Rejuvenation of N2 Layer Reservoir with Shift Paradigm from Gas Reservoir into Oil Reservoir: A Case Study from Benuang Structure, South Sumatera

Abstract

This paper focuses on developing a substantial oil reservoir in South Sumatra, Indonesia, characterized by a large gas cap and a bottom water aquifer. The reservoir comprises thick, clean sandstone from the Miocene Talang Akar Formation, deposited in a transitional estuarine complex with good porosity and lateral continuity due to an accreted channel complex. Initial production was gas-dominated, leading to its classification as a non-potential reservoir with a recovery factor (RF) of up to 79%. The recent success in infill drilling has resulted in nearly 4000 BOPD from the N2 layer, revealing the ongoing potential for oil reserves. The shift from gas to oil production in the reservoir is an intriguing phenomenon. To understand this shift, static and dynamic data from the latest infill drilling, along with current saturation logs from existing wells, are evaluated. This comprehensive analysis forms the basis for constructing a reservoir model. The model is expected to describe the reservoir's conditions from its initial state to the current condition and assess the potential for incremental production. The comprehensive reservoir model estimates gas in place at 41 BSCF with an RF of 79%, based on cumulative production of 33 BSCF. The estimated oil in place is 11 MMSTB with the current RF at only 10%, from 1.1 MMSTB. The results of the recent study present opportunities for further development in the N2 layer using clustering infill drilling, workovers, and pressure maintenance to optimize oil production. This is estimated to have a potential additional oil production of 3.9 MMSTB, representing a ‘New Life’ for the N2 layer by shifting the paradigm from developing the gas reservoir to developing the oil reservoir. The evaluation of the N2 layer can change the paradigm for gas reservoirs with oil rims. While the common rule in the oil and gas industry is to produce oil first to optimize oil recovery, the case of the N2 layer challenges this norm. Despite the reservoir producing a large volume of gas, the oil potential remains high and can be developed by considering various reservoir parameters, as explained in this paper.