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1.

Tight heterogeneous glutenite reservoir is typically not easy to form complex hydraulic fracture (HF) due to its poor physical properties, poor matrix seepage capacity, and small limit discharge radius and undeveloped natural fracture system. To improve the HF complexity and the stimulated reservoir volume (SRV), a novel stimulation technology called CO2 miscible fracturing has been introduced and its fracturing mechanism has been studied. The CO2 miscible fracturing modifies the in situ stress field by injecting low viscosity fluid to increase the HF complexity and SRV. Therefore, a series of numerical simulations based on a hydro-mechanical-damage model were carried out to study the effects of low viscosity fluid pre-injection on pore pressure, stress field, and fracturing effect in tight heterogeneous glutenite reservoirs. The results indicate that the low viscosity fluid injection can effectively increase the pore pressure around the wellbore and reduce the effective stress of the glutenite. The FCI and SRV increase with the increase of the pre-injection amount of the low viscosity fluid. The HF complexity and SRV can be improved by pre-injecting low viscosity fluid to transform the in situ stress field. The field application of this technology in a well of Shengli Oilfield showed that low-viscosity fluid pre-injection can effectively increase the width of the fractured zone, improve the SRV, and optimize the fracturing effect.

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2.
Li  Wen  Liu  Yuming  Yao  Xin  Chu  Jinwang  Chen  Xiaowei  Tan  Zhuoying 《Geotechnical and Geological Engineering》2021,39(2):909-917

Pre-conditioning by hydraulic fracturing is a vital work in block caving mining of hard-rock metal mine, and breakdown pressure is one of the important parameters in rock hydraulic fracturing process, which determines the equipment selection and job-parameter control. Based on numerical simulation method, a pre-holed cylindrical hard rock sample was built for simulation of hydraulic fracturing process to investigate the effect of stress condition, rock properties and water injection rate on breakdown pressure. The results show that the breakdown pressure is increased with the confining stress, and the breakdown pressure is 2.28 times confining stress based on rock tension strength. The variation of rock tension strength affects the breakdown pressure and a positive linear relation exists. Pre-formed notch could obviously reduce the breakdown pressure and control the orientation of hydraulic fracture. And the borehole diameter has a reverse influence on rock breakdown pressure.

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3.
Borehole Stability Analysis of Horizontal Drilling in Shale Gas Reservoirs   总被引：1，自引：1，他引：0  
Serious wellbore instability occurs frequently during horizontal drilling in shale gas reservoirs. The conventional forecast model of in situ stresses is not suitable for wellbore stability analysis in laminated shale gas formations because of the inhomogeneous mechanical properties of shale. In this study, a new prediction method is developed to calculate the in situ stresses in shale formations. The pore pressure near the borehole is heterogeneous along both the radial and tangential directions due to the inhomogeneity in the mechanical properties and permeability. Therefore, the stress state around the wellbore will vary with time after the formation is drained. Besides, based on the experimental results, a failure criterion is verified and applied to determine the strength of Silurian shale in the Sichuan Basin, including the long-term strength of gas shale. Based on this work, horizontal well borehole stability is analyzed by the new in situ stress prediction model. Finally, the results show that the collapse pressure will be underestimated if the conventional model is used in shale gas reservoirs improperly. The collapse pressure of a horizontal well is maximum at dip angle of 45°. The critical mud weight should be increased constantly to prevent borehole collapse if the borehole is exposed for some time.  相似文献   

4.
Rock is a heterogeneous geological material. When rock is subjected to internal hydraulic pressure and external mechanical loading, the fluid flow properties will be altered by closing, opening, or other interaction of pre-existing weaknesses or by induced new fractures. Meanwhile, the pore pressure can influence the fracture behavior on both a local and global scale. A finite element model that can consider the coupled effects of seepage, damage and stress field in heterogeneous rock is described. First, two series of numerical tests in relatively homogeneous and heterogeneous rocks were performed to investigate the influence of pore pressure magnitude and gradient on initiation and propagation of tensile fractures. Second, to examine the initiation of hydraulic fractures and their subsequent propagation, a series of numerical simulations of the behavior of two injection holes inside a saturated rock mass are carried out. The rock is subjected to different initial in situ stress ratios and to an internal injection (pore) pressure at the two injection holes. Numerically, simulated results indicate that tensile fracture is strongly influenced by both pore pressure magnitude and pore pressure gradient. In addition, the heterogeneity of rock, the initial in situ stress ratio (K), the distance between two injection holes, and the difference of the pore pressure in the two injection holes all play important roles in the initiation and propagation of hydraulic fractures. At relatively close spacing and when the two principal stresses are of similar magnitude, the proximity of adjacent injection holes can cause fracturing to occur in a direction perpendicular to the maximum principal stress.  相似文献   

5.
The Tseng-Wen Reservoir Transbasin water tunnel has broken the historical record of the maximum overburden of a tunnel within Taiwan. The three-dimensional in situ stresses at the tunnel were evaluated using acoustic emission (AE) and deformation rate analysis (DRA) with an under-coring technique. This is the first time that the AE and DRA have been used in evaluating an in situ stress in Taiwan. As cores drilled in different directions are required for the three-dimensional stress evaluation, but the in situ borehole drilling is usually limited to the vertical direction, small samples 2.28 cm in diameter and 4.56 cm in length were under-cored from the borehole cores in six directions to evaluate the in situ stresses. Cyclic uniaxial compressive loadings were artificially applied to the Changchikeng sandstones under-cored from the outcrops. The experimental results validate the definitiveness of using under-cored samples at different pre-stresses with AE and DRA. Then, cores taken from the Tseng-Wen Reservoir Transbasin water tunnel site at a great depth of 416–739 m underground were under-cored. The experimental results indicate that the major principle in situ stress is from 30.8 to 39 MPa and is in the directions of SE–NW and SW–NE at different depths.  相似文献   

6.
动扭剪试验中砂土液化后流动特性分析   总被引：2，自引：0，他引：2  
刘汉龙  陈育民 《岩土力学》2009,30(6):1537-1541
土体在初始液化后仍然可能承受动荷载作用发生大变形。引入流体力学中的剪应变率和表观黏度的概念,对振动扭剪试验中饱和砂土液化后的流动特性进行了分析。分析中将砂土的状态分为0有效应力状态和非0有效应力状态。结果表明,砂土在0有效应力状态下表现出与静扭剪试验类似的“剪切稀化”非牛顿流体的特征,表观黏度随着剪应变率的增大而减小。加载周数对“剪切稀化”状态下的剪应变率幅值有影响,随着加载周数的增加,剪应变率幅值逐渐增大,而对流动曲线的形状没有影响。在非0有效应力状态下,砂土的表观黏度随应变的增大而增大,随孔压比的减小而增大,且所有的试验得到的表观黏度与孔压比具有一致关系。  相似文献   

7.
In this paper, the results of laboratory studies of fracture initiation, early propagation and breakdown are reported. Three experiments were conducted on a low permeability sandstone block, loaded in a polyaxial test frame, to representative effective in situ stress conditions. The blocks were instrumented with acoustic emission (AE) and volumetric deformation sensors. In two experiments, fluids of different viscosity were injected into the wellbore, fluid injection was interrupted soon after the breakdown pressure had been reached. This allowed us to investigate hydraulic fracture initiation. In the third test, fracture initiation criteria were applied to stop hydraulic fracture propagation significantly earlier, prior to breakdown, and as it propagated a short distance from the wellbore. The analysis of AE results shows an increase in AE activity and a change in the AE spatial correlation, during the fracture initiation. This early stage of fracturing correlates strongly with the onset of rock volumetric deformation, and is confirmed by the analysis of ultrasonic transmission monitoring. The rock microstructure, after the test, was investigated by analysis of scanning electron microscope images. These indicated the development of leak-off zone near the wellbore and a dry hydraulic fracture at the farther distance from the wellbore.  相似文献   

8.
In this research, the hydraulic conductivity changes in uniformly graded sands, due to injection pressure increase, were experimentally evaluated using a cell-type radial model. Conducted tests, simulating variation of media permeability at different depths along a recharge well, were monitoring variations of the samples’ hydraulic conductivity at predetermined three different overburden pressures. The startup low pressure inflow was afterward altered by increasing the injection pressure up to the point at which hydraulic conductivity started to change at each run; we called it the threshold injection pressure. The corresponding hydraulic conductivity at such pressure was measured. As the increased permeability was a function of distance to the simulated recharge, it was deemed too beneficial to develop an equation to enable predicting this new hydraulic conductivity at different distances. Findings indicate that in uniformly graded sands under overburden pressure up to 68.64 kPa, the hydraulic conductivity in the threshold injection pressure—compared to its primary amount up to 45 cm from borehole wall—show a remarkable growth. However, this growth rate for greater distances up to 60 cm is negligible. Furthermore, in the threshold injection pressure, the hydraulic conductivity seems not to be time dependent. But, in constant injection pressures above the threshold injection pressure, the hydraulic conductivity shows some sort of time dependency.  相似文献   

9.
Windblown methane is an important gas resulting in atmospheric greenhouse effect. Therefore, reduction in windblown methane is one of the important measures to mitigate atmospheric greenhouse effect. In China, weak coal seam of low permeability is common in coal mines, so it is beset with difficulties to decrease the methane emission rate by means of gas drainage from the virgin coal seam, further to decrease the windblown mine gas. Utilizing the pressure relief and permeability and fluidity improvement effect in coal mining an approach to release methane emission through surface borehole was established, for example establishing a comprehensive deformation fracture model of surface borehole in extraction area based on quantitative rule of overlying rock movement in pit and forming a technology to select the surface borehole arrangement site in extraction area on the basis of deformation of bore frame structure and distribution characteristics of extraction flow field. And optimization technology of shape and structure of surface borehole in extraction area on the basis of ultimate stress analysis of surface borehole casing was given. The technology overcomes effectively the problem that surface borehole casing is vulnerable to premature fracture due to impact of strata movement on the surface borehole, and further increases the drainage result of the surface borehole. The technology has been test in China Shanxi Jincheng Sihe Coal Mine, achieving good results, including 12,000 Nm3/d pure methane drainage rate from single borehole, 85% methane concentration and 1.1 million Nm3 accumulative methane drainage, which demonstrate practicability and advanced performance of the technology.  相似文献   

10.
The determination of in situ stresses is very important in petroleum engineering. Hydraulic fracturing is a widely accepted technique for the determination of in situ stresses nowadays. Unfortunately, the hydraulic fracturing test is time-consuming and expensive. Taking advantage of the shape of borehole breakouts measured from widely available caliper and image logs to determine in situ stress in petroleum engineering is highly attractive. By finite element modeling of borehole breakouts considering thermoporoelasticity, the authors simulate the process of borehole breakouts in terms of initiation, development, and stabilization under Mogi-Coulomb criterion and end up with the shape of borehole breakouts. Artificial neural network provides such a tool to establish the relationship between in situ stress and shape of borehole breakouts, which can be used to determine in situ stress based on different shape of borehole breakouts by inverse analysis. In this paper, two steps are taken to determine in situ stress by inverse analysis. First, sets of finite element modeling provide sets of data on in situ stress and borehole breakout measures considering the influence of drilling fluid temperature and pore pressure, which will be used to train an artificial neural network that can eventually represent the relationship between the in situ stress and borehole breakout measures. Second, for a given measure of borehole breakouts in a certain drilling fluid temperature, the trained artificial neural network will be used to predict the corresponding in situ stress. Results of numerical experiments show that the inverse analysis based on finite element modeling of borehole breakouts and artificial neural network is a promising method to determine in situ stress.  相似文献   

11.
Summary Numerical simulations of circular holes under internal hydraulic pressure are carried out to investigate the hydraulic fracture initiation, propagation and breakdown behavior in rocks. The hydraulic pressure increases at a constant rate. The heterogeneity of the rocks is taken into account in the study by varying the homogeneity index. In addition, the permeability is varied with the states of stress and fracture. The simulations are conducted by using a finite element code, F-RFPA2D, which couples the flow, stress and damage analyses. The simulation results suggest that the fracture initiation and propagation, the roughness of the fracture path and the breakdown pressure are influenced considerably by the heterogeneity of rocks. The hole diameter elongation and the stress field evolution around the fracture tip during the fracture propagation can also provide useful information for the interpretation of the hydraulic fracturing behaviour.  相似文献   

12.
Water injection experiments were performed in 1997, 2000 and 2003 at the 1800 m borehole near the fracture zone of the 1995 Hyogo-ken Nanbu earthquake. During these experiments, a contraction of about 10− 8–10− 7 was observed with three-component strainmeters at a bottom of the 800 m borehole, 70 m southwest of the 1800 m borehole. We estimated hydraulic properties of the fracture zone near the Nojima fault by using the strain data to investigate a healing of the fault during the postseismic stage. We calculated pore pressure changes due to the water injection using Darcy's equation and obtained strain changes due to the pore pressure changes as elastic deformations of the crust. The calculated strain changes have a nearly agreement with the observed strain changes. Hydraulic conductivity in 1997, 2000 and 2003 was determined to be 0.9 ± 0.2 × 10− 6, 0.8 ± 0.2 × 10− 6 and 0.4 ± 0.1 × 10− 6 m/s, respectively. The reduced hydraulic conductivities in 2000 and 2003 suggest that the fractures had been healing.  相似文献   

13.
We report laboratory experiments to investigate the role of gas desorption, stress level and loading rate on the mechanical behavior of methane infiltrated coal. Two suites of experiments are carried out. The first suite of experiments is conducted on coal (Lower Kittanning seam, West Virginia) at a confining stress of 2 MPa and methane pore pressures in the fracture of 1 MPa to examine the role of gas desorption. These include three undrained (hydraulically closed) experiments with different pore pressure distributions in the coal, namely, overpressured, normally pressured and underpressured, and one specimen under drained condition. Based on the experimental results, we find quantitative evidence that gas desorption weakens coal through two mechanisms: (1) reducing effective stress controlled by the ratio of gas desorption rate over the drainage rate, and (2) crushing coal due to the internal gas energy release controlled by gas composition, pressure and content. The second suite of experiments is conducted on coal (Upper B seam, Colorado) at confining stresses of 2 and 4 MPa, with pore pressures of 1 and 3 MPa, under underpressured and drained condition with three different loading rates to study the role of stress level and loading rate. We find that the Biot coefficient of coal specimens is <1. Reducing effective confining stress decreases the elastic modulus and strength of coal. This study has important implications for the stability of underground coal seams.  相似文献   

14.
Damage and Permeability Development in Coal During Unloading   总被引：1，自引：1，他引：0  
One of the key issues in protective seam mining is the pressure relief and permeability improvement effect. In this paper, the results of X-ray CT scanning experiments and permeability experiments using reconstituted coal specimens subjected to the same stress path and the same effective confining pressure (confining pressure minus pore pressure) are combined using the stress–strain relationship to study the damage to reconstituted coal specimens and its influence on permeability during the unloading process. When the effective confining pressure (σ 3 ? p) is unloaded from 8 to 6 MPa and the deviatoric stress increases, the damage variables will increase by 0.0351 and 0.084, respectively, compared with the unloading point under the fixed axial displacement with unloading confining pressure (FADUCP) and fixed deviatoric stress with unloading confining pressure (FDSUCP) stress paths. At the same time, the permeability increased by 1.7 and 16.7 %, respectively. Therefore, the damage variable and permeability increased notably little in this process. After the effective confining pressure is unloaded to approximately 5 MPa, together with the decrease in the deviatoric stress, the growth of the damage variable and permeability begins to accelerate. In addition, the relative decrease in the deviatoric stress with appearing damage cracks, and the relative increase in permeability with the same amount of effective confining pressure being unloaded, shows that the damage to specimens under the FDSUCP stress path is larger than that from the FADUCP stress path.  相似文献   

15.
水力压裂扩展特性的数值模拟研究   总被引：7，自引：0，他引：7  
采用ABAQUS建立了水力压裂计算模型,模拟了地应力、岩石力学特性、压裂液流体特性等各种复杂因素对水力压裂扩展的影响。通过计算分析得到一些有益结论：（1）在注入压力一定的情况下起裂压力与最小水平地应力、临界应力、初始孔隙压力成正比,而与压裂液黏度、最大水平地应力、弹性模量无关;（2）裂缝扩展长度和最大缝宽与最小水平地应力、初始孔隙压力、弹性模量成反比,而与最大水平地应力无关;（3）水力压裂作业中,缝长的扩展过程可分为无扩展阶段、快速扩展阶段、稳定扩展阶段以及缓慢扩展阶段等4个阶段。研究结论对于水力压裂作业优化具有参考价值。  相似文献   

16.
We report laboratory experiments that investigate the permeability evolution of an anthracite coal as a function of applied stress and pore pressure at room temperature as an analog to other coal types. Experiments are conducted on 2.5 cm diameter, 2.5-5 cm long cylindrical samples at confining stresses of 6 to 12 MPa. Permeability and sorption characteristics are measured by pulse transient methods, together with axial and volumetric strains for both inert (helium (He)) and strongly adsorbing (methane (CH4) and carbon dioxide (CO2)) gases. To explore the interaction of swelling and fracture geometry we measure the evolution of mechanical and transport characteristics for three separate geometries — sample A containing multiple small embedded fractures, sample B containing a single longitudinal through-going fracture and sample C containing a single radial through-going fracture. Experiments are conducted at constant total stress and with varied pore pressure — increases in pore pressure represent concomitant (but not necessarily equivalent) decreases in effective stress. For the samples with embedded fractures (A and C) the permeability first decreases with an increase in pressure (due to swelling and fracture constraint) and then increases near-linearly (due to the over-riding influence of effective stresses). Conversely, this turnaround in permeability from decreasing to increasing with increasing pore pressure is absent in the discretely fractured sample (B) — the influence of the constraint of the connecting fracture bridges in limiting fracture deformation is importantly absent as supported by theoretical considerations. Under water saturated conditions, the initial permeabilities to all gases are nearly two orders of magnitude lower than for dry coal and permeabilities increase with increasing pore pressure for all samples and at all gas pressures. We also find that the sorption capacities and swelling strains are significantly reduced for water saturated samples — maybe identifying the lack of swelling as the primary reason for the lack of permeability decrease. Finally, we report the weakening effects of gas sorption on the strength of coal samples by loading the cores to failure. Results surprisingly show that the strength of the intact coal (sample A) is smaller than that of the axially fractured coal (sample B) due to the extended duration of exposure to CH4 and CO2. Average post-failure particle size for the weakest intact sample (A) is found to be three times larger than that of the sample B, based on the sieve analyses from the samples after failure. We observe that fracture network geometry and saturation state exert important influences on the permeability evolution and strength of coal under in situ conditions.  相似文献   

17.
The coupled heat-fluid-stress problem of circular wellbore or spherical cavity subjected to a constant temperature change and a constant fluid flow rate is considered. Transient analytical solutions for temperature, pore pressure and stress are developed by coupling conductive heat transfer with Darcy fluid flow in a poroelastic medium. They are applicable to low permeability porous media suitable for liquid-waste disposal and also simulating reservoir for enhanced oil recovery, where conduction dominates the heat transfer process. A full range of solutions is presented showing separately the effects of temperature and fluid flow on pore pressure and stress development. It is shown that injection of warm fluid can be used to restrict fracture development around wellbores and cavities and generally to optimize a fluid injection operation. Both the limitations of the solutions and the convective flow effect are addressed. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

18.
Geological storage of CO2 is considered a solution for reducing the excess CO2 released into the atmosphere. Low permeability caprocks physically trap CO2 injected into underlying porous reservoirs. Injection leads to increasing pore pressure and reduced effective stress, increasing the likelihood of exceeding the capillary entry pressure of the caprocks and of caprock fracturing. Assessing on how the different phases of CO2 flow through caprock matrix and fractures is important for assessing CO2 storage security. Fractures are considered to represent preferential flow paths in the caprock for the escape of CO2. Here we present a new experimental rig which allows 38 mm diameter fractured caprock samples recovered from depths of up to 4 km to be exposed to supercritical CO2 (scCO2) under in situ conditions of pressure, temperature and geochemistry. In contrast to expectations, the results indicate that scCO2 will not flow through tight natural caprock fractures, even with a differential pressure across the fractured sample in excess of 51 MPa. However, below the critical point where CO2 enters its gas phase, the CO2 flows readily through the caprock fractures. This indicates the possibility of a critical threshold of fracture aperture size which controls CO2 flow along the fracture.  相似文献   

19.
Fluid‐driven fractures of brittle rock is simulated via a dual‐graph lattice model. The new discrete hydromechanical model incorporates a two‐way coupling mechanism between the discrete element model and the flow network. By adopting an operator‐split algorithm, the coupling model is able to replicate the transient poroelasticity coupling mechanism and the resultant Mandel‐Cryer hydromechanical coupling effect in a discrete mechanics framework. As crack propagation, coalescence and branching are all path‐dependent and irreversible processes, capturing this transient coupling effect is important for capturing the essence of the fluid‐driven fracture in simulations. Injection simulations indicate that the onset and propagation of fractures is highly sensitive to the ratio between the injection rate and the effective permeability. Furthermore, we show that in a permeable rock, the borehole breakdown pressure, the pressure at which fractures start to grow from the borehole, depends on both the given ratio between injection rate and permeability and the Biot coefficient.  相似文献   

20.
Underground borehole drilling usually causes instability in the surrounding coal due to in situ stress redistribution (including stress concentration and stress release). However, the mechanisms of unloading-induced coal strength reduction are still poorly understood. The primary objective of this study is to investigate the effect of confining pressure unloading on soft coal strength reduction for borehole stability analysis. A series of mechanical tests were conducted on both the traditionally and newly reconstituted coal samples under two different experimental stress paths, including conventional uniaxial/triaxial compression and triaxial compression with confining pressure unloading. The unloading stress path was obtained by analyzing the stress redistribution around a borehole, to capture a more accurate coal mechanical response. According to our experimental results, plastic deformation generated before failure under the unloading stress path is smaller than that generated under the conventional loading stress path. Furthermore, the cohesion of the traditionally and newly reconstituted samples diminishes approximately by 44.77 and 29.66%, respectively, with confining pressure unloading, indicating that there is a significant reduction in coal strength due to confining pressure unloading. The mechanism for unloading-induced coal strength reduction comes from confining pressure unloading-induced increase in shear stress on the fracture surface and a decrease in shear strength. This effect increases the shear slipping potential, whose driving force generates tension fractures at both ends of the preexisting fractures.  相似文献   

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