Time-Lapse Monitoring of SAGD Reservoirs, with Superconducting Gravimetry

Abstract:

Monitoring steam-assisted gravity drainage (SAGD) projects is necessary to assess fluid flow during production and determine bitumen recoverability, and can be constrained by measuring gravity and gravity gradient signatures. There is currently a void in monitoring fluid migration during SAGD operations, which is limited to expensive 3D seismic surveys, and sparse borehole surveys. The GWR iGrav portable superconducting instrument, with sub-microGal sensitivity, has the ability to measure time-lapse gravity and gravity gradient with high spatial focus.

The objective of this study is two-fold. The first part of the study is to develop geological models for a SAGD project, and forward model specific gravity signatures (g_z, g_zz, g_zx, g_zy) related to fluid migration during production. The sedimentary structures and background density models will be integrated with time-lapse gravity and gravity gradient measurements, to isolate fluid migration patterns. As the steam chamber expands in those forward models, it alters the density distribution of the sedimentary layers, and the gravity responses are calculated/measured. Inversions will be performed on the gravity observations to assess density changes incorporating knowledge of sedimentary features. The second part of the study is to execute sensitivity studies to evaluate if a single or two relative gravimeters can achieve the required sub-microgal resolution considering the noise conditions at the site, and to develop an optimal survey strategy for a pair or superconducting gravimeters.

The feasibility of applying the proposed technique to SAGD reservoirs was hindered by the lack of the required sensitivity, and high noise levels. These limitations can be overcome using a pair of superconducting gravimeters in time-lapse gravity gradient surveys. The ultimate goal is to monitor spatial and temporal fluid migration at all stages of reservoir development, to improve recovery of bitumen and detect any out of zone migration.