Table of Content

25 April 2021, Volume 28 Issue 2

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Summary

Present Situation and Progress of Geosteering Drilling Pre-prospecting Technology

Lin Xin, Yuan Renguo, Qin Lei, Liu Suzhou, Su Chaobo, Lu Zhongyuan, Yu Zhongtao, Tan Weixiong

2021, 28(2):  1-10.  DOI: 10.3969/j.issn.1006-6535.2021.02.001

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Geosteering drilling pre-prospecting technology can effectively improve drilling efficiency and success rate. There are three types of geosteering drilling pre-prospecting technology: near-bit pre-prospecting, seismic-while-drilling pre-prospecting, and azimuth electromagnetic wave pre-prospecting. With case study of exploration and development of complex oil and gas reservoirs, methods widely used in the geosteering drilling pre-prospecting technology were analyzed and summarized, including near-bit measurement, R-VSP, VSP-WD, bed boundary mapping, reservoir mapping while drilling, and looking ahead while drilling. All kinds of geo-steering pre-prospecting technologies can be used for measurements in different ways, but they all have certain limitations. There is a need for pre-prospecting tehcnologies to conduct further studies on the improvement of prospecting accuracy and the combined application of multiple methods, so as to provide strong technical support for casing setting depth design, formation pressure prediction, mud density window setting, detailed description of complex oil and gas reservoirs, and drilling trajectory optimization for the application of pre-prospecting technologies. The study can provide a strong reference for domestic research and field application of geosteering drilling pre-prospecting technologies.

Geologic Exploration

Study on the Characteristics and Reservoir Quality of Permian Igneous Rocks in Southern Sichuan Basin

Jiang Yuliang, Qin Qirong, Gao Ruiqi, Cui Jing, Geng Chao, Fan Cunhui

2021, 28(2):  11-19.  DOI: 10.3969/j.issn.1006-6535.2021.02.002

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In order to find out the geological characteristics and exploration potential of igneous rocks in the Southern Sichuan Basin, the lithology, lithofacies, reservoir characteristics and distribution of igneous rocks were analyzed according to the field outcrops, cores, cuttings, logging and seismic data in the study area. The results showed that the igneous rocks in the Southern Sichuan Basin mainly developed basalt and tuff, the basalt was gradually thinning out from southwest to northeast, and the tuff was most developed in Qianwei County; lithofacies were mainly overflow and explosive facies, and unique reservoir spaces such as tectonic fractures and denudation pores were developed in different vertical areas of each facies belt; the reservoirs mainly included dissolved tuff reservoir and fractured basalt reservoir, which were developed in the upper section of explosive facies and the fracture area of overflow facies. The study results provide technical support for the further study of igneous rocks in the Southern Sichuan Basin and have guiding significance for the sustainable exploration and development in the Southern Sichuan Basin.

Study on Characteristics and Main Controlling Factors of Nano-pores in Low-maturity Shale Reservoirs in Member 2 of Kongdian Formation in Cangdong Sag

Zhang Panpan, Liu Xiaoping, Guan Ming, Sun Biao

2021, 28(2):  20-26.  DOI: 10.3969/j.issn.1006-6535.2021.02.003

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In order to deeply study the characteristics and main controlling factors of nano-pores in lacustrine low-mature shale reservoirs, 12 samples was taken from the Member 2 of Kongdian Formation in Cangdong Sag, Huanghua Depression, and tested for X-ray diffraction, organic geochemical testing, electron microscopy scanning, and low-temperature nitrogen adsorption to analyze the shale reservoirs in Member 2 of Kongdian Formation. The results indicated that the total content of organic carbon of the shale in the Member 2 of Kongdian Formation in Cangdong Sag was 0.48% to 7.23%, and the thermal evolution of organic substances was 0.45% to 0.84%, indicating that the shale was in the low-mature stage as a whole; the nano-pores in the shale were developed with an abundance of intercrystal pores, dissolved pores and shrinkage joints of organic substances, followed by organic substance pores; the nano-pores were relatively open as a whole, and mainly composed of mesopores and macropores in structure; the shale pore volume was changed mainly by the comprehensive effects of mechanical compaction, hydrocarbon generation of organic substances, transformation of clay minerals, and other factors; there was a positive correlation between pore volume and mineral content in clay and total content of organic carbon; the occurrence form of organic substances was the key factor for pore development of organic substances in shale. The results provide a theoretical basis for the evaluation of lacustrine low-maturity shale reservoirs, and serve as references for the evaluation of lacustrine low-maturity shale resources.

Key Controlling Factors of Natural Gas Accumulation in the Ordovician Ma Member 4 in the Central Ordos Basin

Ren Haijiao, Wu Weitao, Zhao Jingzhou, Chen Mengna

2021, 28(2):  27-33.  DOI: 10.3969/j.issn.1006-6535.2021.02.004

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There is certain potential of exploring the carbonate gas in the Ordovician Ma Member 4 in the Central Ordos Basin. However, the key controlling factors for natural gas accumulation are not clear, limiting the exploration in this area. In response to this problem, the gas reservoir characteristics of the Ordovician Ma Member 4 was analyzed with reference to drilling, logging, geochemistry, gas testing and other data. The key controlling factors of hydrocarbon accumulation were studied from the aspects of gas source rock conditions, reservoir characteristics, migration conditions, and shielding conditions. The results showed that the gas reservoirs of the Ordovician Ma Member 4 were mainly lithological gas reservoirs; the richness of natural gas was controlled by the gas-source rock conditions; the natural gas of the Ordovician Ma Member 4 was the gas mixture generated by the upper Paleozoic coal-bearing source rocks and the Lower Paleozoic carbonate source rocks, dominated by the former, and the overall gas produced was less due to insufficient gas source; the characteristics of the reservoir made the natural gas mainly accumulated in the lime dolomite flat facies and dolomite lime flat facies with medium physical properties; the three-dimensional network system composed of pores, fractures and faults provided channels for natural gas migration; under the control of limestone shielding conditions, the natural gas was mainly distributed in the section with lithologic change of thick dolomite and limestone, and the central section with increased dolomite content of thick limestone. The understanding of the key controlling factors of natural gas accumulation in the Ordovician Ma Member 4 can provide a theoretical foundation for the exploration of marine carbonate rocks in the Ordos Basin.

Petrological Characteristics of Shale Gas Reservoirs in Long₁¹ Sub-member of Longmaxi Formation in W202 Block of Weiyuan Structure

Yang Guang, Tian Weizhi, Lyu Jiang, Xu Shouhui, Kang Le, Li Hanfei

2021, 28(2):  34-40.  DOI: 10.3969/j.issn.1006-6535.2021.02.005

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In order to identify the relationship between the petrochemical composition and mineral properties and the gas-bearing property of the shale reservoirs in Weiyuan Structure in the low-steep fold belt in the southwestern Sichuan Basin. Core samples were taken from the lower Silurian Longmaxi Formation in the W202 Block of the Weiyuan Structure, and studied on rock chemical elements, shale mineral types and characteristics, organic carbon content and adsorption capacity. It was found in the study that the lithology of Long₁¹ Sub-member of Longmaxi Formation was mainly lime (dolomitic) shale, siliceous shale, carbonaceous shale, and clay shale; mineral type was dominated by terrigenous minerals, authigenic minerals, bioclasts and a little amount of organic matter; there were 4 layers in the vertical direction, but their petrochemical characteristics were obviously different; the Layer I of the Long₁¹ Sub-member was developed with quartz, with hight brittleness, rich pyrite, high organic matter abundance and strong adsorption capacity, and it was a quality reservoir for shale gas. This study provides a technical support for the adjustment of exploration and development plans for shale gas reservoirs.

The Law of Hydrocarbon Accumulation in Carbonate Reservoirs in Halahatang Area, Tarim Basin

Zhu Weiping, Sun Dong, Yao Qingzhou, Fang Qifei, Dai Dongdong

2021, 28(2):  41-48.  DOI: 10.3969/j.issn.1006-6535.2021.02.006

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The Halahatang area is the main oil-bearing zone in the Tarim Basin. It is difficult to predict the law of hydrocarbon accumulation due to the influence of multi-period tectonic movements. For this reason, based on the interpretation of seismic data, a systematic study was conducted for the fault distribution and paleostructure in hydrocarbon accumulation period of the deep carbonate reservoirs in the Rewapu block in the Halahatang area. As indicated by the analysis results of reservoir distribution and productivity data, the secondary migration of oil and gas was adjusted by branch faults in NE-SW, NW-SE and near SN direction in the study area, and oil and gas were accumulated in the reservoir development termination area at the updip end of the faults; oil and gas migrated along the dominant migration direction, and the reservoirs that have long been located in the nose-shaped uplift area of the paleostructure and the overlapping area of local tectonic high points were conducive to oil and gas preservation. Based on the analysis of faults and paleostructures, combined with the characteristics of the reservoir, a hydrocarbon accumulation model was established with an “X”-shaped strike-slip fault used to channel oil and gas from south to north, and its associated secondary branch faults to adjust the secondary migration of oil and gas along the direction of the dominant oil and gas migration, finally enabling oil and gas to accumulate in reservoir development area located in the uplift area of the paleostructure and the overlapping area of local tectonic high points.

Geochemical Characteristics of Shale Gas with Moderate Thermal Evolution in Chang₇ Member, Central Ordos Basin

Huo Pingping, Fan Bojiang, Wang Yanyan, Liang Quansheng, Li Yating, Wang Xia, Shi Liang

2021, 28(2):  49-56.  DOI: 10.3969/j.issn.1006-6535.2021.02.007

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In view of the low degree of thermal evolution and complicated shale gas occurrence of the Chang₇ Member in the central Ordos Basin, a comparative study was conducted with basic geochemical methods and gas analysis experiments to discover the geochemical characteristics of shale gas and explore the occurrence process of shale gas. The results showed that the shale mainly contained Type Ⅰ and Type Ⅱ₁ organic substances, with moderate thermal evolution, average organic carbon content of 4.52%, free hydrocarbon content of 4.09 mg/g, and pyrolysis hydrocarbon content of 8.70 mg/g; the shale gas in the study area was petroliferous gas, and its geological origin was thermal decomposition; the occurrence state of shale gas was coexistence of free gas, dissolved gas and adsorbed gas, and the sum of adsorbed gas content and dissolved gas content was greater than free gas content; the adsorption of shale for different gases was quite different, and its adsorption for nitrogen was the weakest; as for the hydrocarbon gases, the initial migration capacity of small molecule gases was greater than that of macromolecular gases. Due to the low thermal evolution, on the one hand, oil, gas and water coexisted in the shale, generally low gas content in shale is not high. On the other hand, a large amount of oil and gas was preserved in geological movement. In the development process of Chang₇ Member, joint development of shale oil and gas is of the most practical and commercial significance. There is much for reference of the study results to the further exploration and development of Chang₇ Member.

Reservoir Engineering

Calculation of Production Depth of Oil Wells in Ultra-Deep Fault-Karst Reservoirs Based on Well Temperature

Gu Hao, Shang Genhua, Li Huili, Wang Qiang, Zhu Lianhua, Zhao Rui, Kang Zhijiang, Li Wangpeng

2021, 28(2):  57-62.  DOI: 10.3969/j.issn.1006-6535.2021.02.008

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Due to good vertical interconnection in the ultra-deep fault-karst reservoirs, oil and gas under the well bottom can easily flow to the well bottom along the fractured channel with high conductivity in oil well production. In order to identify the production depth of oil wells, Well W9 well in Z Oilfield was taken as an example to compare and analyze the characteristics of flow temperature and static temperature of ultra-deep fault-karst reservoirss, and establish the calculation method of oil well production depth with different temperature-depth relationships based on temperature characteristics. It was found in the study that the flow temperature at the same depth was greater than the static temperature, which had nothing to do with the nozzle diameter; when the depth was constant, the larger the nozzle diameter, the higher the flow temperature; as the depth increased, the difference between the flow temperature and the static temperature generally decreased, and the flow temperature gradient was low near the well bottom; the temperature characteristics of the ultra-deep fault-karst reservoirs were affected by heat loss, oil well production depth and heat transfer mode; the production depth of Well W9 was about 52 to 62 m under the current production conditions with Ф4.5 mm nozzle. This study can provide a basis for the judgment of oil column height, reserve calculation and formulation of reasonable production systems for ultra-deep fault-karst reservoirs.

Evaluation and Development Countermeasures for Nonproducing Reserves of Heavy Oil Reservoirs in Shengli Oilfield

Li Weizhong

2021, 28(2):  63-71.  DOI: 10.3969/j.issn.1006-6535.2021.02.009

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Shengli Oilfield, rich in heavy oil resources, still has nearly 3.20×10⁸t proven reserves that have not been effectively developed after years of technical research and development. In order to realize the effective development of nonproducing reserves of different types of heavy oil reservoirs, the reserves characteristics and development difficulties were systematically analyzed, the heavy oil reservoirs were divided into five types, namely sensitive heavy oil, deep low-permeability heavy oil, super heavy oil, edge-bottom water heavy oil and ultra-thin layer, and development countermeasures were prepared for different types of nonproducing reserves with comprehensive application of physical modeling, numerical simulation, laboratory testing, and other methods. It was found in the study that sensitive heavy oil reservoirs could be developed with technologies such as moderate sand production, heavy oil viscosity reduction and cold recovery, and oil layer combustion; deep low-permeability heavy oil reservoirs could be developed with technologies of fracturing assisted solubilization and viscosity reduction, and viscosity reduction and displacement; super heavy oil reservoirs could be developed with E-SAGD, HECS and other EOR technologies to improve the fluidity of crude oil; edge-bottom water heavy oil reservoirs could be developed with bottom-water steam flooding, viscosity reduction and cold recovery, and microbial oil recovery to reduce the effect of edge-bottom water on development; the thin heavy oil reservoirs could be developed mainly with short-radius horizontal well, and fracturing + viscosity reduction and cold recovery to increase the controlled reserves of single well. The technologies and findings obtained in the study are of great significance in guidance and reference for the development of similar reserves at home and abroad.

Prediction Method of Key Development Indicators of Fracture-cavity Carbonate Reservoirs and Its Application

Chang Baohua, Li Shiyin, Cao Wen, Liu Zhiliang, Lu Linlin

2021, 28(2):  72-77.  DOI: 10.3969/j.issn.1006-6535.2021.02.010

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In view of the heavy difficulties in prediction of key development indicators of fractured-cavity carbonate reservoirs, a prediction method was established for the key development indicators from isolated fracture-cavity reservoirs to the whole block on the basis of the theories of material balance, well-controlled dynamic reserves evaluation, and regression analysis of the dynamic and static parameters of the reservoir bodies in production. With the aid of this method, relatively accurate predication results were obtained for key development indicators of single well, even the whole block, including available reserves, recoverable reserves, recovery efficiency, and gas lift measures for oil enhancement. This method applied to the Zhonggu I Block, and the results indicated that the recoverable oil and gas reserves of the block were 85.8×10⁴m³ and 26.6×10⁸m³ respectively, and the oil and gas (decreasing) recovery rates were 27.8% and 59.2% respectively, the recoverable reserves of remaining reservoir bodies not put in production in the block are 36.4×10⁸m³, the oil and gas recovery rates after gas lift were 32.6% and 68.4% respectively, increased by 4.8、9.2 percentage points. This method can effectively predict the key development indicators of single-well reservoir bodies and blocks of such oil and gas reservoirs, providing a basis for the formulation of development plans and technical policies.

Testing Device and Method for Unsteady Gas-water Relative Permeability under High Temperature and High Pressure

Zhang Guangdong, Wu Zheng, Li Yichi, Chen Yijian, Yang Qingsong, Pan Yi

2021, 28(2):  78-82.  DOI: 10.3969/j.issn.1006-6535.2021.02.011

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At present, gas-water relative permeability curves of gas reservoirs with high temperature and high pressure are mostly obtained by unsteady testing method. The testing device is mainly equipped with back pressure valve to control fluid pressure. However, the pressure is easy to change under high temperature and high pressure, resulting in deviation in gas-water measurement. For this reason, this paper proposed a testing device and method for unsteady gas-water relative permeability under high temperature and high pressure based on the capacitance method. The testing device was fixed a self-developed capacitance liquid meter. The experimental temperature and pressure were increased to 200.0 ℃ and 80.00 MPa, respectively. The water production rate was measured by the change of liquid volume and capacitance. An electric turbo-metering pump was used to control the fluid control and measure the gas volume. The influence of fluid pressure and oil reservoir temperature on the gas-water relative permeability curve was analyzed on the basis of experimental test data. The results showed that under the same conditions, when the experimental temperature was increased, the irreducible water saturation was lower and the gas-water co-permeation area was wider; when the experimental fluid pressure was decreased, the irreducible water saturation was higher and the relative permeability of the gas phase was lower. The study results are of great significance for guidance in the development scheme design, dynamic analysis and understanding of gas-water seepage rules of gas reservoirs with high temperature and high pressure.

Variation Rules of Driving Energy of Deep Aqueous Carbonate Gas Reservoirs in Moxi Block of Anyue Gas Field

Ruan Jifu, Pang Jin, Yuan Quan, Yi Jin, Ou Jiaqiang

2021, 28(2):  83-88.  DOI: 10.3969/j.issn.1006-6535.2021.02.012

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Deep aqueous carbonate gas reservoirs are developed with pores and fractures. The strong compressibility of rocks and the complex gas-water relationship affect the variation of driving energy in the gas reservoirs, leading to the great difference in dynamic analysis and development mode optimization of gas reservoirs. In response to this problem, taking the Longwangmiao gas reservoir in the Moxi block of the Anyue Gasfield as an example, the variation rules of the compressibility of deep carbonate rocks were studied through experiments, and the equations were established for the compressibility sensitivity to stress of different types of reservoirs. Combined with the material balance equation of water drive gas reservoirs, the pressure drop indicator curve was employed to study the influence of the compressibility of different types of reservoir on the driving energy variation of variable-volume confined gas reservoirs, and the influence of different water saturations of the connate water area and gas-water transition zone on the driving energy variation of variable-volume confined gas reservoirs, and the influence of compressibility on the driving energy of water drive gas reservoirs. Taking Well 009-3-X2 as an example, the driving indicator in different periods was calculated and its variation rules were analyzed. The results of the study showed that there was quite difference in the compressibility of different types of carbonate reservoirs, the compressibility of fracture-cavity reservoirs was sensitive to stress, the elastic energy of rocks and pore water wars strong in the early stage of development, the elastic driving indicator was 0.317 to 0.535, and the elastic energy was mainly released when the recovery percent was less than 20%; when the water saturation of the reservoir reached 60%, the elastic driving indicator of free water in the formation reached 0.359; when there was edge-bottom water, the driving energy of the edge-bottom water pressure was higher in the initial stage, and gradually weakened later. The study results provide significant basis for the optimization of carbonate gas reservoir development modes, productivity evaluation and reserves calculation.

Experimental study on the influence of movable water on reservoir stress sensitivity

Yang Guohong, Li Xiuqing, Li Mingqiu, Wu Zheng, Zhu Xun, Zou Juan, Li Xuesong, Feng Yan

2021, 28(2):  89-95.  DOI: 10.3969/j.issn.1006-6535.2021.02.013

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In view of the problem that the pressure is difficult to reach a steady state and the test duration is long when measuring gas-water seepage under the condition of movable water, a experimental method for evaluating stress sensitivity under high temperature, high-pressure and gas-water seepage was proposed in this paper. Capacitance method was introduced to control core water saturation with liquid meter, adjust gas-water circulating flow with peristaltic pump to obtain stress sensitivity data, then the effect of movable water on reservoir stress sensitivity was analyzed. This method only breaks away from the restrictions on independent adjustment of gas-water proportion at the inlet and joint control with back pressure valve, but also provides more technologies for movable water control in gas-water seepage. The results showed that the stress sensitivity of reservoirs with movable water was much stronger than that of dry cores and reservoirs with irreducible water; with the continuous increase in core water saturation, the reservoir stress sensitivity continued to increase; the better the physical properties of the core, the smaller the effect of movable water on stress sensitivity; a productivity model under movable water conditions was established with consideration of stress sensitivity, and it was found that the higher the reservoir water saturation, the greater the effect of stress sensitivity on gas well productivity. The experimental study has an important guiding role in the seepage mechanism and productivity evaluation of water-bearing gas reservoirs.

Evaluation on Productivity of Asymmetric Hydraulic Fractured Vertical Wells in Coalbed Methane Reservoirs

Li Chen

2021, 28(2):  96-101.  DOI: 10.3969/j.issn.1006-6535.2021.02.014

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The coalbed methane reservoirs are characterized by strong heterogeneity, uneven distribution of fracturing fractures, and usually asymmetric distribution. At present, there is no evaluation model for productivity of coalbed methane reservoirs with asymmetric fractures. In view of the above problem, Langmuir isotherm adsorption equation and Fick diffusion law were used to explain the adsorption, desorption and diffusion in coalbed methane reservoirs, and a special pseudo-time function was introduced to solve the strong nonlinearity in the material balance equation considering adsorption and desorption. The new defined parameters were used to characterize the asymmetry of hydraulic fractures in the fracturing process, and the productivity evaluation model and its analytical solutions under different symmetry of fractures were obtained. With the principle of time superposition, the actual production data was fitted with the theoretical data. It was found in the study that the asymmetry of fractures greatly affected the results of productivity evaluation and prediction of gas reservoirs; the energy utilization rate of symmetric fractures was the highest; the asymmetry of fractures mainly affected the productivity evaluation in the early production; with the increase of fracture conductivity, the effect of the fracture asymmetry on gas well productivity became lower. The study results are of great important guidance for the accurate evaluation and productivity prediction of fractured vertical wells and the efficient development of coalbed methane reservoirs.

Integrated Numerical Simulation of Carbon Dioxide Displacement and Sequestration

Gao Ran, Lyu Chengyuan, Lun Zengmin, Wang Rui

2021, 28(2):  102-107.  DOI: 10.3969/j.issn.1006-6535.2021.02.015

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At present, numerical simulations cannot calculate the distribution of CO₂ in oil, gas, and water and the effect of CO₂ dissolved in water on the physical properties of the reservoir. In response to this problem, the innovative application of the integrated numerical simulation of CO₂ displacement and sequestration can simulate the distribution of CO₂ in oil, gas and water, the effect of dissolved CO₂ on sequestration, and the whole process of CO₂ displacement and sequestration on the basis of the coupled oil-gas flash evaporation and the solution and dissolution of CO₂ in water. It was found in the study that CO₂ dissolved in formation water developed into carbonic acid dissolved rock, leading to an increase in reservoir permeability, and the longer the action duration, the more obvious the dissolution effect; the solution and dissolution of CO₂ in the water affected the cumulative oil production and CO₂ sequestration. In the study of CO₂ displacement and sequestration, it was necessary to consider both the solution and dissolution effects in order to reflect the actual production. The numerical simulation study was conducted in the Lower Sha 1 reservoir in Pucheng Oilfield, and proved that the integrated numerical simulation method of CO₂ displacement and sequestration is accurate. This study can become certain reference and guidance for scheme design of CO₂ displacement and sequestration project and the evaluation of CO₂ sequestration potential.

Study on Reverse Water Plugging Technology of Nanoparticle Reinforced AM/AMPS Composite Hydrogel

Wang Encheng

2021, 28(2):  108-111.  DOI: 10.3969/j.issn.1006-6535.2021.02.016

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In order to solve the problems of low strength and poor temperature and salt resistance of common polymer profile control and displacyment agent, AM/AMPS polymer composite hydrogel plugging agent with nano SiO₂ particles was developed and its plugging mechanism was analyzed. It was found in the experimental study that AM/AMPS+SiO₂ composite hydrogel had high temperature and salt resistance, erosion resistance and plugging performance, and its plugging mechanism was to improve the wettability and change the rock formation from lipophilic to hydrophilic. This composite hydrogel was used in reverse water plugging and oil displacement in oil wells in the Wenliu Oilfield, the oil displacement effect was significantly improved, realizing accumulative oil enhancement of 4 244.0 t, with high economic benefits. This study has a strong guiding significance in solving the problem of low recover efficiency of oil wells with high water cut, and opens a new prospect in application.

Study and Application of 3D Physical Modeling Experiment of Reservoir Stimulation

Li Li, Zhang Xingyong, Qin Li, Tang Jian

2021, 28(2):  112-119.  DOI: 10.3969/j.issn.1006-6535.2021.02.017

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The analysis on the influencing factors of reservoir stimulation has always made reference to experience and theory. Although there were some small-scale rock sample experiments occasionally conducted, the results obtained cannot reflect the actual conditions of reservoir stimulation. In order to discover the influence law of construction parameters on the reservoir stimulation, large-sized experimental samples were prepared with reference to the rock mechanics parameters of typical high-stress reservoirs in Shunbei Oilfield, three-dimensional hydraulic fracturing experiments were conducted with a hydraulic fracturing physical modeling system, and the effects of pumping displacement and liquid viscosity on hydraulic fracturing and the relationship between acoustic emission amplitude and pumping pressure were studied. The results showed that with the increase in initial pumping displacement, the fracture pressure required for reservoir stimulation was gradually increased; after reaching a certain value, the displacement had little effect on the fracture pressure; the pumping displacement had an effect on the fracture size at the moment of fracturing, but had little effect on the entire fracturing process; the application of low-viscosity slick water at the beginning of stimulation could significantly reduce the net fracture pressure, in favor of reducing the construction pressure; the amplitude of acoustic emission was positively correlated with the net pumping pressure, and the greater the net pressure at the moment of fracturing, the greater the amplitude. The study results provide a reference for the design of fracturing construction parameters optimization for ultra deep well fracturing under high stress.

Study on Well Spacing Limit for Conventional Water Flooding and Carbon Dioxide Flooding in Low-permeability Reservoirs

Yu You, Wei Jianguang, Zhang Baozhong, Li Jiangtao

2021, 28(2):  120-125.  DOI: 10.3969/j.issn.1006-6535.2021.02.018

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In order to realize the optimal design of well spacing for low-permeability reservoirs in Jilin Oilfield, based on a case study of Fuyu oil reservoir in Jilin Oilfield, pressure gradient test was conducted, a curve was prepared to indicate the relation between starting pressure gradient and permeability of water flooding and carbon dioxide flooding, and a reasonable well spacing design drawing was drawn for water flooding and carbon dioxide flooding with reference to reservoir pressure distribution law in a mode of "one for injection and one for production". the results of the study show that the minimum starting pressure gradient of the low-permeability Fuyu reservoir had a negative power exponential relation with the permeability, and the minimum starting pressure gradient of the same core water flooding was 2 to 4 times that of carbon dioxide flooding under the condition that the injection-production pressure difference was 15 to 25 MPa; when the reservoir permeability was lower than 0.10 mD, the upper limit of the reasonable well spacing for carbon dioxide flooding was greater than that of water flooding, which was more beneficial for development; when the permeability was from 0.05 to 0.10 mD, the associated reasonable well spacing range was from 120 to 380 m; when the reservoir permeability was greater than 0.10 mD, the water flooding cost was much lower and much favorable for development; when the permeability is from 0.10 to 0.20 mD, the reasonable well spacing range was from 70 to 120 m. The study results provide an important reference for the selection of reasonable well spacing with different medium flooding in Fuyu reservoir in Jilin Oilfield.

Analysis on Contribution to Productivity of SRV-fractured Horizontal Wells in Tight Reservoirs Based on Simulation of Fracture Network Propagation

Mu Lijun, Wu Shunlin, Xu Chuangchao, Su Yuliang, Ren Long

2021, 28(2):  126-132.  DOI: 10.3969/j.issn.1006-6535.2021.02.019

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On account of difficulties in prediction of fracture network propagation caused by stimulated reservoir volume (SRV) fracturing and in simulation of coupled flow in multi-pore media in tight reservoirs, a study was conducted on fluid flow rules in tight reservoirs based on fracture propagation mechanism with SRV fracturing, a mathematical model for transient seepage flow in multiple media and a propagation model for fracture network produced by fracturing under mutual interference of multiple fractures were established, and finite element method was used for solving. A production simulation was performed based on a case study of tight oil in the Ordos Basin to analyze the contribution of different pore media to the productivity of SRV-fractured horizontal wells in tight oil reservoirs. The findings show that if the spacing of SRV-fractured horizontal well clusters is shortened, the hydraulic fracture propagation length is shorter, the average belt length is decreased, the stimulated area first is decreased and then tends to be stable, and the average belt width and fracture width first are increased and then tend to be stable; the construction to productivity of SRV-fractured horizontal well is dominated by natural and and hydraulic fractures, and the matrix has little contribution to productivity. The results of the study provide a theoretical foundation for fracture propagation simulation and productivity prediction of fractured horizontal wells in tight reservoirs.

Study on the Activation of Natural Fractures in Ultra-deep Fractured Clastic Reservoirs

Zhang Hui

2021, 28(2):  133-138.  DOI: 10.3969/j.issn.1006-6535.2021.02.020

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In view of the lack of specific method of activating natural fractures in the ultra-deep fractured clastic reservoir in Cretaceous Bashijiqike Formation in Tarim Oilfield, the natural fractures of the reservoir in the study area was analyzed for conditions of realizing shear-based activation and tension-based activation according to the Moore-Coulomb failure criterion, and the fracture conductivity in hydraulic fracturing and acid fracturing was simulated and tested. The results of the study show that the conductivity of fractures with mutual dislocation in hydraulic fracturing was 100 to 1 000 times that of fractures without mutual dislocation, the conductivity of fractures without mutual dislocation was increased by more than 1 000 times after sand addition and the shear slip of natural fractures in fracture network fracturing could also achieve stimulation effects, so sand addition had an important influence on the stimulation effect of fractures without mutual dislocation; moreover,if sand was added, whether fracture with mutual dislocation had no essential effect on the conductivity. In fracture network acid fracturing, the filled natural fractures were under a lower closure stress (less than 20 MPa), the acid-rock reaction time had little effect on the conductivity; under higher closure stress, the longer the acid-rock reaction time, the greater the dissolution degree of fillings and the higher the retention degree of conductivity. Effective conductivity could be obtained by filled natural fractures through acid erosion, but did not obtained by non-filled natural fractures through acid erosion. The study results provide technical support for the stimulation of fracture networks in the ultra-deep reservoir in the Kuqa Piedmont.

Analysis and Discussion on Well Test of Gas Field Storage Facility in Gas Injection Period

Liu Xiaoxu, Zhou Yuan, Wang Xia, Luo Xin, Zhou Daoyong

2021, 28(2):  139-143.  DOI: 10.3969/j.issn.1006-6535.2021.02.021

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In view of the unclear and inadequate basis for the preparation of gas injection scheme for gas field storage facility, based on the well test results of downhole coiled tubing of Xiangguosi gas storage facility, the concept of "temperature drop effect" and "variable skin effect" in gas injection period of the gas storage facility was proposed, and the influence of temperature drop effect, variable skin effect and reservoir stress sensitivity on injection and production in the injection and production of gas storage facility was analyzed. With the technology of analyzing the well test in injection period of gas field storage facility, the response characteristics of various factors on the well test curve and the influence on the well test interpretation parameters were studied. The results show that: ①Compared with the well test during the gas production period, the reservoir physical parameters obtained during the gas injection period have the same reference value; ②the temperature drop effect of gas storage facility has a negligible influence on the well test interpretation results, while the influence of variable skin effect and stress sensitivity effect varies greatly in different injection, production and operation periods; ③the influence of variable skin effect and stress sensitivity should be fully considered when preparing the injection, production and operation scheme of the gas storage facility, and well test should be conducted in different injection, production and operation periods to obtain accurate reservoir parameter values. The study results provide important study basis and theoretical guidance for the well test and interpretation of gas storage facility.

Design Method and Application of ASP Flooding System Based on Emulsion Seepage Mechanism

Yue Qing

2021, 28(2):  144-149.  DOI: 10.3969/j.issn.1006-6535.2021.02.022

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In the field experiment of ASP (alkali, surfactant, polymer) flooding in Daqing Oilfield, the produced liquid of crude oil was seriously emulsified, but the migration law of the produced emulsion and its effect on enhancing recovery efficiency are unclear. Therefore, the long core model experiment was conducted to clarify the mechanism of emulsion seepage and flooding. The results show that after injection based on the actual formula of ASP flooding system, the emulsifying capacity was continuously weakened with the increase of the migration distance, the emulsifying slug was produced when the pore volume was 0.40 time and ended when the pore volume was 0.60 time, and the length of the emulsifying slug was 0.20 time the pore volume. The results of flooding experiment show that the optimal displacement slug size was 0.30 time the pore volume, and the actual emulsifying slug did not reach the optimal size, so the injection time of ASP system should be extended. The experiment truly reflected the emulsification process of the ASP flooding under the formation conditions, and clarified the emulsification law and the influence of the emulsion slug on the flooding effect, with important guiding significance for on-site adjustment.

Study on the Stress Sensitivity Characteristics and Quantitative Characterization of Oil Reservoir with High Formation Saturation Pressure Difference

Jia Pin, Li Zhuang, Yin Hengfei, Cheng Linsong

2021, 28(2):  150-155.  DOI: 10.3969/j.issn.1006-6535.2021.02.023

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Oil reservoirs with high formation saturation pressure difference are usually characterized by medium porosity, mid-to-high permeability and wide variation of reservoir pressure, and stress sensitivity is a common problem in the development of such reservoirs. In view of the stress sensitivity of oil reservoirs with high formation saturation pressure difference, taking Kenli Oilfield as an example, the experiment for gas logging permeability of natural core in net confining stress was conducted to obtain the change rule of effective stress and permeability of the core in single and multiple times of pressure rise and fall, and a new stress sensitivity coefficient was defined to develop a power-based stress sensitivity quantitative characterization method for such oil reservoirs. The results showed that the maximum permeability loss of 30, 100, 300 mD cores during single pressure rise and fall was 35%, 27% and 15%, respectively. The change trend of core permeability of multiple times of pressure rise and fall is similar to that of single time of pressure rise and fall. The permeability loss increases with the increase of pressure rise and fall times. When the reservoir permeability is lower than 100 mD, the maximum permeability loss can be up to more than 30%. The study on change rule and characterization method of stress sensitivity is of great important guidance for the formulation of reasonable working system, reasonable productivity and reservoir protection scheme design for oil reservoir with high formation saturation pressure difference.

Drilling & Production Engineering

Study on Flow Rules of Herschel-Bulkley Fluid in Concentric Annulus of Elliptical Wellbore and Simplified Model for Pressure Drop Calculation

Zhang Jie, Tang Ming, Jiang Zhenxin, Gan Yifeng, Zeng Dezhi

2021, 28(2):  156-162.  DOI: 10.3969/j.issn.1006-6535.2021.02.024

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The wellbore is easy to become elliptical due to the uneven rocks and in-situ stress, leading to great deviation in the existing annulus flow model of regular circular wellbore. In order to identify the flow rules of Herschel-Bulkley fluid in the concentric annulus of the elliptical .wellbore, an analytical model and a Fluent simulation model were established to analyze the influence of the elliptical .wellbore on the annulus flow velocity and pressure drop gradient. A simplified model was established for pressure drop calculation of concentric annulus of elliptical wellbore by correcting the hydraulic diameter and introducing effective viscosity to realize accurate and rapid prediction of flow pressure drop. The effectiveness of the simplified model was verified by the analytical model, simulation model and measured results. The results showed that when the flow rate was constant, the axial average velocity and maximum velocity increased linearly and exponentially with the increase of the ratio of the long and short axis of the ellipse; the dimensionless pressure drop gradient increased linearly with the increase of the ratio of the long and short axis of the ellipse; the simplified model was well fitted with the simulation model, analytical model and measured results, and the deviations were ±8%, ±5% and ±5%, respectively. The study results can be used as a theoretical support for accurate and rapid prediction of flow pressure drop in concentric annulus in elliptical wellbore.

Optimization and Performance Evaluation of High-grade Steel Casing Section Milling Tools

Miao Juan, Huang Bing, Xie Li, Tang Ming

2021, 28(2):  163-170.  DOI: 10.3969/j.issn.1006-6535.2021.02.025

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Casing section milling technology has applied widely to old wells sidetracking and abandoned wells, with broad application prospects. Aiming at solving the problems of long cutting time, slow milling speed and short footage of conventional section milling tools in the section milling process of high-grade steel casing, high-performance section milling tools were designed for Φ177.8 mm casing. The cutting blade of section milling tool was made of imported tungsten carbide with high strength and high stability. The tooth structure of the cutting blade was optimized as concave teeth + sharpened diamond teeth, and the tooth structure of the milling blade was optimized as concave teeth + non-evenly arranged teeth. The cutting and section milling effects of the designed section milling tool were evaluated by ground experiment and LS-DYNA simulation system. The results showed that the high-grade steel casing section casing cut off the casing within 5.00 min, the section milling speed was more than 0.8 m/h, and the average footage of single section milling was about 10 m. The casing section milling efficiency was improved significantly. Φ177.8 mm high-grade steel casing section milling tool is advantaged by short cutting time, high section milling speed, and long footage of single milling, with wide promotion and application prospects.

Damage Mechanism of Casing for Thermal Recovery of Heavy Oil Well and Casing Coupon Technology Experiment

Li Zongkun

2021, 28(2):  171-174.  DOI: 10.3969/j.issn.1006-6535.2021.02.026

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In view of the long-term high temperature and high pressure of the casing and frequent casing damage in the process of steam flooding, a statistical analysis of the casing damage in the steam flooding of the Qi 40 Block in Liaohe Oilfield was conducted, and the mechanical and thermodynamic study was conducted for the casing damage process. The results showed that when steam was injected into the oil well, the casing temperature raised, resulting in thermal expansion force which increased with temperature; when the thermal expansion force was greater than the cementing resistance of the casing, the casing broke away from the cement sheath and expanded freely, and an inclined plane effect was produced on the contact surface of them, making the thermal expansion force change from the longitudinal elastic force to the increasing lateral squeezing force that brought extrusion damage to the casing, which is the complete casing damage mechanism. Based on the damage mechanism, the casing coupon technology was developed. Preliminary tests have shown that the casing coupon can greatly increase its cementing force and reduce the probability of casing damage. The study results are of important significance for improving the development benefits of heavy oil old areas.