Modeling the resolution of inexpensive, novel non-seismic geophysical monitoring tools to monitor CO2 injection into coal beds

A sensitivity study of gravity and electromagnetic (EM) techniques, and amplitude vs. angle (AVA) analysis for CO{sub 2} movement in coal beds was based on the SECARB pilot test planned in the Black Warrior basin in Alabama. In the area of interest, coalbed methane is produced mainly from the Black Creek, Mary Lee, and Pratt coal zones at depths between 400 and 700 m and approximately 3 m thick on average. The permeability of coal in the Black Warrior basin decreases exponentially with depth as overburden stress increases. The permeability of the top layer is 100 mD, while the permeability of the deepest layer is around 1 mD. The pilot field test will include injecting a total of 1000 tons of CO{sub 2} into these three coal zones ({approx}300 tons to each zone). The density, sonic and resistivity well-logs from a deep disposal well a couple of miles from the pilot test site were used to create background (pre-injection) models. Our laboratory measurements of seismic velocity and electrical resistivity as a function of CO{sub 2} saturation on coal core samples were used to provide a link between the coalbed CO{sub 2} flow simulation models and the geophysical models. The sensitivity studies showed that while the response to the 300 tons of CO{sub 2} injected into a single layer wouldn't produce measurable surface response for either gravity or EM, the response due to an industrial-size injection would produce measurable surface signal for both techniques. Gravity inversion results illustrated that, provided we can collect high-quality gravity data in the field and we have some a priori information about the depth of the reservoir, we can recover the spatial location of CO{sub 2} plume correctly, although with the smoothing constraint of the inversion, the area was slightly overestimated, resulting in an underestimated value of density change. AVA analysis showed that by inverting seismic and EM data jointly, much better estimates of CO{sub 2} saturation can be obtained, especially in the third injection zone, where seismic AVA data fail to detect the high CO{sub 2} saturation. Analysis of spatial resolution and detectability limits show that gravity and EM measurements could, under certain circumstances, be used as a lower-cost alternative to seismic measurements