Spontaneous potential (SP) is commonly measured during reservoir characterization. SP signals are also generated during hydrocarbon production due to the streaming potential occurrence. Measurement of SP could be used to detect and monitor water encroaching on a well. SP signals could also be monitored during production, with pressure support provided by water alternate gas (WAG) process. The objectives of this study are to quantify the magnitude of the SP signal during production by WAG injection and to investigate the possibility of using SP measurements to monitor the sweep efficiency.
Measurement of streaming potential has been previously proposed to detect the water encroachment towards a production well. The peak of the signal corresponds to the waterfront where there is a change of saturation from ionic water to non-polar hydrocarbon. Similar trend is predicted in the case of WAG where we have several interfaces between the injected water and the injected gas.
The results indicate that the magnitude of the SP generated in hydrocarbon reservoir during WAG process can be large and peaks at the location of the moving water front. Gas, which can be assumed to be non-polar, exhibit no electrokinetic effect. These observations suggest that WAG displacement process can be monitored indirectly from the signal acquired. Water or gas override can be detected and controlled if wells were equipped with inflow-control valves. As a conclusion, the SP measurement is a promising method to monitor the effectiveness of a WAG process.
This study is significant because monitoring the progress of water and gas in a WAG process is key in the effectiveness of this enhanced oil recovery method. Measurement of the streaming potential provides another method besides using tracers to monitor the WAG profile. Better monitoring will lead to more efficient displacement and great benefits in term of economy and environment.