Differences of fracture propagation behavior for two typical fractured formations

Abstract

Hydraulic fracturing is an important stimulation measure for the development of coalbed methane and shale gas. At present, the fracture initiation and propagation mechanism in coal and shale formations are still unclear, which brings difficulties to hydraulic fracturing constructions. To solve the problems about the complexity and randomness of bedding planes and cleats in natural rock samples, the true triaxial hydraulic fracturing experiments were conducted with artificial coal and shale samples. The geometry of hydraulic fractures and the influencing factors were analyzed. The results showed that there were obvious differences in the hydraulic fracture geometry between the coal and shale. Hydraulic fractures propagated in the direction of minimum resistance, minimum energy dissipation, and minimum propagation path. Hydraulic fractures propagated along discontinuous weak planes and then branched or diverted. When the fractures propagated in shale and coal, there were obvious multiple fluctuations in the pump pressure curve. Hence, it was possible to preliminarily judge whether the fractures encountered discontinuous weak planes according to the pump pressure curve.

Keywords

• Hydraulic fracturing
• Shale
• Coal
• Fracture geometry
• Fracture propagation