oa Properties of Tight Gas Sand from Digital Images

Basin-centered gas deposits are abundant in North America. A key property required to plan and optimize production in these reservoirs is permeability. Extremely low and often disconnected porosity results in small permeability, often in the nano-Darcy domain. An alternative to a physical measurement is a numerical simulation of fluid flow in a 3D digital pore space accurately imaged by high-resolution CT scanning. We present examples of this digital technique for two samples from the Williams Fork formation with the total porosity ranging from 0.01 to 0.10. The porosity of the first sample was about 0.08, dominated by fairly large pores connected by narrow conduits, presumably due to secondary dissolution of carbonate inclusions. This pore space was imaged in a micro-CT machine with a voxel resolution of 2.2 microns. The simulated permeability was in the 1 to 5 mD range (Table 1). The pore space of the second sample was not discernible at the micro-CT resolution. It was imaged in a nano-CT machine with a voxel resolution of 0.065 microns. Disconnected spherical pores of approximately 1 micron in size were discovered.