Table of Content

15 October 2021, Volume 28 Issue 5

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Summary

Advances in the Application of Temporary Plugging Agents for Fracturing in Unconventional Reservoirs

Wang Jiwei, Kang Yuzhu, Zhang Dianwei, Feng Dongjun, Chen Gang, Tian Lingyu

2021, 28(5):  1-9.  DOI: 10.3969/j.issn.1006-6535.2021.05.001

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It is difficult to achieve economic and efficient development of unconventional resources by conventional fracturing technologies which only make single symmetric fractures due to extremely low permeability, so it is necessary to forcibly expand the stimulated reservoir volume by fracturing and make the fracture network more developed to enhance the produce of a single well. Temporary plugging is an effective method to improve fracture complexity. In this paper, a summarization was conducted on the existing studies and application effects of temporary plugging balls, conventional temporary plugging agents, fiber temporary plugging agents and other new synthetic temporary plugging agents used in the fracturing stimulation of unconventional reservoirs such as shale at home and abroad. Moreover, applicable conditions, advantages and disadvantages of the temporary plugging agents were explained, and the future development of temporary plugging agents was also stated clearly. The study results provide a significant reference and guidance for the studies on temporary plugging agents for fracturing stimulation of unconventional reservoirs in the future.

Geologic Exploration

Water Contact Angle and Its Stability Influencing Factors of Shale Reservoirs in Longmaxi Formation, Southern Sichuan

Gao Zhiye, Xiong Shuling, Cheng Yu, Fan Yupeng

2021, 28(5):  10-16.  DOI: 10.3969/j.issn.1006-6535.2021.05.002

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In response to the problems that the characteristics that water contact angle in shale reservoirs changes over time are unclear and the influencing factors are unknown, water contact angle experiments were conducted to analyze the influencing factors of water contact angle and its stability in shale reservoirs. The results of the study showed that the water contact angle of shale varied exponentially with time; the inter-bed water contact angle of the dry shale sample penetration was mainly affected by the TOC content, followed by quartz, and the inter-bed stability index (CASI) of dry shale sample was mainly affected by the clay mineral content, followed by TOC; the fluid-shale interaction affected the shale surface properties, and the surface properties of vertical bedding plane of shale were more homogeneous after the fluid was saturated, while the surface properties of the parallel bedding plane more inhomogeneous. Finally, a prediction formula for the stability index of shale contact angle in different bedding direction and different fluid saturation experiment environment was proposed. The study is indicative of the clarification of oil-gas migration in shale reservoirs.

Accumulation mechanism of deep tight sandstone gas reservoir in Kelasu structural belt, Kuqa Depression

Li Baoshuai

2021, 28(5):  17-22.  DOI: 10.3969/j.issn.1006-6535.2021.05.003

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In order to better understand the accumulation mechanism of deep tight sandstone gas in Kelasu structural belt, a study was conducted on the history of hydrocarbon charging and porosity evolution. Taking the Dabei 1 gas reservoir in the Kuqa Depression as an example, the history of hydrocarbon charging was determined by fluid inclusion petrography and micro-temperature measurement technology, and the pore evolution history of the reservoir was restored by quantitative back-stripping method. The accumulation mechanism of Dabei1 gas reservoir was clarified according to the coupling relationship between them. The study results showed that there were three stages in hydrocarbon charging process in Dabei 1 gas reservoir. The first stage was the mature crude oil charging from Neogene Kangcun period to Kuqa period, forming a conventional reservoir, with the reservoir porosity from 12% to 14%. The second stage was highly mature hydrocarbon charging in Neogene Kuqa period, with the reservoir porosity from 8% and 10% and coexistence of conventional gas reservoir and deep-basin tight gas reservoir. The third stage was dry gas filling from Late Neogene Kuqa period to Quaternary western region period, forming tight conventional and deep-basin tight composite gas reservoirs, with reservoir porosity of about 5%. Since the western region period, a large number of fractures have been developed in the reservoir due to the late tectonic movement, and the gas reservoir has been relocated, that is, the natural gas migrated along the fractures under the action of buoyancy and accumulated in the high part of the structure. The study results further prove the prospects of tight gas exploration and development in the Kelasu structural belt in the Kuqa Depression, and provide a theoretical basis for the prediction of favorable areas in the study area.

On Integration of Geological Modeling and Engineering Design of Fractured Oil Reservoirs in Well Block Che21

He Li, Xiao Yang, Sun Yicheng, Liu Shouyu, Liu Li, Liu Zilong

2021, 28(5):  23-29.  DOI: 10.3969/j.issn.1006-6535.2021.05.004

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To study the connection between artificial and natural fractures in the formation and the actual fracture distribution pattern in Well Block Che21, and to realize the economic and effective development of Carboniferous fractured oil reservoirs in Well Block Che21, an integrated research procedure from geological modeling to fracture simulation was developed on the basis of the idea of integrated geological engineering research and the establishment of a real 3D spatial model of the reservoir, taking into account the longitudinal lithologic distribution and the non-homogeneity of plane attribute distribution in the reservoir. The results showed that the real 3D geomechanical model could realistically characterize the mechanical properties of reservoir rocks, and the average minimum horizontal principal stresses of Wells CHHW2101 and CHHW2102 were 22.0 MPa and 26.0 MPa respectively, which are consistent with the downhole pressure 23.0 MPa and 27.0 MPa of the two wells after pump shutdown. The real 3D fracture simulation technology can evaluate the fracture simulation results more efficiently, accurately and directly, and judge the connectivity with natural fractures. The study results can provide theoretical and technical support for subsequent fractured wells in Well Block Che21, filling the gap of integrated fracturing in China.

Identification and Prediction of Effective Fractures in Volcanic Rocks by Maximum Likelihood Method Based on Forward Modeling

Xiong Ting, Jia Chunming, Tuo Junjun, Li Sheng, Shang Chun

2021, 28(5):  30-36.  DOI: 10.3969/j.issn.1006-6535.2021.05.005

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To address the problem of unclear identification and prediction of effective fractures in volcanic rocks of Chepaizi Uplift on the northwest margin of Junggar Basin, a seismic normalization model was developed for three types of fractures to compare the effects of identifying volcanic rock fractures with coherence, curvature and maximum likelihood attributes, and to quantify the density of developed effective fractures based on the maximum likelihood method by using the modeling technology of micro-scaling effective fractures by core and imaging logging. It was found in the study that the maximum likelihood attribute method could more accurately predict the volcanic fractures and achieve better imaging effect on the details of fracture development in the ault development zone; the high-angle tectonic fracture development zone where the fracture orientation was consistent with the current maximum principal stress direction was more favorable to the formation of volcanic reservoirs; it was predicted that the distribution area of the four effective fracture development zones of volcanic rocks was about 76 km², which can be used as a favorable area for further exploration. The technology in the study can accurately characterize and predict the effective fractures in volcanic rocks, which is of guiding significance to the optimization of favorable targets in volcanic rocks.

Analysis of the Characteristics and Exploration Potential of Volcanic Hydrocarbon Reservoir in Qijia Area

Gao Rongjie

2021, 28(5):  37-44.  DOI: 10.3969/j.issn.1006-6535.2021.05.006

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In view of existing problems of poor stable production and reduced areas covered by prospecting rights in Huanxiling Oilfield, it is increasingly important to seek for commercially economic reserves of oil and gas reservoirs and to enhance the support to sustainable stable production, and it is imperative to actively study the exploration of special lithological reservoirs. The genesis and reservoir characteristics of volcanic rock in Qijia Area were described in detail based on the data of drilling, logging, core and seismic interpretation, and the exploration potential of volcanic reservoir in Qijia Area was discussed from the aspects of hydrocarbon accumulation factors and completion well productivity patterns. It was found in the study that the lithology of the volcanic rocks in Qijia Area was complex, with three stages of texture, namely, andesite porphyritic texture, intermatrix intergranular hidden texture, basalt stomatal porphyritic texture, gabbro texture and tuff volcanic tuffaceous texture. The reservoir characteristics of the volcanic reservoir were dominated by weathered crust, and the reservoir space included both pore and fracture media; the exploration potential of the volcanic reservoir was concentrated in the northern section near the western fault; the reservoir enrichment was controlled by lithology, tectonic morphology, weathered crust development and other factors. The study concluded that the volcanic reservoir in Qijia Area had a good productivity which was strongly correlated with lithology, tectonics and reservoir space development. The study results can be used as a reference for the in-depth exploration of the volcanic rocks in Qijia Area and as guidance for the exploration and development of volcanic rocks in other similar areas.

Application of Natural Gamma Spectrometry Logging in Mineral Resources Exploration in Sichuan Basin

Wu Xiaoguang, Miao Xiangxi, Wang Zhiwen, Kang Jianyun

2021, 28(5):  45-52.  DOI: 10.3969/j.issn.1006-6535.2021.05.007

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In view of the difficulties in identification and logging evaluation of highly radioactive reservoirs in Sichuan Basin, the causes of high radioactivity in the reservoirs were summarized and sorted out in combination with geological background, energy-spectrum logging characteristics and core data. The combined characteristics of the uranium, thorium and potassium curves in the spectrometry logging data and the cross plot analysis method were adopted in the effective research and analysis related to lithology, reservoir identification, mud content calculation and hydrocarbon source rock quality evaluation in the exploration of highly radioactive terrestrial dense sandstone gas reservoir, marine carbonate gas reservoir, shale gas reservoir and miscellaneous halite solid minerals. There is much for reference of the study to the exploration of highly radioactive mineral resources in the basin and other work areas with similar geological conditions

Seismic Petrophysical Characteristics of Dense Sandstones in Qingshui Sag and Dominant Reservoir Prediction

Gao Rongjin

2021, 28(5):  53-59.  DOI: 10.3969/j.issn.1006-6535.2021.05.008

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In view of the issue that conventional post-stack inversion fails to accurately predict the fluid in the reservoir, a pre-stack AVO fluid prediction method was used to analyze the petrophysical characteristics and AVO response characteristics of the dense sandstone in Qingshui Depression and to carry out a study on pre-stack fluid factor inversion. Petrophysical experiment was conducted to determine the elastic modulus of dense sandstones and the relationship between p-wave velocity and s-wave velocity of sandstone when the sandstones were saturated with fluids of different properties. Fluid substitution and pre-stack AVO analysis were also carried out. The results of the study showed that the density and Poisson′s ratio of dense sandstone in Qingshui Sag changed significantly under the condition of saturated oil, gas and water, especially after containing gas, the density and Poisson′s ratio decreased rapidly, and the AVO reflection amplitude decreased rapidly with increasing offset. This feature and the measured mudstone baseline were used to constrain the inversion of pre-stack AVO fluid factor, effectively improves the accuracy of oil and gas prediction in the tight sandstone of the area, laying a foundation for the accurate prediction of the oil-gas-water relationship in this area.

Application of Reservoir Scale Sequence Method in Hydrocarbon Resources Assessment in the Low-Exploration Area of Mahu Sag

Li Ting, Wang Tao, Jiang Wenlong, Wang Shuo, Guo Wenjian, Yang Haibo

2021, 28(5):  60-67.  DOI: 10.3969/j.issn.1006-6535.2021.05.009

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To address the large errors of statistical method in the assessment of hydrocarbon resources in low-exploration area, starting from the method analysis, the resources assessment in the low-exploration area was improved with the reservoir scale sequence through repeated optimization of the parameter values and the assessment process. The results showed that there were multiple parameters involved in the prediction with reservoir scale sequence method; the prediction results were affected by the change rate k_sequence of reservoir scale, the truncation of possible maximum reservoir scale Q_max-cut and the truncation of possible minimum reservoir scale Q_min-cut, with multiplicity of solutions; the method was simple and easy to operate, and the reliability of resource assessment results could be effectively improved with reasonable assessment ideas and values. This study accumulates experience for similar practice but also provides a basis for future exploration, deployment and planning in the study area.

Diagenesis and Reservoir Formation Effect of Tight Sandstone in Lower Permian Shihezi Formation, Dongsheng Gasfield

Tan Dongping, Luo Long, Tan Xianfeng, Wang Jia, Gao Xuanbo, Cao Tongsheng, Zhou Huanhuan, Zha Xiaojun

2021, 28(5):  68-76.  DOI: 10.3969/j.issn.1006-6535.2021.05.010

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In response to the poor understanding of the diagenesis and reservoir formation effect of sandstones in Dongsheng Gasfield, Ordos Basin, the lithological characteristics, pore-permeability characteristics, reservoir space types and diagenetic characteristics of the reservoirs in Dongsheng Gasfield were studied in depth through core observation and description on the basis of slice observation, cathode luminescence, scanning electron microscope, X-ray diffraction and physical characteristic analysis, and other materials, the diagenetic evolution stages and evolutionary sequence of reservoir were determined, and the reservoir formation effect was discussed systematically. The results of the study showed that the reservoir sandstones of the lower Shihezi Formation in Dongsheng Gasfield, Ordos Basin were mainly in the Stage A of mesozone rock, and partly in Stage B of mesozone rock, with medium compositional maturity and low textural maturity; the secondary vugular pore was the main storage space, and the reservoir was a typical low-porosity and low-permeability reservoir; the sandstone diagenesis in the study area mainly included mechanical compaction, cementation and dissolution, among which mechanical compaction, carbonate cementation and siliceous cementation reduced the porosity and were important causes of reservoir densification, while the secondary pores formed by dissolution was conducive to improving the physical properties of the reservoir. The characteristics, diagenetic evolution and reservoir formation relationship of lower Shihezi Formation, Dongsheng Gasfield were studied in order to provide some reference and guidance for further exploration of tight sandstone gas in the study area.

Sedimentary Model of Lacustrine Small Fine-grained Shallow Water Delta

Guan Xutong, Li Shengli, Ma Shuiping, Liu Sheng, Huang Xiaodi

2021, 28(5):  77-85.  DOI: 10.3969/j.issn.1006-6535.2021.05.011

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Lacustrine shallow water delta has become major target for lithologic hydrocarbon reservoir exploration in continental basins in China. To clarify its sedimentary characteristics and sedimentary models, a comprehensive study of the sedimentary characteristics of small fine-grained shallow water deltas in Member 2 of Shahejie Formation in the upthrown block of Southern Dagang Fault, Qikou Sag was carried out on the basis of rock, electrical and seismic data, and a sedimentary model was established. With analysis of the denudation range and distribution characteristics of heavy minerals in Member 2 of Shahejie Formation, the sediment source was mainly from the northeast; the sedimentary microfacies, such as overwater distributary channels, estuarine dams, submerged distributary channels and sheet sands were identified based on rock, electrical and seismic characteristics; finally, the distribution characteristics of planar sedimentary facies were determined in combination with the sand-to-formation ratio. It was found in the study that the main controlling factors of small fine-grained lacustrine shallow water deltas were tectonic conditions, climate, provenance supply, water depth, lake level variation and topographic slope; under the background of stable tectonic conditions, relatively arid climate, low provenance supply, shallow water depth, declining lake level and gentle topography, local uplift provided sediment sources, and small fine-grained shallow water deltas were developed mainly in delta front and more developed at the estuarine dams. The results of the study provide a new perspective for a comprehensive understanding of the sedimentary characteristics of lacustrine shallow deltas and prospects for hydrocarbon exploration.

Reservoir Engineering

Study on Transform Boundary of High-temperntwe and Low-Temperature Oxidation in In-situ Combustion

Hu Changhao

2021, 28(5):  86-92.  DOI: 10.3969/j.issn.1006-6535.2021.05.012

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There are two combustion states of in-situ combustion, namely low-temperature and high-temperature oxidation of crude oil. To address the problems of unstable low-temperature oxidation process, low thermal efficiency and low recovery percent, indoor physical simulation and numerical simulation experiment were carried out with the in-situ combustion in Du 66 Block as the study object, a kinetic model of in-situ combustion reaction was established through the historical fitting of oxygen consumption and different-component exhaust gas data, and a transform boundary chart of high- and low-temperature oxidation was constructed based on the initial temperature and ventilation intensity at the front edge of the in-situ combustion. The results of the study showed that in high-temperature oxidation, the apparent H/C atomic ratio of crude oil in Du 66 Block was 0.5 to 2.0 and the CO/CO₂ volume ratio was 0.13 to 0.40; the combustion during low-temperature oxidation was unstable and easily extinguished; the initial temperature and ventilation intensity at the critical front edge of high- and low-temperature oxidation of in-situ combustion in Du 66 Block were 280°C and 1.0 m³/(m²·h) respectively; if lower than or close to the critical value, there may be a risk of low-temperature oxidation or even extinction; development countermeasures were put forward based on in-situ combustion in Du 66 Block, including maintaining the minimum ventilation intensity and stratified in-situ combustion. The study provides a theoretical basis for the engineering design of in-situ combustion reservoirs.

Study on Optimization of Position Adjustment Occasion of Steam Injection Screen Pipe for Steam Stimulation in Horizontal Wells

Li Zhaomin, Zhao Xinru, Ban Xiaochun, Lu Teng, Zhang Dingyong, Zhang Jiang, Shi Mingming, Sun Chao

2021, 28(5):  93-99.  DOI: 10.3969/j.issn.1006-6535.2021.05.013

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In order to solve the problem of low thermal efficiency caused by uneven steam injection of steam stimulation in horizontal wells in heavy oil reservoirs, numerical simulation methods were used to study the steam injection law of horizontal wells in heterogeneous reservoirs, the impact of steam injection screen pipe position on the steam injection law in horizontal wells was analyzed, the temperature variation coefficient was introduced to compare the degrees of uneven steam injection in horizontal sections in multiple stages, the method for optimizing the position adjustment occasion of the steam injection screen pipe was proposed, and the impact of the position adjustment of the steam injection screen pipe at different stages of the curve on the steam stimulation oil production was compared according to the "V"-shaped law of the temperature variation coefficient firstly decreasing and then increasing. The results showed that the temperature variation coefficient was decreased continuously in the initial stage of steam stimulation; according to the principle of thermodynamic additivity, the specific heat capacity of reservoirs with better steam injection at this stage was increased, proving that the water cut was increased continually, the remaining oil was produced in large amounts, and the adjustment could not fully utilize the remaining oil in the high-permeability reservoir at this moment; if the position of the screen pipe was adjusted in the late stage of steam stimulation, the steam was injected and accumulated in the high-permeability area with low oil content after a large amount of oil was recovered from the high-permeability area until the screen pipe was adjusted, resulting in low heat utilization efficiency; when the temperature variation coefficient reached the minimum value, indicating that the specific heat capacity of the high-permeability reservoir system raised to a higher value, with high water saturation and less remaining oil, the adjustment of screen pipe position at this moment could improve the uneven development of heterogeneous reservoir. Therefore, the earlier the temperature variation coefficient reached the minimum value, the better the effect of the screen pipe position adjustment. The study results were applied to Lean Oilfield. After selecting the occasion to adjust the screen pipe position according to the temperature variation coefficient method, the steam injection profile was more balanced, and the average daily oil production was increased from 0.5 t/d in the previous cycle to 3.4 t/d. The study results have no guidance for improving the uneven steam injection in steam stimulation.

Improved Numerical Simulation Equation Based on Heavy-oil Fire Flooding Mechanism

Wang Taichao, Zhu Guojin, Tan Xianhong, Wang Kai, Xie Haojun

2021, 28(5):  100-106.  DOI: 10.3969/j.issn.1006-6535.2021.05.014

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In order to solve the problems of complex conversion relationship between components, poor convergence and low accuracy of results during the numerical simulation of the in-situ combustion, the reaction equation was improved to heavy oil cracking and coke combustion from the perspective of fuel deposit of in-situ combustion, and a repetitive and accurate indoor experimental method for fuel deposit rate was established to calculate the deposit rate in heavy oil cracking and obtain an improved simulation equation by fitting. The results showed that the improved model was more reasonable, and the simulation results of the roughened model were basically consistent with these of indoor experimental results, as well as the temperature field, combustion front distribution and other simulation results of the encrypted classical equation model, and with this model, the high-temperature oxidation process could be described factually and the calculation time was greatly reduced. There is much for reference of the study results to the preparation of in-situ combustion scheme.

Dynamic Characteristics of Development with Steam Flooding of Middle and Deep Heavy Oil

Shang Ce

2021, 28(5):  107-112.  DOI: 10.3969/j.issn.1006-6535.2021.05.015

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In response to the problems that the controlling countermeasures of steam flooding in middle and deep heavy oil are greatly influenced by the dynamic characteristic parameters of the development stage and it is difficult to clearly divide the development stage of steam flooding, taking the steam injection in Qi 40 Block as an example, the dynamic and static monitoring data analysis, reservoir engineering calculation, numerical simulation and other means were comprehensively used according to the analysis of variation law of steam chamber to define the recovery law of different development stages of steam flooding, and select the dynamic parameters such as injection pore volume, recovery percent of recoverable reserves, water content and temperature as the limit parameters for the division of steam flooding stages. The results of the study showed that the injection pore volume at the steam breakthrough in Qi 40 Block was linearly related to the oil saturation before the flooding conversion; the injection pore volume multiplier was a second-order function with zero intercept with the recovery percent of recoverable reserves. The study results are highly operational in the classification of steam flooding development, and provide a reference for the development of control countermeasures for different types of steam flooding well clusters.

Quantitative Evaluation and Application of Main Control Factors for High Production of Horizontal Shale Gas Wells

He Chang, Wan Yujin, Geng Xiaoyan, Su Yunhe, Zhang Xiaowei

2021, 28(5):  113-119.  DOI: 10.3969/j.issn.1006-6535.2021.05.016

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In view of the great difference in the productivity of horizontal shale gas wells under the same geological conditions or construction technologies, taking the WH platform of the Weiyuan shale gas field as an example, 12 parameters were selected to determine the main production control factors with the principal component analysis method. The results showed that the two principal components selected by the principal component analysis method reflected 94.09% information of the original parameters comprehensively; fracturing length, high-quality reservoir thickness, number of fracturing sections, fracturing fluid consumption, and drilling length and fracturing length of high-quality reservoir were the main control factors for horizontal wells in WH platform. A comprehensive parameter “SRV factor” considering the three dimensions of SRV at the same time was established with high-quality reservoir thickness, high-quality reservoir fracturing length and fracturing fluid consumption as representative parameters. A gas well production prediction model was established through calibration with production to realize fast and accurate production prediction. This study provides a reference for gas well construction, production prediction and reasonable development technology.

Optimal Design of Horizontal Wells in Volcanic Hydrocarbon Reservoir with Bottom Water

Teng Weiwei, Li Xiang

2021, 28(5):  120-125.  DOI: 10.3969/j.issn.1006-6535.2021.05.017

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In view of the characteristics of low matrix permeability, fracture development, fast water occurrence and low production of volcanic reservoirs with bottom water, the reservoir numerical simulation and the oil-gas-water model with dual porosity and dual permeability were adopted to study the influences of horizontal well length, well spacing, water avoidance height, fracture spacing, fracture half-length and fracture conductivity on horizontal well production in the study case of Well Block J10 in Jinlong Oilfield, Junggar Basin, Xinjiang. The optimal solution for horizontal well development parameters was worked out, that is, the reasonable horizontal well spacing in Well Block J10 was 300 to 350 m, the length of horizontal section 630 m, the optimal water avoidance height 225 m, the fracture spacing 90 m, the fracture half-length 105 m, and the fracture conductivity 60 D·cm. The results of this study have been applied in the field with good production results, and can provide a reference for the development deployment of volcanic hydrocarbon reservoir with bottom water.

Evaluation Method and Application of ESRV Fracturing Effect

Ma Junxiu, Lan Zhengkai, Wang Lirong, Yi Yonggang

2021, 28(5):  126-133.  DOI: 10.3969/j.issn.1006-6535.2021.05.018

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It is critical for effective implementation of stimulated reservoir volume fracturing in unconventional oil and gas reservoirs to accurately characterize the effective stimulated reservoir volume (ESRV) and clarify the main controlling factors affecting the stimulated volume. However, there is still no effective methods to accurately characterize the effective stimulated reservoir volume, and the analysis on main controlling factors is too simple to meet the demand of practical engineering. In response to this problem, an ESRV characterization method based on unstructured network was proposed to accurately characterize the fracture network system in combination with actual production, followed by the analysis of main controlling factors that affects the stimulated volume by random forests algorithm. The evaluation method is more objective, meeting the field demand. The results of the field application showed that the stimulated volume calculated by the new method was more accurate than that estimated by the traditional method, with an average difference of 16%; the fracturing fracture network was controlled by multiple variables; the influence of fracture construction parameter accounted for 53.8% and that of geological and rock mechanics parameters accounted for 46.2% among the primary main controlling factors affecting the stimulated volume. This method provides a reference for the optimal design of multi-stage fracturing of horizontal wells and the prediction of post-fracturing production.

Key Technology and Application of Reconstruction of Existing Brine Extraction Caverns into Salt Cavern Gas Storage

Shi Yue, Guo Wenpeng, Xu Ning, Hou Xiaolu, Zhao Yuan, Du Caixia

2021, 28(5):  134-139.  DOI: 10.3969/j.issn.1006-6535.2021.05.019

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In order to reconstruct the existing brine extraction caverns in convection well into underground gas storage, the experience of the reconstruction of existing brine extraction caverns into salt-cavern gas storage was summarized, its applicability in the reconstruction of brine extraction caverns in convection well into gas storage was analyzed, the more accurate basic conditions and the calculation formula of sealing test pressure were determined for reconstruction, and the qualification evaluation conditions, the improved test method for numerical simulation method in stability analysis, the new technology for wellbore reconstruction and the technical requirements of the operation were given. The study results were applied to the reconstruction of the old caverns in Huaian Area, and the selected convection well met the reconstruction conditions; the pressure change in the injection well was 3.258 to 3.478 MPa during the sealing test with 10 injection pressure ramps, and the wellhead pressure and leakage decreased significantly, satisfying the gas storage conditions; the construction plan of sealing the old well and drilling another new well was determined, and the production casing size was 244.5 mm; a pressure test was conducted at a pressure 1.1 times the upper limit of operating pressure of the gas storage. The results of the study are of guidance to the reconstruction of a large number of existing brine extraction caverns of convection wells in domestic salt mines.

A Method to Identify the Combustion State Based on Characteristics of Chromatographic Fingerprint of Fluid Generated From In-situ Combustion of Heavy Oil

Liu Qicheng, Yan Hongxing, Yang Junyin, Yang Pengcheng, Zhang Chonggang

2021, 28(5):  140-145.  DOI: 10.3969/j.issn.1006-6535.2021.05.020

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In order to clarify the variation rules of physical and chemical properties of crude oil and tail gas in the process of in-situ combustion of heavy oil, an in-situ combustion experiment on heavy oil was conducted using an indoor three-dimensional physical model, and a study was carried out on the characteristics of chromatographic fingerprint of saturated hydrocarbons, aromatic hydrocarbons, olefins and tail gas components of the crude oil and tail gas after high-temperature oxidation by in-situ combustion. It was found in the study that The physical properties of the crude oil were improved and the contents of saturated hydrocarbons and aromatic hydrocarbons in the group components were increased after in-situ combustion; the content of n-alkanes in the chromatographic fingerprint of saturated hydrocarbons was increased significantly, and the hydrocarbon weight ratio was increased; the crude oil, new olefin compounds were generated in pairs with saturated hydrocarbons with the same carbon number, and the ratio of olefins to saturated hydrocarbons with the same carbon number was always less than 0.5; the phenanthrene series of compounds in the chromatographic fingerprint of aromatic hydrocarbons showed significant methyl transfer and demethylation; the multidimensional gas chromatogram of the exhaust gas showed olefins, hydrogen, carbon monoxide, and other characteristic components of high-temperature oxidation in in-situ combustion. This study provides an effective technical method to indicate the combustion state during the in-situ combustion, and supports the identification of the combustion status of in-situ combustion of crude oil.

Gas Seepage Mechanism of Deep Gas Reservoirs with High CO₂ and Water Content in Yinggehai Basin

Lei Xiao, Wang Wenjuan, Luo Jihui, Yang Liu, Li Biao, Wang Lu, He Yongming

2021, 28(5):  146-153.  DOI: 10.3969/j.issn.1006-6535.2021.05.021

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The deep gas reservoirs in Yinggehai Basin, South China Sea is featured by typical characteristics such as high temperature, high pressure, high water content and high CO₂ content, resulting in a very special seepage mechanism. In order to disclose the seepage characteristics of these gas reservoirs and analyze the influencing factors of gas productivity, a experimental system of ultra-high-temperature and high-pressure long core displacement was established to simulate the temperature and pressure conditions of the actual reservoir, and gas seepage experiments were conducted with different water cuts and different CO₂ contents. It was found in the study that the seepage characteristics of the gas reservoir could be divided into four stages: initiation pressure, low-velocity non-Darcy seepage, Darcy seepage and deviation from Darcy seepage; bound water would cause low-velocity non-Darcy seepage, moving water might result in initiation pressure, and bound water would turn into moving water under high pressure difference, making gas seepage deviate from Darcy seepage, causing or aggravating gas-water flowing and reducing gas seepage at high pressure difference; CO₂ in the gas not only prolonged the low-velocity non-Darcy seepage, but also promoted the conversion of bound water into moving water at high content, forming initiation pressure and bringing forward the deviation from Darcy seepage at high pressure difference in advance. Therefore, the increase in water saturation and CO₂ content in these reservoirs will inhibit gas production, and production pressure differential should be strictly controlled to avoid low-velocity non-Darcy seepage and deviation from Darcy seepage at high pressure difference. The results of the study may provide a theoretical basis for the efficient development of these gas reservoirs.

Experimental Determination and Application of Hydrate Formation Conditions for Sour Natural Gas

Liu Huang, Li Ruijing, Wang Zhouhua, Wang Qingwen

2021, 28(5):  154-160.  DOI: 10.3969/j.issn.1006-6535.2021.05.022

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To address the problem that hydrate generation is prone to blocking wellbores and production pipes in the development of sour natural gas reservoirs, the hydrate formation conditions were experimentally determined with constant-temperature pressure search method for gas samples taken from two natural gas reservoirs with high CO₂ content in Western China. It was found in the study that high CO₂ content in natural gas increased the HCO₃^- ion content of formation water, thereby improving the salinity of formation water; compared with pure CH₄ gas, the existing CO₂ significantly reduced the formation pressure required for gas hydrate generation under the same temperature conditions compared with pure CH₄ gas. For the target gas fluid, there was no obvious difference in hydrate formation conditions between low-salinity formation water and pure water; the pressure of gas hydrate formation in formation water with high salinity (16 970.7 mg/L) was significantly higher than that in pure water system. Further, the decomposition enthalpy of the synthesized hydrate was calculated based on the Clausius-Clapeyron equation as 62 kJ/mol, and the gas hydrate formed was of Type I structure. Finally, the characteristics of hydrate formation in the target gas reservoir development process were analyzed in combination with experimental data and production performance data, and the hydrate formation in the wellbore and the location of the wellbore where the hydrate formed were predicted. There is much for reference of the results of the study to the prevention of hydrate formation in field production.

Imbibition Mechanism and Model of Bedding Fractures in Shale Oil Reservoir

Xie Jianyong, Yuan Zhenzhu, Dai Bing, Wu Chengmei, Jia Chun, Xu Feng, Luo Qun, Cui Qian

2021, 28(5):  161-167.  DOI: 10.3969/j.issn.1006-6535.2021.05.023

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Shale oil is mainly developed in line with the concept of making longitudinal fractures (tectonic fractures) by staged fracturing in horizontal well, without consideration of the imbibition effect of opening bedding fractures. To further study the role of bedding fractures on shale oil development, the imbibition efficiency of bedding and tectonic fractures was analyzed by means of core imbibition simulation experiment, cast slice observation and CT scanning, taking shale oil in Jimusaer Area, Xinjiang as an example. The results showed that the imbibition depth of tectonic fracture model was 1.5 cm, the imbibition depth of bedding fracture model was 1.0 cm, and the imbibition efficiency of bedding fracture was higher in surface layer; the imbibition efficiency can be increased by about 4.3% by opening the bedding fractures with the same area on the basis of opening tectonic fractures; according to the physical simulation experiment of imbibition, different oil-water imbibition conversion modes were established for tectonic fracture and bedding fracture, indicating that the development efficiency of shale oil can be greatly improved by the imbibition effect of bedding fracture on the basis of structural fracture imbibition. This study provides a theoretical basis for fracturing method optimization and shale oil recovery enhancement.

Drilling & Production Engineering

Study and Application of Blowoff Device with Combined Nozzle for Ultra-Heavy Oil Well

Gao Zhongmin, Yang Bo, Chen Weidong

2021, 28(5):  168-174.  DOI: 10.3969/j.issn.1006-6535.2021.05.024

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Five-stage nozzle is used to control the blowoff rate of fluids produced in the ultra-heavy oil wells. The fluid production rate fluctuates greatly before and after nozzle replacement, and as water content decreases, the viscosity of oil increases, the blowoff may be suspended, which affects the specific time for oil well blowoff and the time to run in pump. For this purpose, a blowoff technology with combined nozzle was developed. The technology was designed with special shaped overflow outlet and cylindrical spool. The special shaped overflow outlet was designed with different curvatures in the front, middle and rear sections, and the overflow area could be adapted to the blowoff rate in smooth, rapid and slow periods. The valve opening was linearly matched to the blowoff rate by matching the special shaped overflow outlet with the cylindrical spool to achieve smooth stepless adjustment of the blowoff rate. The energy loss of wellhead during blowoff could be reduced by optimizing the position of special shaped overflow outlet. The technology has been applied 113 times on site in Liaohe Oilfield, cumulatively reducing the total blowoff time by 4 123.0h. The technology can be used as a reference for optimizing the blowoff system in ultra-heavy oil wells.