煤层气近井煤缝壁面滤饼的结构与硬度特征及工程意义

煤层气近井部位钻完井液侵入形成的煤缝壁面滤饼结构,对压裂裂缝的起裂和延展具有较为复杂的制约关系。为了弄清煤缝壁面滤饼的结构与力学性状及其对压裂裂缝起裂延展的控制作用机制,基于矿井解剖、界面区微观结构SEM及元素EDS分析、矿相XRD测试以及界面硬度压痕法试验等手段,分析研究了煤层气近井煤缝壁面滤饼的组合方式、界面搭接结构、界面力学性状等制约压裂裂缝延展的关键因素。研究结果表明:①煤层气近井煤缝壁面与滤饼界面主要有煤岩—固井水泥滤饼搭接型、固井水泥滤饼—钻完井液滤饼—煤岩搭接型两种形式,前者界面结合相对更为紧密;②上述两种滤饼界面区的基材没有发生显著的物理化学变化,界面清晰、分界明显,均属于惰性接触界面;③压痕结果显示,煤岩—固井水泥滤饼搭接型界面材料维氏硬度规律为固井水泥滤饼煤岩搭接界面,而固井水泥滤饼—钻完井液滤饼—煤岩搭接型界面材料维氏硬度规律为固井水泥滤饼钻完井液滤饼搭接界面,硬度值能够较好地指示界面组合中的力学薄弱面;④从矿井解剖现场采集的样品证实,水力压裂期间压裂裂缝优先突破煤岩—钻完井液滤饼的界面。结论认为,该研究成果可以为煤储层水力压裂裂缝延展机制理论深化、压裂裂缝起裂延展模式的精准构筑、压裂效果评价等提供新的思路。

Structure and hardness characteristics of the filter cake–coal wall interface near a CBM well and its engineering significance

The filter cake–coal wall interfacial structure formed with the invasion of drilling and completion fluid near a coalbed methane (CBM) well plays a complex restriction role in the initiation and propagation of hydraulic fractures. In order to figure out the structure and mechanical property of the filter cake–coal wall interface and its controlling mechanism on the initiation and propagation of hydraulic fractures, this paper analyzed the key factors restricting hydraulic facture propagation, such as combination mode, interfacial connection structure and interfacial mechanical property of the filter cake–coal wall interface near a CBM well, by means of coal mine observation, interfacial zone microstructure SEM and EDS analysis, mineral XRD test and indentation method test. And the following research results are obtained. First, the filter cake–coal wall interface near a CBM well is mainly classified into two types, including the coal–cementing cement filter cake interface and the cementing cement filter cake–filter cake of drilling and completion fluid–coal interface, and the first one is much tighter. Second, these two types of filter cake–coal wall interfaces are basically constant in the physical and chemical properties of base materials and have clear interfaces and distinct boundaries, so both of them are inactive contact interfaces. Third, the indentation method test results show that the Vickers hardness of the material in the coal–cementing cement filter cake interface follows the law of cementing cement filter cake ＞ coal ＞ connection interface, while that of the cementing cement filter cake–filter cake of drilling and completion fluid–coal interface is cementing cement filter cake ＞ filter cake of drilling and completion mud ＞ connection interface. The hardness values can indicate the mechanical weakness of the interface combination. Fourth, the samples collected on site during the coal mine observation verify that during hydraulic fracturing, hydraulic fractures firstly break through the interface of coal–filter cake of drilling and completion fluid. In conclusion, this research result provides a new idea of deepening the theories on the propagation mechanism of hydraulic fractures in coal reservoirs, accurately establishing a hydraulic fracture initiation and propagation model and evaluating fracturing effects.