The Effect of Anhydrite on Oil Production in Dolomite Reservoirs -- Some Early Observations

Abstract

Results from wireline log analysis of recently drilled wells in the San Andres dolomite of the Slaughter and Levelland fields, west Texas, suggested a correlation between the presence of anhydrite and poor oil production. Log data from wells in the North-Central Levelland, Wright and Boyd Mallet units show that a high percentage of wells that perform poorly all have significant amounts of anhydrite in their reservoirs. Preliminary indications suggest that more than 20% anhydrite in the reservoir will severely reduce porosity and permeability and cause higher than average water saturations. In addition, the spacing of new wells indicates that reservoir degradation by anhydrite may be sporadic and areally restricted, allowing a poorly performing well to be located no more than 1500 ft from better performing wells that have low amounts of anhydrite. These observations imply that uneconomic infill well locations could be minimized by mapping the percent of anhydrite within the reservoir section. On a practical basis, the only way to acquire a sufficiently large enough database for good anhydrite prediction in west Texas is to refine log analysis to yield accurate mineralogy from cased hole logs.