Evaluating the use of seismic methods to map the extent of an in-situ lignite-gasification cavity

Abstract

Seismic reflection data were collected in 1981 at the In-Situ Lignite-Gasification site located near Rockdale, Texas managed by the Petroleum Engineering Department at Texas A&M University. The three zero-offset-VSP data sets recorded at the site were used to derive a P-wave velocity profile. The data from 10 offset sources recorded in one of the VSP wells were used to map the shape of the interpreted fracture chimney resulting from the break-up of the strata over the gasification zone. P-to-S converted waves gave the most information about the geometry of the fracture chimney. The S-wave velocity profile and geometry of the fracture chimney were determined by iterative ray tracing. Traveltimes of the P-to-S events in the model data were matched to those in the field data to within ± 10 milliseconds. The traveltime errors seemed to have been more sensitive to the S-wave velocity profile than to changes in the geometry of the fracture chimney. The useable bandwidth for the two surface lines was increased from 30 Hertz to 130 Hertz by applying spectrum equalization and deconvolution processes to the data. The effects in the surface data, resulting from the cavity and/or fracture chimney, were not obvious, but could be interpreted. For future applications, the sources and receivers should be spaced at a maximum of ten percent of the expected size of the gasification cavity. Also, sufficient areal extent to image the gasification must be obtained. The fact that converted P-to-S wave energy was the most dominant energy in the VSP data dictates the need to acquire orthogonal, three-component data (one vertical, and two horizontal components). Zero-offset. P-wave and S-wave VSP data need to be acquired to accurately determine the velocity profiles for both wave types at the site. Given accurate velocity profiles, the VSP data can be used to determine the effects of gasification in the subsurface by matching traveltimes between field and model data via iteration on the geometry of the cavity using a ray-tracing procedure.