An integrated quantitative approach for determination of net reservoir cutoffs: A case study of Q oil field, Lake Albert, Uganda

Petrophysical cutoff of net reservoir plays a very important role in reservoir characterization for the evaluation of hydrocarbons in place and the estimation of ultimate hydrocarbon recovery. There have been many approaches to quantify cutoffs, yet each of these approaches yields a different reservoir model with some amount of uncertainties due to lack of enough data, insufficiency of knowledge and the heterogeneous nature of petroleum reservoirs. Conventionally, net reservoirs cut-offs are evaluated by applying static petrophysical well-logs without consideration of dynamic performance, which results in a crisp classification of reservoir or non-reservoir zones. This paper takes Q oilfield in Albert Basin of Uganda as an example, provides a new structured quantitative approach which integrates all core, petrophysical interpretation, modular dynamics test data, reservoir fluid data and development wells pattern together to determine a reasonable net reservoir cutoffs. Three dependent variables including oil initially in place (OIIP), total oil production (FOPT) and total water production (FWPT) were selected to test the sensitivity with two independent variables, i.e. volume of clay (Vclay) and porosity. Then experiment design and response surface method were used in constructing the proxy models that are related to the dependent variable. After running 5000 realizations, probability density function (PDF) was utilized to locate P50 value on the cumulative probability curve. Finally, the cutoffs of Vclay and porosity were determined by the arithmetic mean of corresponding P50 value. The case study clearly illustrates how all available data from a reservoir should be integrated for appropriate determination of the net reservoir cutoffs.