Case Histories (1997)

An experiment testing in-situ combustion with oxygen as an enhanced oil recovery process was run at the Holt Sand Unit in Montague County, Texas, from 1981 to 1983. This report integrates the interpretation of geological and geophysical data collected by the Exploration and Production Research Department of ARCO Oil and Gas Company during the experiment. The report illustrates monitoring of the burn in time using 3-D seismic data and describes the factors in the detailed geology of the reservoir which controlled the propagation of the burn. The most important feature of a reservoir involved in a flooding type recovery program is formation permeability. The Holt Sandstone was found to contain a complex array of vertical and horizontal barriers and enhancements to permeability. The reservoir complexity required that a proper description of permeability include not only measured permeabilities from core samples, but also a detailed study of the depositional history and post-depositional alteration. The original permeability of the Holt, which was controlled by depositional processes, has been overprinted by natural flushing, secondary cementation, and fracturing. A geologic model of reservoir-scale permeability is shown to be useful in interpreting the details of the combustion process. Study of pre-burn and post-burn core samples by thin-section description and rock property measurements showed that the high temperatures of the thermal recovery process significantly altered the character of the reservoir rock, lowered its bulk density, and decreased its acoustic velocity. The change in velocity was especially significant since it was much greater than could be explained by the replacement of pore fluid with gas. Three sets of 3-D seismic data were collected for comparison of seismic characteristics of pre-burn, mid-burn, and post-burn signals reflected from the Holt Sandstone and the Palo Pinto Limestone below it. The seismic data show that the burn propagated predominantly in a southwestern direction. The oxygen injection was maintained for nine months, but the comparison of the mid-burn to post-burn seismic data indicates that the majority of the burned zones were generated during the first three months of combustion. A seismic dim spot anomaly observed in the data was interpreted as indicative of the burned zone. A map view of this anomaly compared to the contour map of net burn thickness measured in post-burn cores correlates quite well in the shape and region of the burned zone. However, the seismic anomaly predicts locations of maximum burn thickness further away from the injection well than the core data suggests. If the amplitude of the seismic anomaly is interpreted in relative terms of no burn, burn, and maximum burn, the seismic data maps the distribution of the burned zone in a more continuous manner than the sparse population of core data. On the other hand, the precise details of vertical distribution and measured thicknesses of burned zones can only be retrieved from core data. The experiment shows that simple geologic models can be used to predict reservoir scale permeability and reflection seismic data can be used to monitor in-situ combustion in future enhanced oil recovery projects.