Waterfront imaging using self-potential: feasibility assessment for a Kuwaiti oil reservoir
Creators: name: Alarouj, Mutlaq
Publisher: name: Imperial College London
Year: 2021

description: Monitoring water movement towards production wells through downhole measurements of self-potential (SP) is a promising reservoir monitoring technology. However, over the past few years, there has been a concern on the applicability of this technology in real hydrocarbon reservoirs. Therefore, in this work, through the use of laboratory experiments and numerical modelling, we demonstrate and assess the feasibility of SP water monitoring in an actual hydrocarbon reservoir in Kuwait known as ‘Burgan Upper Sand’. The experimental work comprises of measuring two main properties: (i) electrokinetic (EK) potential; and (ii) exclusion-diffusion (ED) potential, which are components of SP that are generated in response to pressure and concentration gradients, respectively. In the experiments, we use sandstone and shale rock samples with natural brines and crude oil collected from Burgan Upper Sand reservoir. We observe, for the first time, positive EK coupling coefficients and zeta potentials on fully water-saturated sandstone samples, caused by a positive zeta potential on dolomite and siderite mineral surfaces occupying part of the pore area, the adsorption of divalent ions in the natural brine onto quartz mineral surfaces and/or ion exchange with clay minerals. At the residual oil saturation, the measured EK coupling coefficients and zeta potentials exhibit different behaviour to fully water-saturated conditions, where the polarity was reversed in some samples. The measured ED potential is diffusion dominated at high water saturations (i.e., full saturation and residual oil saturation), with the shale samples being slightly exclusive, consistent with previous studies. On the other hand, the measurements at low water saturations (i.e., irreducible water saturation) display a larger contribution from the exclusion potential. We also observe that the ion exclusion is dependent on the charge at the oil-brine interface, where a positively charged interface leads to positive ions exclusion and vice versa. These results have not been observed in previous ED potential studies. Our results highlight the importance of using realistic rock, water compositions and crude oil in experimental measurements and show that using experimentally-determined trends obtained from quartz-dominated samples saturated with simple NaCl brines can induce significant errors in the data interpretation process. To numerically assess the technology, we develop an SP solver that can simulate SP in subsurface reservoirs. Using that solver and the results from the experimental work, we simulate the SP in Burgan Upper Sand and test the effect of several production and reservoir parameters. The modelling results suggest that: (i) the magnitude of the SP signals are extremely low due to the high reservoir permeability in the clean sand channels; and (ii) this technology fails to locate SP anomalies in multi-layered reservoirs, caused by the high SP arising in the fastest front, which highly dissipates in the high conductivity bounding shale layers and masks any SP in other approaching fronts. Therefore, the feasibility assessment does not encourage proceeding to field implementation for Burgan Upper Sand and suggests that this technology can be more promising in lower permeability reservoirs.