Evaluation of Acid Fracturing Treatments in Shale Formation

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.