Examination of Potential Mechanisms Responsible for the High Treatment Pressures Observed During Stimulation of Coalbed Reservoirs

Abstract

ABSTRACT Treatment pressures observed during hydraulic fracturing of coal seams are often different than for conventional formations. In particular, there is usually an early, steep pressure rise and overall treatment pressures often exceed the lithostatic pressure, but produce surprisingly little vertical fracture extension. In addition, instantaneous shut-in pressures are high following a fracture treatment, while closure pressures after treatment are in good agreement with in-situ stresses determined prior to initiating hydraulic fracture stimulation. These responses cannot be explained by conventional modeling approaches. On the other hand, there are cases for coal treatments where pressures remain at levels comparable to other formations. An investigation into the physical causes of the unusual behavior of coal seam stimulations resulted in the identification of a new physical mechanism for further consideration. The accumulation of coal chips at the wellbore and partial plugging of an advancing fracture tip by coal fines carried ahead of or within the fluid treatment pad are mechanisms which consistently explain all of the observed behavior. Hydraulic fracture design codes, which include the description of the coal fines accumulation at the fracture periphery, were used to assist in the design of hydraulic fracture treatments at the Gas Research Institute, Multiple Coal Seam Field Project, at Rock Creek in Alabama, and were used to analyze the actual treatments.