring and reservoir engineering method is 500 m; in comparison, in Daqing oilfield, horizontal section is about 1 200−1 500 m, reasonable well spacing is 700−1 400 m. Wellpad drilling is still at the exploratory stage in China at present. In tight oil pilot area of Daqing oilfield, reserves controlled by single horizontal well are (12−27)×104 t, 6 to 14 times of fractured vertical wells, and drilling and completion of 4 wells from one wellpad has been done. In Changqing oilfield, reserves controlled by single horizontal well reach (30−50)×104 t, 6 wells can be drilled from one wellpad, which means saving more than 60% of the surface area if popularized. 4.4.3
Massive volume fracturing
Massive volume fracturing is to optimize configuration of matrix pores and artificial fractures and to increase controlled volume in order to improve the producing degree of reserves. In the fracturing design, five points should be followed: (1) Increase fracturing stages and clusters to realize effective producing of controlled reserves across the whole horizontal section. Now, fracturing stages can reach more than 22; (2) Divide the horizontal section into many small blocks according to matrix permeability, and then design fracturing
scale of every block, including fracture space, length, and conductivity and perforation parameters, according to reservoir distribution characteristics. The ideal result is to realize uniform stimulation (Fig. 5). (3) Increase the fracturing fluid injection rate to expand fracture scale and effective drainage radius. At present, the fracturing fluid injection rate can reach 15 m3/min and drainage radius can be more than 2 times that of conventional fracturing. (4) Increase fracturing fluid injection volume. Continental tight oil reservoirs in China generally have low pressure, so large injection volume can maintain formation energy effectively. (5) Adding a big volume of sand at low sand fluid ratio. In order to reduce the risk of fracture closure, rear supporting agent will be injected to enhance crack support ability. In field operation, in blocks where horizontal stress difference and permeability are higher, cutting volume fracturing is recommended to reduce the interference between the fractures; for low permeability blocks, “breaking the block” fracturing is suggested to form fracture network connecting pores; in reservoirs with low horizontal principal stress difference, complex fractures are likely to form, so less stages and more clusters can be designed to reduce fracturing costs. In addition, most continental tight oil reservoirs are made up of stacked thin layers, how to increase horizontal well drilling encounter rate is a problem, so it is necessary to strengthen the reservoir geological evaluation for layers adjacent to the target layer, develop multilayer fracturing technology, in order to expand the stimulation scope of individual wells. Massive volume fracturing can greatly increase the contact area between matrix and fractures, and improve horizontal wells’ productivity. Table 2 shows the results of different fracturing process in a pilot of Changqing Oilfield. The results show fracturing stages, fracturing fluid injection rate and fracturing scale are positively correlated with tested oil production to some extent; YP10 had the most stages, and highest initial oil production, but it also saw rapid bottomhole pressure drop and production decline, so its cumulative production in 300 days is not the highest. YP6, YP9 had the least stages, but high fracturing fluid injection rate, and sand volume, Their 300 d cumulative production is close to YP10. YP7 with more fracturing stages, high fluid injection volume and fracturing scale, ranks first in 300 d cumulative production. So, more stages, high injection rate, and large scale fracturing are the fracturing direction.
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Fig. 5
Schematic diagram of volume fracturing by blocks
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Table 2
Statistics on volume fracturing horizontal wells in Ch7 tight oil reservoir of Xi-233 block
Length of hori- Stage Well name zontal well/m number YP3 1 523 11 YP4 1 461 10 YP5 1 874 10 YP10 1 535 21 YP6 1 536 9 YP9 1 535 8 YP8 1 535 14 YP7 1 535 15
Cluster number 22 20 20 42 44 39 28 30
Injection rate/ Total amount Fracturing fluid volume/m3 (m3·d−1) of sand/m3 6.0 483 6 560 6.0 439 7 653 6.0 439 5 899 6.0 1 058 16 047 15.0 1 274 12 876 15.0 1 019 10 270 7.5 1 448 12 582 7.6 1 488 13 611
Massive volume fracturing can not only increase the producing reserves in the horizontal section substantially, but also help maintain the formation energy. In Changqing oilfield, flow-back ratio of fracturing fluid is low, large amount of fracturing fluid is left around the horizontal well, forming a high pressure zone of 116%−128% of the original formation pressure (Table 3), which works as advanced water injection. 4.4.4
Re-fracturing
Re-fracturing can change artificial fracture direction to form a new fracture network and develop the remaining oil zone. Meanwhile, re-fracturing can increase formation pressure, which would be helpful for well production, and achieve “secondary recovery”. Re-fracturing mainly includes the following aspects: firstly, closed invalid fractures and “V” shaped fractures affected by local stress (Fig. 6a) in initial Table 3 Block
Well number
Xi233 An83
10 13
Well production in formation testing/(m3·d−1) 133 154 141 184 142 128 152 137
Cumulative oil production/t 2 923 2 714 2 242 3 826 3 592 3 886 3 722 4 154
fracturing needs re-fracturing, at this time single well fracturing design needs to be carried out. Secondly, the remaining high oil saturation areas on both sides of the fracture need re-fracturing (Fig. 7a). After long term production, re-fracturing of several wells can be carried out (Fig. 6b), changing artificial fracture direction, and improving the development effect (Fig. 7b). Thirdly, remaining oil reserves near the heel or toe of horizontal wells, and those not reached by the initial well trajectory, can be effectively developed by re-fracturing (Fig. 7c). According to “re-fracturing and production” process, and improvement of horizontal well anhydrous re-fracturing technology, CO2/N2 dry fracturing process has great advantages. As sand carrying agent, gas can be injected into reservoir easily to supply formation energy and reduce the oil-gas interfacial tension effectively and enhance oil recovery. Jilin
Horizontal well fracturing parameters and formation pressure prediction results
Length of Average fracturing parameters Before production horizontal Fracturing fluid Total flow-back Flow-back Remaining liquid Formation presStage/Cluster well/m volume/m3 volume/m3 ratio/% volume/m3 sure increase/% 1 537 13/26 9 665 4 671 48.3 4 994 16 760 10/23 7 681 2 753 35.8 4 928 28
Fig. 6
Fig. 7
Schematic diagram of re-fracturing technology
Residual oil distribution before and after re-fracturing
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Pressure coefficient 1.00 0.98
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oilfield has completed the development of related equipment for CO2/N2 dry fracturing process and the large scale application will be implemented in the near future. 4.4.5
Controlled oil production
Controlled oil production uses reasonable production regime to get a long-term and stable production rate, realizing uniform producing of single well controlled reserves, and thus higher cumulative production. After Multi-stage fracturing, tight oil wells may reach high production rate, if not controlled, production will decline rapidly, leaving many problems: firstly, the initial fast flow rate easily cause fracture closure which will result in failure of fracturing. Secondly, production regime changes frequently, which not only causes inconvenience to operation management, but also the change of wellbore pressure and flow rate, and in turn, the instability of fracturing proppant, even plugging wellbore and affecting normal well production. Thirdly, quick pressure drop near wellbore could cause pressure-sensitive damages. Controlled production can effectively avoid the above problems and ensure high permeability of fractures. According to wellhead pressure and back flow chart, reasonable nozzles were selected to control flow-back rate and prevent fracturing sand from carrying out in Daqing oilfield. With the stable production regime, vertical and horizontal wells in Xinjiang oilfield achieved good performance. Table 4 shows production of two wells in X233 pilot in Changqing Oilfield, Well YP7 produced at above 14 t/d steadily in the first year, with an annual decline rate of only 0.7%; while Well YP10’s initial production was higher, but the wellbore fluid level decreased significantly, and the annual decline rate was 48.7%. 4.4.6
“Factory-like” operation
“Factory-like” operation refers to the assembly line-like operation including drilling, completion, fracturing and production one after another with a series of mature technologies and standard equipment. Wells deployed from a wellpad Table 4 Well name YP7 YP10
Length of horiFracturing zontal section/m stage/Cluster 1535 1535
15/30 21/42
Fig. 8
should be similar in structure, same in completion process. Operation efficiency can be greatly improved and tight oil reservoir development cost will be reduced, based on “wellpad” drilling, and mature engineering technology. In China, “factory-like” operation is still at the stage of process testing and equipment matching. Fig. 8 shows the staggering fracturing technology in Changqing oilfield. A pilot of horizontal well volume fracturing shows that the fracturing operation time reduced from an average of 62 days in 2012 to 31 days in 2013, that is an improvement of operation efficiency by one time. A new type of fracturing fluid was developed to reduce the friction and cost of fracturing fluid after several years’ of research. Now, this technology has been applied in 95 wells with 14 070 m3 fracturing fluid recovered. 4.4.7
Centralized surface construction
Centralized surface construction means a centralized processing station in a certain region, which integrates three functions, fracturing water supply, fracturing fluid recovery and treatment, and crude oil gathering and transportation (Fig. 9). Every wellpad is connected to the centralized processing station by one set of pipeline system, which carries fracturing water to the wellpad, and transport oil and water to the station at fracturing fluid flow-back and production stage. This method can reduce the operation cost effectively, save energy and reduce emission, so it is a greener technology. In Changqing Oilfield, water shortage is a serious problem for “factory-like” operation, which is solved by designing and setting up multiple water wells supplying the processing station. Overall planning and unified construction of fracturing water supply, waste water processing and crude oil gathering and processing made it possible to realize centralized production and digitalized management, which results in improvement in production efficiency and reduction in labor intensity. In some special conditions, the well platform is too far away to design a processing station. Small skid mounted treatment system for single platform may be more economic benefit.
Comparison of fracturing and producing results in a tight oil pilot Initial phase Oil producWellbore tion/(t·d−1) fluid level/m 14.5 0 18.1 540
Cumulative Present Production oil producOil producWellbore days/d tion/t tion/(t·d−1) fluid level/m 14.0 0 338 5065 8.4 1300 457 5376
Comparison of normal fracturing and staggering fracturing
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Annual production decline rate/% 0.7 48.7
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RESEARCH PAPER
Discussion on effective development techniques for continental tight oil in China DU Jinhu1,*, LIU He2,3, MA Desheng2,3, FU Jinhua4, WANG Yuhua5, ZHOU Tiyao2,3 1. PetroChina Exploration & Production Company, Beijing 100007, China; 2. PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China; 3. State Key Laboratory of Enhanced Oil Recovery, Beijing 100083, China; 4. PetroChina Changqing Oilfield Company, Xi’an 710018, China; 5. Daqing Oilfield Co. Ltd., Daqing 163453, China
Abstract: Based on the main geological features and technical breakthroughs made in tight oil exploration, the major challenges facing tight oil development are analyzed, and the key technical trend for tight oil development is discussed in this paper. Mainly found in continental deposits, tight oil reservoirs in China feature small area, poor physical properties, big differences in geological characteristics between different basins, but low porosity, low permeability and pressure in general. In contrast to marine tight oil, tight oil in continental deposits faces such challenges as low production and recovery, and poor economics. Through nearly three years of research and pilot test, an integrated development mode with repeated fracturing of horizontal wells as the principal technique has been proposed, which includes integrated design, platform long horizontal well drilling, massive volume fracturing, re-fracturing stimulation, controlled production, factory-like operation, concentrated surface construction etc. It is recommended that study be strengthened on basic tight oil development theory, practical development technologies, and economic evaluation of tight oil development over the whole life cycle. Key words: continental tight oil; geological characteristics; effective development; key techniques; repeated fracturing of horizontal wells
Introduction Borrowing unconventional technologies used in shale gas development, such as sweet spot identification, long horizontal well drilling, volume fracturing and factory-like operation, etc, tight oil development is booming in the U.S. Tight oil production jumped from 30 million tons in 2011 to 96.9 million tons in 2012 [1−2]. Yanchang Formation in the Ordos Basin, Lucaogou Formation in the Junggar Basin, the Cretaceous in the Songliao Basin, the Jurassic in the Sichuan Basin, the Paleogene in the Qaidam Basin, etc in China have abundant tight oil resources, with the geologic resources of about 8−10 billion tons [1, 3−6]. Based on the main geological features of tight oil reservoirs and recent achievements in the tight oil exploration, the major challenges in efficient development of tight oil are analyzed and the key techniques are discussed in this paper, which will be of valuable guidance for tight oil development in China.
1 Progress in tight oil exploration and development The geological evaluation methods for tight oil have taken
shape in China after nearly three years of research, and systematic understandings have been achieved in reservoir types, relationship between sources and reservoirs, main factors controlling sweet spots, and accumulation types, etc. At the same time, major breakthroughs have been made in tight oil exploration: preliminarily control and predicted reserves are approaching billions of tons in the Ordos, Junggar, and Songliao Basin, confirmed control reserves exceed 600 million tons. Prospective areas of 100 million tons oil reserves have also been found in Santanghu Basin and the Huabei, Liaohe, and Dagang oilfields. Major progresses have been made in research on exploration and development techniques for tight oil, including four sets of key technologies: (1) The seismic prediction technique for tight oil reservoirs has been established based on the multi-parameter petrophysical chart; tight oil sweet spot selection method has been created based on comprehensive evaluation of geology, engineering and economy, which laid foundation for target selection in tight oil exploration and deployment of wellpads for cluster horizontal well drilling. (2) A system of evaluation methods named “Seven Proper-
Received date: 23 Jan. 2014; Revised date: 10 Feb. 2014. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the Accumulation Conditions and Key Supporting Techniques Research for PetroChina Tight Oil Reservoirs (101013kt1009). Copyright © 2014, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
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ties Relationship” have been created based on well logging techniques to offer references for selection of drilling formation and well trajectory and fracturing design for horizontal wells. (3) A series of techniques of fast drilling and drilling fluid systems for long horizontal wells have been established to support the safe and fast drilling and high formation encounter ratio of horizontal wells. (4) A number of multi-stage fracturing techniques for horizontal wells have also been developed, including sand jet fracturing, hydraulic bridge plug pumping fracturing, and open-hole packer sleeve fracturing etc. Innovations have been made in fracturing design optimization and development of fracturing liquid system, realizing large scale volume fracturing with thousands of cubic meters of sand, and ten thousands of cubic meters of fracturing fluid. Tight oil development has obtained some accomplishments. All horizontal wells tested in the West 233 demonstration pilot block of Ordos Basin produced over 100 cubic meters of oil per day after volume fracturing. Another pilot block, An-83 has been built with production capacity of 300 thousand tons per year, where average production of horizontal wells is about 8 times of vertical wells; 3 tight oil pilots with a production capacity of 139 thousand tons in Songliao Basin also have been set up, and over 10 thousand cubic meters of oil has been produced from horizontal wells cumulatively; an oil production capacity of 65 thousand tons in Jilin Oilfield has been established and the daily production of horizontal wells after multi-stage fracturing reaches 26−53 tons, which is 7 times that of vertical wells; in the Jimusar sag, the production of horizontal wells was 71 cubic meters per well per day, which is 7 times the production of vertical wells in the surrounding areas.
2
Geological characteristics Mainly found in continental deposits, tight oil reservoirs in Table 1
Basin
Ordos
Horizon
Ch7
Source- reservoir relationship Inside source
SonFuyu Below source gliao Gaotaizi Inside source JungLuInside souce gar caogou SiDaanzhai Inside source chuan Bohai Sha-3 Inside source Bay QaiLower Inside source dam Miocene Santanghu
Tiaohu
Inside souce
China span a wide range of horizons, and are quite different in reservoir types, physical properties and rock properties in different basins (Table 1). Main geological characteristics are as follows. (1) Continental tight oil reservoirs in China are smaller in area than marine ones abroad. Limited by the sedimentary characteristics of lake basins, the tight oil source beds in China cover several hundred to tens of thousands of square kilometers, in contrast, the Bakken Shale Formation in the Williston basin, Northern America covers an area of 7×104 km2. But the China tight oil source beds have a bigger thickness of more than 30 m in general, and medium-high total organic carbon content, which provided solid material foundation for the forming of tight oil reservoirs. (2) The tight oil reservoirs in China are diverse in types, poor in physical properties and strong in heterogeneity. The tight reservoirs in China include a variety of rocks, such as tight sandstone, conglomerate, limestone, dolomite, tuff and transitional rocks etc. Tight oil reservoirs and sources are in a number of combination modes: reservoir-source in one, reservoir above source, and reservoir below source. Reservoirs in Ch7 member of Ordos Basin are thick (about 10 to 30 m) and rich in micro-fractures. Fuyu layer of Songliao Basin features thin single layers of 3−5 m, multi-layer stacking, and fast change laterally. In contrast, tight oil reservoirs in North America are simple in lithology and better in physical properties, and mainly sandstone and carbonate ones. (3) The porosity and permeability of tight oil reservoirs in China are very low. The tight oil reservoirs in China is generally less than 10% in porosity, and less than 0.1×10−3 μm2 in permeability, while porosity of North American tight oil reservoirs is about 10%−13%. Reservoir pore throat radius in China is of micro-nano scale. For example, the throat radius of CH7 tight oil reservoir mainly distributes from 0.10 to 0.75 μm (Fig. 1).
Typical genetic types of tight oil reservoirs in China’s continental basins Main controlling factors of Tight oil accu“sweet spots” mulation patterns Total thickness/m TOC/% Facies control Fracture Gravity Tight sandThick sandstone 10−30 5−20 Diagenetic Structural flow stone porous Tight sand- Sedimentary Thin sandstone 30−90 3−9 Delta Structural stone -Diagenetic porous Saline Dolomitic Thick dolomitic 50−150 2−15 Diagenetic Dissolution lacustrine sandstone sandstone porous Marl and lime＜2 Lacustrine Limestone Diagenetic Structural stone fracture Marl and lime＞200 1.6 Lacustrine Limestone stone fracture Gravity Tight sandThin sandstone 0.4−1.1 flow stone porous
Source rock Type High abundance shale High abundance shale High abundance shale Low abundance shale High abundance shale Low abundance shale High abundance sedimentary tuff
150
8.1
Sedimentary facies
Reservoir lithology
Lacustrine
Limestone
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Thick tuff porous
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(7) Reservoir buried depth differs widely in different basins. For example, Yanchang Formation in Ordos and Fuyu Formation in Songliao Basin are buried at around 2 000 m deep, while the depth of Lucaogou Formation in Jimusaer is about 3 000 m, and Sha3 member tight oil reservoir in Shulu is about 3 500−4 000 m deep.
3
Fig. 1 Pore throat radius distribution results obtained by constant rate mercury injection
(4) Affected by deposition mode, oil saturation differs quite a bit between reservoirs inside and outside sources. Tight oil reservoirs in source beds are generally higher in oil saturation. Oil saturation in CH7 formation is about 65%−85% (Fig. 2), Lucaogou Formation in Jimusaer Basin has an oil saturation of 70%−95%; in contrast, Fuyu Formation, a typical tight oil reservoir below source bed has an oil saturation of less than 50%, besides, wells there generally produce some water. (5) Tight oil reservoirs in different basins of China are quite different in oil density and GOR, and normal or low in pressure, but there are a few high pressure reservoirs. CH7 tight oil reservoir has an oil density of 0.70 to 0.85 g/cm3, GOR of 100 to 200 m3/m3, and pressure coefficient of 0.60−0.80; in comparison, Qingshankou tight oil reservoir in Songliao Basin has an oil density of 0.85 g/cm3, GOR about 40 m3/m3, and pressure coefficient of above 1.20; Lucaogou Formation tight oil reservoir in Jimusaer has an oil density of 0.80 to 0.90 g/cm3, and pressure in normal range. The tight oil reservoirs in the U.S., have an oil density of 0.80 to 0.82 g/cm3, GOR of about 90−250 m3/m3, pressure coefficient of 1.35 to 1.80, representing abnormal high pressure reservoirs. (6) Rock brittleness and stress difference are quite different in different basins. CH7 tight oil layer has a rock brittleness of about 40%, and a fairly small stress difference of about 4 to 7 MPa; Lucaogou tight oil layer has higher rock brittleness, and stress difference of less than 6 MPa in general; Fuyu tight oil layer in Songliao Basin has strong plasticity, and stress difference of around 10 MPa. High brittleness is favorable for the creation of reticular fractures, and the principal stress difference is one of the key factors controlling fracture direction [7−8].
Fig. 2
Porosity vs. oil saturation of Ch7 member
Challenges facing tight oil development
Featuring low porosity, low permeability and low pressure, continental tight oil reservoirs in China are facing problems like fast production decline, difficult energy compensation and poor producing degree in development. (1) The pore radius in tight oil reservoirs is of micro-nano scale, posing challenges to improving producing degree of reserves. Fig. 3 shows that about 30% to 50% of movable oil is stored in the submicron pores with radius of about 0.1−1 μm. The oil production after fracturing generally can not be very high because the reservoir properties are very poor, and the pore connectivity improvement by conventional fracturing is limited. For example, Ch7 reservoir in Changqing Oilfield, wells obtained industrial oil production after conventional fracturing is only about 40% of the total fractured wells, making it hard to produce the reserves. For the Fuyu reservoir in Daqing Oilfield, the fluid supply radius of conventional fracturing well is only 90 m, the control reserves of a vertical well is only 1.9×104 t, and the control reserves of conventional horizontal wells with a 600 m long horizontal section, is only about (7−9)×104 t. (2) The tight oil reservoirs are very low in pressure and permeability, making it very difficult to enhance single well production. Due to low formation pressure and low reserves controlled by individual wells, the natural energy is very limited, so tight oil wells universally suffer rapid production decline after the initial short-term high oil production. Fluids in tight oil reservoirs flow slowly due to low transmissibility of formation pressure, as a result, individual wells features long term low production, and low cumulative production. For example, Ch7 reservoir in Changqing Oilfield, oil production was about 5.8 t/d in formation testing phase, but only 0.6−0.9 t/d in production test phase. (3) It is hard to maintain reservoir pressure, posing challenges to enhance oil recovery. The recovery factor of tight oil
Fig. 3 Distribution of movable oil in Ch7 tight oil reservoir of Changqing Oilfield
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reservoirs is generally less than 10% by natural depletion, while wells may suffer quick water breakthrough after massive fracturing, making oil in matrix hard to be produced. (4) High project investment makes it difficult to improve the economic benefits. For instance, the Fuyu reservoir in Daqing Oilfield, the total cost of drilling a horizontal well and volume fracturing is about (3 000−5 000)×104 RMB, but the cumulative production of individual wells predicted is only 1.48×104 t, so the total cost per well must be reduced to less than 3 000×104 RMB to obtain economic benefit. Big buried depth of tight oil reservoirs in the Junggar, Bohai Bay, Qaidam basins etc, rough terrain and lack of water resources in Changqing oilfield, and low oil saturation in Songliao Basin make it difficult to reduce tight oil development cost in these basins. Therefore, only by establishing specific low cost development mode for each basin according to the tight oil reservoir features, can the tight oil in each basin be developed effectively and economically.
4
Effective development technologies
Huge in resource scale, tight oil exploration and development are necessary. However, facing low production and low benefit, the biggest problem is what development approach and technologies should be taken to maximize the production rate, recovery factor and comprehensive benefit. According to the present status of tight oil development technology at home and abroad [9−13], especially the technical research and practice in recent years, this paper puts forward some thoughts on effective development of tight oil. 4.1
Guiding ideology
Shaking off the bound of old concepts and ideas in conventional development, the tight oil development should center around economic benefit, and adhere to the principle of “Five integration”, i.e. “the integration of exploration and development, geology and engineering, economy and technology, surface and subsurface, and research and production”. With “massive re-fracturing and production” as the leading development technology, “factory-like” operation, “centralized” surface facility construction and refined management organization model should be adopted to improve single well production, reduce cost and enhance economic benefit of thigh oil development. 4.2
Connotation of development technology
“Massive re-fracturing and production” for horizontal wells mainly has three meanings: first, improvement flow condition and formation pressure by long horizontal section, multistage multi-cluster and high fluid volume fracturing, increasing single well production rate and cumulative recovery in “primary production”; Second, application of re-fracturing technology to increase the single crack length, or changing fracture direction to form new fracture network, to develop the remaining oil zone and increase formation pressure in “secondary recovery”; third, using integrated technology, includ-
ing integrated design, wellpad long horizontal well drilling, massive volume fracturing and re-fracturing, “factory-like” operation and centralized surface facility construction etc. 4.3
Technology mechanism
Because of low porosity and poor connectivity and low formation pressure, when produced by natural depletion, tight oil reservoir will see fast pressure drop and fast driving force reduction, in turn rapid decline of single well production and low oil recovery efficiency. Therefore, improving reservoir connectivity, complementing formation energy, and creating effective flow driving system are at the heart of tight oil development. Massive re-fracturing and production technology has a scientific theoretical base: during the fracturing process, fracturing fluid is forced into the pores because of fracturing pressure is greater than formation pressure, and balanced before blowout; during the producing process, under the pressure differential, oil in the pores flowing through the fractures into the wellbore, forming industrial oil production. When oil production becomes uneconomic, re-fracturing technology is used to increase the single fracture length, or changing fracture direction to form new fracture network, the formation energy will be re-supplied, and the next new production stage begins. 4.4 4.4.1
Key technologies Integrated design
Integrated design combines the optimization of exploration, development, engineering, and management etc. By extending exploration to the following operation, and early stepping-in of surface engineering design optimization and adjustment, an integrated information processing platform should be set up to meet the evaluation needs of the whole production process (fig. 4). Following the principle of reverse thinking and forward implementation, plan of all aspects will be optimized timely as according to the changes in reservoir parameters in real time. Integrated design mainly includes the following two connotations: first ‘reverse thinking’, based on the surface environment, and the optimization design of surface construction and system management, as well as tight oil resource evaluation results, the position, scale and number of drilling wellpads are optimized, then the well type and well trajectory are optimized according to platform position and reservoir stress direction; finally the horizontal well direction, fracturing scale and well spacing are optimized according to stress direction and fracture characteristics, to complete the well pattern optimization. Second, ‘forward implementation’, as optimized plans of all aspects are completed, well drilling sequence will be optimized according to the geological understandings. After new well data acquisition, geological model, well pattern and surface construction plans will be adjusted immediately until the completion of the overall plan. The goal of integrated design is to maximize the benefits and reserves producing degree.
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Fig. 4 Schematic diagram of interactive information integration processing platform
4.4.2
Long horizontal well drilling from wellpad
Reserves controlled by single well will be increased greatly with long horizontal wells, and adding more wells drilled from one wellpad can increase reserves controlled by the wellpad. One advantage of wellpad is to reduce the surface area significantly, and provide prerequisite conditions for the “factory-like” operation and centralized surface construction. Length of horizontal section, well orientation and well space are the key contents of wellpad horizontal well design. In different basins, tight oil reservoir conditions are different, so the design parameters are not the same. For example, tight oil reservoirs in Changqing oilfield, horizontal section length of horizontal wells is about 1 500 m. Reasonable well spacing calculated by crack monitoring and reservoir engineering method is 500 m; in comparison, in Daqing oilfield, horizontal section is about 1 200−1 500 m, reasonable well spacing is 700−1 400 m. Wellpad drilling is still at the exploratory stage in China at present. In tight oil pilot area of Daqing oilfield, reserves controlled by single horizontal well are (12−27)×104 t, 6 to 14 times of fractured vertical wells, and drilling and completion of 4 wells from one wellpad has been done. In Changqing oilfield, reserves controlled by single horizontal well reach (30−50)×104 t, 6 wells can be drilled from one wellpad, which means saving more than 60% of the surface area if popularized. 4.4.3
Massive volume fracturing
Massive volume fracturing is to optimize configuration of matrix pores and artificial fractures and to increase controlled volume in order to improve the producing degree of reserves. In the fracturing design, five points should be followed: (1) Increase fracturing stages and clusters to realize effective producing of controlled reserves across the whole horizontal section. Now, fracturing stages can reach more than 22; (2) Divide the horizontal section into many small blocks according to matrix permeability, and then design fracturing
scale of every block, including fracture space, length, and conductivity and perforation parameters, according to reservoir distribution characteristics. The ideal result is to realize uniform stimulation (Fig. 5). (3) Increase the fracturing fluid injection rate to expand fracture scale and effective drainage radius. At present, the fracturing fluid injection rate can reach 15 m3/min and drainage radius can be more than 2 times that of conventional fracturing. (4) Increase fracturing fluid injection volume. Continental tight oil reservoirs in China generally have low pressure, so large injection volume can maintain formation energy effectively. (5) Adding a big volume of sand at low sand fluid ratio. In order to reduce the risk of fracture closure, rear supporting agent will be injected to enhance crack support ability. In field operation, in blocks where horizontal stress difference and permeability are higher, cutting volume fracturing is recommended to reduce the interference between the fractures; for low permeability blocks, “breaking the block” fracturing is suggested to form fracture network connecting pores; in reservoirs with low horizontal principal stress difference, complex fractures are likely to form, so less stages and more clusters can be designed to reduce fracturing costs. In addition, most continental tight oil reservoirs are made up of stacked thin layers, how to increase horizontal well drilling encounter rate is a problem, so it is necessary to strengthen the reservoir geological evaluation for layers adjacent to the target layer, develop multilayer fracturing technology, in order to expand the stimulation scope of individual wells. Massive volume fracturing can greatly increase the contact area between matrix and fractures, and improve horizontal wells’ productivity. Table 2 shows the results of different fracturing process in a pilot of Changqing Oilfield. The results show fracturing stages, fracturing fluid injection rate and fracturing scale are positively correlated with tested oil production to some extent; YP10 had the most stages, and highest initial oil production, but it also saw rapid bottomhole pressure drop and production decline, so its cumulative production in 300 days is not the highest. YP6, YP9 had the least stages, but high fracturing fluid injection rate, and sand volume, Their 300 d cumulative production is close to YP10. YP7 with more fracturing stages, high fluid injection volume and fracturing scale, ranks first in 300 d cumulative production. So, more stages, high injection rate, and large scale fracturing are the fracturing direction.
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Fig. 5
Schematic diagram of volume fracturing by blocks
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Table 2
Statistics on volume fracturing horizontal wells in Ch7 tight oil reservoir of Xi-233 block
Length of hori- Stage Well name zontal well/m number YP3 1 523 11 YP4 1 461 10 YP5 1 874 10 YP10 1 535 21 YP6 1 536 9 YP9 1 535 8 YP8 1 535 14 YP7 1 535 15
Cluster number 22 20 20 42 44 39 28 30
Injection rate/ Total amount Fracturing fluid volume/m3 (m3·d−1) of sand/m3 6.0 483 6 560 6.0 439 7 653 6.0 439 5 899 6.0 1 058 16 047 15.0 1 274 12 876 15.0 1 019 10 270 7.5 1 448 12 582 7.6 1 488 13 611
Massive volume fracturing can not only increase the producing reserves in the horizontal section substantially, but also help maintain the formation energy. In Changqing oilfield, flow-back ratio of fracturing fluid is low, large amount of fracturing fluid is left around the horizontal well, forming a high pressure zone of 116%−128% of the original formation pressure (Table 3), which works as advanced water injection. 4.4.4
Re-fracturing
Re-fracturing can change artificial fracture direction to form a new fracture network and develop the remaining oil zone. Meanwhile, re-fracturing can increase formation pressure, which would be helpful for well production, and achieve “secondary recovery”. Re-fracturing mainly includes the following aspects: firstly, closed invalid fractures and “V” shaped fractures affected by local stress (Fig. 6a) in initial Table 3 Block
Well number
Xi233 An83
10 13
Well production in formation testing/(m3·d−1) 133 154 141 184 142 128 152 137
Cumulative oil production/t 2 923 2 714 2 242 3 826 3 592 3 886 3 722 4 154
fracturing needs re-fracturing, at this time single well fracturing design needs to be carried out. Secondly, the remaining high oil saturation areas on both sides of the fracture need re-fracturing (Fig. 7a). After long term production, re-fracturing of several wells can be carried out (Fig. 6b), changing artificial fracture direction, and improving the development effect (Fig. 7b). Thirdly, remaining oil reserves near the heel or toe of horizontal wells, and those not reached by the initial well trajectory, can be effectively developed by re-fracturing (Fig. 7c). According to “re-fracturing and production” process, and improvement of horizontal well anhydrous re-fracturing technology, CO2/N2 dry fracturing process has great advantages. As sand carrying agent, gas can be injected into reservoir easily to supply formation energy and reduce the oil-gas interfacial tension effectively and enhance oil recovery. Jilin
Horizontal well fracturing parameters and formation pressure prediction results
Length of Average fracturing parameters Before production horizontal Fracturing fluid Total flow-back Flow-back Remaining liquid Formation presStage/Cluster well/m volume/m3 volume/m3 ratio/% volume/m3 sure increase/% 1 537 13/26 9 665 4 671 48.3 4 994 16 760 10/23 7 681 2 753 35.8 4 928 28
Fig. 6
Fig. 7
Schematic diagram of re-fracturing technology
Residual oil distribution before and after re-fracturing
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Pressure coefficient 1.00 0.98
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oilfield has completed the development of related equipment for CO2/N2 dry fracturing process and the large scale application will be implemented in the near future. 4.4.5
Controlled oil production
Controlled oil production uses reasonable production regime to get a long-term and stable production rate, realizing uniform producing of single well controlled reserves, and thus higher cumulative production. After Multi-stage fracturing, tight oil wells may reach high production rate, if not controlled, production will decline rapidly, leaving many problems: firstly, the initial fast flow rate easily cause fracture closure which will result in failure of fracturing. Secondly, production regime changes frequently, which not only causes inconvenience to operation management, but also the change of wellbore pressure and flow rate, and in turn, the instability of fracturing proppant, even plugging wellbore and affecting normal well production. Thirdly, quick pressure drop near wellbore could cause pressure-sensitive damages. Controlled production can effectively avoid the above problems and ensure high permeability of fractures. According to wellhead pressure and back flow chart, reasonable nozzles were selected to control flow-back rate and prevent fracturing sand from carrying out in Daqing oilfield. With the stable production regime, vertical and horizontal wells in Xinjiang oilfield achieved good performance. Table 4 shows production of two wells in X233 pilot in Changqing Oilfield, Well YP7 produced at above 14 t/d steadily in the first year, with an annual decline rate of only 0.7%; while Well YP10’s initial production was higher, but the wellbore fluid level decreased significantly, and the annual decline rate was 48.7%. 4.4.6
“Factory-like” operation
“Factory-like” operation refers to the assembly line-like operation including drilling, completion, fracturing and production one after another with a series of mature technologies and standard equipment. Wells deployed from a wellpad Table 4 Well name YP7 YP10
Length of horiFracturing zontal section/m stage/Cluster 1535 1535
15/30 21/42
Fig. 8
should be similar in structure, same in completion process. Operation efficiency can be greatly improved and tight oil reservoir development cost will be reduced, based on “wellpad” drilling, and mature engineering technology. In China, “factory-like” operation is still at the stage of process testing and equipment matching. Fig. 8 shows the staggering fracturing technology in Changqing oilfield. A pilot of horizontal well volume fracturing shows that the fracturing operation time reduced from an average of 62 days in 2012 to 31 days in 2013, that is an improvement of operation efficiency by one time. A new type of fracturing fluid was developed to reduce the friction and cost of fracturing fluid after several years’ of research. Now, this technology has been applied in 95 wells with 14 070 m3 fracturing fluid recovered. 4.4.7
Centralized surface construction
Centralized surface construction means a centralized processing station in a certain region, which integrates three functions, fracturing water supply, fracturing fluid recovery and treatment, and crude oil gathering and transportation (Fig. 9). Every wellpad is connected to the centralized processing station by one set of pipeline system, which carries fracturing water to the wellpad, and transport oil and water to the station at fracturing fluid flow-back and production stage. This method can reduce the operation cost effectively, save energy and reduce emission, so it is a greener technology. In Changqing Oilfield, water shortage is a serious problem for “factory-like” operation, which is solved by designing and setting up multiple water wells supplying the processing station. Overall planning and unified construction of fracturing water supply, waste water processing and crude oil gathering and processing made it possible to realize centralized production and digitalized management, which results in improvement in production efficiency and reduction in labor intensity. In some special conditions, the well platform is too far away to design a processing station. Small skid mounted treatment system for single platform may be more economic benefit.
Comparison of fracturing and producing results in a tight oil pilot Initial phase Oil producWellbore tion/(t·d−1) fluid level/m 14.5 0 18.1 540
Cumulative Present Production oil producOil producWellbore days/d tion/t tion/(t·d−1) fluid level/m 14.0 0 338 5065 8.4 1300 457 5376
Comparison of normal fracturing and staggering fracturing
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Annual production decline rate/% 0.7 48.7
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