Abstract

The presence of compressible fluid, gas, in the pore space of a rock is known to have a strong influence on acoustic velocities. The Vp/Vs ratio obtained from wireline sonic logs vs. DTc crossplot technique is often used qualitatively and quantitatively to differentiate the presence of gas from water. For gas the Vp/Vs value tends to be around 1.58 - 1.6 and for water it varies with degree of compaction and effective stress among other factors. Lack of compaction and low effective stress conditions in sandstone reservoirs tend to increase the ratio for water and vice versa. Hence in under compacted sands, gas and water points can be easily identifiable on Vp/Vs vs. DTc crossplots due to increase in Vp/Vs ratio for water whereas in sands of compacted nature and at higher effective stress regimes the distinction is not accurate as the Vp/Vs value of water decreases and almost approaches the value of gas which is around 1.6. These findings have been brought out in the present study by analyzing the data from 3 wells of Krishna-Godavari Basin and generating the Vp/Vs vs. DTc crossplots against potential gas and water bearing zones. The sands belong to three different geological ages, Pliocene, Miocene and Cretaceous, having different degrees of compaction, pore pressure and effective stress.