Time-Lapse Gravity and Gravity Gradient Reservoir Monitoring

Abstract

Fluid flow that leads to a density change in reservoirs can be tracked and monitored by measuring gravity and gravity gradient signatures. Practical targets include steam-assisted gravity drainage (SAGD) projects, tight-shale gas reservoirs, and water pumping from aquifers. There is currently a void in monitoring fluid migration, which is limited to 3D seismic surveys, and borehole surveys. The iGrav portable superconducting gravimeter, with sub-microGal sensitivity, has the ability to measure time-lapse gravity and gravity gradient with high spatial focus, and has the potential to be applied to multi-scale and diverse reservoirs. The focus of this study is two-fold, modeling and field implementation. The first part of the study is to develop geological models for projects that are undergoing SAGD production, water pumping, and gas extraction, and calculate specific gravity signatures related to fluid migration. The rock structures and background density models will be integrated with time-lapse gravity and gravity gradient measurements, to isolate fluid migration patterns. As the fluids are extracted in the reservoirs, they alter the density distribution of the layers, and the gravity responses can be measured. The forward models reveal the required sensitivity needed for the practical implementation of gravimetry. Inversions will be performed on the gravity observations to assess the resolvability of density change in the reservoirs, using auxiliary information on rock properties and sedimentology. The second part of the study is to implement time-lapse gravimetry for selected reservoirs. The objective is to execute sensitivity studies considering site-specific noise levels and optimal survey design, in order to assess if the required sub-microgal sensitivity can be reached. Initial results indicate that a pair of iGrav superconducting gravimeters meet the sensitivity requirements and the spatial focusing desired to monitor fluid migration at the reservoir scale.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016