NMR Determination Of Porosity And Permeability Of Western Tight Gas Sands

Abstract

Abstract Samples of fine-grained sandstone from the Colorado Interstate Gas Exploration (CIGE), Natural Buttes No. 21 core, Uinta Basin, Utah were studied using pulsed nuclear magnetic resonance (NMR) and standard mineralogical techniques. Brine-saturated rock porosities varied from 1 to 13 percent and were found deducible from the magnitude of the proton NMR. she complex pore geometry and presence of authigenic carbonate and clay minerals in these samples precluded he use of standard flow models for predicting brine permeabilities from T1 decays. A network model of permeabilities from T1 decays. A network model of the pore system is proposed and shown capable of accurately reproducing measured rock permeabilities, which varied from 10(-4) to 1 millidarcy. Introduction The Rocky Mountain region contains large accumulations of gas in nonmarine unconventional (low-permeability) reservoir rocks of Tertiary and Cretaceous age. Most of the gas occurs in the pores of fine-grained sandstone and siltstone. These sequences represent individual and composite channel-form beds that were deposited in alluvial (stream) and lacustrine (lake) environments. Although the volume of gas estimated to be contained In these rocks is very large, individual sandstone reservoirs that contain the gas are lenticular; i.e., of limited lateral and vertical extent. Variation in the geometry of these reservoir rocks is due to relatively impermeable claystone and carbonate beds complexly inter-bedded with the sandstones, abundant cross-cutting channel-formed sandstones, and a complex diagenetic history. Potential reservoir sandstones in these geologic settings are commonly referred to as being unconventional or tight because measured porosities are highly variable and measured porosities are highly variable and measured permeabilities are characteristically very low. As a permeabilities are characteristically very low. As a result, gas production rates from these lenses tend to be low. An understanding of the reservoir properties of tight sandstones is necessary in order to accurately employ stimulation techniques for enhanced gas recovery. The proton nuclear magnetic resonance (NMR) signal detected from wet sandstone is produced by fluids contained within the pores of the rock. This signal is sensitive to the presence of fluid-pore interfaces and thus has potential for probing the pore structure. The utility of the NMR technique for deducing information about the pore geometry and the volume and kind of fluid contained in the pore structure is not new. Nuclear magnetism logging of residual oil in uncased bore holes was developed in the 1960's and early 1970's. The logging tools developed are capable of detecting hydrocarbons and water in rocks with large pores. Laboratory measurements on conventional, marine, sandstone samples have yielded some successes in relating the NMR to porosity, pore size, wettability, water saturation, porosity, pore size, wettability, water saturation, and rock permeability. p. 437