Summary

During the past 5 years, we have applied high-resolution geophysical methods [crosswell seismic and electromagnetics (EM) and passive seismic] to map and characterize petroleum reservoirs in the San Joaquin Valley and to monitor changes during secondary-recovery operations. The two techniques provide complementary information. Seismic data reveal the reservoir structure, whereas EM measurements are more sensitive to the pore-fluid distribution. Seismic surveys at the South Belridge field were used to map fracture generation and monitor formation changes caused by the onset of steamflooding. Early results show possible sensitivity to changes in gas saturation caused by the steamflooding. Crosswell EM surveys were applied at a shallow pilot at Lost Hills for reservoir characterization and steamflood monitoring. Images made from baseline data clearly show the distribution of the target oil sands; repeated surveys during the steamflood allowed us to identify the boundaries of the steam chest and to predict breakthrough accurately. Applications of the EM techniques in steel-cased wells are at an early stage, but preliminary results at Lost Hills show sensitivity to formation resistivity in a waterflood pilot.

Introduction

Although large quantities of petroleum are produced through water and steamflooding, the process is typically poorly understood. This leads to inefficient recovery and associated production problems, such as premature water/steam breakthrough, fracturing of reservoir rock, and well failures. In a new effort to understand these fluid-displacement processes and associated reservoir changes, we are applied crosswell geophysical methods to monitor secondary-recovery processes.

The goal of this project is to use high-resolution geophysical methods jointly to map and characterize petroleum reservoirs during secondary-recovery operations. We view the introduction of steam- and waterfloods in petroleum reservoirs as natural tracers to map fluid flow and to define the reservoir structure. Efficient use of such tools can help determine flow mechanisms, map creation and destruction of fracture porosity, and track injected flow through natural channels that connect (and isolate) petroleum deposits. It is an ideal mechanism for detailed reservoir characterization; the reservoir is defined (and redefined) as it is produced.

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