重复压裂压新缝力学机理研究

【作者】 邓燕；

【导师】 赵金洲；

【作者基本信息】 西南石油学院 ， 油气田开发工程， 2005， 博士

【摘要】 重复压裂是老油气田或老油气区进一步增产挖潜和进一步提高采收率的关键技术,特别是压新缝重复压裂更是中高含水油田综合治理和增油控水的有效手段。重复压裂裂缝能否重定向、重复压裂能否造新缝一直是学术界和工程界争论的焦点。本文从井筒附近应力场变化规律研究入手,对重复压裂造新缝的力学机理进行了较为系统的研究,取得了如下的主要成果和认识: (1) 系统研究了垂直裂缝井重复压裂前应力场的分布特征及其主要影响因素。研究表明,油气井中初次人工裂缝产生了诱导应力:油气井生产和注水等作业导致地层孔隙压力变化而诱导地应力重新分布;注入等作业导致油藏温度变化而诱导产生热弹性应力场等,这些诱导应力改变了重复压裂井井底附近的应力场分布。 (2) 应用弹性力学理论,建立了油气井中初次人工裂缝的诱导应力模型;并采用半逆解法得到初次人工裂缝诱导的应力场的解析解。 (3) 将渗流力学和岩土力学相结合,建立了含水力压裂裂缝的储层渗流和固相平衡的数学模型;然后对渗流数学模型采用有限差分法求解,而岩土变形数学模型则采用有限元法求解,并采用显式交替求解方式将二者结合起来的储层流固耦合油藏数值模拟求解技术求解;得到流固耦合作用时由于油气井的生产活动导致的孔隙压力变化而引起的应力场。实现了垂直裂缝井重复压裂前生产过程导致孔隙压力变化而诱导的应力场的定量分析和模拟。 (4) 利用扩展的虎克定律和平面弹性理论以及能量守恒原理,建立了推导无裂缝注水井附近由于注水而产生的热弹性应力的数学模型;通过转化和叠加已有经验公式,建立了有裂缝注水井附近温度场的变化引起的应力场的计算模型。实现了垂直裂缝井重复压裂前由于长期注水温度场发生变化导致应力场的定量分析和模拟。 (5) 数值模拟结果表明,初次人工裂缝和孔隙压力的变化等改变了油气藏中的应力场,最大水平主应力变化较快,最小水平应力变化较慢,二者都是时间和空间的函数。因此,当重复压裂井中的诱导应力差足以改变地层中的初始应力差时,井筒附近区域有一个应力重定向区域,那么重复压裂就可以产生新缝。 (6) 在应力场分布的研究成果的基础上,运用岩石力学、断裂力学和水力压裂力学的基本原理,采用位移不连续法,模拟了在储层内各点应力不同的复杂应力条件下,重复压裂新裂缝的起裂方位、裂缝扩展和延伸轨迹。 新裂缝的造缝机理和延伸规律是重复压裂研究中的难点和应用价值的核心问题,也是制约重复压裂优化设计的关键。本文的研究无疑可以为重复压裂优化设计提供一定的理论基础,用以在指导大量的重复压裂施工,提高其工艺可行性和经济可行性,对于老油气田综合治理、稳产增产具有非常重要的意义。更多还原

【Abstract】 Refracturing is a key way for enhancing productivity and recovery for the maturing oil/gas field. It is also an effective means to integrated management and steady oil and control-water, especially for high water-cut oil field. For a long time, it is always a argument focus in the academic and project that whether the reorientation and new fracture can be achieved or not. Base on the stress changes nearby the wellbore ,in this paper, the mechanical mechanism of fracture creation for refracturing is systematically studied, the main achievements and understands is as follows:(1) Systematically study the characteristic of pre-refracturing stress field of well with vertical fracture and its main influencing factors. As the study shows, the induced stress resulted from primarily fracture, oil/gas production and some treatments such as water-injection will change pore pressure and further lead to the stress redistributing; injecting water into the formation for a long time depresses the reservoir temperature and also change the induced stress field. All of these factors change the distribution of in-situ-stress field of reservoir.(2) With the theory of elasticity, a model of induced stress field is established for primary fracture and the analytic solution is achieved by semi- retro-solution method.(3) Integrating percolating mechanics with rock mechanics and hypothesizing the reservoir is elastic-plastic, a mathematical model for percolation-solid phase balance is abstractly presented; as solving, adopting method of finite difference for percolating model, finite element method (FEM) for rock-change model while explicit alternation for fluid-solid coupling model and a stress field resulted from the change of pore pressure by oil/gas production is obtained. Quantitative analysis and simulation are also did for this induced stress field(4) With extended Hook’s Law and plane elastic theory as well as the principle of energy conversation, a mathematical model for calculating thermoelastical stress is put forward. This stress is resulted from water injection nearby water injection well; based on the published empirical equations and by transforming and superpositing the thermal stress which caused by a cylinder in the center of infinite elastic formation, a calculation model of stress field for water injection well with fracture is developed. This stress field is resulted from the change of temperature. Quantitative analysis and simulation are also did for this induced stress field(5) As the simulation results shows, as the existence of the induced stress resulted from primarily fracturing and the variation of pore pressure etc.. the stress field in reservoir hasbeen changed with faster change for maximum horizontal main stress and slower change for minimum horizontal main stress, both of them are the functions of time and space. Therefore, when the induced stress differential is abundant enough to change the initial stress differential, there will be a stress-redirected zone near the wellbore and new fracture can be formed by refracturing(6) Based on the above researches on distribution of stress field and with rock mechanics, fracture mechanics and hydrofracturing mechanics, using the method of displacement discontinuity, initiating azimuth of fracture, fracture propagation and its extended track for refracturing are simulated, under the condition of complex stress that reservoir is of different stress values anywhereFor refractuing, the mechanism of new fracture’s formation and its propagation rule are not only the keys and noduses, but also the crux impacting the refracturing optimal design. It is no doubt that the research in this paper indeed offers certain theory base to refracturing optimal design and a guidance to field treatment as well as enhance the technical and economical feasibility. It is of great significance for integrated management and for steady production and stimulation of maturing oil/gas reservoir.更多还原