A carbon dioxide flood pilot is being conducted in a section of Chevron's McElroy field in Crane County, west Texas. Prior to CO 2 injection, two high-frequency crosswell seismic profiles were recorded to investigate the use of seismic profiling for high-resolution reservoir delineation and CO 2 monitoring. These preinjection profiles provide the baseline for timelapse monitoring. Profile #1 was recorded between an injector well and an offset observation well at a nominal well-to-well distance of 184 ft (56 m). Profile #2 was recorded between a producing well and the observation well at a nominal distance of 600 ft (183 m). The combination of traveltime tomography and stacked CDP reflection amplitudes demonstrates how highfrequency crosswell seismic data can be used to image both large and small scale heterogeneity between wells: Transmission traveltime tomography is used to image the large scale velocity variations; CDP reflection imaging is then used to image smaller scale impedance heterogeneities. The resolution capability of crosswell data is clearly illustrated by an image of the Grayburg-San Andres angular unconformity, seen in both the P-wave and S-wave velocity tomograms and the reflection images. In addition to the imaging study, cores from an observation well were analyzed to support interpretation of the crosswell images and assess the feasibility of monitoring changes in CO 2 saturation. The results of this integrated study demonstrate (1) the use of crosswell seismic profiling to produce a high-resolution reservoir delineation and (2) the possibility for successful monitoring of CO 2 in carbonate reservoirs. The crosswell data were acquired with a piezoelectric source and a multilevel hydrophone array. Both profiles, nearly 80 000 seismic traces, were recorded in approximately 80 hours using a new acquisition technique of shooting on-the-fly. This paper presents the overall project summary and interpretation of the results from the near-offset profile.
When square pixels are used to parameterize the slowness field in traveltime tomography, the problems of discretization for inversion purposes and discretization for display purposes are inextricably mixed. A "high resolution" result demands many pixels or model parameters, thus burdening tomography with the inversion of a large and sparse projection matrix though simplifying the display problem by using regions of constant slowness. In this paper, we present a method of separating the tomography problem into two distinct steps -first inversion, then imaging. To reduce the complexity of the inversion step, we select a more natural set of pixels which are derived from the raypaths used to model the process of creating the traveltime data. The support of the string function is the raypath itself, thus the strings and raypaths are orthogonal (except when both equal the same line) and the projection matrix is diagonal and invertable in closed form. We are then left to image, i.e., synthesize the inversion result for display purposes. The method is tested on cross-well synthetic and field data requiring iterative curved raytracing. It is demonstrated to be a fast and robust technique of traveltime tomography.
Reliable crosswell reflection imaging is a challenging task, even after the data have been wavefield‐separated in the time domain. Residual, strong coherent noise is still present in the data. Stacking is complicated by the wide range of reflection incidence angles available for imaging. With wavelengths of a few feet, small misalignments as a result of velocity or geometric errors produce destructive interference and degrade the quality of the stacked image. We present an imaging sequence that addressed these complications and allowed us to produce high‐quality stacked images for both P‐ and S‐waves from a large‐volume crosswell data set. A very good tie was achieved at both wells. Heterogeneities imaged from well to well included very thin beds [less than 5 ft (1.5 m) thick] within the reservoir, pinchouts, and a major angular unconformity—the Grayburg/San Andres—that could not be observed reliably with any other technique (log correlation, surface seismic imaging, or tomography). In fact, the produced crosswell reflection images exhibit dramatically higher resolution and continuity than the P‐wave traveltime tomogram.
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