Advanced Borehole Seismology

Abstract

Abstract Single-Well Seismology (SWS), Reverse Vertical Seismic Profiling (RVSP) and Cross-Well Seismology (CWS) are three new borehole seismic techniques. These techniques will provide much higher resolution images of oil and gas reservoirs than can be obtained with surface seismic techniques. Borehole seismology involves inserting the source and/or the seismic receivers in oil or gas wells. In the past these methods have been limited to relatively short distances between source and receivers in wells less than 10,000 ft deep. The past limitation of borehole seismology was due to the available downhole seismic sources, which were all impulsive and fluid coupled. The impulsive sources put all the energy into the borehole over a few milliseconds generating large stresses on the fragile casing-cement interface. Some nondestructive impulsive sources, such as the piezo electric sources, suffer from low energy, narrow bandwidth and therefore limited transmission range. In February 1997 we deployed a new borehole seismic source and receiver system which was designed for long source-receiver spacing in deep wells. The advanced downhole source now has generated reflections with source to receiver raypath length of 16,000 feet. The source is also designed to operate to well depths of 20,000 feet at 400° F extending the capability of borehole seismology. This advanced borehole seismic data acquisition system consists of a powerful, clamped, swept-frequency, nondestructive, vibratory source; a multi-level receiver string of clamped, three-component geophones; and an acquisition system for Reverse VSP's. The downhole vibrator produces high quality P-wave direct arrivals and reflections as well as S-waves with a known polarization. The receivers used with the source operate using fiber optic technology for low noise and fast data transmission from the well to the surface. We anticipate that borehole seismology will soon be used in management of both new and mature reservoirs. There are two different classes of applications for the borehole seismic techniques. The first is monitoring and evaluation of primary production as well as improved or enhanced oil recovery efforts such as water, steam or C02 floods. Another application is reservoir characterization for improved location of in-fill drilling, or the completion of new wells. This technology will address many problems associated with reservoir definition such as the determination of the continuity of bedding, the direction of fractures, the determination of lithologies, mapping fluid saturation, and the mapping of fault planes. Initial tests of the vibrator were conducted at Chevron's borehole test site in La Habra, California. The data quality is excellent, supporting the concept behind the vibrator design. Despite the fact that the sediments at the test site are poorly consolidated, low-velocity, low-Q sands and silts, very broad band, high amplitude direct P and S waves were recorded over travel distances of 1,100 feet, and there are indications of reflections with travel paths up to 4,000 feet. This suggests that the vibrator can be used for deep reverse VSP's and crosswell surveys with large interwell distances. Subsequent commercial surveys also produced excellent data confirming the initial results obtained at the test site. Introduction The high cost of data acquisition and the limitations of borehole seismic sources have impeded the widespread use of borehole seismic techniques.