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Repeatability Evaluation of the Time-Lapse Technology Using Ultra-Stable Seismic Source

Abstract

The seismic time-lapse technology to monitor the temporal changes of subsurface has been an essential technique for understanding structural changes in the subsurface in EOR works and shale gas production. We have used Accurately Controlled and Routinely Operated Signal System (ACROSS) with an ultra-stable seismic source to monitor the change continuously. To discuss the true temporal change of subsurface, it is necessary to evaluate the repeatability of the measurement system. Kragh and Christie (2002) proposed the normalized root mean square (NRMS) repeatability of the time-lapse measurements and they obtained 18–30% for the study in the Gulf of Mexico. Eiken et al. (2003) obtained approximately 40% NRMS. In order to clarify the repeatability of our time-lapse study using ACROSS, we examined the data obtained by 31 seismic stations in Al Wasse field, Saudi Arabia in 2012–2013 and 2015. In the source-gather seismic records using the ACROSS seismic source, the first arrivals decayed at the offset distance further than 700 m. It is thought to be caused by the presence of velocity inversion just below the top layer. We calculated NRMSs of the first arrivals at the stations within 700 m from the source. We found that the NRMS repeatability in the ACROSS source room was less than 2% for 12 days periods. The NRMS variations of two stations among 31 ones were smaller than 5% during two months in 2012–2013 and 2015. Considering the NRMS at the source room, the NRMS of source itself is small (~less than 2%) and the environmental changes such as water pumping and temperature change affect the NRMSs of the field stations. The NRMSs of the other stations show very large variation. It is probably not due to the source signature change, because the geophones were located at the ground surface and the test field has more than 64 water-pumping stations. The water-pumping stations pump up water from aquifer at ~400 m depth and it affected the seismic records. The presence of aquifer and weak P first arrivals could degrade the NRMS estimation. The NRMSs obtained by the source-room geophone and two field stations showed the greatest repeatability of the measurement system with ACROSS for at least two months. The excellent repeatability of our seismic monitoring technology could accelerate the use of the time-lapse technique in the unconventional oil and gas exploration and productions. The authors greatly appreciate the aggressive assistances of JCCP officers for this project.