Crosswell seismic imaging for reservoir management

Local variations of reservoir thickness and quality across oil and gas fields cause a significant portion of hydrocarbon reserves to be left in place in the inter-well reservoir volume. It is estimated that about 500 Bbo and 450 Tcf of gas remain trapped in existing reservoirs in the U.S. Production of this in-field reserve requires accurate information on reservoir geometry and flow unit properties in the inter-well space. This article will explain how to obtain the necessary information to exploit these interwell reserves.

Reservoir management data requirements. The need for reliable reservoir information is not limited to re-exploration and development of untapped in-field reserves. Prudent gas and oil reservoir management, efficient secondary recovery process control and reliable reserve estimation all require highly precise and accurate information at various stages in the life of a reservoir. Duration of these stages varies from a few months, in the case of aquifer gas storage fields, to several years for large producing structures.

To date, tools available to reservoir engineers and reservoir geologists for resolving reservoir properties have been seismic images and well log data. However, despite new developments in these technology areas, a fundamental resolution-matching problem has limited the synergy of these two approaches. The problem is that formation evaluation tools have high resolution but limited depth of investigation, while seismic is a deep investigating tool with low resolving power relative to inter-well reservoir definition requirements. For example, a modern well log can provide reservoir information for every inch of the wellbore but cannot "see" for more than a few feet into the reservoir.

Conversely, surface seismic can see for hundreds of feet but cannot resolve better than 10 ft in depth or thickness, Fig. 1. In short, there is an information void exactly at the scale required for interwell reservoir mapping and characterization. The data needed for optimal field development, secondary recovery and reservoir management falls exactly within this information gap.

The technology development. Research organizations, academic institutes, major producers and service companies have been working on this problem for a long time. Meeting this industry need has also been a main R & D objective of the Gas Research Institute (GRI) since the late 1980s. Specifically, GRI's strategy has been to focus on development of a borehole seismic system that can provide high definition imaging with resolution better than five feet and a depth of investigation that reaches inter-well distances of up to one mile (640-acre spacing). In pursuing this strategy, GRI initiated a project aimed at developing a crosswell tomography system utilizing a piezoelectric downhole source and a variety of receiver elements. The technology has achieved proof of concept in numerous actual field surveys and is presently offered by TomoSeis as a commercial service.

Crosswell seismic technology uses one well as the source well and one or more wells as receiver well(s), as shown in Fig. 2. Seismic energy emanating from the source reaches a number of receiver elements in the receiver well(s). In field operations, receiver and source arrays are placed at predetermined positions, the source is activated and the transmitted seismic waves are recorded.

This process is repeated with various source/receiver configurations, enabling the same volume of reservoir rock to be covered by different paths of sound waves. In this fashion, the data can be processed similar to tomographic imaging in the medical field whereby differences in transmission properties of materials, e.g., flesh and bone, are used to "map" inner boundaries of different materials. Obviously, in the case of crosswell tomography, the objective is to map boundaries of heterogeneous volumes, e.g., channel fills, sand lenses, fault planes, etc., in the inter-well space. Fig. 3 shows a crosswell tomogram depicting an interwell reservoir discontinuity resulting from a fault displacement.

Realizing that rock properties governing transmission and reflection of seismic waves are influenced by porosity, pore fluid and pressure; crosswell tomograms can be used not only for delineation of changes in inter-well reservoir geometry, but also as an indicator of reservoir content, i.e., gas, oil or water. In view of the high resolving power and extended depth of investigation of crosswell seismic imaging, this technology provides an effective tool for delineation and characterization of inter-well reservoir geometry and properties at nearly well log scale. As such, the technology can play a major role in locating sweet spots, avoiding poor producers and dry holes, optimizing placement of horizontal and extended reach wells and finding bypassed pay zones within existing wells.

Additional uses. Crosswell seismic imaging can also be used as an effective tool for reservoir management, particularly for monitoring gas cap expansions and water encroachment. Time-lapse maps of reservoir fluid boundaries will provide reservoir engineers with snap shots of reservoir conditions on which they can base strategies to optimize production, avoiding damage due to water influx or gas cap expansion. It is obvious that while the tool can play a significant role in monitoring of pressure maintenance and secondary recovery operations, its application expands to production management in water-drive gas bearing reservoirs as well as aquifer gas storage fields.

In the area of secondary oil recovery, crosswell seismic imaging is capable of providing an accurate image of the C[O.sub.2] front. In this case, C[O.sub.2] injection causes significant changes in seismic properties of the reservoir volume due to increased pressure and changes in elastic properties of the host reservoir rock. Using time lapsed images of the C[O.sub.2] front, and detailed reservoir geometry, engineers can project future position and behavior of the front and modify field operation parameters for high efficiency sweep while preventing undesired breakthrough.

For years, reservoir engineers and development geologists have had to contend with measurements that did not image, at proper scale, important variations in reservoir properties. With crosswell seismic imaging, they now have a tool to bridge the gap between detailed core and well log data, and large-scale seismic images.

COPYRIGHT 1998 Gulf Publishing Co.
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