Experimental investigation on proppant transport and distribution characteristics in coal hydraulic fractures under true triaxial stresses

Abstract

Fracturing is one of the key technologies for unconventional oil and gas production. However, the closure stress will cause the hydraulic fractures to close, and proppant is often injected into the fracture to maintain the effective conductivity of the fractures for a long time. The proppant distribution in the fractures is one of the key factors affecting the efficient exploitation of unconventional natural gas. However, there are few studies on the proppant transport and distribution characteristics in the real hydraulic fractures. A self-developed high-pressure sand injection triaxial hydraulic fracturing experimental system was used to study the proppant migration and distribution in hydraulic fractures. The results show that the proppant migration process mainly includes four stages: water filling stage, fracture reopening stage, proppant transport stage, and pressure relief stage. The basic characteristics of proppant transport in horizontal fractures is mainly along slope gradient transport or dispersion, and reverse slope gradient or low valley deposition. The sedimentary areas mainly include peak climbing deposition, ring peak deposition, and convection-diffusion deposition. The surface roughness of horizontal fractures is one of the key factors affecting the proppant transport and distribution. The fracture inlet roughness determines the proppant transport direction, and the overall roughness of the fracture surface determines the proppant distribution form. The proppant deposition is the most in the open hole parallel area, followed by the sealed hole parallel area in vertical fractures. The stress concentration at the hole bottom forms horizontal wing branch fractures, and a little proppant is deposited in the fractures. Generally, the stress required for proppant transport in horizontal fractures is smaller than that in vertical fractures. The findings of this study can help us better understand the migration and distribution of proppant in hydraulic fractures, and provide theoretical guidance for the efficient exploitation of unconventional natural gas.