Eslinger, Eric¹, Robert Everett², Scotty Tuttle³
(1) Eric Geoscience, Inc. and The College of Saint Rose, Glenmont, NY
(2) Consultant, Victoria, BC (3) Dominion E&P, Inc. (currently Mission Resources, Inc.), Houston, TX

ABSTRACT: A Mineralogy-Based Model for Limits to Gas Production Drawdown in the Vicksburg Formation, Nueces County, TX

A core characterization was done on one well and petrophysical analyses were done on thirteen wells in the Vicksburg interval (8000 to 11,000 feet) of the Bluntzer Field. Core-calibrated results included profiles for porosity, permeability, and water saturation. A primary goal was to aid interpretation of possible causes of casing collapse during or after completion. The petrophysical analyses were done using output from a software program (GAMLS) coupled with a forward modeling procedure (MFBG) that uses a dual-water model to compute water saturation. Reservoir sands are very fine-grained sandstones and siltstones that are on the low end of the spectrum of Vicksburg sand reservoir quality. The sandstones contain a high percentage of clay minerals (18 - 38 wt % of matrix).
Triaxial stress test measurements were made (by Core Laboratories) on selected whole core samples and the results were used for Mohr-Coulomb analyses. This test data in conjunction with available mineralogic data indicated that the clay mineral content and the calcite content affects the strength of the rocks. Using this information, a predictive model was generated that estimates the abandonment pressure, which is the reservoir pressure below which the reservoir rocks will become unstable due to long-term gas production. The model relies on knowing the mineralogy and the static elastic modulii of all rock types present. In the absence of mineralogic data, application of this model can be applied to wells via a GAMLS-predictive "transform" that relates wireline well logs to mineralogy and rock strength.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.