Reservoir stress path and induced seismic anisotropy： results from linking coupled fluid-flow/geomechanical simulation with seismic modelling

We present a workflow linking coupled fluid-flow and geomechanical simulation with seismic modelling to predict seismic anisotropy induced by non-hydrostatic stress changes. We generate seismic models from coupled simulations to examine the relationship between reservoir geometry, stress path and seismic anisotropy. The results indicate that geometry influences the evolution of stress,which leads to stress-induced seismic anisotropy. Although stress anisotropy is high for the small reservoir, the effect of stress arching and the ability of the side-burden to support the excess load limit the overall change in effective stress and hence seismic anisotropy. For the extensive reservoir, stress anisotropy and induced seismic anisotropy are high. The extensive and elongate reservoirs experience significant compaction, where the inefficiency of the developed stress arching in the side-burden cannot support the excess load.The elongate reservoir displays significant stress asymmetry,with seismic anisotropy developing predominantly along the long-edge of the reservoir. We show that the link betweenstress path parameters and seismic anisotropy is complex,where the anisotropic symmetry is controlled not only by model geometry but also the nonlinear rock physics model used. Nevertheless, a workflow has been developed to model seismic anisotropy induced by non-hydrostatic stress changes, allowing field observations of anisotropy to be linked with geomechanical models.

Stress sensitivity of formation during multi-cycle gas injection and production in an underground gas storage rebuilt from gas reservoirs

Permeability sensitivity to stress experiments were conducted on standard core samples taken from Wen 23 Gas Storage at multi-cycle injection and production conditions of the gas storage to study the change pattern of stress sensitivity of permeability.A method for calculating permeability under overburden pressure in the multi-cycle injection and production process was proposed,and the effect of stress sensitivity of reservoir permeability on gas well injectivity and productivity in UGS was analyzed.Retention rate of permeability decreased sharply first and then slowly with the increase of the UGS cycles.The stress sensitivity index of permeability decreased with the increase of cycle number of net stress variations in the increase process of net stress.The stress sensitivity index of permeability hardly changed with the increase of cycle number of net stress variations in the decrease process of net stress.With the increase of cycle number of net stress variation,the stress sensitivity index of permeability in the increase process of net stress approached that in the decrease process of net stress.The lower the reservoir permeability,the greater the irreversible permeability loss rate,the stronger the cyclic stress sensitivity,and the higher the stress sensitivity index of the reservoir,the stronger the reservoir stress sensitivity.The gas zones with permeability lower than 0.3’10-3 mm2 are not suitable as gas storage regions.Stress sensitivity of reservoir permeability has strong impact on gas well injectivity and productivity and mainly in the first few cycles.

Influence of the Earth Stress on the Development of Gas Reservoirs

Gas reservoirs are located kilometers deep beneath the earth's surface under great earth stresses, including the overburden stress and the horizontal stress. After a well is drilled, the stress condition around the well bore will be changed. During the development, a pressure funnel forms around the hole, with the rock stress redistributed. In this paper, the influence of the earth stress on the gas reservoir development, including the output, the period of the steady output, the recovery and ratio, is researched thoroughly with the theory of reservoir seepage dynamics and clarified with the calculation method. The research shows that the earth stress produces impacts on the development of a gas reservoir when the stress is great.

Stress sensitivity of tight reservoirs during pressure loading and unloading process

Laboratory experiments were conducted on laboratory-made tight cores to investigate the stress-dependent permeability hysteresis of tight reservoirs during pressure loading and unloading process. Based on experiment results, and Hertz contact deformation principle, considering arrangement and deformation of rock particles, a quantitative stress dependent permeability hysteresis theoretical model for tight reservoirs was established to provide quantitative analysis for permeability loss. The model was validated by comparing model calculated results and experimental results. The research results show that during the early pressure-loading period, structural deformation and primary deformation worked together, rock permeability reduced dramatically with increasing effective stress. When the effective stress reached a certain value, the structural deformation became stable while the primary deformation continued; the permeability variation tended to be smooth and steady. In the pressure unloading process, the primary deformation recovered with the decreasing effective stress, while the structural deformation could not. The permeability thus could not fully recover, and the stress-dependent hysteresis was obvious.

Influences of lithology on in-situ stress field in low permeability reservoirs in Bonan Oilfield, Bohai Bay Basin, China

The differences of rock mechanical properties were analyzed based on triaxial compression test in low permeability reservoirs of the Bonan Oilfield. Through the analysis of reservoir mechanics, the influence mechanisms of different mechanical properties of rocks on reservoir in-situ stress were studied. By means of stress ellipse and finite element simulation, the influence rules of different mechanical properties of rocks on in-situ stress field were discussed. For the low permeability reservoirs of the Bonan Oilfield, the coarser rock has a larger Young’s modulus value and a lower Poisson’s ratio. The rock mechanical parameters and stress-strain relationship of sandstone facies and mudstone facies are different. Different rocks have different mechanical properties, which cause extra stress at the lithological contact interface, and the existence of extra stress affects the reservoir in-situ stress. Without considering the influence of structural features on the in-situ stress field, the reservoir in-situ stress is controlled by the magnitude of extra stress and the angle between lithological contact surface and boundary stress.

Effects of effective stress and temperature on permeability of sandstone from CO2-plume geothermal reservoir

Rock is generally complex and heterogeneous,therefore the heterogeneity effects of effective stress and temperature on permeability should be taken into account.In this study,two-part Hooke’s model（TPHM） is introduced to understand the influences of effective stress and temperature on permeability of soft and hard parts（two parts） of rock based on coupling thermo-hydro-mechanical tests.Under a fixed temperature level（25 ℃.35 ℃.50 ℃.65 ℃.80 ℃.90 ℃ and 95 ℃）.the tests were carried out in a conventional triaxial system whereas the confining pressure was remained at 50 MPa.and the pore pressure was increased to the specified levels step by step.i.e.8 MPa,18 MPa.28 MPa.38 MPa.41 MPa,44 MPa.46 MPa and 48 MPa.The temperature-dependent relationships for two parts permeabilities are proposed on the basis of the initial test results.We point out that temperature of 65 ℃-90 ℃ is the threshold for the development of CO2-plume geothermal（CPC） reservoir sandstone cracking under low effective stress（2-9 MPa） based on the relationship between temperature and soft part permeability.Furthermore,we discuss the effect of temperature on the two parts in the rock.The results indicate that as the temperature increases from 25 ℃ to 65 ℃.the flow channel in the hard part has a stronger response to temperature than that in the soft part at a fixed effective stress level,which is opposite to the situation of effective stress.Considering that natural rock is generally heterogeneous with non-uniform pore structure,we suggest a physical interpretation of the phenomenon that before the thermal cracking threshold the two parts have different responses to temperature.

A well test model for stress-sensitive and heterogeneous reservoirs with non-uniform thicknesses

In view of the anisotropy,heterogeneity and stress-sensitive permeability in low permeability reservoirs,an analytical well test model was established by introducing the concept of permeability modulus.This model considered the permeability stress-sensitivity,wellbore storage effect,and the skin effect.The perturbation technique and Laplace transformation were used to solve the mathematical model analytically in Laplace space,and the bottom-hole pressure type curves were plotted and analyzed in real space by using the Stehfest numerical inversion.

The Relationship between Fractures and Tectonic Stress Field in the Extra Low-Permeability Sandstone Reservoir at the South of Western Sichuan Depression

The formation and distribution of fractures are controlled by paleotectonic stress field, and their preservative status and effects on development are dominated by the modern stress field. Since Triassic, it has experienced four tectonic movements and developed four sets of tectonic fractures in the extra low-permeability sandstone reservoir at the south of western Sichuan depression. The strikes of fractures are in the S-N, NE-SW, E-W, and NW-SE directions respectively. At the end of Triassic, under the horizontal compression tectonic stress field, for which the maximum principal stress direction was NW.SE, the fractures were well developed near the S-N faults and at the end of NE-SW faults, because of their stress concentration. At the end of Cretaceous, in the horizontal compression stress fields of the NE-SW direction, the stress was obviously lower near the NE-SW faults, thus, fractures mainly developed near the S-N faults. At the end of Neogene-Early Pleistocene, under the horizontal compression tectonic stress fields of E-W direction, stress concentrated near the NE-SW faults and fractures developed at these places, especially at the end of the NE-SE faults, the cross positions of NE-SW, and S-N faults. Therefore, fractures developed mostly near S-N faults and NE-SW faults. At the cross positions of the above two sets of faults, the degree of development of the fractures was the highest. Under the modern stress field of the NW-SE direction, the NW-SE fractures were mainly the seepage ones with tensional state, the best connectivity, the widest aperture, the highest permeability, and the minimum opening pressure.

Application of Heterogeneous Composite Model with Consideration of Stress Sensitive in Low Permeability Gas Reservoir

Stress sensitivity is a key factor affecting the productivity of single wells in low permeability gas reservoirs. A well test model for heterogeneous composite gas reservoirs under the influence of stress-sensitive effects was established. Based on the theoretical model, the well test was designed by gradually increasing the pressure difference. The relationship between abnormal high pressure and reservoir stress sensitivity was analyzed. Theoretical research shows that stress sensitivity will cause permeability damage during the production process, and the pressure drop test curve shows that the physical properties of the reservoir have gradually deteriorated. The pressure recovery test curve shows that the physical properties of the reservoir are getting better. Field practice shows that stress sensitivity is related to the formation of abnormally high pressure in the formation without considering the micro-cracks in the formation. Stress-sensitive reservoirs are generally unbalanced and compacted due to overpressure, for fluid expansion/conduction overpressure in Ledong Area. For these reservoirs, there is almost no stress sensitivity. The research results have significance for guiding the design and data interpretation of stress-sensitive reservoir.

夹层型陆相页岩油储层压裂裂缝扩展实验研究

鄂尔多斯盆地延长组沉积了一套泥页岩及细粒砂质岩,具有丰富的页岩油资源,勘探开发评估资源量合计达十几亿吨。但页岩油储层可动性差、地层埋深浅,水平段层理、断缝、断层发育,裂缝扩展形态未知,体积压裂改造难度大。针对长7段井下储层获取的全直径致密砂泥岩岩心和页岩岩心,利用水泥包裹岩心方法开展真三轴室内压裂物模实验,获取水力裂缝形态,揭示弱应力场下页岩油储层水力裂缝扩展机理。实验发现,页岩油储层层状结构较密集,岩石颗粒之间胶结性较弱,压裂液容易沿着层理渗滤,在垂向应力与最小水平主应力之差小于2 MPa时,水力裂缝形态多为水平缝,压裂液主要沿层理或水平的天然裂缝渗滤;垂向应力与最小水平主应力之差增加到7 MPa时,会出现垂向穿层缝,形成局部台阶,最后被弱胶结层理面捕获后,沿层理渗滤延展。因此,压裂施工时,优选垂向应力与最小水平主应力差值较大的区域进行施工(井口位于山顶),有利于水力裂缝垂向延展,增加储层的体积压裂效果,提高页岩油产量和增大经济效益。

裂缝性储层天然缝与人工缝耦合机理及水窜预防井网调整

裂缝性致密砂岩储层开发调整方案的制定需系统开展天然缝、人工缝耦合关系评价。以南襄盆地东南部裂缝性致密油砂岩储层为例,开展了致密砂岩储层岩石力学、地应力、动静态天然裂缝及人工缝压裂扩展模拟等研究,并对水窜诱因及井网调整措施进行了讨论。研究结果表明,根据差应变及压裂测试结果,工区目的层现今地应力为张性应力状态,水平最大及最小主应力差(σ_(H)-σ_(h))值主要分布在4~11 MPa之间;此外,动、静态裂缝分析均显示,发育近WE向及近NE向2组天然缝。压裂模拟结果显示,水力裂缝明显受天然裂缝发育特征的控制。当储层中发育WE向及近NE向2组共轭天然缝时,随着天然缝密度的增加,人工缝长度近乎线性降低;当天然缝发育程度较低时可形成长水力缝,而当天然缝发育程度较高时,水力缝主要为短缝。当储层中仅发育WE向1组天然缝时,随着天然缝密度的增加,水力缝长度与改造面积先快速增加,后趋于稳定;且当天然缝发育程度较低时主要形成短缝,而当天然缝发育程度较高时,水力缝主要为长缝。天然缝的组系、方向及发育程度影响水力缝扩展规模及方向。另外,引起核桃园组三段水窜的原因包括天然缝带的连通性、构造因素、天然缝与人工缝的连通。早期压裂人工缝在后期压驱中会起到类似于天然缝的作用。综合考虑已有井网形式、原油分布、构造、天然缝及人工缝耦合关系,提出针对不同区域天然缝特征差异使用不同井网调整措施的技术方案。

Analysis on small-earthquake composite fault plane solutions from 1961 to 1999 in Xinfengjiang reservoir area

To understand the earthquake characteristics in Xinfengjiang (XFJ for short) reservoir area, we collected the small earthquakes occurred in the area from 1961 to 1999. We segmented this 40-year period, parted the research region and calculated the composite fault plane solution of each block, disscussed the effect characteristics of stress field of water pressure using Mohrs stress circle. The final result shows that the main rupture pattern was very different before and after the M = 6.1 main shock, changing from strike slip to normal rupture. The maximum principal stress axes of composite fault plane solutions are characterized by synchronous change with water level.

Stress-strain analysis of Aikou rockfill dam with asphalt-concrete core

Aikou rockfill dam with asphalt-concrete core is situated in a karst area in Chongqing City, China. In order to study the operative conditions of the rockfill dam, especially those of the asphalt-concrete core, the Duncan model is adopted to compute the stress and strain of both the rockfill dam and the asphalt-concrete core after karst grouting and other treatments. The results indicate that the complicated stress and deformation of both the dam body and the core are within reasonable ranges. It is shown that structure design and foundation treatment of the dam are feasible and can be used as a reference for other similar projects.

Numerical simulation of hydraulic fracture propagation in tight oil reservoirs by volumetric fracturing

Volumetric fracturing is a primary stimulation technology for economical and effective exploitation of tight oil reservoirs. The main mechanism is to connect natural fractures to generate a fracture network system which can enhance the stimulated reservoir volume. By using the combined finite and discrete element method, a model was built to describe hydraulic fracture propagation in tight oil reservoirs. Considering the effect of horizontal stress difference, number and spacing of perforation clus- ters, injection rate, and the density of natural fractures on fracture propagation, we used this model to simulate the fracture propagation in a tight formation of a certain oil- field. Simulation results show that when the horizontal stress difference is lower than 5 MPa, it is beneficial to form a complex fracture network system. If the horizontal stress difference is higher than 6 MPa, it is easy to form a planar fracture system; with high horizontal stress differ- ence, increasing the number of perforation clusters is beneficial to open and connect more natural fractures, and to improve the complexity of fracture network and the stimulated reservoir volume （SRV）. As the injection rate increases, the effect of volumetric fracturing may be improved; the density of natural fractures may only have a great influence on the effect of volume stimulation in a low horizontal stress difference.

Analysis on small-earthquake composite fault plane solutions from 1961 to 1999 in Xinfengjiang reservoir area

To understand the earthquake characteristics in Xinfengjiang (XFJ for short) reservoir area, we collected the small earthquakes occurred in the area from 1961 to 1999. We segmented this 40-year period, parted the research region and calculated the composite fault plane solution of each block, disscussed the effect characteristics of stress field of water pressure using Mohrs stress circle. The final result shows that the main rupture pattern was very different before and after the M = 6.1 main shock, changing from strike slip to normal rupture. The maximum principal stress axes of composite fault plane solutions are characterized by synchronous change with water level.

Coalbed Methane-bearing Characteristics and Reservoir Physical Properties of Principal Target Areas in North China

The coalbed methane (CBM) resources in North China amounts up to 60% of total resources in China. North China is the most important CBM accumulation area in China. The coal beds of the Upper Paleozoic Taiyuan and Shanxi formations have a stable distribution. The coal reservoir of target areas such as Jincheng, Yanquan-Shouyang, Hancheng, Liulin, etc. have good CBM-bearing characteristics, high permeability and appropriate reservoir pressure, and these areas are the preferred target areas of CBM developing in China. The coal reservoirs of Wupu, Sanjiaobei, Lu'an, Xinmi, Anyang-Hebi, Jiaozuo, Xinggong and Huainan also have as good CBM-bearing characteristics, but the physical properties of coal reservoirs vary observably. So, further work should be taken to search for districts with high pressure, high permeability and good CBM-bearing characteristics. Crustal stresses have severe influence on the permeability of coal reservoirs in North China. From west to east, the crustal stress gradient increases, while the coal reservoirs permeability decreases.

Discussion on 'Was the 2008 Wenchuan earthquake triggered by Zipingpu Reservoir?'

Some recent publications presented a result suggesting that Zipingpu reservoir hastened the occurrence of the 2008 Ms8.0 Wenchuan earthquake by tens to hundreds of years. Their researches calculated the Coulomb stress change induced by Zipingpu reservoir on the rupturing fault of Wenchuan earthquake. Their results, however, are critically dependent upon the 3-D event location, reservoir location, and the fault plane orientation. We repeated Ge et al.＇s work in this paper and found that an improper dip angle parameter of their 2-D fault model might lead to a wrong conclusion. Both the modeling results based on the 2-D model and 3-D model with proper fault parameters will show Coulomb stress changes alone were neither large enough nor had the correct orientation to affect the occurrence of Wenchuan earthquake, which supports our recent argument based on the local seismicity analysis and the induced Coulomb stress change calculation with a 3-D model.

Productivity Analysis Method of Abnormal High-Pressure Gas Reservoir in Ying-Qiong Basin

Ying-Qiong Basin in the west of South China Sea contains plenty of abnormal high-pressure gas reservoirs, whose stress sensitivity is crucial for well productivity. To explore the influence of stress sensitivity on production, the variable outlet back pressure stress sensitivity experiments were applied to test core sample permeability under different burden pressure and obtain the relational expression of power function of core stress sensitivity. Afterwards, new productivity equation is deduced in consideration of reservoir stress sensitivity, and the affection of stress sensitivity on production is analyzed. The result demonstrates close link between stress sensitivity and productivity, since single well production decreases dramatically when reservoir stress sensitivity has been taken into account. This research is constructive for well-testing data interpretation in stress sensitive gas reservoirs.

A Comprehensive Evaluation Method for Low-permeability Reservoirs

According to the geological characteristics and their influential factors of the low-permeability reservoirs, a comprehensive method for evaluation of low-permeability reservoirs is put forward. The method takes a matrix system as the basis, a fracture system as the focus and a stress field system as the restricted factor. It can objectively reflect not only the storage capability and seepage capability of low-permeability reservoirs, but also the effect on development as well. At the same time, it can predict the seepage characteristics at different development stages and provide a reasonable geological basis for the development of low-permeability reservoirs.

三轴应力下CO_(2)置换开采天然气水合物实验研究

为模拟海域真实环境,在三轴应力、氯化钠体系以及水合物储层富水状态的条件下,开展针对氯化钠体系、储层初始饱和度以及置换压应力对液态CO_(2)置换开采海域天然气水合物的实验。研究表明:在三轴应力以及孔隙度约46.70%的条件下,氯化钠体系对CH4置换效率影响较小,但对CO_(2)水合物合成表现出抑制作用;储层初始饱和度与CH4置换效率之间是负相关关系且高的储层含水率更有利于CO_(2)封存;CH4置换效率随着置换压应力的增长有一定幅度的提高,置换压应力的增大为CO_(2)水合物合成提供了高的驱动力,进而提高了CO_(2)封存效率。