AN EFFICIENT METHOD FOR MEASURING TRAPPED GAS SATURATION UNDER CO-CURRENT CONDITIONS

The amount of gas trapped by water influx is a critical parameter for the evaluation of the recoverable reserves of a large gas reservoir in Mozambique. We compare trapped gas saturation measurement results obtained by using various methods. All the tests were performed on the same six core samples. They differed from one another in the liquid used, the boundary conditions and the gravity force applied to the samples. The tests included: 1) spontaneous imbibition measurements on unsealed cores totally immersed in either toluene or water, 2) spontaneous imbibition measurements on laterally sealed cores with one end in contact with water and the other end exposed to air, 3) forced imbibition measurements by centrifuging unsealed cores under water with 1D NMR saturation profiling run in combination. The latter method is very efficient because it produces an exhaustive compilation of initial gas saturation vs. trapped gas saturation data. The lab data are compared to the log data acquired in an interval below the gas-water contact, where current gas saturations are the result of the rise of a paleo-gas-water contact. We take these saturations as the reference. Tests 1) and 2) provided slightly high trapped saturations, probably due to the presence of countercurrent flow (mainly in test 1) and evaporation effects (only in test 2). For the centrifuge/NMR experiments a good match to the log data is obtained for Bond numbers in the 4×10 -6 – 5×10 -5 range.