CO2 Leak Detection and Conformance Verification Using Borehole Gravity

Abstract

Abstract Borehole gravity measured from a vertical Carbon Capture and Storage (CCS) injection well, or nearby observation well, provides a cost-efficient solution for monitoring the reservoir for CO2 flow and the overburden for leaks. In this paper, two CO2 injection reservoir models using piston flow and Buckley-Leveret theory are built to simulate and evaluate surface and borehole gravity signatures associated with CO2 flow through thin and thick reservoirs in deep and shallow scenarios. The sensitivity of gravity to plume parameters is also analyzed. The results demonstrate that the lateral gradient in vertical surface gravity after a year of injection is effective for monitoring shallow reservoirs over relatively long time periods and ineffective for deep reservoirs. However, borehole gravity after only a few months of injection is sensitive to both time-lapse density variations and the lateral extent of the saturation plume in all reservoir thickness and depth scenarios. A competition exists for the dominant parameter controlling the borehole gravity signal between density contrast and plume radius. At the bottom of the borehole above the reservoir, sensitivity is dominated by density contrast. As one ascends the borehole, the distance to the reservoir below increases, which lowers the gravity while increasingly adding contributions to the vertical component of gravity, resulting in the plume radius becoming the most sensitive parameter. It is shown that this enhanced sensitivity enables inversions of borehole gravity, using weighted damped least squares, to sufficiently image the radial variation in density contrast associated with the CO2 plume. Analysis of survey parameters on inversion accuracy shows the benefit of conducting surveys that penetrate the reservoir. The results also show that while surface gravity is not capable of seeing small leaks, borehole gravity can detect the depth of leaks within hundreds of feet from a wellbore by a distinctive cross over along the gravity profile. The estimated plume radius based on a reservoir filling model and the comparison of that predicted smooth gravity profile to the observed profile that is sensitive to leak cross-overs in the confining zone provides a cost-effective measurement that can trigger higher cost surveys for deeper levels of investigation.