Experimental Study of Hydraulic Fracture Conductivity Demonstrates the Benefits of Using Proppants

Abstract

Abstract Hydraulic fracturing treatments using treated water and very low proppant concentrations ("waterfracs") have been successful in stimulating low-permeability reservoirs. However, the mechanism by which these treatments provide sufficient conductivity is not well understood. To understand the effects of hydraulic fractures on conductivity, a series of laboratory conductivity experiments were performed with hydraulically fractured cores from the East Texas Cotton Valley sandstone formation. Jordan sand and sintered bauxite proppants were used at concentrations of 0, 0.1 and 1.0 lbm/ft2, and the conductivity was measured at effective closure stresses ranging from 1,000 to 7,000 psi. The results of this study demonstrate that fracture displacement is required for surface asperities to provide residual fracture width and sufficient conductivity in the absence of proppants. However, the conductivity may vary by at least two orders of magnitude depending on formation properties such as the degree of fracture displacement, the size and distribution of asperities, and rock mechanical properties. In the presence of proppants, the conductivity can be proppant or asperity dominated, depending on the proppant concentration, proppant strength and formation properties. Under asperity dominated conditions, the conductivity varies significantly and is difficult to predict. Low concentrations of high-strength proppant reduce the effects of formation properties and provide proppant dominated conductivity. At conventional proppant concentrations, conductivity experiments performed with flat, parallel core faces tend to overestimate the conductivity observed with hydraulic fractures. Actual hydraulic fracture conductivity may be as much as an order of magnitude lower in the presence of low strength proppant. An important implication of this study is that the success of a "waterfrac" treatment is difficult to predict because it will depend significantly on formation properties. This dependence can be overcome by using high strength proppants or proppants at conventional field concentrations.