水压作用下套管井的应力分析与流体窜槽机制研究

发布时间：2018-09-09 17:15

【摘要】：水力压裂的目的在于增加地层的裂缝,但压裂也可能导致压裂液沿套管井的水泥环及其周围空间流动,俗称窜槽。窜槽会使储层之间相互污染,导致试井时油层出水等现象,并且影响压裂的顺利进行。本文研究水力压裂引起窜槽的条件,并考虑避免窜槽的对策。论文分别按强度理论和断裂力学方法研究了压裂过程中引起套管井失去水力封隔效能的两类通道的形成机制。这两类通道是:套管井水泥胶结良好,但水泥环因强度不足而发生脆性破坏,形成窜槽通道;水泥与套管之间(第一界面)或者水泥与地层之间(第二界面)在压裂之前就存在微小流体间隙,压裂过程中微间隙扩展而形成窜槽通道。针对界面胶结完好的套管井段,利用三维有限元模拟了水泥环脆性破坏导致的窜槽行为。分析了水压作用下套管井射孔附近区域的应力状态,计算了使地层或水泥环破坏的临界注水压力,并研究了不同介质参数、射孔条件和地应力条件下水泥环的抗窜能力。研究表明,用低杨氏模量、高泊松比的水泥进行固井,可以有效防止水泥环窜槽。在低杨氏模量、高泊松比的地层进行压裂会导致水泥环窜槽的发生。射孔条件以及地应力的偏应力对水泥环窜槽机制影响不大。而地应力的静水压会影响水泥环的水力封隔能力。本文将界面流体间隙视为含水界面裂纹,研究了套管井界面裂纹在水压作用下的扩展。压裂过程中界面裂纹的扩展会导致窜槽发生。本文分别针对第一界面和第二界面存在裂纹的套管井,利用内聚力单元模拟界面的力学行为,分析了水泥属性、地层属性和地应力对界面窜槽的影响。结果表明,高杨氏模量、高泊松比的水泥会提高固井界面的抗窜能力。而在地层杨氏模量低、泊松比低的井段进行压裂,界面不易窜槽。水平地应力的提高可提高界面的抗窜能力,但垂直地应力的变化对界面抗窜能力影响很小。本文考虑综合两种窜槽机制,给出避免压裂过程中发生窜槽的方案。结果表明,固井时应采用高泊松比的水泥,而水泥杨氏模量应在一个合理的范围内;选择进行压裂的地层,其泊松比应尽量低,而杨氏模量不能过高或过低;应尽量选择水平地应力低而垂直地应力高的位置进行压裂。
[Abstract]:The purpose of hydraulic fracturing is to increase the formation fracture, but fracturing may also lead to the flow of fracturing fluid along the cement ring of casing well and its surrounding space, commonly known as channeling channel. Channeling will pollute each other between reservoirs and lead to reservoir effluent during well testing and affect the smooth fracturing. In this paper, the conditions of channeling caused by hydraulic fracturing are studied, and the countermeasures to avoid channeling are considered. According to strength theory and fracture mechanics method, this paper studies the formation mechanism of two kinds of channels which cause casing well to lose its hydraulic sealing efficiency during fracturing. The two kinds of channels are: cement cementation is good in casing well, but the cement ring is brittle and broken because of insufficient strength, forming channeling channel; Between cement and casing (the first interface) or between cement and formation (the second interface) before fracturing there is a tiny fluid gap. The channeling behavior caused by brittle failure of cement ring is simulated by three dimensional finite element method for casing section with well-cemented interface. The stress state in the area near perforation of casing well under water pressure is analyzed, the critical water injection pressure that causes formation or cement ring failure is calculated, and the anti-channeling ability of cement ring under different medium parameters, perforation conditions and in-situ stress conditions is studied. The results show that cement cementing with low Young's modulus and high Poisson's ratio can effectively prevent cement ring channeling. Fracturing in low Young's modulus and high Poisson ratio will lead to cement ring channeling. The perforation condition and the deflection stress of in-situ stress have little effect on the channeling mechanism of cement ring. The hydrostatic pressure of in-situ stress will affect the hydraulic sealing capacity of cement ring. In this paper, the interfacial fluid gap is regarded as the water-bearing interface crack, and the propagation of the interfacial crack in casing well under the action of water pressure is studied. During fracturing, the growth of interfacial crack will lead to channeling. In this paper, for the casing wells with cracks at the first and second interfaces, the effects of cement properties, formation properties and in-situ stresses on the interfacial channeling are analyzed by using cohesive force unit to simulate the mechanical behavior of the interface. The results show that cement with high Young's modulus and high Poisson's ratio can improve the anti-channeling ability of cementing interface. But in the formation where Young's modulus is low and Poisson's ratio is low, the interface is not easy to be channeled. The anti-channeling ability of the interface can be improved by the increase of horizontal in-situ stress, but the change of vertical in-situ stress has little effect on the anti-channeling ability of the interface. In this paper, two kinds of channeling mechanisms are considered, and the scheme of avoiding channeling during fracturing is given. The results show that the cement with high Poisson's ratio should be used in cementing, and the Young's modulus of cement should be within a reasonable range, the Poisson's ratio should be as low as possible in the fracturing formation, and the Young's modulus should not be too high or too low. Fracturing should be carried out in a position where horizontal stress is low and vertical stress is high.
【学位授予单位】：哈尔滨工业大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TE256

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