页岩压裂裂缝渗透率的测试与分析

为了提高页岩气体积压裂的有效性,有必要对不同类型页岩压裂裂缝渗透率及其影响因素开展进一步的评价研究。为此,在研究页岩矿物组构特征、储层孔隙与裂缝特征的基础上,对3种不同类型(原位闭合型、剪切自支撑型、单层支撑型)的页岩压裂裂缝渗透率进行测试,并系统研究了页岩储层压裂裂缝类型、裂缝面粗糙度、碳酸盐矿物含量、页岩层理、围压等因素对页岩压裂裂缝渗透率的影响。结果表明:(1)原位闭合型裂缝渗透率与压力的关系符合Walsh理论,随围压增大而减小,其渗透率介于0.13~16.75m D,该类型裂缝不论有没有支撑剂充填和错位,对于提高页岩气储层产能都同样的重要、有效;(2)相对于原位闭合型裂缝,剪切自支撑型裂缝渗透率可提高1~2个数量级(渗透率介于7.53~88.48 m D)、单层支撑型裂缝渗透率可提高2~3个数量级(渗透率介于9.98~771.82 m D);(3)裂缝面粗糙度越大,裂缝渗透率越高,裂缝面分维数与渗透率具有较好的正相关关系;(4)剪切自支撑、单层支撑型裂缝渗透率和压力的关系与Walsh理论存在着一定的偏差,该偏差反映了自支撑点破碎、支撑剂嵌入和破碎等的影响。结论认为,该实验研究成果对于页岩气压裂工艺优选、参数优化具有指导意义。

Test and analysis on the permeability of fractured fractures in shale reservoirs

In order to improve the effectiveness of shale gas SRV (stimulated reservoir volume), it is necessary to evaluate and study the permeability of different types of fractured fractures in shale and its influential factors. In this paper, the mineral composition characteristics, reservoir pore and fracture characteristics of shale were investigated, and the permeability of three types of fractured fractures in shale (i.e., in-situ closed type, shear self-propped type and single-layer propped type) was tested. Besides, the effects of fracture type, fracture surface roughness, carbonate content, shale bedding and confining pressure on the permeability of fractured fractures in shale reservoirs were studied systematically. The following research results were obtained. First, the permeability–pressure relationship of in-situ closed fracture is in accordance with the Walsh theory. The permeability decreases with the increase of confining pressure and it is in the range of 0.13–16.75 mD. In-situ closed fracture plays the same role in increasing the productivity of shale gas reservoirs with or without proppant filling or dislocation. Second, compared with in-situ closed fracture permeability, the shear self-propped fracture permeability is 1–2 orders of magnitude (7.53–88.48 mD) higher, and single-layer propped fracture permeability is 2–3 orders of magnitude (9.98–771.82 mD) higher. Third, the larger the fracture surface roughness, the higher the fracture permeability. And there is a better positive correlation between the fractal dimension and the fracture permeability. Fourth, the permeability–pressure relationship of shear self-propped fracture and single-layer propped fracture is, to some extent, deviated from the Walsh theory, which reflects the influence of self-propped point crushing, proppant embedding and crushing. In conclusion, the experimental results can be used as the reference for the selection of shale fracturing technologies and the optimization of parameters.