Development of Tight Gas Sand Core Analysis Techniques for the Pinedale Field, Sublette County, Wyoming

Abstract

Routine core analysis techniques used on the very low permeability “tight” sandstone reservoirs in Pinedale field failed to give reliable results sufficient to design and justify field development or to calculate gas productivity and the field's original gas in place. In part these problems arose because of the complex variety and textures of the clay minerals lining the pores in the producing intervals. Early in the core evaluation program, it became apparent that mineralogy, clay composition and texture, and the rock's low porosity and permeability warranted an unconventional approach to core analysis. Thus, new core analysis protocols had to be developed and tested to provide representative, meaningful data in a timely fashion. Examination of the effects of cleaning and drying core samples led to the adoption of “fresh state” core analysis methods. Core tests were employed to provide not only thorough characterization of clay-bound water but also an understanding of how this clay-bound water affected various rock and petrophysical properties. Almost every rock property was examined by multiple techniques, including well-documented traditional core analysis methods and newly introduced technologies and methods. Triplicate core plug sampling and fresh-core screening tests expedited the test programs. Rapid and extensive clay characterization resulted from simple staged drying. Routine core water saturations were supported with corrections via filtrate tracers, mainly tritium, in some wells. Special core analyses included updated core water salinity determinations with additional fresh-state tests for electrical properties, capillary pressure, and relative permeability. Fortunately, operators in Pinedale field understood the importance of reliable core analyses and provided 21 conventional cores, each 4 in (10 cm) in diameter, totaling more than 1000 ft (300 m) in length from 10 wells distributed along the anticline. These cores were cut from 2002 to 2005 early in the development of the field using a water-based mud system. Results of the core analysis program greatly improved understanding of the field's reservoir system, and allowed for quantitative and petrophysical characterization of the reservoir rocks. Fresh-state analysis techniques provided both routine and special core data to develop log models for gas in place. Furthermore, special core analysis revealed that the formation-water salinity typically ranged from 30,000 to 40,000 mg/l NaCl, which is substantially higher than earlier estimates of about 13,000 mg/l. All else being held equal in a resistivity model, these higher salinity values very favorably impacted the estimate of the field's gas in place.