A Deterministic Method for Gas Reservoir Evaluation Using Dual Wait-Time NMR and Density Log Data

Abstract

Abstract Accurate estimates of porosity and fluid saturations are critical for the proper evaluation of a gas reservoir. These properties are determined by combining data from a dual wait-time (DTW) nuclear magnetic resonance (NMR) log and a density log. The novel deterministic method additionally yields several in-situ gas properties. The density and dual wait-time (DDTW) technique is applicable to reservoirs where the pore-filling fluid consists of one liquid phase and one gas phase. The low proton density of the gas phase causes a reduction in the NMR signal strength resulting in underestimation of the apparent porosity. The amount of polarization for different wait-times depends on the specific spin-lattice relaxation time of each fluid and may cause additional NMR porosity underestimation. The density log, however, delivers a porosity that is overestimated because of the presence of a gas phase. Published correlations for gas properties are used to establish the new deterministic method. DDTW primarily yields the total porosity f, and the flushed zone gas saturation, Sg, xo. Other derived properties are the in-situ gas density, rg, as well as the two NMR-related properties hydrogen index, HIg, and spin-lattice relaxation time, T1g. Two field examples illustrate the method and an error propagation study supports its stability.