Use of Vapor Desorption Data to Characterize High Capillary Pressures in a Basin-Centered Gas Accumulation with Ultra-Low Connate Water Saturations

Abstract

Abstract This paper presents results of a laboratory study of capillary pressure characteristics of core samples from the Bossier sands in the Mimms Creek Field located in Freestone Co., TX. The Bossier tight gas sand play is believed to be part of a basin-centered gas system (BCGS) in the East Texas Basin. Similar to other BCGSs, the Bossier sands are sometimes characterized by an abnormally high capillary pressure profile associated with an ultra-low connate water saturation distribution. We use a vapor desorption technique to measure capillary pressures as high as 10,000 psi and corresponding to water saturations as low as 1%. We also compare capillary pressures measured with a centrifuge and demonstrate our inability to fully define the pressure-saturation range observed in the Bossier sands using a centrifuge. Introduction Tight gas sands constitute a significant percentage of the U.S. natural gas resource base and offer tremendous potential for future reserve growth and production. A recent study by the Gas Technology Institute1 indicated tight gas sands comprise almost 70% of gas production from all unconventional gas resources and account for 19% of the total gas production from both conventional and unconventional sources in the U.S. The same study1 estimated total producible tight gas sand resources exceed 600 Tcf, while economically recoverable reserves are 185 Tcf. Basin-centered gas systems (BCGS) represent an important component of the tight gas sand resource base.2 A BCGS is generally defined as a regionally pervasive, low-permeability sand accumulation that is gas saturated, is abnormally pressured, and lacks a down dip water contact.3 Similar to conventional oil and gas systems, BCGSs are often described by complex geological and petrophysical systems as well as heterogeneities at all scales. Unlike conventional reservoirs, however, BCGSs often exhibit unique gas storage and producing characteristics. Moreover, many BCGSs, especially those in deeper basins, are also characterized by very low connate water saturations and the associated high capillary pressures. Consequently, effective exploitation of these resources requires accurate description of key reservoir parameters, particularly capillary pressures, to quantify the vertical water saturation distribution and resource-in-place. Unfortunately, limited capillary pressure data from BCGSs have been published in the petroleum literature. Previous studies of Bossier sands in the Mimms Creek Field and in deeper parts of the East Texas Basin have suggested that these sands are part of a basin-centered gas system.4,5 In addition, these previous studies documented the presence of an ultra-low (i.e., sub capillary-equilibrium) water saturation distribution in the Bossier sands.5 The goal of our current laboratory study, therefore, was to describe the capillary pressures associated with these low saturations. Numerous techniques have been developed for measuring capillary pressures, including porous plate, centrifuge, and mercury injection. The primary advantage of the porous plate and centrifuge methods is the ability to use reservoir fluids during the pressure measurements; however, limitations on the maximum achievable pressure preclude application in most tight gas sands. High-pressure mercury-injection can reach the necessary pressures, typically 5,000 to 10,000 psi, but the use of non-reservoir fluids to compute capillary pressures accurately at reservoir conditions has been questioned. Accordingly, we used the vapor desorption technique7–11 to measure capillary pressures, especially at ultra-low water saturations.