Hydraulic fracturing by stimulated reservoir volume (SRV) is a necessity to realize commercial development of shale gas, and its stimulation mechanism still needs further study, and the effectiveness of supplementary stimulation measures needs further exploration. The shale always contains some carbonate minerals. This paper tests the permeability of acid-etched fracture in shale to explore the influence of carbonate mineral content, acid fluid types and concentration, fracture plane roughness, proppant, and confining pressure on the acid-etched effects in shale, and uses CT scanning to conduct research on variation of microscopic pore-throat texture in shale before and after acid-etching. The test shows that the roughness of the fracture plane perpendicular to the bedding plane is higher than the roughness of that paralleled to the bedding plane, and the roughness in both fracture planes perpendicular to and paralleled to the bedding plane increases as the carbonate minerals content increases. In same group of shale samples, the permeability of self-propped fracture before and after acid-etching respectively is positively correlated with the fractal dimension of the fracture plane before and after acid-etching, and the variation of permeability of self-propped fracture before and after acid-etching is also positively correlated with the variation of fractal dimension of fracture plane before and after acid-etching, which is not shown in different groups of shale samples. When the content of carbonate minerals in shale is between 10% and 30%, the relation between optimum HCL concentration and carbonate mineral content is expressed as Y_{(OptimumHCLconcentration)} = −0.5X_{(Carbonatemineralcontent)} + 0.15. If the shale has a high carbonate mineral content (>30%), the effect of acid-etching is not easily controlled, so the technique of acid fracturing should be carried out cautiously. The permeability of single-layer proppant and self-propped fracture after acid-etching conforms to Walsh theory within certain pressure, and variation and migration of curve slope reflects unstable arrangement, imbedding, and crush of proppant, and nonreactive filled impurity of clay and quartz desquamated and migrated, which coincides well with constant variation of permeability. Applying proper acid fluids and optimum concentration in shale with varying carbonate contents will increase pore size, fracture width, and fracture number. For the shale with abundant calcite-cemented fractures, the optimum acid fluid concentration should be increased properly. Due to ultralow permeability of the matrix in shale, even high concentration acid fluid could not penetrate the core with barren natural fractures or calcium-filled fractures. The results of research provide valuable information for design of acid fracturing in shale play.

中文翻译：

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页岩地层酸压裂处理评价

利用页岩气压裂法进行水力压裂是实现页岩气商业化发展的必要条件，其页岩气的增产机理尚需进一步研究，补充增产措施的有效性尚需进一步探索。页岩中总是含有一些碳酸盐矿物。本文测试了页岩中酸蚀裂缝的渗透率，以探讨碳酸盐矿物含量，酸液类型和浓度，裂缝面粗糙度，支撑剂和围压对页岩酸蚀作用的影响，并利用CT扫描进行分析。进行酸蚀前后页岩微观孔喉结构变化的研究。试验表明，垂直于顺层平面的断裂面的粗糙度高于平行于顺层平面的断裂面的粗糙度，随着碳酸盐矿物含量的增加，垂直于和平行于顺层面的两个断裂面的粗糙度也增加。在同一组页岩样品中，酸蚀前后自支撑裂缝的渗透率与酸蚀前后裂缝面的分形维数，自支撑裂缝渗透率的变化呈正相关。酸蚀后和酸蚀前后裂缝面的分形维数变化也呈正相关，这在不同组的页岩样品中均未发现。当页岩中碳酸盐矿物含量在10％到30％之间时，最佳HCL浓度与碳酸盐矿物含量之间的关系表示为Y _{（最佳HCL浓度）} = −0.5 X _{（碳酸盐矿物含量）}+ 0.15。如果页岩中碳酸盐矿物含量高（> 30％），则酸蚀作用难以控制，因此应谨慎进行酸裂技术。酸蚀后单层支撑剂和自支撑裂缝的渗透率在一定压力下符合沃尔什理论，曲线斜率的变化和迁移反映了支撑剂的不稳定排列，嵌入和压碎，以及粘土和石英的非反应性填充杂质脱层和迁移，这与渗透率的恒定变化非常吻合。在碳酸盐含量不同的页岩中使用适当的酸性流体和最佳浓度会增加孔径，裂缝宽度和裂缝数。对于方解石胶结裂缝丰富的页岩，应适当提高最佳酸性流体浓度。由于页岩中基质的超低渗透性，即使是高浓度的酸性流体也无法渗透具有贫瘠的天然裂缝或钙填充裂缝的岩心。研究结果为页岩气中酸压裂的设计提供了有价值的信息。