A Case Study Demonstrating How Nmr Logging Reduces Completion Uncertainties In Low Porosity, Tight Gas Sand Reservoirs

Nuclear Magnetic Resonance (NMR) logging in low permeability gas reservoirs has been used to assist standard formation evaluation techniques in identifying productive reservoirs from those that lead to tight tests or produce formation water. By incorporating NMR logging into the standard logging suite, improved completion decisions are made regarding perforation intervals, hydraulic fracture program design and accurate estimates of producible gas. The deep gas reservoirs of the Vicksburg trend in this study contain complex clastic mineralogy derived from igneous rocks. Transport, deposition, and diagenesis play an important role in the producing characteristics of these reservoirs, Burial and diagenesis lead to low-porosity reservoirs with permeability in the range of 0.01 to 1 mD. Diagenesis of lithic rock fragments and feldspars creates significant quantities of micro-porosity, which degrades reservoir quality. The micro-porous rock holds large amounts of non-producible formation water, yet shows up as high water saturation in standard log estimates, Therefore, when conventional logging estimates of porosity and water saturation are used, it is not clear which reservoirs will produce gas free of formation water or not produce at all because of low permeability. NMR technology provides additional information on irreducible water-filled porosity and quantitative reservoir permeability not available from standard logging tools. In cases where the wells are drilled with oil-based mud and formation water resistivity is not known accurately, NMR reduces the risk of completing zones, which produce water while identifying tight gas zones, by the absence of oil-based mud filtrate in the flushed zone. When NMR measurements are combined with log-derived measurements of porosity and water saturation, both producible porosity and permeability thickness for these reservoir sands can be quantified. This paper is a case study showing the benefits of NMR logging and core analysis in low porosity, gas-bearing sandstones.