考虑酸损伤效应页岩压裂裂缝扩展机理研究

发布时间：2018-05-16 09:19

本文选题：页岩酸损伤 + 岩石力学　；参考：《西南石油大学》2017年博士论文

【摘要】：页岩气资源开发技术发展对解决我国常规天然气产量不足和缓解能源对外依存度高的现状具有重大意义。与北美主要页岩气储层相比,四川盆地龙马溪组页岩储层压裂工程地质条件相对较差。高地应力差(15～20 MPa)、低脆性指数(50)及天然裂缝相对不发育的特点使得常规压裂施工后难以保证形成复杂裂缝网络,导致该地区页岩气增产改造难度大、单井开发成本高。可以通过酸处理改变页岩力学特征提高压后形成复杂缝网的可能。然而酸损伤页岩内部微孔隙和微裂缝结构复杂,水力压裂过程中储层岩体变形破坏特征对复杂裂缝扩展影响机理不明确。本文以川南地区龙马溪组储层页岩为对象,基于室内试验手段研究酸处理后页岩矿物组分、微观结构和力学性质变化。综合运用塑性力学和损伤力学理论,建立龙马溪组页岩酸损伤弹塑性本构模型。通过引入酸损伤弹塑性本构方程建立龙马溪组页岩压裂裂缝扩展数值模型,研究考虑酸损伤效应的页岩裂缝扩展规律及影响因素。主要研究工作及取得的认识如下:(1)盐酸溶蚀页岩中的碳酸盐矿物后改变岩石颗粒胶结强度和微观孔隙结构,在宏观上表现为酸损伤页岩孔渗特征和力学特征改变。由于裂缝扩展受储层岩体应力-渗流耦合过程的影响,因此酸损伤页岩孔渗特征和力学特征的改变将引起页岩复杂裂缝扩展机理的改变。有必要开展酸损伤效应对页岩压裂裂缝扩展的影响规律研究,明确回答页岩酸处理是否能为形成复杂裂缝网络服务。(2)针对研究区页岩开展加压饱和酸处理实验,分析酸处理对页岩矿物组分、微观结构及孔渗特征的影响。龙马溪组页岩碳酸盐含量随着酸处理时间增加而减少。酸处理过程中,酸对岩石胶结物的溶蚀作用破坏了岩石固有结构,削弱岩石骨架颗粒之间的胶结作用。测试结果表明页岩内部碳酸盐矿物被酸溶蚀后形成许多次生孔隙和微裂隙,导致龙马溪组页岩孔隙度和渗透率增加。(3)针对酸处理前后页岩岩样开展了三轴压缩试验测试,分析酸损伤效应对页岩力学特征的影响。结果表明:随酸反应时间的增加页岩峰值抗压强度和杨氏模量明显降低,力学性质参数与孔渗参数之间的关系满足线性回归拟合。分析酸处理前后页岩的变形特征和破坏特征,认为龙马溪组页岩酸损伤后具有塑性变形特征,且破裂形态复杂,是压裂后地层形成复杂裂缝网络的有利条件。(4)根据酸损伤页岩破坏的物理过程分析变形破坏特征。运用塑性力学理论,建立基于Drucker-Prager屈服准则的页岩弹塑性本构模型。利用损伤力学理论,基于酸损伤后页岩孔隙度增加的现象提出考虑孔隙度变化的损伤因子和损伤演化方程。将损伤变量引入弹塑性本构模型中建立酸损伤弹塑性本构模型,编写Fortran程序对模型求解。对不同酸反应时间下的酸损伤页岩三轴压缩试验进行了数值验证,结果表明酸损伤弹塑性本构模型能够比较准确地模拟经过酸处理后页岩的受载荷变形过程和峰后应变软化特征。(5)本文提出利用粘性区单元建立二维裂缝相交扩展模型,模拟不同天然裂缝展布和不同地应力状态下的裂缝扩展结果。通过真三轴水力压裂物模实验结果与数值模拟结果对比验证了该方法的正确性。引入酸损伤弹塑性本构方程研究页岩酸损伤效应下的裂缝扩展规律。通过与弹性岩体模型结果对比分析,认为考虑酸损伤的裂缝扩展数值模型计算诱导应力场更复杂、地层水平应力比更低,是形成复杂缝网的有利条件。(6)根据川南龙马溪组某口井工程地质特征建立考虑酸损伤效应的页岩三维多裂缝相交扩展数值模型,模拟不同施工参数下缝网扩展的形态。基于损伤力学理对酸处理后的天然裂缝和层理面损伤进行模拟,基于酸损伤弹塑性本构模型模拟储层岩体变形。分析施工参数对多裂缝扩展复杂程度的影响,对比分析该井现场微地震监测结果,认为考虑酸损伤效应的多裂缝扩展模型能反映龙马溪组页岩酸处理压裂扩展规律。本文通过试验测试和理论模型相结合,建立酸损伤页岩本构模型并引入龙马溪组页岩储层水力压裂裂缝扩展数值模型,研究考虑酸损伤效应下的多裂缝扩展机理。相比常规压裂,在高应力差条件下酸损伤页岩储层压裂后裂缝更容易扩展,裂缝复杂程度更高。本文通过建立描述龙马溪组酸损伤页岩力学性质特征的本构关系,分析考虑酸损伤效应下龙马溪组页岩储层裂缝扩展机理,为该类通过常规压裂不易形成复杂缝网的页岩储层提供增产改造思路和理论支撑。
[Abstract]:The development of shale gas resources development technology is of great significance to solving the current situation of the shortage of conventional natural gas production and the high dependence of energy on energy. Compared with the main shale gas reservoirs in North America, the fracturing engineering geological conditions of the Sichuan basin Long Maxi formation shale reservoir are relatively poor. The high geostress difference (15~20 MPa), low brittleness index (50) and The characteristics of natural fractures are relatively undeveloped, which makes it difficult to guarantee the formation of complex fracture network after conventional fracturing, which leads to the difficulty of shale gas production and transformation in this area, and high cost of single well development. The energy of shale mechanics can be changed by acid treatment to improve the ability to form complex seams after pressure. However, acid damage shale internal pore gap and microcrack are damaged by acid. In the process of hydraulic fracturing, the mechanism of the deformation and failure of the reservoir rock in the process of hydraulic fracturing is not clear. In this paper, the shale of the long Ma Creek Reservoir in South Sichuan Province is taken as the object. Based on the laboratory test, the mineral composition, microstructure and mechanical properties of the shale after acid treatment are studied. Plastic mechanics and damage mechanics are used synthetically. The theory is to establish the elastoplastic constitutive model of shale acid damage in the Long Maxi formation. Through the introduction of the elastoplastic constitutive equation of acid damage, the numerical model of the fracture expansion of the shale fracture in the long Maixi formation is established, and the law and the influence factors of shale fracture considering the effect of acid damage are studied. The main research work and the knowledge obtained are as follows: (1) in the hydrochloric acid dissolved shale After carbonate minerals, the cementation strength and micropore structure of the rocks are changed, and the pore and permeability characteristics and mechanical characteristics of the shale are changed in macroscopic view. Because the fracture expansion is influenced by the stress seepage coupling process of the reservoir rock, the change of pore and permeability characteristics and mechanical characteristics of acid damaged shale will cause the expansion of the shale complex crack. It is necessary to carry out the study of the effect of acid damage effect on the fracture expansion of shale fracturing, and clearly answer whether shale acid treatment can serve as a network service for the formation of complex fractures. (2) the effects of acid treatment on shale mineral composition, microstructure and pore and permeability characteristics are analyzed by pressure saturated acid treatment experiments on shale in the study area. The shale carbonate content in the long Ma Creek formation decreases with the increase of acid treatment time. In the process of acid treatment, the corrosion of acid to rock cementation destroys the natural structure of rock and weakens the cementation between the rock skeleton particles. The test results show that the carbonate minerals in the shale are corroded by acid and form many secondary pores and micro cracks. The porosity and permeability of shale in the long Ma Creek formation increase. (3) three axial compression tests were carried out for shale samples before and after acid treatment, and the effect of acid damage effect on shale mechanical characteristics was analyzed. The results showed that the peak compressive strength and Yang Shimo quantity of shale decreased obviously with the increase of acid reaction time, and the mechanical properties and pore permeability parameters were observed. The relationship between the shales is satisfied with the linear regression fitting. The deformation characteristics and damage characteristics of shale before and after the acid treatment are analyzed. It is considered that the shale acid after the damage of the shale is characterized by plastic deformation and complex fracture morphology, which is a favorable condition for the formation of complex fracture network after fracturing. (4) analysis of deformation and damage according to the physical process of acid damaged shale damage. Characteristics. Using the theory of plastic mechanics, the elastoplastic constitutive model of Shale Based on Drucker-Prager yield criterion is established. Based on the damage mechanics theory, the damage factor and the damage evolution equation with the porosity change are proposed based on the increase of shale porosity after the acid damage. The damage variable is introduced into the elastoplastic constitutive model to establish the acid damage. The elastoplastic constitutive model and the Fortran program are compiled to solve the model. The three axial compression tests of acid damaged shale under different acid reaction time have been verified. The results show that the acid damage elastoplastic constitutive model can accurately simulate the deformation process of the shale after acid treatment and the strain softening characteristics after the peak. (5) this paper is proposed in this paper. A two-dimensional fracture propagation model is established by using the viscous region element to simulate the fracture propagation of different natural fractures and different stress states. The correctness of the method is verified by the results of real three axis hydraulic fracturing experiment and numerical simulation. The acid damage elastoplastic constitutive equation is introduced to study shale acid loss. The crack propagation law under the injury effect is compared with the results of the elastic rock mass model. It is considered that the calculation of the induced stress field is more complex and the horizontal stress ratio of the stratum is lower. (6) considering the acid damage according to the engineering geological characteristics of a certain well in the Long Ma Creek formation in South Sichuan, the calculation of the induced stress field is more complex and the horizontal stress ratio of the stratum is lower. A three-dimensional multi fracture propagation numerical model of shale is used to simulate the expansion of seams under different construction parameters. The damage of natural fractures and bedding surfaces after acid treatment is simulated on the basis of damage mechanics, and the deformation of the reservoir rock is simulated based on the acid damage elastoplastic constitutive model. The complexity of the construction parameters for multiple fracture expansion is analyzed. The results of the field microseismic monitoring in this well are compared and analyzed. It is believed that the multi fracture expansion model considering the acid damage effect can reflect the fracture expansion law of the shale acid treatment in the Long Maxi formation. In this paper, an acid damaged shale constitutive model is established by combining the experimental test and the theoretical model, and the fracture expansion of the shale reservoir of the Long Maxi formation is introduced. The numerical model is used to study the mechanism of multiple crack propagation under the effect of acid damage. Compared with conventional fracturing, the fracture is more easy to expand and the fracture complexity is higher after the acid damaged shale reservoir under the high stress difference. This paper establishes the constitutive relation of the mechanical properties of the acid damaged shale of the Long Maxi group, and analyzes the effect of acid damage. The fracture expansion mechanism of the shale reservoir of the maxi formation in Xialong is provided for the shale reservoir which is not easily formed by the conventional fracturing.

【学位授予单位】：西南石油大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TE377

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