23 February 2022, Volume 49 Issue 1
    

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    PETROLEUM EXPLORATION
  • Practice and theoretical and technical progress in exploration and development of Shunbei ultra-deep carbonate oil and gas field, Tarim Basin, NW China
    MA Yongsheng, CAI Xunyu, YUN Lu, LI Zongjie, LI Huili, DENG Shang, ZHAO Peirong
    Petroleum Exploration and Development, 2022, 49(1): 1-17. https://doi.org/10.11698/PED.2022.01.01
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    In this review on the exploration and development process of the Shunbei ultra-deep carbonate oil and gas field in the Tarim Basin, the progress of exploration and development technologies during the 13th five-year plan has been summarized systematically, giving important guidance for the exploration and development of ultra-deep marine carbonate reservoirs in China and abroad. Through analyzing the primary geological factors of “hydrocarbon generation-reservoir formation-hydrocarbon accumulation” of ancient and superposed basin comprehensively and dynamically, we point out that because the Lower Cambrian Yuertusi Formation high-quality source rocks have been located in a low-temperature environment for a long time, they were capable of generating hydrocarbon continuously in late stage, providing ideal geological conditions for massive liquid hydrocarbon accumulation in ultra-deep layers. In addition, strike-slip faults developed in tectonically stable areas have strong control on reservoir formation and hydrocarbon accumulation in this region. With these understandings, the exploration focus shifted from the two paleo-uplifts located in the north and the south to the Shuntuoguole lower uplift located in between and achieved major hydrocarbon discoveries. Through continuing improvement of seismic exploration technologies for ultra-deep carbonates in desert, integrated technologies including seismic acquisition in ultra-deep carbonates, seismic imaging of strike-slip faults and the associated cavity-fracture systems, detailed structural interpretation of strike-slip faults, characterization and quantitative description of fault-controlled cavities and fractures, description of fault-controlled traps and target optimization have been established. Geology-engineering integration including well trajectory optimization, high efficiency drilling, completion and reservoir reformation technologies has provided important support for exploration and development of the Shunbei oil and gas field.
  • Geological characteristics, evaluation criteria and discovery significance of Paleogene Yingxiongling shale oil in Qaidam Basin, NW China
    LI Guoxin, ZHU Rukai, ZHANG Yongshu, CHEN Yan, CUI Jingwei, JIANG Yinghai, WU Kunyu, SHENG Jun, XIAN Chenggang, LIU He
    Petroleum Exploration and Development, 2022, 49(1): 18-31. https://doi.org/10.11698/PED.2022.01.02
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    Major breakthroughs of shale oil exploration have been made recently in the upper member of Paleogene Lower Ganchaigou Formation of Yingxiongling area, Qaidam Basin. However, the low total organic carbon content of saline-lacustrine source rock, and unclear genetic mechanism, evaluation criteria and resources potential of the shale oil have restricted the exploration and evaluation of Yingxiongling shale oil. Through analysis of large amounts of core, well drilling, seismic, laboratory test data and integrated study, focusing on the shale and mixed types of shale oil reservoirs characterized by high-frequency interbedded organic-rich laminated shale and limy dolomite, it is concluded that the shale oil in the upper member of Lower Ganchaigou Formation in the Yingxiongling area have six geological characteristics: (1) two-stage hydrocarbon generation of hydrogen-rich source rock and large amount of retained oil; (2) multiple types of reservoir space and good reservoir properties; (3) source-reservoir integration, thick “sweet spot” and high oil-bearing grade; (4) high pressure coefficient between and under salt layers, and sufficient formation energy; (5) high content of light components, high gas-oil ratio, and good quality of the crude oil; (6) high content of brittle minerals and good fracability. The evaluation criterion of shale oil is preliminarily established based on the eight parameters: total organic carbon content, maturity, effective porosity, oil saturation, brittle mineral content, pressure coefficient, lamellation density, and burial depth. Combined with parameters of E32 source rock thickness, area, vertical distribution of oil layers, and free hydrocarbon content, the shale oil resources have been preliminarily estimated as 21×108 t. The structurally stable area is the predominant objective of shale oil exploration and the favorable exploration area for Yingxiongling shale oil is nearly 800 km2.
  • Basic characteristics of key interfaces in Upper Ordovician Wufeng Formation - Lower Silurian Longmaxi Formation in Sichuan Basin and its periphery, SW China
    WANG Yuman, WANG Hongyan, QIU Zhen, SHEN Junjun, ZHANG Qin, ZHANG Leifu, WANG Canhui, LI Xinjing
    Petroleum Exploration and Development, 2022, 49(1): 32-44. https://doi.org/10.11698/PED.2022.01.03
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    Based on anatomy of key areas and data points and analysis of typical features of shell layer in Guanyinqiao Member, basic characteristics of key interfaces, mainly bentonite layers, in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in the Sichuan Basin and its surrounding areas and the relationship between these key interfaces with the deposition of organic-rich shale have been examined systematically. The Wufeng Formation-Longmaxi Formation has four types of marker beds with interface attributes, namely, the characteristic graptolite belt, Guanyinqiao Member shell layer, section with dense bentonite layers, and concretion section, which can be taken as key interfaces for stratigraphic division and correlation of the graptolite shale. The shell layer in Guanyinqiao Member is the most standard key interface in Wufeng Formation-Longmaxi Formation, and can also be regarded as an important indicator for judging the depositional scale of organic-rich shale in key areas. There are 8 dense bentonite sections of two types mainly occurring in 7 graptolite belts in these formations. They have similar interface characteristics with the shell layer in Guanyinqiao Member in thickness and natural gamma response, and belong to tectonic interfaces (i.e., event deposits). They have three kinds of distribution scales: whole region, large part of the region, and local part, and can be the third, fourth and fifth order sequence interfaces, and have a differential control effect on organic-rich shale deposits. The horizon the characteristic graptolite belt occurs first is the isochronous interface, which is not directly related to the deposition of organic-rich shale. Concretions only appear in local areas, and show poor stability in vertical and horizontal directions, and have no obvious relationship with the deposition of the organic-rich shale.
  • Characteristics and main control factors of Ordovician shoal dolomite gas reservoir in Gucheng area, Tarim Basin, NW China
    FENG Jun, ZHANG Yajin, ZHANG Zhenwei, FU Xiaofei, WANG Haixue, WANG Yachun, LIU Yang, ZHANG Junlong, LI Qiang, FENG Zihui
    Petroleum Exploration and Development, 2022, 49(1): 45-55. https://doi.org/10.11698/PED.2022.01.04
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    Based on seismic, drilling data and experimental analysis, the characteristics and main controlling factors of shoal dolomite gas reservoir in the third member of Ordovician Yingshan Formation of Gucheng area, Tarim basin were examined. The study shows that the dolomite gas reservoir in Gucheng area is lithologic gas reservoir controlled by shoal and fault jointly, and its formation is mainly attributed to the following factors: (1) The continuously developing paleotectonic structure has been in the direction of gas migration and accumulation; (2) The large area of medium-high energy grain bank deposited in gentle slope environment is the material basis for the formation of dolomite reservoir; (3) Atmospheric water leaching and dolomitization and fluid dissolution in fault zone are the key factors for the formation of high-quality dolomite reservoir; (4) The natural gas comes from cracking of the ancient oil reservoir and hydrocarbon generation of dispersed organic matter in source rocks, and the NNE-trending strike-slip fault is the dominant channel for vertical migration of natural gas; (5) Limestone cap rocks in the first and second members of Yingshan Formation provide direct sealing for the formation of gas reservoir there. On the basis of comprehensive analysis, it is pointed out that the Gucheng area has three grain shoal zones in the third member of Yingshan Formation in nearly S-N direction, which together with seven strike-slip fault zones in NNE direction control the development of shoal dolomite gas reservoir.
  • Development characteristics and petroleum geological significance of Permian pyroclastic flow volcanic rocks in Western Sichuan Basin, SW China
    PENG Hao, YIN Cheng, HE Qinglin, XIA Guoyong, LIU Yong, MA Tinghu, CHEN Kang, LIU Ran, SU Wang
    Petroleum Exploration and Development, 2022, 49(1): 56-67. https://doi.org/10.11698/PED.2022.01.05
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    By examining field outcrops, drilling cores and seismic data, it is concluded that the Middle and Late Permian “Emeishan basalts” in Western Sichuan Basin were developed in two large eruption cycles, and the two sets of igneous rocks are in unconformable contact. The lower cycle is dominated by overflow volcanic rocks; while the upper cycle made up of pyroclastic flow volcanic breccia and pyroclastic lava is typical explosive facies accumulation. With high-quality micro-dissolution pores and ultra-fine dissolution pores, the upper cycle is a set of high-quality porous reservoir. Based on strong heterogeneity and great differences of pyroclastic flow subfacies from surrounding rocks in lithology and physical properties, the volcanic facies and volcanic edifices in Western Sichuan were effectively predicted and characterized by using seismic attribute analysis method and instantaneous amplitude and instantaneous frequency coherence analysis. The pyroclastic flow volcanic rocks are widely distributed in the Jianyang area. Centering around wells YT1, TF2 and TF8, the volcanic rocks in Jianyang area had 3 edifice groups and an area of about 500 km2, which is the most favorable area for oil and gas exploration in volcanic rocks.
  • Lithofacies paleogeography restoration and its significance of Jurassic to Lower Cretaceous in southern margin of Junggar Basin, NW China
    GAO Zhiyong, SHI Yuxin, FENG Jiarui, ZHOU Chuanmin, LUO Zhong
    Petroleum Exploration and Development, 2022, 49(1): 68-80. https://doi.org/10.11698/PED.2022.01.06
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    In view of the difficulties in the study of lithofacies paleogeography and the low reliability of the distribution range of sedimentary sand bodies in the prototype basin caused by less deep drilling, complex seismic imaging and low degree of exploration in the southern margin of Junggar Basin. A new method based on the source to sink idea was used to restore lithofacies paleogeography and predict glutenite distribution. In the restoration, apatite fission track age was used to define range and uplift time of macro-provenance; the range of provenance area and the migration process of lake shoreline were restored based on the quantitative relationship between gravel diameter and transportation distance, tectonic shortening and other geological parameters; drilling cores and field outcrop sedimentary structures were analyzed, and a series of maps of lithofacies paleogeographic evolution and distribution range of glutenite bodies were compiled. It is concluded that from Early Jurassic to Early Cretaceous, in the southern margin of Junggar Basin, the provenance area gradually expanded from south to north, the lake basin expanded, shrunk and expanded, and the paleoclimate changed from humid to drought to humid. The western section always had proximal fan delta deposits from the southern ancient Tianshan provenance developed, and in the middle and eastern sections, the provenance areas evolved from far source to near source, mainly river-delta, braided delta, fan delta and other sediments developed. The boundary between provenance areas of the western and middle sections is speculated to be Hongche fault zone. In an angle open to the northwest with the current basin edge line, the restored ancient lake shoreline controlled the heterogeneity of reservoirs in the delta plain belt and delta front belt on its both sides. The ancient lake shoreline, current stratigraphic denudation line and current basin margin line limit the types and scope of favorable reservoirs. This understanding provides an important geological basis for oil and gas exploration in the deep lower source-reservoir assemblage at the southern margin of Junggar basin.
  • Saturation evaluation of microporous low resistivity carbonate oil pays in Rub Al Khali Basin in the Middle East
    WANG Yongjun, SUN Yuanhui, YANG Siyu, WU Shuhong, LIU Hui, TONG Min, LYU Hengyu
    Petroleum Exploration and Development, 2022, 49(1): 81-92. https://doi.org/10.11698/PED.2022.01.07
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    To solve the problem that it is difficult to identify carbonate low resistivity pays (LRPs) by conventional logging methods in the Rub Al Khali Basin, the Middle East, the variation of fluid distribution and rock conductivity during displacement were analyzed by displacement resistivity experiments simulating the process of reservoir formation and production, together with the data from thin sections, mercury injection and nuclear magnetic resonance experiments. In combination with geological understandings, the genetic mechanisms of LRPs were revealed, then the saturation interpretation model was selected, the variation laws and distribution range of the model parameters were defined, and finally an updated comprehensive saturation interpretation technique for the LRPs has been proposed. In the study area, the LRPs have resistivity values of less than 1 Ω·m, similar to or even slightly lower than that of the water layers. Geological research reveals that the LRPs were developed in low-energy depositional environment and their reservoir spaces are controlled by micro-scale pore throats, with an average radius of less than 0.7 μm, so they are typical microporous LRPs. Different from LRPs of sandstone and mudstone, they have less tortuous conductive paths than conventional reservoirs, and thus lower resistivity value under the same saturation. Archie's formula is applicable to the saturation interpretation of LRPs with a cementation index value of 1.77-1.93 and a saturation index value of 1.82-2.03 that are 0.2-0.4 lower than conventional reservoirs respectively. By using interpretation parameters determined by classification statistics of petrophysical groups (PGs), oil saturations of the LRPs were calculated at bout 30%-50%, 15% higher than the results by conventional methods, and basically consistent with the data of Dean Stark, RST, oil testing and production. The 15 wells of oil testing and production proved that the coincidence rate of saturation interpretation is over 90% and the feasibility of this method has been further verified.
  • Characteristics and impacts on favorable reservoirs of carbonate ramp microfacies: A case study of the Middle-Lower Ordovician in Gucheng area, Tarim Basin, NW China
    LIU Yini, HU Mingyi, ZHANG San, ZHANG Junlong, GAO Da, XIAO Chuantao
    Petroleum Exploration and Development, 2022, 49(1): 93-105. https://doi.org/10.11698/PED.2022.01.08
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    Based on the latest drilling core, thin section, 3D seismic, well logging data as well as exploration practice, the sequence stratigraphy and sedimentary microfacies of the Middle-Lower Ordovician carbonates in Gucheng area, and their controlling effects on the development of reservoir were examined by the theory and methods of fine carbonate sedimentological. The results show that the Middle-Lower Ordovician in Gucheng area is a set of typical carbonate ramp deposits, which can be divided into 10 microfacies in 4 subfacies as follows: back ramp, inner shallow ramp, outer shallow ramp and deep ramp. The back ramp subfacies consists of muddy-dolomitic flat and dolomitic lagoon microfacies, and is dominated by lamellar micrite dolomite tight in lithology. The inner shallow ramp subfacies includes dolomitic shoal and dolomitic flat in shoal top and dolomitic flat between shoals microfacies, and is mainly composed of crystal dolomite with metasomatic residual structure; with abundant karst vugs and intercrystalline pores, the crystal dolomite has an average porosity of 4.36%. The outer ramp subfacies includes medium-high and low energy grain shoal and inter-shoal microfacies, and is dominated by sandy limestone, oolitic limestone and micrite limestone with few pores. The deep ramp subfacies is dominated by low-energy argillaceous deposits, with storm shoal microfacies in local parts. The Lower-Middle Ordovician has six third-order sequences from bottom to top, among which SQ3 sequence (the third member of Yingying Formation), the main reservoir, has three fourth-order high-frequency sequences. Apparently, the favorable reservoir in the study area is jointly controlled by sedimentary microfacies and high frequency sequence in the carbonate ramp, the former controls the primary pore structure, and the latter controls the intensities of karstification exposure and dolomitization in the penecontemporaneous period. The dolomitic shoals and top dolomitic flats of different stages, superimposed and connected into favorable reservoirs of considerable scale like "platform margin", are favorable exploration facies in the carbonate ramp.
  • Discussion on classification and naming scheme of fine-grained sedimentary rocks
    PENG Jun, ZENG Yao, YANG Yiming, YU Ledan, XU Tianyu
    Petroleum Exploration and Development, 2022, 49(1): 106-115. https://doi.org/10.11698/PED.2022.01.09
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    Based on reviews and summaries of the naming schemes of fine-grained sedimentary rocks, and analysis of characteristics of fine-grained sedimentary rocks, the problems existing in the classification and naming of fine-grained sedimentary rocks are discussed. On this basis, following the principle of three-level nomenclature, a new scheme of rock classification and naming for fine-grained sedimentary rocks is determined from two perspectives: First, fine-grained sedimentary rocks are divided into 12 types in two major categories, mudstone and siltstone, according to particle size (sand, silt and mud). Second, fine-grained sedimentary rocks are divided into 18 types in four categories, carbonate rock, fine-grained felsic sedimentary rock, clay rock and mixed fine-grained sedimentary rock according to mineral composition (carbonate minerals, felsic detrital minerals and clay minerals as three end elements). Considering the importance of organic matter in unconventional oil and gas generation and evaluation, organic matter is taken as the fourth element in the scheme. Taking the organic matter contents of 0.5% and 2% as dividing points, fine grained sedimentary rocks are divided into three categories, organic-poor, organic-bearing, and organic-rich ones. The new scheme meets the requirement of unconventional oil and gas exploration and development today and solves the problem of conceptual confusion in fine-grained sedimentary rocks, providing a unified basic term system for the research of fine-grained sedimentology.
    • OIL AND GAS FIELD DEVELOMENT
  • Theoretical exploration of water injection gravity flooding oil in ultra-deep fault-controlled fractured-cavity carbonate reservoirs
    YANG Xuewen, WANG Rujun, DENG Xingliang, LI Shiyin, ZHANG Hui, YAO Chao
    Petroleum Exploration and Development, 2022, 49(1): 116-124. https://doi.org/10.11698/PED.2022.01.10
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    Based on the analysis of geological characteristics of ultra-deep fault-controlled fracture-cavity carbonate reservoirs and division of reservoir units, two physical models were made, and physical simulations of oil displacement by water injection were carried out to find out water flooding mechanism in the fault-controlled fracture-cavity carbonate reservoir under complex flow state. On this basis, a mathematical model of fault-controlled carbonate reservoir with coexisting seepage and free flow has been established. Pilot water injection tests have been carried out to evaluate the effects of enhancing oil recovery by water injection. The results show that: fault-controlled fracture-cavity carbonate reservoir units can be divided into three types: the type connected and strong in natural energy, the type connected and weak in natural energy, and the type isolated and weak in natural energy; the fault-fracture activity index of the fault-controlled fractured-cavity body can effectively characterize the connectivity of the reservoir and predict the effective direction of water injection; the mathematical model of fault-controlled carbonate reservoir with coexisting seepage and free flows can quantitatively describe the fluid flow law in the fracture-cavity body; the water injected into the fault-controlled fracture-cavity body is weakly affected by the capillary force of the lithologic body, and the oil-water movement is mainly dominated by gravity. The development modes of single well water injection, unit water injection, and single well high pressure water injection proposed based on the connection structure of fracture- cavity space and well storage space configuration are confirmed effective by pilot tests, with obvious water injection gravity flooding effect.
  • A reservoir drying method for enhancing recovery of tight gas
    ZHANG Liehui, XIONG Yu, ZHAO Yulong, TANG Hongming, GUO Jingjing, JIA Chunsheng, LEI Qiang, WANG Binghe
    Petroleum Exploration and Development, 2022, 49(1): 125-135. https://doi.org/10.11698/PED.2022.01.11
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    Based on the study of damage mechanisms of generalized water lock and related water lock removal methods, drying agents for enhancing tight gas reservoir recovery were developed, and the basic properties, injection mode and drying effect of the drying agents were evaluated. The chemical effect, thermal effect, salt resistance, salt resistance formulas and delay mechanism of the drying agent systems for different types of tight reservoirs developed through lab experiment were investigated. The solubility and solubilization properties of supercritical carbon dioxide on drying agent systems were tested. The injection mode of dissolving drying agent in supercritical carbon dioxide was proposed. The mechanisms of supercritical carbon dioxide with water in formation matrix micropores were analyzed. Micro-pore structures and seepage characteristics of reservoir before and after drying were compared. Based on the characterization combining NMR and laser etched pore structure model, drying effects of the drying agents on bound water of different occurrences were evaluated qualitatively and quantitatively. Lattice Boltzmann method was used to evaluate the influence of drying effect on gas micro-seepage ability. The influence of drying effect on productivity and production performance of gas well was analyzed by numerical simulation. The drying can greatly reduce water saturation of tight reservoir and improve the seepage capacity of gas near wellbore and fractures. The new measures enhancing recovery of tight gas reservoirs on the basis of reservoir drying could be developed.
  • 4D-stress evolution of tight sandstone reservoir during horizontal wells injection and production: A case study of Yuan 284 block, Ordos Basin, NW China
    ZHU Haiyan, SONG Yujia, LEI Zhengdong, TANG Xuanhe
    Petroleum Exploration and Development, 2022, 49(1): 136-147. https://doi.org/10.11698/PED.2022.01.12
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    To investigate the 4D stress change during injection and production in tight sandstone reservoirs, a multi-physical fields modeling method is proposed considering the reservoir heterogeneity, hydraulic fracture and complex injection-production system. The 4D stress evolution of tight sandstone reservoir in Yuan 284 block of Huaqing oilfield, Ordos Basin, during injection-production in horizontal wells is investigated by modeling coupled flow and geomechanics. Results show: (1) Induced by injection and production, the 3D stress increases near the injectors but decreases near the producers, and the horizontal stresses are distributed in obvious strips along their respective stress directions. (2) The horizontal stress difference is the highest at the horizontal wellbore beside injectors during injection and production, while it is the lowest in undeveloped zone between the injectors, and the orientation of maximum horizontal principal stress changes the most near the injectors, which is distributed radially. (3) The hydraulic fracture in re-fracturing well was observed to be asymmetrical in geometry and deflected as the stress changed. The results provide theoretical guidance for horizantal well network modification and re-fracturing optimization design in tight sandstone reservoir.
  • Mathematical model of dynamic imbibition in nanoporous reservoirs
    TIAN Weibing, WU Keliu, CHEN Zhangxing, LEI Zhengdong, GAO Yanling, LI Jing
    Petroleum Exploration and Development, 2022, 49(1): 148-155. https://doi.org/10.11698/PED.2022.01.13
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    The oil-water imbibition equation in the nano-scale pores considering the dynamic contact angle effect, nanoconfinement effect, inertia effect, and inlet end effect was established, and the relation between the friction coefficient of solid-oil-water three-phase contact line and the fluid viscosity in the interface zone was derived. In combination with the capillary bundle model and the lognormal distribution theory, the imbibition model of tight core was obtained and key parameters affecting imbibition dynamics were analyzed. The study shows that in the process of nanopore imbibition, the dynamic contact angle effect has the most significant impact on the imbibition, followed by nanoconfinement effect (multilayer sticking effect and slippage effect), and the inertia effect and inlet end effect have the least impact; in the initial stage of imbibition, the effect of inertial force decreases, and the effect of contact line friction increases, so the dynamic contact angle gradually increases from the initial equilibrium contact angle to the maximum and then remains basically stable; in the later stage of imbibition, the effect of contact line friction decreases, and the contact angle gradually decreases from the maximum dynamic contact angle and approaches the initial equilibrium contact angle; as the pore radius decreases, the dynamic contact angle effect increases in the initial stage of imbibition and decreases in the later stage of imbibition; as the oil-water interfacial tension increases, the imbibition power increases, and the dynamic contact angle effect increases; there is a critical value for the influence of interfacial tension on the imbibition dynamics. In improving oil recovery by imbibition in tight oil reservoir, interfacial tension too low cannot achieve good imbibition effect, and the best interfacial tension needs to be obtained through optimization.
  • An analysis method of injection and production dynamic transient flow in a gas field storage facility
    WANG Jieming, LI Chun, SUN Junchang, TANG Ligen, ZHONG Rong, LIU Xianshan, ZHENG Shaojing
    Petroleum Exploration and Development, 2022, 49(1): 156-165. https://doi.org/10.11698/PED.2022.01.14
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    A dynamic transient flow analysis method considering complex factors such as the cyclic injection and production history in a gas field storage facility was established in view of the limitations of the existing methods for transient flow analysis and the characteristics of the injection-production operation of strongly heterogeneous gas reservoirs, and the corresponding theoretical charts were drawn. In addition, an injection-production dynamic transient flow analysis model named "three points and two stages" suitable for an underground gas storage (UGS) well with alternate working conditions was proposed. The "three points" refer to three time points during cyclic injection and production, namely, the starting point of gas injection for UGS construction, the beginning and ending points of the injection-production analysis stage; and the "two stages" refer to historical flow stage and injection-production analysis stage. The study shows that the dimensionless pseudo-pressure and dimensionless pseudo-pressure integral curves of UGS well flex downward in the early stage of the injection and production process, and the dimensionless pseudo-pressure integral derivative curve is convex during the gas production period and concave during the gas injection period, and the curves under different flow histories have atypical features. The new method present in this paper can analyze transient flow of UGS accurately. The application of this method to typical wells in Hutubi gas storage shows that the new method can fit the pressure history accurately, and obtain reliable parameters and results.
    • PETROLEUM ENGINEERING
  • Progress and prospects of horizontal well fracturing technology for shale oil and gas reservoirs
    LEI Qun, XU Yun, CAI Bo, GUAN Baoshan, WANG Xin, BI Guoqiang, LI Hui, LI Shuai, DING Bin, FU Haifeng, TONG Zheng, LI Tao, ZHANG Haoyu
    Petroleum Exploration and Development, 2022, 49(1): 166-172. https://doi.org/10.11698/PED.2022.01.15
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    By systematically summarizing horizontal well fracturing technology abroad for shale oil and gas reservoirs since the "13th Five-Year Plan", this article elaborates new horizontal well fracturing features in 3D development of stacked shale reservoirs, small well spacing and dense well pattern, horizontal well re-fracturing, fracturing parameters optimization and cost control. In light of requirements on horizontal well fracturing technology in China, we have summarized the technological progress in simulation of multi-fracture propagation, horizontal well frac-design, electric-drive fracturing equipment, soluble tools and low-cost downhole materials and factory-like operation. On this basis, combined with the demand analysis of horizontal well fracturing technology in the "14th Five-Year Plan" for unconventional shale oil and gas, we suggest strengthening the research and development in the following 7 aspects: (1) geology-engineering integration; (2) basic theory and design optimization of fracturing for shale oil and gas reservoirs; (3) development of high-power electric-drive fracturing equipment; (4) fracturing tool and supporting equipment for long horizontal section; (5) horizontal well flexible-sidetracking drilling technology for tapping remaining oil; (6) post-frac workover technology for long horizontal well; (7) intelligent fracturing technology.
  • The biodiesel-based flat-rheology drilling fluid system
    JIANG Guancheng, SHI He, HE Yinbo
    Petroleum Exploration and Development, 2022, 49(1): 173-182. https://doi.org/10.11698/PED.2022.01.16
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    As the base oil of the current flat-rheology synthetic drilling fluid is high in cost and not renewable, the biodiesel-based flat-rheology drilling fluid with low-cost, environmental protection and renewable advantage was studied. Based on the optimization of raw materials, a cheap, environment-friendly biodiesel of soybean oil ethyl ester with good fluidity at low temperature was selected as the base oil. By selecting high oil-water ratio and introducing cationic surfactant into the auxiliary emulsifier, the thickening of biodiesel-based emulsion caused by hydrolysis and saponification after high-temperature aging was effectively eliminated. The organoclay prepared with cationic modifier of hexadecyl trimethyl ammonium chloride was used to improve the rheologic properties, stability and fluid loss of the drilling fluid while preventing low-temperature thickening. A flat-rheology modifier was synthesized with dimer fatty acid and cocoanut fatty acid diethanolamide, which could form strong network structure in the biodiesel-based drilling fluid to adjust effectively rheological properties of the drilling fluid. A biodiesel-based flat-rheology drilling fluid system with the density of 1.2 g/cm3 has been formulated which has constant rheology in the temperature range of 2-90 ℃, temperature tolerance of 160 ℃, seawater salinity tolerance of 5%, shale cuttings tolerance of 10%, and is environmentally friendly.
  • Evolution mechanism of optical fiber strain induced by multi-fracture growth during fracturing in horizontal wells
    CHEN Ming, GUO Tiankui, XU Yun, QU Zhanqing, ZHANG Shicheng, ZHOU Tong, WANG Yunpeng
    Petroleum Exploration and Development, 2022, 49(1): 183-193. https://doi.org/10.11698/PED.2022.01.17
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    A forward model for optical fiber strain was established based on a planar 3D multi-fracture model. Then the forward method calculating distributed fiber strain induced by multi-fracture growth was proposed. Based on this method, fiber strain evolution during fracturing of horizontal well was numerically simulated. Fiber strain evolution induced by fracture growth can be divided into three stages: strain increasing, shrinkage convergence, and straight line convergence; whereas the evolution of fiber strain rate has four stages: strain rate increasing, shrinkage convergence, straight line convergence, and strain rate reversal after pumping stop. Fiber strain does not flip after pumping stop, while the strain rate flips after pumping stop, so strain rate can reflect injection dynamics. The time when the fracture extends to the fiber and inter-well pressure channeling can be identified by the straight line convergence band of distributed fiber strain or strain rate, and the non-uniform growth of multiple fractures can be evaluated by using the instants of fractures reaching the fiber monitoring well. When the horizontal section of the fiber monitoring well is within the height range of a hydraulic fracture, the instant of the fracture reaching the fiber can be identified, otherwise, the converging band is not obvious. In multi-stage fracturing, under the influence of stress shadow from previous fracturing stages, the tensile region of fiber strain may not appear, but the fiber strain rate can effectively show the fracture growth behavior in each stage. The evolution law of fiber strain rate in single-stage fracturing can be applied to multi-stage fracturing.
  • Dynamic fluid transport property of hydraulic fractures and its evaluation using acoustic logging
    LI Huanran, TANG Xiaoming, LI Shengqing, SU Yuanda
    Petroleum Exploration and Development, 2022, 49(1): 194-202. https://doi.org/10.11698/PED.2022.01.18
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    The existing acoustic logging methods for evaluating the hydraulic fracturing effectiveness usually use the fracture density to evaluate the fracture volume, and the results often cannot accurately reflect the actual productivity. This paper studies the dynamic fluid flow through hydraulic fractures and its effect on borehole acoustic waves. Firstly, based on the fractal characteristics of fractures observed in hydraulic fracturing experiments, a permeability model of complex fracture network is established. Combining the dynamic fluid flow response of the model with the Biot-Rosenbaum theory that describes the acoustic wave propagation in permeable formations, the influence of hydraulic fractures on the velocity dispersion of borehole Stoneley-wave is then calculated and analyzed, whereby a novel hydraulic fracture fluid transport property evaluation method is proposed. The results show that the Stoneley-wave velocity dispersion characteristics caused by complex fractures can be equivalent to those of the plane fracture model, provided that the average permeability of the complex fracture model is equal to the permeability of the plane fracture. In addition, for fractures under high-permeability (fracture width 10~100 μm, permeability ~100 μm2) and reduced permeability (1~10 μm, ~10 μm2, as in fracture closure) conditions, the Stoneley-wave velocity dispersion characteristics are significantly different. The field application shows that this fluid transport property evaluation method is practical to assess the permeability and the connectivity of hydraulic fractures.
    • COMPREHENSIVE RESEARCH
  • Silurian hydrocarbon exploration breakthrough and its implications in the Shajingzi structural belt of Tarim Basin, NW China
    ZHANG Junfeng, ZHANG Yuanyin, GAO Yongjin
    Petroleum Exploration and Development, 2022, 49(1): 203-214. https://doi.org/10.11698/PED.2022.01.19
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    The Silurian hydrocarbon exploration in the northwest Tarim Basin had long been fruitless, till Well XSD1 drilled in 2018 in the Shajingzi structural belt, northwest Tarim Basin tapped industrial gas flow from the Silurian for the first time. The reservoir-forming model and resource extent need to be made clear urgently. Based on the comprehensive research of drilling, formation testing, geochemical data, and sedimentary and accumulation history, in combination with field surveys, experiments, structure interpretation and reconstruction of structure evolution, it is found that: (1) The northwest Tarim Basin had widespread tidal deltaic deposits in the Silurian period, which contain good reservoir-cap combinations; (2) the Shajingzi fault and associated faults connected the Cambrian-Ordovician source rocks in the Awati sag, and controlled the formation of Silurian structural traps, hence, the traps turned up along the structural belt in an orderly pattern and came together into contiguous tracts; (3) the Silurian petroleum in Shajingzi structural belt was dominated by gas, and the major accumulation period was the Himalayan period when the traps fixed in shape; (4) the Silurian gas resources in the Shajingzi belt were estimated at around 2.018×1011 m3, and Silurian gas resources of the northwest Tarim Basin were estimated at 2.03×1012 m3, implying huge exploration potential, so this area will become a major area for reserve and production increase from clastic strata in the basin; (5) with the Shajingzi fault of large scale and long active time connecting deep source rock layers, multiple formations in Lower Paleozoic of Shajingzi structural belt may have breakthroughs in hydrocarbon exploration.
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