压痕法评价煤层气井固井水泥石断裂力学性质

深部煤层气井高压注入压裂、水平井分段压裂、CO_2地质封存等复杂工况对井筒环空封隔能力提出了更高的要求。为了探究煤层气井固井水泥石本体破坏原貌特征、水泥石断裂力学性质对环空封隔失效的影响,通过实地解剖沁水盆地晋城矿区煤层气井的固井情况,基本查明了煤层气井井筒环空水泥环的破坏情况,并利用压痕法(IM)对该矿区煤层气井固井水泥石原样进行维氏硬度、断裂韧度的实验测试,研究其在不同载荷下的变化规律。研究结果表明:①该区煤层气井井筒封隔失效的主要形式包括固井水泥环的第一、第二界面的开裂形成流体通道,以及水泥环本体脆性断裂形成高角度垂向裂缝和断块体;②水泥石的高脆性导致硬度随载荷增加先降低再升高,现用固井水泥浆体系的高脆性是其在射孔、压裂环节封隔失效的关键内在因素;③断裂韧度随着载荷的增大逐渐升高,后期维持在0.71 MPa·m~(1/2),抗裂性能较差;④射孔冲击在水泥石中形成辐射裂纹,压裂期间高压流体的挤注导致射孔辐射裂纹失稳扩展并形成垂向裂缝窜槽和断块。结论认为,采用抗冲击型高韧性复合水泥可以控制射孔阶段固井水泥石的辐射裂纹尺寸,有利于防止煤层气井井筒环空封隔失效。

Evaluation of fracture mechanical properties of set cement in CBM wells by the indentation method

The sealing capacity of wellbore annulus has to satisfy stricter requirements under complex working conditions, such as high-pressure injection fracturing of deep coalbed methane (CBM) well, multi-stage fracturing of horizontal well and CO2 geological sequestration. In order to explore the original appearance damage characteristics of set cement in CBM wells and the effects of the fracture mechanical properties of set cement on the failure of annulus sealing, this paper analyzed the downhole cementing situations of CBM wells in the Jincheng mining area of the Qinshui Basin on the basis of field observation. And the damage situations of the cement sheath in the wellbore annulus of CBM wells were basically figured out. Then, the Vickers hardness and fracture toughness tests were carried out on the original set cement samples of CBM wells by using the indentation method (IM). Finally, the change laws of hardness and fracture toughness under different loads were analyzed. And the following research results were obtained. First, the main wellbore sealing failure forms of CBM wells include the fluid channels which are generated by the cracking of the first and second interfaces of cement sheath and the high-angle vertical fractures and fault blocks which are formed by the brittle fracturing of cement sheath. Second, due to the high brittleness of set cement, the hardness decreases firstly and then increases with the increase of indenter load. And the high brittleness of existing cementing slurry systems is a key intrinsic factor leading to the sealing failure in the stages of perforation and fracturing. Third, the fracture toughness increases gradually with the increase of indenter load, and then maintains at 0.71 MPa·m1/2 in the late stage when the crack resistance is poorer. Fourth, radiation cracks are formed in set cement under the impact of perforating projectile. During the fracturing, the injection of high-pressure fluid leads to the instability and propagation of radiation cracks and the formation of vertical fracture channeling and fault blocks. It is concluded that high-toughness, impact-resistance composite cement can control the size of the radiation cracks in the stage of cementing, which is beneficial to preventing the annulus sealing failure of CBM wells.