A Novel Approach to Water-cut Prediction in Deep Cambro-Ordovician Tight reservoir with Complex Hydrocarbon Charge, Diagenetic and Structural Growth Histories

In deep tight shaly-sand reservoir, with complex hydrocarbon charge and structural growth history, it is difficult to characterize reservoir zones with high water cut from others with low water cuts and high hydrocarbon production rate, due to high resistivity readings of tight reservoirs. Deep reservoir “B” in Abu Butabul Field, NW Oman has been charged via two genetically and chronologically different hydrocarbon phases (oil then gas); during a complex deep burial digenesis and structural trap history. Due to the variation of Hydrocarbon properties of such tight reservoir, it has been difficult to analysis such a reservoir using conventional petrophysical evaluation methods. In this study, a modified version of the approach published by Ibrahim, Abd_Elmoula, Said Hasani, Sultan, Jahwari in SPE 130261 in which Ro (Resistivity of Rock saturated with Formation water) and SWirr (irreducible water saturation) used to distinguish water from hydrocarbon zones has been taken a step further in order to predict water cut in tight reservoirs (illustrated in the workflow below). After calculating wireline logs derived permeability, there are two elements to be calculated: KHw (Permeability of sands units filled with water * thickness of these sand units), and KHhc (Permeability of sands units filled with hydrocarbon *thickness of these sand units). The ratio of KHw/(KHhc+KHw) provides a mean of estimating percentage of the expected water production. This approach has been validated with actual water cut from production data. Using mathematical product of KHhc*Phie (effective porosity)*(1-SW) provides a mean to rank the wells in terms of expected Hydrocarbon productivity, which then can be contoured and utilized for reservoir fracturing program of the reservoirs in promising wells. Supporting case-studies and production test results are discussed in order to demonstrate the validity and rate of success of this workflow.Moreover the workflow has been integrated with seismic inversion derived porosity maps in order to delineate areas of larger hydrocarbon reserves, which consequently has obvious implications on reserves evaluation and subsequent field development plan.