Pressure Core Measurements in Tight Sandstone Lenses During the Multiwell Experiment

Abstract

A pressure coring operation was performed in two tight sandstone lenses of the Mesaverde group during the drilling of one well of DOE's Multi-Well Experiment. The goals were:to obtain accurate water saturation data;to measure the amount and composition of gas on a foot-by-foot basis; andto recover fluids for water chemistry analysis. This application of pressure coring bad not been tried before; it was felt to be the best approach for accurate water saturation measurements in tight sandstone core. Several changes to usual pressure coring procedures were adopted. These included the procedures were adopted. These included the development of an organic noninvasive coring fluid, compatible with the oil-based drilling fluid used, that would help preserve water saturation. Three 15-ft pressure cove barrels were taken in two sandstone lenses 20 and 30 ft thick; all three barrels were filled with sandstone. Goals 1 and 2 were met. Water saturation values are reported and discussed. The type and duration of the water extraction processes ave shown to be very important in determining water saturations in tight sands. Gas species and amount are available on a foot-by-foot basis. Introduction The Multi-Well Experiment (MWX) is a research-oriented field laboratory and it is the major production technology activity of the Department of Energy sponsored Western Gas Sands Project. The overall MWX objectives are to Project. The overall MWX objectives are to thoroughly characterize lenticular gas sand reservoirs and evaluate technology for their production. A major activity of the Multi-Well production. A major activity of the Multi-Well Experiment is the core program. This paper deals with one major aspect of the M" core program, the design and execution of a pressure core operation in tight sands with accompanying pressure core analysis. The effect of water saturation is extremely important in tight sands; permeability is very strongly dependent upon water saturation. For example, water saturations of 15%, 30%, 40% and 50% reduced the permeabilities of a Multi-Well Experiment core sample by factors of 0.61, 0.33, 0.25 and 0.05 respectively. Thus, an unambiguous technique to accurately measure water saturation becomes extremely important. There is general concern with the accuracy of water saturations measured in core because fluids may be excluded when core is brought to the surface. Additionally, drilling fluids may penetrate into the cove, especially if water-based penetrate into the cove, especially if water-based fluids are used. It was believed that pressure coring would circumvent the first problem and the core could be brought to the surface intact with all its fluids. The use of organically based drilling fluids, with suitable tagging, would alleviate the second problem. Therefore, a pressure coring operation was undertaken in the tight sandstone of the Mesaverde group in the second well, MWX-2, of the Multi-Well Experiment. At the time it was undertaken it was felt to be the best approach for water saturation measurements in tight sandstone core. This application of pressure coring had not been tried before. Specifically, the goals of the project were to:obtain accurate water saturation data.obtain both the amount and species of gas foreach foot of pressure core taken.Obtain fluids from the thawing of the pressurecore for water chemistry analysis. P. 229

Core analysis in a low permeability sandstone reservoir: Results from the Multiwell Experiment

“…high water saturation at site, is not well studied. Two earlier papers were written about the MWX core program (Sattler, 1983(Sattler, , 1984. Detailed presentations of the core data by depositional environment are to be found in the Sandia/CER MWX Final Report Series, I to…”

“…Water saturations averaged around 65 percent at the top of the Mesaverde and 30 to 35 percent in some of the productive sands. Sattler, 1984).…”

“…A pressure core operation was performed in the E and F fluvial sands, at 5485 to 5500 and 5551 to 5581 ft, during the drilling of MWX 2 (Sattler, 1984). The goals of this operation were (1) to obtain accurate water saturation data, (2) to measure the amount and composition of gas on a foot-by-foot basis, and (3) to recover fluids for water-chemistry analyses.…”

“…Not only was there core weight loss from the Dean Stark distillation but additional weight loss occurred from the vacuum ovendrying. Analyses of the data (Sattler, 1984) suggests the added weight loss is due mainly to removal of additional connate water from tight core.…”

“…No water was recovered during the thawing of the pressure core prior to the Dean Stark analyses, probably because permeabilities of the samples were too low. Gas was recovered during the thawing process and the gas was analyzed on a foot-by-foot basis (Sattler, 1984).…”