Abstract

The Delaware formation is a very fine grained sandstone, often 4000 feet thick, located in West Texas and SE New Mexico. Resistivity based log interpretation in the Delaware has proven to be ineffective in most circumstances. Deep invasion and high irreducible water volumes lead to water saturation calculations that rarely reflect future production. Recent work in this formation demonstrates that Nuclear Magnetic Resonance (NMR) is an effective evaluation tool.

Lab NMR measurements on core plugs accurately predict pore size distributions that have been used to estimate permeability and irreducible water volume (Bvirr). Lab NMR also demonstrates that water saturated rock has a distinctly different NMR response to partially oil saturated rock. This leads to an estimate of residual oil saturation from NMR.

This success is then duplicated using downhole measurements made by the Nuclear Magnetic Tool (NMT) and the Combinable Magnetic Resonance Tool (CMR). Downhole NMR estimates of porosity, pore size, permeability and residual oil saturation on five wells are then validated by comparison to mudlogs, sidewall cores and production results.

Limitations of Log Interpretation in the Delaware

The Delaware formation is a very fine grained sandstone reservoir located in the western edge of the Permian Basin in West Texas and SE New Mexico. Like many very fine grained clastic reservoirs, it has a tendency to be laminated in nature. At first glance it appears to be a very typical clastic reservoir. However, when it comes to formation evaluation, the Delaware is far from typical. The mud log is the most reliable technique for identification of oil zones in the Delaware. Sidewall cores are also commonly used. Unfortunately, water saturation calculations from logs correlate poorly with production. The use of the resistivity log is limited to an analysis of the invasion profile to help understand reservoir quality (permeability) and occasionally to verify hydrocarbon shows on the mudlog.

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