Table of Content

25 April 2024, Volume 31 Issue 2

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Summary

A Review of Methods for Measuring the Wettability of Reservoir Minerals

Shi Kanyuan, Chen Junqing, Pang Xiongqi, Jiang Fujie, Pang Hong, Hui Shasha, Ma Kuiyou, Cong Qi

2024, 31(2):  1-9.  DOI: 10.3969/j.issn.1006-6535.2024.02.001

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The wettability of the reservoir rock is crucial to the distribution and flow of fluids in the pore, which is related to the selection of measures to improve the recovery rate of oil and gas fields and the effect of their implementation. Due to the complexity of the mineral composition and the diversity of the fluid composition in the underground oil and gas reservoir, there is no uniform standard for measuring the wettability of reservoir rock, and the selection of a suitable wettability measurement method according to the specific conditions of the reservoir is crucial to the oil production. Based on this, the most recent and widely used methods for rock wettability measurement at home and abroad were discussed in details in terms of physical experiments and numerical simulations, and the principles, advantages and disadvantages, and applicable conditions of these methods were summarized, and an outlook of the wettability measurement methods was also given. The study helps to select the applicable wettability measurement methods, which can provide a reference for improving the accuracy of rock wettability characterization in reservoirs.

Geologic Exploration

Microscopic Characteristics and Main Controlling Factors of Tight Sandstone Reservoirs in the Upper Paleozoic Shanxi Formation,Yan′an Gas Field

Guo Shun, Zhou Jinsong, Guo Lan, Chen Xipan, Zhao Zidan, Wen Fenggang

2024, 31(2):  10-18.  DOI: 10.3969/j.issn.1006-6535.2024.02.002

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The reservoirs of the Shanxi Formation in the main gas-bearing layers of the Yan′an Gas Field exhibit strong heterogeneity and poor physical properties, typical of tight sandstone gas reservoirs. It is difficult to predict favorable exploration areas and exploit gas reservoirs. To clarify the causes of low porosity and permeability in the reservoir and identify the main controlling factors, various experimental analyses such as core thin sections, scanning electron microscopy, and whole-rock X-ray diffraction were conducted. Based on differences in plastic particle content, five lithofacies were identified within the tight sandstone of the Shanxi Formation. Phase control was used to guide modeling, and three-dimensional geological modeling software was employed for stochastic lithofacies modeling. The results indicate that sedimentary facies, hydrodynamic conditions, and diagenesis collectively influence sandstone pore development and reservoir performance. Mechanical compaction of plastic materials is identified as the fundamental cause of poor reservoir properties. Pure quartz sandstone and quartz-rich, low-plasticity lithic sandstone exhibit well-developed pores and better physical properties, representing effective reservoirs. These lithofacies are mainly distributed in the middle to lower parts of underwater distributary channels and deltaic barrier microfacies with strong hydrodynamic conditions. Lithofacies modeling provides a better understanding of the three-dimensional spatial distribution patterns, enhancing the accuracy of predicting effective reservoir distribution. This approach has significant implications for the fine development of the Yan′an Gas Field.

Cenozoic Uplift Processes and Hydrocarbon Accumulation Effects in the Yingxiongling Tectonic Belt in Qaidam Basin

Wei Xuebin, Ma Xinmin, Yang Mei, Sheng Jun, Wang Duo, Wei Wei, Shi Qi

2024, 31(2):  19-27.  DOI: 10.3969/j.issn.1006-6535.2024.02.003

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Yingxiongling Tectonic Belt is the largest positive tectonic unit within the Qaidam Basin, and the tectonic zone and its periphery are rich in oil and gas resources. To address the problems of unclear understanding of the geological structure of the tectonic belt, inconsistent understanding of the initial uplift time, and unclear formation process and mechanism, the following understandings were reached by means of tectonic analysis, tectonic evolution, tectonic simulation, and restoration of the burial and hydrocarbon history, based on the drilling wells and the continuous 3D seismic data, and by using the time as the main line, the tectonic activity period as the node, the main seismic section as the outline, and the specific tectonics as an item to reveal the 3D spatial structure and the formation process of the Yingxiongling Tectonic Belt from the geometrical and kinematical perspectives: Yingxiongling Tectonic Belt is characterized by spatial spreading of north-south zoning, east-west segmentation, and vertical up-and-down layering superposition; since the Cenozoic era, the tectonic zone has gone through three phases: Paleoproterozoic fault depression, the initial uplift in the early Neoproterozoic, and the late Neoproterozoic-Quaternary adjustment and stereotyping, and the evolution process has been characterized by the expansion from the south to the north and from the west to the east, and the tectonic evolution has been closely related with the hydrocarbon accumulation; the special boundary conditions of the basins, the internal structure, and the transformations of the stress field in the background of the uplift of the Tibetan Plateau can provide references for the establishment of the hydrocarbon accumulation model of Yingxiongling Tectonic Belt and the selection of a favorable direction of the next step of the exploration.

Recognition and Exploration Potential Analysis of Triassic Baikouquan Formation in Mahu Sag

Zhong Houcai, Zhu Junmei, Lin Yu, Deng Yong, Liu Zuoqiang, Huang Youhua, Zhang Shan, Qu Lin

2024, 31(2):  28-36.  DOI: 10.3969/j.issn.1006-6535.2024.02.004

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In order to further explore the exploration potential of the nearly 4 000.0 km² exploration blank area in central platform of Mahu Sag, expand the exploration potential of Baikouquan Formation in Mahu Sag, and realize the breakthrough of Mahu Sag as a whole, the stratigraphic delineation of Baikouquan Formation in Mahu Sag has been refined by the comprehensive use of geological data, drilling data, and high-density seismic data. The sand bodies of the lowstand system tract in central platformof Mahu Sag have been defined as Baikouquan Formation 0 Member (T₁b₀). On this basis, combined with the regional sedimentary and tectonic backgrounds, the hydrocarbon accumulation conditions and controlling factors of the lowstand system tract T₁b₀ in Mazhong plateau area were analyzed in terms of the hydrocarbon source rock conditions, migration system, and depositional system. The study shows that the hydrocarbon source rocks of the Permian Fengcheng Formation have the bimodal characteristics of oil generation in an alkali lake conditions, the hydrocarbon potential is large, and T₁b₀ is closer to the source and more accessible to capture the deep oil and gas. The strike-slip faults in central platform of Mahu Sag directly inserted into the deep hydrocarbon source rocks during the Indo-Chinese epoch and formed an efficient migration system; the top and bottom plates of T₁b₀ have superior conditions, and T₁b₀ is superimposed on a large stratigraphical superposed background and develops multiphase lake invasion and retreating sand bodies, which form a large scale and effective reservoir-caprock association. T₁b₀ develops four large-scale fan bodies, and the tight zones, mudstones between fans, and tectonics of the fan delta plain subfacies are configured with each other to form a composite multi-faceted barrier, which lays the foundation for the large-scale formation of reservoirs in the front edge of the fan delta. The analogous study concluded that the subfacies of the leading edge of the central abduction fan has good upward-dipping and lateral barrier conditions and has a macro-integrated basis for the formation of reservoirs. The lithological target is superimposed and developed with a high degree of implementation, and it is the key exploration target for seeking breakthrough of the new stratigraphic system in T₁b₀ at present. The breakthrough of T₁b₀ is expected to open up the exploration situation in central platform of Mahu Sag, thus realizing the overall breakthrough of Baikouquan Formation in Mahu Sag.

Mechanism of Overpressure Formation in the Youquanzi Oilfield in western Qaidam Basin

Han Tianhua, Liu Chenglin, Tian Jixian, Yang Taozheng, Feng Dehao, Li Guoxiong

2024, 31(2):  37-46.  DOI: 10.3969/j.issn.1006-6535.2024.02.005

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Overpressure is widely developed in the Cenozoic formation of the Youquanzi Oilfield in western Qaidam Basin, and the identification of overpressure genesis has great significance for the study of oil-gas migration and reservoir formation. The measured pressures, logging curves, acoustic velocity - density cross plot, overpressure genesis comprehensive analysis method and overpressure compartment mode for the Youquanzi Oilfield were comprehensively analyzed. The results show that the negative pressure, normal pressure, overpressure and strong overpressure systems were longitudinally developed in the Cenozoic Formation of the Youquanzi Oilfield; the pressure coefficient of the overpressure system was generally greater than 1.3, and the pressure coefficient of some overpressure points was greater than 1.73, showing the characteristics of strong overpressure; overpressure in this area, the main controlling factors were the uneven compaction, tectonic extrusion and hydrocarbon generation; the oil-gas migration was in line with the overpressure compartment mode; the oil and gas were migrated to the Nanyi Mountain Area due to overpressure. The study result clarifies the oil-gas migration direction of the Yauquanzi Oilfield and is of guiding significance to the study of oil and gas reservoirs in western Qaidam Basin and similar regions.

Reservoir Characteristics of the second Member of Sinian Dengying Formation in Penglai Gas Area of Sichuan Basin and Their Controlling Factors

Xu Shaoli, Ma Kui, Yang Qiang, Xie Changqiong, Zhang Xin, Yan Wei, Wu Luya, Wang Wenzhi

2024, 31(2):  47-56.  DOI: 10.3969/j.issn.1006-6535.2024.02.006

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Significant breakthroughs have been made in the exploration of natural gas in the second Member of the Sinian Dengying Formation in Penglai gas area, with single well production reaching new highs repeatedly, demonstrating the excellent scale reserve resources of Dengying Formation in Penglai gas area. However, the sedimentary and reservoir characteristics and their main controlling factors in the area are not yet clarified, and there are great difficulties in predicting high-quality reservoirs, which seriously hinder the next exploration deployment and reserve upgrading. In this context, in order to further deepen the study of sedimentary reservoir characteristics in Penglai gas area and reveal the main controlling factors of high quality reservoirs, a new round of sedimentary reservoirs research is carried out on the second Member of Penglai gas area using the latest rock-resistivity, seismic and testing data. The results show that the reservoir lithologies of the second Member of Dengying Formation are thrombolites and doloarenites of mound-shoal facies. The reservoir spaces include pores, karst caves and fractures. The distribution of the reservoirs varies significantly in the vertical direction, and the continuity is strong in the plane. Sedimentary facies belts, karst, and diagenesis jointly control the high-quality reservoirs. Among them, sedimentary facies belts are the basis for the formation of high-quality reservoirs in the second Member of Dengying Formation; karst determines the reservoir property. Late diagenesis is an essential factor affecting the formation of high-quality reservoirs in the second Member of Dengying Formation. The overall gas content of the second Member of Dengying Formation is good, with the development of multiple large lithological traps. Research suggests that the lithological trap bodies of mound-shoal facies under karst reformation are favorable target areas for further exploration and also new positions for increasing natural gas reserves and production in the basin by trillions of cubic meters. This study provides a foundation and research examples for clarifying the sedimentary and reservoir characteristics of the second Member of Penglai gas field and its main controlling factors to provide a reference for subsequent exploration and development work.

Evaluation Techniques of Bedrock Reservoir Effectiveness in the Northern Songliao Basin

Gao Tao, Wang Chunyan, Wang Haiyan, Jin Xueying, Qin Hao

2024, 31(2):  57-65.  DOI: 10.3969/j.issn.1006-6535.2024.02.007

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To address the problem such as the complex bedrock lithology, dense reservoirs, fracture development, low calculation accuracy of reservoir parameter and difficult evaluation of reservoir effectiveness in the northern Songliao Basin, a new three-dimensional lithology identification method of "original rock+composition+structure" was proposed; the effective porosity model for variable skeletal density based on M and N was established by calculating the slope M using the acoustic-density rendezvous plot and the slope N using the neutron-density rendezvous plot; the gas saturation model of fracture-containing reservoir was established by studying the conductivity mechanism of bedrock and correcting the additional conductivity of fractures; the effective thickness standard of bedrock reservoir and the classification standard of effective reservoir were determined by considering the effective porosity, secondary porosity and the degree of fracture development. The application results show that the lithologic interpretation compliance rate of this set of technology can reach 87.5%, the gas-bearing formation identification compliance rate reaches 88.5%, and the calculation accuracy of porosity and saturation meets the requirements of the reservoir specification. The study results can provide technical guarantee for the geological exploration and reserve evaluation of bedrock reservoirs in the northern Songliao Basin.

Lower Limit and Main Controlling Factors of Effective Reservoir Physical Properties in He 7 - He 8 Members, central Linxing Area, Ordos Basin

Zhao Fei, Wang Lingbo, Wu Peng, Xu Yanyong

2024, 31(2):  66-75.  DOI: 10.3969/j.issn.1006-6535.2024.02.008

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In response to the problem that the unclear lower limit of the physical properties of the effective reservoir in He 7 - He 8 Members of Linxing Area, based on the physical properties data, logging interpretation results, gas test data, sedimentary phases, casting thin section and other information, the analysis of the main controlling factors for the development of effective reservoir was carried out by combining the cumulative frequency method, the gas recovery index method, and the rendezvous chart analysis method. The study shows that the lower limits of porosity for He 7 - He 8 Members are 6.0% and 5.5%, respectively, and the lower limits of permeability are 0.05 mD and 0.04 mD, respectively. Effective reservoirs are controlled by the combination of sedimentation, diagenesis and tectonics, and the sedimentation controls the spatial spreading and development scale of reservoirs, which is the main factor affecting the physical properties of reservoirs; the diagenesis has a more complicated influence on the physical properties of reservoirs, among which, the compaction intensifies the degree of densification of reservoirs, secondary pores produced by dissolution and erosion are conducive to the formation of effective reservoirs, and the cementation has two sides to the development of effective reservoirs; late tectonics facilitates the formation of microcracks of tectonic origin and promotes the mineral dissolution, which is of positive significance to the improvement of local reservoir connectivity. The above understanding is instructive for effective reservoir prediction as well as oil and gas exploration in coal seams in the Linxing Area.

Research and Application of Overpressure Prediction Model in Carbonate Formations

Chen Yunqiao, Wang Linlin, Wang Xiaolong, Xu Wen, Liu Yukun, Wan Zexin, Bao Yiyao, Cai Ningbo

2024, 31(2):  76-83.  DOI: 10.3969/j.issn.1006-6535.2024.02.009

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The prediction of overpressure in carbonate formation is a basic problem that has not been solved at home and abroad. Taking the overpressure carbonate formation in northeast Sichuan as the research object, the variation law of elastic parameters of carbonate rocks with confining pressure and pore pressure is studied through physical simulation experiments, a prediction model of overpressure in carbonate formation based on pore compressibility is proposed. The experimental results show that there is a certain correlation between the increase of porosity of carbonate rocks and the change of elastic parameters of rock samples and the increment of pore pressure, based on this, the theoretical relationship between pore compressibility, pore pressure and pore volume of rock is established, and the overpressure prediction model is derived, using density, Sonic and neutron logging data to predict the overpressure in typical drilling, the predicted results are in good agreement with the measured pressure, the new overpressure prediction model provides a new idea for overpressure prediction of carbonate formation.

Sequence Characteristics of Dense Sandstone Reservoirs and Favorable Zone Preference in Zhongtaishan Area, Central Sichuan

Zheng Chao, Qiu Yuchao, Wen Long, Li Nan, Wu Changjiang, Zhang Xiaoli, Chen Shuangling, Yuan Chun

2024, 31(2):  84-92.  DOI: 10.3969/j.issn.1006-6535.2024.02.010

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In the lower sub-member of the third member of Xujiahe Formation in the Zhongtaishan Area of central Sichuan is characterized by obvious differences in stratigraphic composition and rapid lateral changes of fluvial-phase sands, which makes the delineation of sequence stratigraphy difficult, and thus affects the identification and characterization of high-quality reservoirs. For this reason, the sequence characteristics of the lower sub-member of the third member of Xujiahe Formation in the Zhongtaishan Area were analyzed based on the logging, mud logging and seismic data, and the distribution range of different lobes was circled in order to make a preferential selection of favorable zones for reservoirs. The results show that: for the stratigraphic deposition of the lower sub-member of the third member of Xujiahe Formation in the Zhongtaishan Area of Central Sichuan, the stratigraphic features of the upper sub-member of the second member of Xujiahe Formation are inherited, which is a gently sloping paleotectonic background with high terrain in the southeast, low terrain in the northwest and elevated in the middle of the depression, and three phases of sequence boundary and two phases of flooding surfaces have been identified; the mineral components of the dense sandstone reservoir are mainly rock chips and feldspar, and the pore types are dominated by dissolution pores and microfractures; there are three main types of lithologic assemblages: thick sandstone assemblages, thick sand interbedded with thin mudstone assemblages, and thick mud interbedded with thin sandstone assemblages; combined with the fact that the high-yield wells have the characteristics of fracture, crack and reservoir development, and are located in the relatively high tectonic parts, the reservoirs in the study area are divided into 3 types of favorable zones. The research results can provide important reference and theoretical support for the subsequent exploration and development.

Evolutionary Law and Geological Significance of Migration and Superposition of Underwater Distributary Channels in the Bashijiqike Formation, Yaha Area, Tarim Basin

Yang Huatong, Lin Xiaobing, Zhang Xuan, Wang Zhuangsheng, Fan Junming, Wang Guihua, Liu Yuansong

2024, 31(2):  93-102.  DOI: 10.3969/j.issn.1006-6535.2024.02.011

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In order to clarify the migration characteristics of the underwater distributary channel at the front of the seasonal braided river delta under the arid climatic conditions in the Yaha Area Tarim Basin, the contact relationship and migration evolutionary characteristics of the underwater distributary channel were discussed on the basis of the core observation, thin-section identification, grain-size analysis, and logging curve analysis in conjunction with the sedimentary background by taking the Bashijiqike Formation as an example. The results show that the Bashijiqike Formation in the study area is a front subfacies of the braided river delta, where the microfacies such as underwater distributary channel, mouth bar and interdistributary bay are developed; the width of the underwater distributary channel ranges from 200 to 1 500 m, the sand-to-ground ratio of the sediments is high, and the channel is migrated frequently, and the migration and superposition styles are of interdistributary bay separated, spliced and cut type; the physical properties of the sand body of the underwater distributary channel are better in general, and the sand body can be formed into continuous favorable reservoirs with good pore permeability after migration and superposition. It is of great significance to clarify the migration and superposition law of the underwater distributary channel for further oil and gas exploration in the Yaha Area.

Reservoir Engineering

Air-assisted microbial enhanced oil displacement in Thin Sandstone Reservoir with Edge and Bottom Water

Liu Xiaoli, Li Yang, Bai Lei, Wu Congwen, Wang Hongbo, Ma Ting, Jiang Zhenxue, Wan Yunyang

2024, 31(2):  103-111.  DOI: 10.3969/j.issn.1006-6535.2024.02.012

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Considering the challenges associated with effectively producing scattered residual oil in reservoirs with edge and bottom water during the “dual high-stage of high degree of reserve recovery and high water cut”, as well as developing a cost-effective and efficient microbial oil displacement system, evaluating the effectiveness of microbial flooding, and identifying key supporting technologies, this study establishes a screening chart for microbial flooding and evaluates its adaptability.The microbial community structure of the reservoir was comprehensively analyzed using microbial molecular ecology technology, and strain H3 with excellent emulsifying effect for oil recovery was screened.Through screening of key nutrients including carbon source, nitrogen source, and phosphorus source, single-factor experiments and orthogonal experiments were conducted to determine a composite powder activation system capable of achieving emulsification dispersion of crude oil while inhibiting the growth of sulfate reducing bacteria.The enrichment characteristics of residual oil of "ship bottom type" in bottom water reservoir were investigated, and a microbial emulsification + air efficiency improvement + thickener plugging EOR mode was developed. It was found that the enhanced oil recovery had a positive correlation with both the injection concentration of bacteria and the injection volume multiple.The research findings were applied to the thin sandstone reservoir with edge and bottom water of Luliang Oilfield in Junggar Basin. A pilot test was conducted to enhance recovery through "4 injectors and 20 producers" microbial flooding pattern. The microbial activator was injected at a rate of 0.13 times the pore volume. From 2017 to 2023, there was an increase in oil production by 5.6×10⁴ t, resulting in a rise of the recovery rate by 4.0 percentage points. It is evident that wells with significant improvement of oil production are primarily located in areas characterized by with high total bacterial concentration, large geological reserves, high structural parts, large reservoir thickness, thin bottom water and high degree of reserve recovery.The research findings can serve as a valuable reference for enhancing the sequencing of microbial flooding in order to optimize recovery efficiency.

Qrthogonal Numerical Simulation Study of the Reservoir Deformation Damage Induced by the Thermal Injection Decomposition of Natural Gas Hydrate

Zhai Cheng, Wu Di, Qin Dongdong

2024, 31(2):  112-119.  DOI: 10.3969/j.issn.1006-6535.2024.02.013

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In response to the problem that the damage mechanism and influencing factors of the reservoir destabilization caused by the thermal decomposition of natural gas hydrate are not clear, the coupled heat-fluid-solid model of the hydrate deposition layer and the elastic-plasticity intrinsic model of the solid skeleton of the reservoir under the action of such coupled model have been established in consideration of the formation heat transfer, pore fluid seepage and deformation damage of solid skeleton due to the thermal decomposition of hydrate. Based on the secondary development of the USDFLD subroutine of ABAQUS software, the orthogonal numerical simulation experiments were carried out to analyze the sensitivity of the thermal injection temperature difference, the absolute permeability of the hydrate deposition layer, and the effective principal stress difference to the influence of the deformation damage of near-well reservoir in the the hydrate deposition layer and the influence mechanism. The results show that the sensitivity to the influence of the deformation damage of near-well reservoir is in descending order of the effective principal stress difference, absolute permeability, and thermal injection temperature difference; the deterioration of the mechanical properties of the near-well reservoir and the decrease of the effective stress caused by the thermal decomposition of hydrate are the main reasons for the deformation damage; the larger the effective principal stress difference is, the larger the maximum value of the equivalent plastic strain is, and the location of the most severe deformation damage in the near-well reservoir is always located in the direction of the minimum horizontal geostress in the borehole under the influence of the the non-uniformity of the horizontal geostress. This study provides a reference for the natural gas hydrate extraction with the thermal stimulation method and drilling operations of hydrate-bearing formations.

Experiments on Influence of Secondary Water Bodies on Fire Flooding Development Effect

Sun Ziqi, Zhao Renbao, Wang Tiantian, Long Haiqing, Huang Tian, Chen Changjian

2024, 31(2):  120-128.  DOI: 10.3969/j.issn.1006-6535.2024.02.014

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Aiming at the problem that the influence of secondary water body generated by steam injection development on fire flooding development effect is still unclear, a combustion tube experimental device was used, and oil sands of different water content were filled into the middle part of the combustion tube to simulate the secondary water body in the reservoir, and to study the influence of the secondary water body on the propagation characteristics of the fireline and the fire flooding development effect. The results show that the secondary water body is favorable for increasing the propulsion velocity of the fireline but would reduce the combustion stability of the fireline. It is difficult for the fireline to advance stably to the production well after passing through the secondary water body when the water saturation of the secondary water body is higher than 75.4%, which leads to a poor development effect; the secondary water body with water saturation lower than 50.0% and a certain amount of oil content does not have much influence on the fire flooding development effect. The fireline could pass through the secondary water body and advance stably. The study results are of great significance to understanding the influence of secondary water bodies in the reservoir on the stable advancement of the fireline and the fire flooding development effect. They could provide theoretical references for the fire flooding technology in steam injection reservoirs.

Microscopic Pore Structure and Coalbed Methane Desorption Law in Low-Permeability Coal Seams

Hu Xiong, Wu Changwu, Yang Xiuchun, Cheng Qianhui, Zhu Wentao, Ma Liang, Zhu Xueguang, Xu Borui

2024, 31(2):  129-135.  DOI: 10.3969/j.issn.1006-6535.2024.02.015

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In order to study the relationship between microscopic pore structure and gas desorption law in low-permeability coal seams, the gas content, porosity, permeability, and pore size distribution of coal samples from No.8 coal seam of Yishang slope in the Ordos Basin were determined by using experimental methods such as direct observation method, nuclear magnetic resonance (NMR) method, and liquid nitrogen adsorption method. The results show that the coalbed methane desorption characteristics are related to the pore size distribution, pore morphology, pore connectivity, specific surface area of the coal, and coal rock composition, etc; the pore characteristics and gas desorption laws of the 2 sets of coal seams less than 10 m apart in the study area have large differences. Among them, the microporous morphology of the glance coal was dominated by cylindrical pores open at one end and ink-bottle pores, with the percentage of micropores and adsorbed pores being 39.2%, and the microcracks in the coal body were relatively developed, but the connectivity between the micropores and mesopores was poor, and the microporous morphology of the dull coal was dominated by cylindrical pores open at one end, with the percentage of micropores and adsorbed pores being 33.2%, the microcracks were undeveloped, and the connectivity between different pores was poor. The gas desorption of glance coal has a large output in the early stage but decays quickly in the late stage, and the gas output has obvious paroxysmal characteristics; the gas desorption of dull coal has a small output in the early stage, which is about 1/2 of that of the glance coal, and the decrease of the output in the late stage is relatively flat. The pore structure, pore type, permeability, stress sensitivity, and types of fracture fillers shall be fully considered in the development strategy for low-permeability coal seams. On the basis of fine characterization of the pore structure, a reasonable development plan should be formulated in accordance with the characteristics of different desorption periods of coalbed methane and in conjunction with geological and development conditions. This study has important guiding significance for understanding the mechanism of coalbed methane gas production and its controlling factors, and improving the mining efficiency of coalbed methane.

A New Method for Productivity Prediction of Offshore Heavy Oil Reservoirs Based on Logging Curves Considering Longitudinal Permeability Ratio

Feng Shasha, Wang Kun, Xie Mingying, Li Li, Wang Danling

2024, 31(2):  136-142.  DOI: 10.3969/j.issn.1006-6535.2024.02.016

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There are large errors between the designed productivity and the actual productivity of directional wells in heavy oil reservoirs with multi-layer combined production. Taking PY10-2 Oilfield in the eastern part of the South China Sea as an example, a reasonable characterization study of flow characteristics of heavy oil reservoirs was carried out. Based on logging curves considering the longitudinal permeability ratio, a productivity model and evaluation method were developed that integrated the influences of factors such as the power-law of heavy oil fluid, longitudinal heterogeneity, start-up pressure, and stress-sensitivity to characterize the strong longitudinal heterogeneity in such reservoirs finely. Good results were achieved in the evaluation of actual oilfield productivity. The research results show that the prediction accuracy of this model is dramatically improved compared with the traditional method, and the average errors of the predicted productivity are less than 10.0%. The productivity of directional wells decreases with the decrease of the power law exponent; the influence of the power-law index on the production capacity increases when the bottom hole flow pressure decreases; the increase in productivity index decreases when the producing pressure drop of the directional wells rises. This study is of great significance for evaluating the reasonable productivity of similar offshore reservoirs.

Evaluation of the Wettability of Carbonate Rock Condensate Gas Reservoirs Modified by Nanoparticles

Gao Zhe, Yang Guoping, Peng Lei, Le Lu, Fu Guomin, Shi Lei, Fu Jianglong, He Tao

2024, 31(2):  143-151.  DOI: 10.3969/j.issn.1006-6535.2024.02.017

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In order to change the wettability of near-well reservoirs in condensate gas reservoirs and effectively relieve condensate gas retrograde condensation damage, the structural characteristics of fluorinated SiO₂ nanoparticles were investigated by using X-ray diffraction, low-temperature N₂ adsorption, and scanning electron microscope observation to evaluate the adsorption degree of fluorinated SiO₂ nanoparticles on carbonate rock cores with different mineral contents; and the changes in the wettability of the rocks, the efficiency of spontaneous permeation, the pressure difference of the single-phase seepage, the oil-gas phase relative permeability curves, and the changes in the condensate recovery before and after the adsorption of the nanoparticles were investigated by carrying out the contact angle measurements, the spontaneous permeation, and the single-phase and two-phase seepage experiments for the core displacement. The results show that The shape of fluorinated SiO₂ nanoparticles is similar to a uniform sphere, the average particle size is 30.34 nm, and the average pore size is 20.00 nm; the adsorption amount of fluorinated SiO₂ nanoparticles on the surface of carbonate rock increases with the increase of porosity, but in the low-permeability rock cores, the change of adsorption amount and the wetting contact angle increases with the increase of calcite content; the oil and gas two-phase co-seepage zone of the rock after nanoparticle adsorption, the oil-gas two-phase co-seepage zone of the rock widens, the isotropic point moves to the upper left, the maximum oil-gas relative permeability increases greatly, and the irreducible water saturation and remaining oil saturation decrease significantly; the fluorinated SiO₂ nanoparticles can be adsorbed on the surface of the rock in about 10 min, transforming the liquid-wetted rock, which is strongly oleophilic and hydrophilic, into an intermediate (gas)-wetted rock, lowering the capillary pressure in pores and the resistance to the seepage of condensate, and raising the relative permeability of gas and oil phases to achieve the goal of blocking removal and increase of production and efficiency. The research results have great value for field application and promotion.

Distribution Characteristics and Main Controlling Factors of Movable Fluid in Tight Sandstone Reservoir of Linxing Gasfield

Du Jia, Guo Jingjing, Liu Yancheng, Zhang Yingchun

2024, 31(2):  152-158.  DOI: 10.3969/j.issn.1006-6535.2024.02.018

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To address the problem of complex microporous structure and unknown distribution characteristics and control factors of movable fluid in the reservoirs of Linxing Area in the Ordos Basin, by comprehensively utilizing low-field nuclear magnetic resonance (NMR), field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) analysis, nuclear magnetic resonance (NMR) imaging, and CT scanning, we quantitatively portrayed the saturation characteristics of the moveable fluids of the Upper Shihezi Formation and the Taiyuan Formation of the Linxing Gasfield, and analyzed the relations among the porosity, permeability, mineralogical composition of the rocks, microscopic porosity connectivity and the saturation of the moveable fluids. The results show that the average movable fluid saturation of the reservoirs of the Shihezi Formation in Linxing Area is 58.05%, and the average fluid saturation of the reservoirs of the Taiyuan Formation is 48.60%, and the average fluid saturation of the reservoirs of the Shihezi Formation is significantly higher than that of the Taiyuan Formation; the permeability and the percentage of porosity connectivity are the important factors influencing the movable fluid saturation of the dense sandstone reservoirs. The results of the study can provide a strong support for the understanding of permeability rules and the preparation of the development plan for dense sandstone reservoirs in Linxing Area.

Drilling & Production Engineering

Predictive Method and its Application of Pressure Drop in Horizontal Wellbore Completion String with Sliding Sleeves

Zeng Quanshu, Hao Xining, Sui Tianyang, Xu Liangbin, Wang Zhiming, Gao Wenlong

2024, 31(2):  159-165.  DOI: 10.3969/j.issn.1006-6535.2024.02.019

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To address the challenge of predicting inflow dynamics in horizontal completion string with sliding sleeves,oil-water single-phase flow tests were conducted inside the wellbore of completion string with sliding sleeves under different mainstream and inflow conditions using a self-developed physical simulation device for production wellbore flow.Calculation formulas for hydraulic friction coefficients and pressure drop equations in the wellbore with sliding sleeve completion string were established.The relationship between wellbore pressure drop distribution and inflow dynamics was studied, and the methods were applied in Panyu Oilfield.The results indicate that for completion string with sliding sleeves,the additional resistance generated by sliding sleeve openings and inflow from perforations is significantly greater than the resistance from the sliding sleeve openings alone,accounting for approximately 10% to 70% of the total pressure drop.Under laminar flow conditions,both sliding sleeve openings and inflow from perforations hinder the mainstream flow in the wellbore.Under hydraulic flow conditions, sliding sleeve openings hinder the flow, while inflow from perforations reduces resistance.The established expressions for hydraulic friction coefficients and pressure drop equations have an error of less than 1.00%,which can effectively characterize the additional resistance caused by factors such as sliding sleeve openings and inflow from perforations.The research findings can be utilized to predict inflow performance of well and provide guidance for adjusting the opening of sliding sleeves in completion string.

Study on the Transition Boundary between Bubbly Flow and Slug Flow in Gas-Water Two-Phase Descending Flow

Feng Yibo, Shi Shuqiang, Wang Jianhai, Ding Baodong, Li Tingting, Xu Ziran, Wang Zhen, Han Yufei

2024, 31(2):  166-174.  DOI: 10.3969/j.issn.1006-6535.2024.02.020

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To address the problem of unclear transition boundary between bubbly flow and slug flow in the process of gas-water mixed injection to displace the "attic oil" in the Tahe Oilfield, the numerical simulation method was used to simulate and analyze the bubbly flow and slug flow in the gas-water two-phase descending flow. The results show that: under the simulation conditions of gas-phase apparent flow rate of 0.01-1.00 m/s, liquid-phase apparent flow rate of 0.03-2.00 m/s, and pipe diameter of 76 mm, the pipeline is mainly characterized by bubbly flow and slug flow; compared with the simulation results, the transition boundary predicted by the Barnea, Kokal, and Xue Yuxing models is small, and that predicted by the Bhagwat and Yijun models is large; with the increase of the gas-phase apparent flow rate, the liquid volume required for the transition from bubbly flow to slug flow gradually increases; under the condition of low liquid volume, the closer to the center of the pipeline, the higher the number of bubbles and the larger the void ratio; with the increase of liquid volume, the volume of individual small bubbles decreases, and the more uniformly the bubbles are distributed throughout the cross-section of the pipeline. Based on the drift model, a new bubbly flow-slug flow transition boundary model was established by considering the bubble group slipping velocity, and 216 sets of literature data validation results show that the accuracy of the new model is 95.37%, which is high. The established bubbly flow-segment plug flow transition boundary model cannot only improve the calculation accuracy of the wellbore pressure and temperature model, but also has a good theoretical significance for the optimization of wellhead parameters, the selection of injection equipment, and the improvement of the efficiency of "attic oil" displacement in the field of Tahe Oilfield.