Applying water alternating gas (WAG) process as an enhanced oil recovery method became a well-known strategy in the industry. In a typical WAG process, either water or gas initially is injected for a designed injection cycle followed by another injection cycle for the other phase. Both cycles are repeated sequentially aiming to optimize the overall sweep efficiency. In this paper, we will explore the benefits of using cross-well EM to monitor the water and CO2 injection in WAG applications and address particularly the following two issues: 1) does cross-well EM has the required sensitivity to evaluate the performance of water and CO2 injection? 2) Cancross-well EM monitor the different cycles of a typical WAG application?

Cross-well EM involves applying inductive physics and 2D/3D inversion to define the inter-well conductivity distribution. Due to its unique spatial configuration, both transmitters and receivers are close to the intended survey targets, the cross-well EM is a proper tool for EOR and time lapse studies. CO2 normally has very low conductivity, which is not a good target for induction method. In a WAG project, both water and CO2 are injected into a reservoir, each has its own cycle (typically three to six months). The water used in WAG normally has much higher conductivity than that of CO2, therefore, by alternating the injection cycle the resulting conductivity change could be large enough to be a good target for cross-well EM.

The model used for simulation consists of injection, production and monitor wells. CO2 and water are injected sequentially, each with three-month cycle. The formation porosity and permeability are selected based on real field results. The water and CO2 floods are simulated using dynamic reservoir simulation software. The water and CO2 injection cycles were continued for 5 years to provide opportunity for time-lapse surveys. The formation resistivity variations due to water and CO2 were calculated using Archie's law. The cross-well EM fields were calculated between injection and monitor wells before and after water/CO2 injection cycles to study the sensitivity. Data inversions were performed to understand how well we could reconstruct the water and CO2 fronts.

The preliminary studies suggested that the cross-well EM is an appropriate tool for tracking CO2 and water flood for the models tested and the technique has an excellent ability to define the reservoir volume being affected by the injected fluid. The modeling results positively answer the sensitivity and efficiency questions for the use of cross-well EM as a surveillance tool for a WAG application.

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