23 April 2021, Volume 48 Issue 2
    

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    PETROLEUM EXPLORATION
  • Tectonic development of subtle faults and exploration in mature areas in Bohai Sea, East China
    XUE Yongan, LYU Dingyou, HU Zhiwei, HUANG Jiangbo, REN Jian
    Petroleum Exploration and Development, 2021, 48(2): 233-246. https://doi.org/10.11698/PED.2021.02.01
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    Based on the theory of structural analysis, the characteristics and structural patterns of subtle faults are studied using 3D seismic data of the Bohai Sea to analyze the development process and origin of the subtle faults. A method of identifying subtle faults is proposed, forming a complete system for analyzing origins of subtle faults in the Bohai Sea. The complex strike-slip fault patterns under the strike-slip and extension stress background, diverse formation rocks, and strong neotectonic movement are the reasons for the development of subtle faults. According to the tectonic origin and development location, the subtle faults in Bohai Sea can be divided into 12 types in the three categories of strike slip dominant, extension dominant and strike slip and extension composite, and the different types of subtle faults occur in different regions of the Bohai Sea. Unreasonable variation of sedimentary stratum thickness, inherited distortion or even abrupt change of stratum occurrence, zonation of plane fault combination, and the variation of oil-water system in the same structure with no-lithologic change are the important signs for identifying subtle faults in Bohai Sea. The subtle faults greatly enlarge the size of the structural trap groups, and areas with dense subtle faults are often active area of hydrocarbon migration and accumulation and favorable exploration zones, which have strong control on the hydrocarbon accumulation. The identification method for subtle faults has guided the exploration in the mature areas of Bohai Sea effectively, with a number of large and medium oil and gas fields discovered, such as Bozhong 29-6 and Penglai 20-2.
  • Deep-water gravity flow deposits in a lacustrine rift basin and their oil and gas geological significance in eastern China
    CAO Yingchang, JIN Jiehua, LIU Haining, YANG Tian, LIU Keyu, WANG Yanzhong, WANG Jian, LIANG Chao
    Petroleum Exploration and Development, 2021, 48(2): 247-257. https://doi.org/10.11698/PED.2021.02.02
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    The types, evolution processes, formation mechanisms, and depositional models of deep-water gravity flow deposits in a lacustrine rift basin are studied through core observation and systematic analysis. Massive transport of slide and slump, fluid transport of debris flow and turbidity currents are driven by gravity in deep-water lacustrine environment. The transformation between debris flow and turbidity current, and the transformation of turbidity current between supercritical and subcritical conditions are the main dynamic mechanisms of gravity flow deposits in a lake basin. The erosion of supercritical turbidity current controls the formation of gravity-flow channel. Debris flow deposition gives rise to tongue shape lobe rather than channel. Deep-water gravity flow deposits are of two origins, intrabasinal and extrabasinal. Intrabasinal gravity flow deposits occur as single tongue-shape lobe or fan of stacking multiple lobes. Extrabasinal gravity-flow deposits occur as sublacustrine fan with channel or single channel sand body. However, the nearshore subaqueous fan is characterized by fan of stacking multiple tongue shape lobes without channel. The differential diagenesis caused by differentiation in the nearshore subaqueous fan facies belt results in the formation of diagenetic trap. The extrabasinal gravity flow deposits are one of the important reasons for the abundant deep-water sand bodies in a lake basin. Slide mass-transport deposits form a very important type of lithologic trap near the delta front often ignored. The fine-grained sediment caused by flow transformation is the potential “sweet spot” of shale oil and gas.
  • Experimental study on attenuation of Stoneley wave under different fracture factors
    LI Ning, WANG Kewen, LIU Peng, WU Hongliang, FENG Zhou, FAN Huajun, SMEULDERS David
    Petroleum Exploration and Development, 2021, 48(2): 258-265. https://doi.org/10.11698/PED.2021.02.03
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    To quantitatively determine the effect of different factors such as fracture width, dip angle, extension and filling material on Stoneley wave amplitude decreasing, the shock tube experiment method was changed from fixing the sample and vertically moving the sensor in the borehole to fixing the sensors along the shock tube wall and vertically moving the sample without drilling the borehole in it. The measurement accuracy and the signal-to-noise ratio of the first Stoneley wave were improved by the time corrections and amplitude corrections of Stoneley wave signals. At the same time, 21 sets of core models with different fracture parameters were processed for this measurement method by using full-diameter carbonate core, and relative amplitudes were defined to characterize Stoneley wave amplitude decreasing. The experimental results show that the relative amplitude of Stoneley wave exponentially decreases with increasing fracture width. The relative amplitude of Stoneley wave linearly decreases with increasing fracture dip angle. The relative amplitude of Stoneley wave exponentially decreases with increasing fracture extension. The relative amplitude of Stoneley wave decreases with increasing the permeability of filling material in the fracture. Under the above four conditions, the fracture width has the greatest effect on the decreasing of Stoneley wave amplitude, followed by the fracture extension and the permeability of filling material, and finally the fracture dip angle.
  • Pore throat structure and classification of Paleogene tight reservoirs in Jiyang depression, Bohai Bay Basin, China
    WANG Yongshi, GAO Yang, FANG Zhengwei
    Petroleum Exploration and Development, 2021, 48(2): 266-278. https://doi.org/10.11698/PED.2021.02.04
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    The pore throat structure characteristics of Paleogene tight sandstone and sandy conglomerate in the Jiyang depression are studied using cast thin section, conventional mercury injection, constant rate mercury injection and micro CT scanning data, and a reservoir classification scheme based on pore throat structure parameters is established. The material composition and structural characteristics of tight reservoirs are analyzed by casting thin section data. The pore throat structure characteristics of tight reservoirs are studied by conventional mercury injection, constant rate mercury injection and micro CT scanning. Ten pore throat structure parameters are analyzed by cluster analysis. Based on the classification results and oil test results, the classification scheme of Paleogene tight reservoirs is established. The Paleogene tight reservoirs in the Jiyang depression have the characteristics of macropores and microthroats, with pores in micron scale, throats in nano-submicron scale, and wide variation of ratio of pore radius to throat radius. The permeability of the tight reservoir is controlled by throat radius, the smaller the difference between pore radius and throat radius, and the more uniform the pore throat size, the higher the permeability will be. The lower limits of average pore throat radius for the tight sandstone and tight sandy conglomerate to produce industrial oil flow without fracturing are 0.6 μm and 0.8 μm, respectively. Reservoirs that can produce industrial oil flow only after fracturing have an average pore-throat radius between 0.2-0.6 μm, and reservoirs with average pore throat radius less than 0.2 μm are ineffective reservoirs under the current fracturing techniques. Different types of tight sandstone and sandy conglomerate reservoirs are classified and evaluated, which are well applied in exploratory evaluation.
  • Build-ups and hydrocarbon accumulation of the isolated carbonate platforms in the eastern Mediterranean
    WEN Zhixin, TONG Xiaoguang, GAO Huahua, WANG Zhaoming, CHEN Ruiyin, SONG Chengpeng, HE Zhengjun, LIU Zuodong, KANG Hailiang
    Petroleum Exploration and Development, 2021, 48(2): 279-289. https://doi.org/10.11698/PED.2021.02.05
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    Based on the data of regional geology, 2D seismic exploration and ocean drilling, the formation of the Eratosthenes Seamount (ESM) and its surrounding isolated platforms, types of organic reefs and hydrocarbon accumulation conditions in the eastern Mediterranean are analyzed through fine tectonic interpretation and seismic facies study, and the future exploration targets are pointed out. The formation and evolution of the ESM and its peripheral isolated platforms are highly related to the open and close of the Neotethyan ocean. The precursors of the ESM and its peripheral isolated platforms are both horst-type fault blocks formed in the Middle Triassic-Early Jurassic intracontinental rift stage. The ESM and its peripheral isolated platforms underwent continued and inherited carbonate build-ups during the Middle Jurassic intercontinental rift stage, the Late Jurassic-Late Cretaceous Turonian passive drift stage, and Late Cretaceous Senonian-Miocene subduction stage, as well as medium-slight inversion transformation beginning in the Late Miocene Messinian caused by the closure of the Neotethyan ocean. Three types of isolated platforms formed controlled by variant paleo-tectonic settings: the first type is composed of a single patch-like reef controlled by a small-scale and narrow horst-type fault block, the second type consists of a single circle-like reef controlled by a middle-scale and wide horst-type fault block, and the third type is comprised of multiple reef-beach complexes controlled by a large-scale and broad paleo-high. The first two types universally developed in the highs of the Aixi and Ainan structure belts, and the third type only developed in the ESM. As a result of fluctuation of sea level, two sequences of reef build-ups, i.e. the Middle Jurassic Bajocian-Upper Cretaceous Turonian and the Miocene, developed in the ESM as well as the highs in the Aixi and Ainan structure belts. Drillings have confirmed that the first two types of isolated platforms with a single patch-like reef and a single circle-like reef have good conditions for natural gas accumulations. The isolated platforms of reef-beach complexes in the ESM also has potentials of natural gas and is worth prospecting.
  • Natural gas geology and exploration direction of the Cambrian Lower Canglangpu Member in central Sichuan paleo-uplift, Sichuan Basin, SW China
    YAN Wei, LUO Bing, ZHOU Gang, CHEN Youlian, ZHONG Yuan, LI Kunyu, ZHOU Hongfei, ZHAO Like, HE Yuan
    Petroleum Exploration and Development, 2021, 48(2): 290-302. https://doi.org/10.11698/PED.2021.02.06
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    Based on drilling data of JT1 and CS1, outcrop profiles and seismic data, the sedimentary pattern, space configuration of source-reservoir-cap combinations and paleouplift evolution characteristics of the Cambrian Lower Canglangpu Member in the central Sichuan paleouplift have been investigated to determine the favorable exploration zones. Controlled by Deyang-Anyue rift, the Lower Canglangpu Member features differential characteristics from east to west in sedimentary framework. In the west side of the Deyang-Anyue rift, this member is composed of clastic sediments, while in the east side of the Deyang-Anyue rift, it is fresh water shelf sediments, with grain beaches occurring along edges of the depressions, shelf and lagoon. Among the beaches, the dolomitic ones are mainly distributed around the depression edge and in the north of the lagoon. The beach sediments could become high quality reservoirs under the effect of karstification. Underlying the Lower Canglangpu Member is the widespread source rock of the Qiongzhusi Formation, so oil and gas generated by the Qiongzhusi Formation source rock can directly charge into the beach reservoirs. The sandy mudstone of the Upper Canglangpu Member can serve as the caprock of the Lower Canglangpu Member reservoir. The dolomitic beaches are located at the higher part of the paleo-uplift during the main accumulation period, with good hydrocarbon accumulation elements. Blocks PT1-CS1, HT1-GT2 and east GT2 are favorable exploration zones.
  • Formation of hoodoo-upland on Ordovician karst slope and its significance in petroleum geology in Tahe area, Tarim Basin, NW China
    ZHANG San, JIN Qiang, SUN Jianfang, WEI Hehua, CHENG Fuqi, ZHANG Xudong
    Petroleum Exploration and Development, 2021, 48(2): 303-313. https://doi.org/10.11698/PED.2021.02.07
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    Based on a large number of geological and geophysical data, the formation, fracture-caves types and hydrocarbon distribution of hoodoo-upland on the Ordovician karst slope in the Tahe area, Tarim Basin, are discussed by analyzing faults and strata thickness. The hoodoo-upland was made of high peaks and narrow valleys in the Ordovician karst slope during the Early Hercynian karst period, which were distributed along the NNE positive flower structure and had inherited evolution. The fault-fractures and fracture-vugs complex were extremely developed, with a thickness of 100 m. The cumulative oil production of 60% oil wells was more than 20×104 t per well in the hoodoo-upland, where the residual thickness of the Ordovician Yingshan Formation was greater than karst depressions. Caves formed by the shelter of collapsed breccias were developed in the valleys. They were 1.6 to 13.5 m high, with a filling rate of 51.6%. The positive flower structure under the settings of strike-slip compression controlled the early formation of the hoodoo-upland on the karst slope, resulting in the differences of drainage distribution and karstification. Compared with the water-rich karst valley, the hoodoo-upland with lean water suffered weaker karstification, had thicker residual stratum, and was higher in terrain. In rainy season, the meteoric water flew and corrode along the cracks, forming a complex network of fractures and caves. Combined with inherited uplift and the effective match of the NNE deep faults, oil and gas continuously charged into the reservoir space in the upland, forming the hoodoo fracture-cave reservoir with vertically quasi continuous distribution, high hydrocarbon abundance and high production.
  • Architecture characterization of Ordovician fault-controlled paleokarst carbonate reservoirs in Tuoputai, Tahe oilfield, Tarim Basin, NW China
    ZHANG Wenbiao, DUAN Taizhong, LI Meng, ZHAO Huawei, SHANG Xiaofei, WANG Yan
    Petroleum Exploration and Development, 2021, 48(2): 314-325. https://doi.org/10.11698/PED.2021.02.08
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    Based on outcrop, core, logging, seismic and production data, and the formation of fault-controlled karst reservoirs, the types and characterization of Ordovician fault-controlled karst reservoir architectures in the Tuoputai area of the Tahe oilfield are studied. According to the concept of genetic geologic body, the fault-controlled karst reservoir is divided into architecture elements of four levels, the strike-slip fault impacting zone is the level-1 architecture element, the fault-controlled karst reservoir the level-2 architecture element, the fracture-cave zone (which can be further subdivided into dissolution cave, dissolution pore and vug, and fracture zones) inside the fault-controlled karst reservoir the level-3 architecture element, and fillings inside caves is the level-4 architecture element (which can be further divided based on the filling degree and lithologic types of the fillings). Specific characterization techniques have been optimized according to the characteristics of various architecture elements. The zone impacted by strike-slip fault is characterized by seismic coherence and artificial interpretation. Under the constraint of zone impacted by strike-slip fault, fault likelihood (FL) property is used to characterize the outline of fault-controlled karst reservoir. Under the constraint of fault-controlled karst reservoir outline, the internal structures are divided based on seismic texture attribute. Finally, the cavern filling pattern is interpreted based on drilling and logging data. The fault-controlled karst reservoirs can be interpreted in 3-dimensional space by architecture element levels, and the characterization technology combining log and seismic data for fault-controlled karst reservoir has been worked out, which has complemented the development theory and technologies for this kind of reservoirs in the Tahe oilfield.
    • OIL AND GAS FIELD DEVELOPMENT
  • “Extreme utilization” development theory of unconventional natural gas
    MA Xinhua
    Petroleum Exploration and Development, 2021, 48(2): 326-336. https://doi.org/10.11698/PED.2021.02.09
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    In the process of unconventional natural gas development practice, the “extreme utilization” concept that focuses on “continuously breaking through the limit of development technology” is gradually formed, and supports the scale benefit exploration of unconventional gas in China. On this basis, the development theory of “extreme utilization” is proposed, its theoretical connotation together with development technologies of unconventional natural gas are clarified. The theoretical connotation is that, aiming at “extreme gas reservoirs”, “extreme techniques” are utilized to build subsurface connected bodies, expand the discharge area, and enlarge the production range, to obtain the maximum single-well production, extreme recovery, and eventually achieve the “extreme effect” of production. The series of development technologies include micro/nano-scale reservoir evaluation, “sweet spot” prediction, unconventional percolation theory and production capacity evaluation, optimization of grid well pattern, optimal-fast drilling and volume fracturing, and working regulation optimization and “integrated” organizing system. The “extreme utilization” development theory has been successfully applied in the development of unconventional gas reservoirs such as Sulige tight gas, South Sichuan shale gas, and Qinshui coalbed methane. Such practices demonstrate that, the “extreme utilization” development theory has effectively promoted the development of unconventional gas industry in China, and can provide theoretical guidance for effective development of other potential unconventional and difficult-to-recovery resources.
  • Change of phase state during multi-cycle injection and production process of condensate gas reservoir based underground gas storage
    TANG Yong, LONG Keji, WANG Jieming, XU Hongcheng, WANG Yong, HE Youwei, SHI Lei, ZHU Huayin
    Petroleum Exploration and Development, 2021, 48(2): 337-346. https://doi.org/10.11698/PED.2021.02.10
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    Based on the differences in production mode and operation process between gas storage and gas reservoir, we established a phase balance test procedure and a theoretical simulation model of phase balance during multi-cycles of injection and production of underground gas storage (UGS) rebuilt from condensate gas reservoir to study the phase characteristics of produced and remaining fluids during multi-cycles of injection and production. Take condensate reservoir gas storage as example, the composition of produced fluid and remaining fluid, phase state of remaining fluid, retrograde condensate saturation and condensate recovery degree in the process of multi-cycles of injection-production were studied through multi-cycle injection-production experiment and phase equilibrium theory simulation. The injected gas could greatly improve the recovery of condensate oil in the gas reservoir, and the condensate oil recovery increased by 42% after 5 cycles of injection and production; the injected gas had significant evaporative and extraction effects on the condensate, especially during the first two cycles; the condensate oil saturation of the formation decreased with the increase of injection-production cycles, and the condensate oil saturation after multi-cycles of injection-production was almost 0; the storage capacity increased by about 7.5% after multi-cycles of injection and production, and the cumulative gas injection volume in the 5th cycle increased by about 25% compared with that in the 1st cycle.
  • A method of reconstructing 3D model from 2D geological cross-section based on self-adaptive spatial sampling: A case study of Cretaceous McMurray reservoirs in a block of Canada
    WANG Lixin, YIN Yanshu, WANG Hui, ZHANG Changmin, FENG Wenjie, LIU Zhenkun, WANG Pangen, CHENG Lifang, LIU Jiong
    Petroleum Exploration and Development, 2021, 48(2): 347-359. https://doi.org/10.11698/PED.2021.02.11
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    An orthogonal 2D training image is constructed from the geological analysis results of well logs and sedimentary facies; the 2D probabilities in three directions are obtained through linear pooling method and then aggregated by the logarithmic linear pooling to determine the 3D multi-point pattern probabilities at the unknown points, to realize the reconstruction of a 3D model from 2D cross-section. To solve the problems of reducing pattern variability in the 2D training image and increasing sampling uncertainty, an adaptive spatial sampling method is introduced, and an iterative simulation strategy is adopted, in which sample points from the region with higher reliability of the previous simulation results are extracted to be additional condition points in the following simulation to improve the pattern probability sampling stability. The comparison of lateral accretion layer conceptual models shows that the reconstructing algorithm using self-adaptive spatial sampling can improve the accuracy of pattern sampling and rationality of spatial structure characteristics, and accurately reflect the morphology and distribution pattern of the lateral accretion layer. Application of the method in reconstructing the meandering river reservoir of the Cretaceous McMurray Formation in Canada shows that the new method can accurately reproduce the shape, spatial distribution pattern and development features of complex lateral accretion layers in the meandering river reservoir under tide effect. The test by sparse wells shows that the simulation accuracy is above 85%, and the coincidence rate of interpretation and prediction results of newly drilled horizontal wells is up to 80%.
  • Effects of salinity and ionic composition of smart water on mineral scaling in carbonate reservoirs during water flooding
    GHASEMIAN Javad, RIAHI Siavash
    Petroleum Exploration and Development, 2021, 48(2): 360-367. https://doi.org/10.11698/PED.2021.02.12
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    This work was conducted to study the risk of formation damage as the result of mineral scales deposition during smart waterflooding into carbonate core sample, as well as the influence of injected water salinity and ionic composition on mineral scaling and precipitation. The reservoir flowing conditions were simulated by a new laboratory core-flooding procedure, which took into count of the effect of in-situ contact time (CT) of injected water and formation water on scaling. After the optimum CT was determined, extent of permeability decline was studied by the change in the salinity and ionic composition of injection seawater. The scaled core sample was analyzed visually by scanning electron microscopy (SEM) to study the crystal morphology of the scale. Under the experimental conditions, extent of permeability decline caused by CaSO4 and CaSO3 composite scales ranged from 61% to 79.1% of the initial permeability. The salinity and the ionic composition of injected smart water, and CT of the mixing waters had significant effects on the co-precipitation of CaSO4 and CaSO3 scales. The SEM images reveal that the loss of permeability is mainly caused by the accumulation and growth perpendicular to the pore wall of scale crystals.
  • Quantitative characterization of irregular micro-fracture network and its effect on the permeability of porous media
    LI Tao, LI Qian, HU Yong, PENG Xian, FENG Xi, ZHU Zhanmei, ZHAO Zihan
    Petroleum Exploration and Development, 2021, 48(2): 368-378. https://doi.org/10.11698/PED.2021.02.13
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    Based on the comprehensive understanding on micro-fractures and matrix pores in reservoir rocks, numerical algorithms are used to construct fractured porous media and fracture-pore media models. Connectivity coefficient and strike factor are introduced into the models to quantitatively characterize the connectivity and strike of fracture network, respectively. The influences of fracture aperture, fracture strike and fracture connectivity on the permeability of porous media are studied by using multi-relaxation-time lattice Boltzmann model to simulate fluid flow in them. The greater the strike factor and the smaller the tortuosity of the fractured porous media, the greater the permeability of the fractured porous media. The greater the connectivity coefficient of the fracture network is, the greater the permeability of the fracture-pore media is, and the more likely dominant channel effect occurs. The fracture network connectivity has stronger influence on seepage ability of fracture-pore media than fracture aperture and fracture strike. The tortuosity and strike factor of fracture network in fractured porous media are in polynomial relation, while the permeability and fracture network connectivity coefficient of the fracture-pore media meet an exponential relation.
  • The effect of inorganic salt precipitation on oil recovery during CO2 flooding: A case study of Chang 8 block in Changqing oilfield, NW China
    YUAN Zhou, LIAO Xinwei, ZHANG Kuaile, ZHAO Xiaoliang, CHEN Zhiming
    Petroleum Exploration and Development, 2021, 48(2): 379-387. https://doi.org/10.11698/PED.2021.02.14
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    Static experiments and dynamic displacement experiments were conducted to quantitatively determine the amount of precipitate generated by the CO2-formation water reaction at different temperatures, pressures, and scaling ion concentrations during CO2 flooding in the Chang 8 block of Changqing Oilfield, the influence of precipitate on the physical properties of reservoirs was investigated, and the corresponding mathematical characterization model was established. The mathematical characterization equation was used to correct the numerical simulation model of E300 module in Eclipse software. The distribution pattern of inorganic salt precipitates during continuous CO2 flooding in Chang 8 block was simulated, and the influence of inorganic salt precipitates on oil recovery was predicted. The inorganic salt precipitate generated during CO2-formation water reaction was mainly CaCO3, and the pressure difference and scaling ion concentration were proportional to the amount of precipitate generated, while the temperature was inversely proportional to the amount of precipitate. The rate of core porosity change before and after CO2 flooding was positively correlated with temperature and flooding pressure difference. The core porosity increase in the CO2-formation water-core reaction experiment was always lower than that of CO2-distilled water-core reaction experiment because of precipitation. The area around the production wells had the most precipitates generated with the injection of CO2. The oil field became poor in development because of the widely distributed precipitate and the recovery decreased to 33.45% from 37.64% after 20-year-CO2 flooding when considering of precipitation.
    • PETROLEUM ENGINEERING
  • Similarity evaluation of stratum anti-drilling ability and a new method of drill bit selection
    YAN Tie, XU Rui, SUN Wenfeng, LIU Weikai, HOU Zhaokai, YUAN Yuan, SHAO Yang
    Petroleum Exploration and Development, 2021, 48(2): 386-393. https://doi.org/10.11698/PED.2021.02.15
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    Considering the stratum anti-drilling ability, drill bit working conditions, drill bit application effect and drill bit economic benefits, the similarity of stratum anti-drilling ability was evaluated by grey relational analysis theory to screen out candidate drill bits with reference values. A new comprehensive performance evaluation model of drill bit was established by constructing the absolute ideal solution, changing the relative distance measurement method, and introducing entropy weight to work out the closeness between the candidate drill bits and ideal drill bits and select the reasonable drill bit. Through the construction of absolute ideal solution, improvement of relative distance measurement method and introduction of entropy weight, the inherent defects of TOPSIS decision analysis method, such as non-absolute order, reverse order and unreasonable weight setting, can be overcome. Simple in calculation and easy to understand, the new bit selection method has good adaptability to drill bit selection using dynamic change drill bit database. Field application has proved that the drill bits selected by the new drill bit selection method had significant increase in average rate of penetration, low wear rate, and good compatibility with the drilled formations in actual drilling. This new method of drill bit selection can be used as a technical means to select drill bits with high efficiency, long life and good economics in oilfields.
  • Hydraulic fracturing induced casing shear deformation and a prediction model of casing deformation
    LU Qianli, LIU Zhuang, GUO Jianchun, HE Le, LI Yanchao, ZENG Ji, REN Shan
    Petroleum Exploration and Development, 2021, 48(2): 394-401. https://doi.org/10.11698/PED.2021.02.16
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    To study the casing deformation (CD) in shale gas well fracturing caused by natural fracture slip, a fracture face stress model is built based on stress analysis, and a CD prediction model is established based on complex function to analyze factors affecting wellbore shear stress and CD. (1) The fracture and wellbore approach angles have significant impacts on the wellbore shear stress. In Weiyuan shale gas field, Sichuan Basin, under the common wellbore approach angle of nearly 90°, the wellbore is subjected to large shear stress and high risk of CD at the fracture approach angle range of 20° to 55° or its supplementary angle range. (2) When the fracture is partially opened, the wellbore shear stress is positively correlated with the fluid pressure, and negatively correlated with the fracture friction coefficient; when the fracture is fully opened, the wellbore shear stress is positively correlated with the natural fracture area. (3) The lower the elastic modulus and the longer the fracture length, the more serious the CD will be, and the Poisson's ratio has a weak influence on the CD. The deformation first increases and then decreases with the increase of fracture approach angle, and reaches the maximum when the fracture approach angle is 45°. (4) At a given fracture approach angle, appropriately adjusting the wellbore approach angle can avoid high shear stress acting on wellbore, and reasonable control of the fluid pressure in the fracture can reduce the CD risk. The shear stress acting on casing is usually much greater than the shear strength of casing, so increasing casing strength or cementing quality have limited effect on reducing the risk of CD. Caliper logging data has verified that the CD prediction model is reliable, so the model can be used to establish risk analysis chart and calculate deformation value, to provide a reference for quick CD risk prediction in fracturing design.
  • Features of fracture height propagation in cross-layer fracturing of shale oil reservoirs
    WANG Yizhao, HOU Bing, WANG Dong, JIA Zhenhua
    Petroleum Exploration and Development, 2021, 48(2): 402-410. https://doi.org/10.11698/PED.2021.02.17
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    Triaxial fracturing modeling experiments were carried out on whole diameter shale cores from different layers of Shahejie Formation in the Dongpu sag, Bohai Bay Basin to find out the vertical propagation shapes of hydraulic fractures in different reservoirs. A numerical simulation method of inserting cohesive elements globally was adopted to build a pseudo-three-dimension fracture propagation model for multiple shale oil reservoirs considering interface strength, perforation location and pump rate, to research the features of hydraulic fracture (HF) penetrating through layers. The hydraulic fracture propagates in a cross pattern in tight sandstone layers, in a straight line in sandstone layers with natural fractures, forms ladder fracture in shale layers with beddings. The hydraulic fracture propagates in a stripe shape vertically in both sandstone and shale layers, but it spreads in plane in shale layers after connecting beddings. Restricted by beddings, the hydraulic fractures in shale layers are smaller in height than those in sandstone layers. When a sandstone layer and a shale layer are fractured at the same time, the fracture extends the most in height after the two layers are connected. Perforating at positions where the sandstone-shale interface is higher in strength, and increasing the pumping rate can enhance the fracture height, thus achieving the goal of increasing the production by cross-layer fracturing in multiple shale oil layers.
    • NEW ENERGY AND EMERGING FIELD
  • The role of new energy in carbon neutral
    ZOU Caineng, XIONG Bo, XUE Huaqing, ZHENG Dewen, GE Zhixin, WANG Ying, JIANG Luyang, PAN Songqi, WU Songtao
    Petroleum Exploration and Development, 2021, 48(2): 411-420. https://doi.org/10.11698/PED.2021.02.18
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    Carbon dioxide is an important medium of the global carbon cycle, and has the dual properties of realizing the conversion of organic matter in the ecosystem and causing the greenhouse effect. The fixed or available carbon dioxide in the atmosphere is defined as “gray carbon”, while the carbon dioxide that cannot be fixed or used and remains in the atmosphere is called “black carbon”. Carbon neutral is the consensus of human development, but its implementation still faces many challenges in politics, resources, technology, market, and energy structure, etc. It is proposed that carbon replacement, carbon emission reduction, carbon sequestration, and carbon cycle are the four main approaches to achieve carbon neutral, among which carbon replacement is the backbone. New energy has become the leading role of the third energy conversion and will dominate carbon neutral in the future. Nowadays, solar energy, wind energy, hydropower, nuclear energy and hydrogen energy are the main forces of new energy, helping the power sector to achieve low carbon emissions. “Green hydrogen” is the reserve force of new energy, helping further reduce carbon emissions in industrial and transportation fields. Artificial carbon conversion technology is a bridge connecting new energy and fossil energy, effectively reducing the carbon emissions of fossil energy. It is predicted that the peak value of China’s carbon dioxide emissions will reach 110×108 t in 2030. The study predicts that China's carbon emissions will drop to 22×108 t, 33×108 t and 44×108 t, respectively, in 2060 according to three scenarios of high, medium, and low levels. To realize carbon neutral in China, seven implementation suggestions have been put forward to build a new “three small and one large” energy structure in China and promote the realization of China's energy independence strategy.
  • Enrichment of tight oil and its controlling factors in central and western China
    SONG Yan, LUO Qun, JIANG Zhenxue, YANG Wei, LIU Dongdong
    Petroleum Exploration and Development, 2021, 48(2): 421-433. https://doi.org/10.11698/PED.2021.02.19
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    Taking the tight oil of the Zhongnan sag in the Ordos Basin, Jimusar sag in the Junggar Basin and Qingxi sag in the Jiuquan Basin as study objects, based on field survey, dissection of tight oil reservoirs, sample test, modeling experiment and comprehensive analysis, this study reveals that the tight oil accumulates at start-up pressure, advances under differential pressure, diffuses at alternating fast and low speeds, charges in stepped large area and migrates rapidly through fractures, and enriches in dominant fractures and pores. The root cause of ladder-like charge is the multiple scales of pores. The widespread source rock with high hydrocarbon generation intensity is the material basis for tight oil enrichment; the dominant source reservoir assemblage is the basic unit for tight oil enrichment; fractures and beddings are conducive to local rapid migration of tight oil; fractures and pores work together to control the enrichment of tight oil. Two typical accumulation models of tight oil are established, namely "source reservoir in coexistence, four optimal factors controlling enrichment around central area, and large-scale continuous distribution" for a large freshwater lake clastic rock basin and "source reservoir integration, four optimal factors controlling enrichment, central area distribution, small in size but high in enrichment degree " for a small salty lake diamictite depression.
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