Trapped-Gas Saturations in Carbonate Formations

Trapped-gas saturations existing after gas displacement by wetting-phase imbibition are presented for selected carbonate reservoirs. presented for selected carbonate reservoirs. Formations representing various rock types were investigated, and samples covering the porosity and permeability range within each field were tested. Cores from Smackover reservoirs located within four states were included to examine differences in trapped gas that might occur within a carbonate deposited over a large geographical area. The trapped gas varied with initial gas in place and with rock type. With gas in place of 80 percent of pore space, trapped gas values ranged from a low of 23 percent of pore space in Type II chalk to a maximum of 69 percent in Type I limestone evaluated. Correlation of trapped-gas saturation values was attempted using several approaches, but none was entirely satisfactory. Essentially no relationship with permeability was found within most reservoirs or between different reservoirs. Within a given field, trapped gas at a common initial gas saturation typically increased as porosity decreased. A general interfield correlation with porosity was noted, but certain anomalous data were observed. Knowledge of rock type was necessary to explain these variations in trapped-gas saturations. It was concluded that the complexity of carbonates necessitates determination of trapped gas on the specific reservoir to be evaluated. Introduction Gas reservoirs with a naturally occurring underlying aquifer and aquifer gas storage projects both offer possibilities for large volumes of gas to be trapped and unrecovered. This trapping results from gas-water capillary forces that become active as production occurs and as water encroaches into pore space that previously contained interstitial pore space that previously contained interstitial water and gas. The magnitude of the trapped gas has been reported by others for sandstones, but essentially no information is available in the technical literature for carbonates. A series of carbonate reservoirs was studied to define the magnitude of trapped gas that existed for the range of porosity and permeability found within each reservoir. Trapped-gas saturation values were developed on each core for an initial gas saturation corresponding to irreducible water. Two cores from each reservoir were tested to yield additional trapped saturations for initial gas values of 20 and 50 percent of pore space. These additional data assist in defining trapped gas within a gas-water transition zone or within a gas storage aquifer where considerable variation in gas saturation may exist. Carbonate formations studied were selected to cover a range in pore geometry. Porosity and permeability were not sufficient to classify the permeability were not sufficient to classify the samples or correlate the data. Archie arrived at a classification of carbonate rocks based on the texture of the rock matrix and the nature of the visible pore structure. Table 1 is a summary of the classification, with slight modifications by Jodry. TABLE 1 - ARCHIE ROCK CLASSIFICATION Texture of Appearance of Appearance Under Matrix Hand Sample 10-Power Microscope Type I Crystalline, hard, dense Compact with smooth face on No visible pore space Crystalline breaking. Resinous between crystals Type II Small crystals are less Chalky Dull, earthy, or chalky than 0.05 mm and are earthy with pore space barely visible. Type III Space indicated Granular or Sandy or sugary between crystals or Sucrosic (sucrose) grains. Oolites are in granular class. Matrix Grain Size Symbol Large (coarse) >0.5 mm 1 Medium 0.25 to 0.5 mm m Fine 0.125 to 0.25 mm f Very fine 0.0625 to 0.125 mm vf Extremely fine < 0.0625 mm xf Pore-Size Classification Pore-Size Classification Visible to Visible Diameter Class Naked Eye 10x Magnification (ml) A No No <0.01 B No Yes 0.01 to 0.1 C Yes Yes 0.1 to 1.0 D Yes Yes >1.0 SPEJ P. 149