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In the Mahakam delta in East Kalimantan, TotalFinaElf E&P Indonésie operates fields with numerous multi-layer reservoirs deposited within a deltaic environment. Formation waters in these reservoirs have very low salinities, which vary with depth and from reservoir to reservoir. When a field is in production, the evaluation and update of initial hydrocarbon net-pay is made more difficult by depletion and fluid level changes, which are related to channel reservoirs connectivity, both laterally and vertically. A new method has been developed, based on acoustic measurements, to help identify the original gas net-pay. This method uses the fact that compressional acoustic waves travel slower in gas than in liquid whereas shear waves are not affected by fluids in the pore space. An empirical correlation is established between Vp/Vs ratio and shear slowness in known liquid-bearing sands. This correlation is used to predict Vp/Vs over the whole logged interval. A large difference between predicted and measured Vp/Vs indicates the presence of gas. Since gas is very compressible, the effect is noticeable even at very low gas saturations, i.e., in gas reservoirs that have already been depleted, or even swept by water. The method was first tried in a mature oil field in order to distinguish gas from liquid and gave encouraging results. It was then applied in a gas field where a 3000-meter interval of dipole sonic log had been recorded in one well. Fluid status identified by this method was crosschecked against all other data (wireline logs, mud logs, wireline fluid samples) and against the geological model. The results helped confirm (or revise) the model, which, in turn, improves mapping, material balance calculations and optimization of production. Based on numerous examples, the conclusion is that very good results are obtained in clean sands, especially where known water and gas bearing intervals are available for calibration. The article also states the limitations of the method, which can give ambiguous results in very shaly reservoirs and also fails in deep reservoirs where porosity falls significantly below 15 percent. |
