Abstract

Laboratory measurements on tar-sand cores saturated with heavy oil show that seismic wave velocity decreases as the temperature increases. We use this physical effect to monitor the progress of a steam flood operation in the Midway Sunset Field in California. Results of seismic wave velocity tomograms acquired both before and after steam flooding show quite dramatically how temperature affects the seismic wave velocity. The heated heavy oil in the reservoir is delineated, and the information can be used by the production engineer for evaluating the progress of the EOR method.

Introduction

Many authors have reported using seismic cross-well tomography for a broad range of reservoir characterization projects. For example, Justice et al. (1989) uses it for projects. For example, Justice et al. (1989) uses it for monitoring steam flooding, Harris et al. (1990) uses it for characterizing reservoir inhomogeneities, and Paulsson et al. (1990) and Lo et al. (1990) use it for imaging a complex fault system in a tar sand reservoir.

Seismic cross-well tomography can provide useful information to reservoir engineers and geologists because it measures reservoir rock velocity (the speed at which seismic waves travel in reservoir rock) in situ, and reservoir rock velocity is in turn related to the reservoir's lithology, fluid saturation, temperature, porosity, permeability, etc.

A typical seismic cross-well tomography survey is illustrated in Figure 1. This figure shows two wells flanking the area under investigation. Solid and open circles represent seismic wave transmitter and detector locations, respectively. Seismic waves are generated at various depths along one well and the target area's response to the probing seismic waves are measured with detectors deployed at probing seismic waves are measured with detectors deployed at various depths along the other well. Thus, the area under investigation is covered by a network of seismic rays and is probed by seismic waves in many different directions. probed by seismic waves in many different directions. Cross-well seismic data collected this way are then analyzed by tomographic inversion techniques such as SIRT (Simultaneous Iterative Reconstruction Technique), which generates a velocity tomogram of the area of interest. Velocity tomograms show a two-dimensional spatial distribution of the reservoir rock velocity, the speed seismic wave travels in reservoir rock. As mentioned earlier, reservoir rock velocity is a function of many reservoir rock properties. Once a velocity tomogram is constructed, then with some simplifying assumptions we can use it to infer two. dimensional distributions of many reservoir rock properties.

Cross-well tomography can also be of benefit for reservoir geology interpretation. Figure 2a is a cartoon representing the true geology between two wells in a producing field. In this cartoon, the producing formation is a tar sand layer overlain by a thinner, less permeable bed. The tar sand is represented by the light shade and the less permeable layer is represented by the dark shade. permeable layer is represented by the dark shade. P. 87

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