Methods For Correcting Porosity Data in a Gypsum-Bearing Carbonate Reservoir
Authors: Tilly H.P. , Gallagher B.J. , Taylor T.D. 
Published: December 2007

Summary
The McElroy field in west Texas is an example of a carbonate reservoir that contains significant quantities of gypsum. Large amounts of reservoir data have been obtained in this field by conventional core analyses and by sidewall neutron porosity (SNP) logs. These data do not account for the effect of gypsum in porosity determination. In an effort to characterize this heterogeneous reservoir more thoroughly, we used special low-temperature core analysis and developed a new method of log analysis to correct old core and log porosity data for gypsum content. These data then were used for a more accurate determination of OOIP.

Introduction
The McElroy field is one of the major reservoirs of west Texas. It is located in Crane and Upton Counties (Fig. 1). The reservoir encompasses about 30,000 acres (121.5×106 m2) and contains about 2,000 wells. Production is from the Grayburg-San Andres dolomite formation of the Permian period at an average depth of about 3,000 ft (900 m). The field was discovered in 1926 and has been in various stages of development since that time.1 Most of the porosity data acquired have been in the form of conventional core analyses and SNP logs. These data are known to be affected significantly by the presence of gypsum. Since this reservoir rock has been found to contain significant amounts of gypsum, direct use of these data would lead to significant errors in porosity estimation.
In early 1959, a method of core analysis was developed that prevented the dehydration of gypsum.2 As new infill wells were drilled, selected wells were cored, and the cores were analyzed by this technique. To utilize as much data as possible in this very large reservoir.
Many of the more recently drilled wells were logged with SNP logs.3 To utilize these data, a method of correcting these logs was developed. The corrections were based on information obtained from special log analysis techniques in nearby infill wells.
Porosity data from logs and cores in the new wells and corrected porosity data from logs and cores in the old wells were used in conjunction with capillary pressure measurements to determine OOIP by the volumetric method.

Theory and Definitions

Core Analysis
Gypsum (CaS04·2H20) is an evaporite mineral found in many carbonate and some sandstone reservoirs. In laboratory measurements, gypsum has been observed to convert to anhydrite and water at temperatures above about 140°F (60°C). Assuming that the conversion from gypsum to anhydrite takes place at approximately this same temperature in the reservoir and that a typical geothermal gradient exists, gypsum general1y is not expected to exist below a depth of about 6,000 ft (1800 m).
Standard core analysis can cause serious errors in the measurement of porosity if gypsum is present in the cores. Vacuum distillation or retort procedures yield porosity values that are too high by as much as 48 % of the bulk volume of gypsum present. Heating the core samples for cleaning or measurement of fluid content is a necessary step in most forms of standard core analysis. The heat applied is high enough for the gypsum to lose its crystallization water and to form anhydrite. It is the loss of this crystallization water that causes the error in the measurement of porosity. This process was described in detail by Hurd and Fitch2 in 1959.

Core Analysis
Gypsum (CaS04·2H20) is an evaporite mineral found in many carbonate and some sandstone reservoirs. In laboratory measurements, gypsum has been observed to convert to anhydrite and water at temperatures above about 140°F (60°C). Assuming that the conversion from gypsum to anhydrite takes place at approximately this same temperature in the reservoir and that a typical geothermal gradient exists, gypsum general1y is not expected to exist below a depth of about 6,000 ft (1800 m).
Standard core analysis can cause serious errors in the measurement of porosity if gypsum is present in the cores. Vacuum distillation or retort procedures yield porosity values that are too high by as much as 48 % of the bulk volume of gypsum present. Heating the core samples for cleaning or measurement of fluid content is a necessary step in most forms of standard core analysis. The heat applied is high enough for the gypsum to lose its crystallization water and to form anhydrite. It is the loss of this crystallization water that causes the error in the measurement of porosity. This process was described in detail by Hurd and Fitch2 in 1959.