Hydraulic Fracture Seismicity in Tight Gas Sands
I have carefully analyzed microseismic data from two tight-gas sand reservoirs and find some striking features common to both data sets. The two field sites are the Carthage Cotton Valley gas field and the Sawyer Canyon Sands gas field. Both fields are located in Texas but are about 640 km apart. These reservoirs are fairly typical of the resource in which gas is produced from thick sequences of multiple, low-permeability sand layers that are interbedded with and isolated by shales. A common feature of both reservoirs is a prevalence of vertical tensile fractures contained within the individual sand layers. The fractures tend to be short (< 100 mm) and vertically discontinuous. These tensile fractures also terminate at shale boundaries and few, if any, occur in the intervening shales.
The microseismicity induced by hydraulic fracturing in these tight gas sands form long, narrow zones isolated within the sand intervals. Vertical fracture growth through the intervening shales occurs without detected seismic signal (aseismic growth). The source mechanisms indicate primarily shearing which occurs as strike-slip displacements along vertical fractures oriented close to the hydraulic fracture trends. Thus, the seismicity detected during stimulations highlights the preexisting fractures contained within the targeted sands. These are fractures that are intersected by or are close enough to the hydraulic fractures to be pressurized and accommodate some of the created volumetric strain. The temporal development of the seismic clouds indicates that the growth of the hydraulic fractures within the clouds are slow, too slow to generate seismically detected signals. The largely aseismic development of tensile failure and volume created is also indicated by the silent growth of the fractures through the intervening shales, where natural fractures are absent.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009