Hydrocarbon Mobility Steering for Optimum Placement of a Power Water Injector above Tar Mats - A Case Study from a Light Oil Carbonate Reservoir in the Middle East

Abstract

Tar mats are encountered in many Middle East carbonate reservoirs including the Kingdom of Saudi Arabia. Tar is considered to be very heavy crude that does not flow or has very limited flow capacity at standard reservoir conditions. In some cases it is found above the aquifer and below a light oil layer. As tar is practically immobile at reservoir conditions it acts as a flow barrier between water and light oil, causing significant challenges for reservoir development. In such cases water injectors work most efficiently when placed horizontally just above the tar mat to maintain pressure support during production. One challenge when drilling for these water injectors is to optimize their position above the tar layer yet as close as possible to the tar to maximize sweep efficiency. A further complication for their placement is the uncertain lateral distribution of the tar in terms of true vertical depth (TVD). The top of the tar layer is often an undulating lateral surface with varying depths. Positioning the well a "safe" distance from the tar is not a good solution as it might leave behind a significant volume of producible oil, ultimately lowering oil recovery. A real-time tar detection method is needed while drilling to quickly respond and modify the wells trajectory if tar is encountered. The low mobility caused by tar can be detected with a formation pressure while drilling (FPWD) tester. A nuclear magnetic resonance (NMR) tool provides oil viscosity estimation and additional tar-related properties. In combination with a conventional logging string, tar can be positively detected while drilling. This case study from a carbonate field in KSA shows the successful placement of a 6.25-in. horizontal well using a logging-while-drilling (LWD) NMR-device and an LWD formation pressure tester to detect tar in combination with conventional resistivity, density and neutron (triple-combo) LWD logs.