Abstract

Patterns of microearthquakes detected downhole defined fracture orientation and extent in the Austin chalk, Giddings field, TX and the 76 field, Clinton Co., KY. We collected over 480 and 770 microearthquakes during hydraulic stimulation at two sites in the Austin chalk, and over 3200 during primary production in Clinton Co. Data were of high enough quality that 20%, 31% and 53% of the events could be located, respectively. Reflected waves constrained microearthquakes to the stimulated depths at the base of the Austin chalk. In plan view, microearthquakes defined elongate fracture zones extending from the stimulation wells parallel to the regional fracture trend. However, widths of the stimulated zones differed by a factor of five between the two Austin chalk sites, indicating a large difference in the population of ancillary fractures. Post-stimulation production was much higher from the wider zone. At Clinton Co., microearthquakes defined low-angle, reverse-fault fracture zones above and below a producing zone. Associations with depleted production intervals indicated the mapped fractures had been previously drained. Drilling showed that the fractures currently contain brine. The seismic behavior was consistent with poroelastic models that predicted slight increases in compressive stress above and below the drained volume.

Introduction

Microearthquakes often accompany reservoir stimulation and production. By collecting high-quality seismic data, the microearthquakes can be mapped, yielding potentially extensive and high-resolution information about the fracture system. Fracture maps can be used to plan infill and horizontal drilling, and to design and evaluate hydraulic stimulation and enhanced recovery, production operations in fracture-dominated oil and gas reservoirs.

Borehole geophones at reservoir depths provide the high-quality data needed to determine microearthquake location patterns. But when special observation wells must be drilled, microseismic studies can be expensive. To demonstrate that high-quality data can be collected inexpensively, we deployed geophones in existing wells and developed techniques for analyzing data from the resulting, sparse array of instruments. We hope the demonstration of inexpensive and effective methods will result in the routine application of microearthquake techniques to study reservoir fracture systems.

Methods currently applied to study fracture systems include tilt-meter surveys that give gross fracture characteristics, and borehole optical, acoustic or resistivity (formation microscanner) surveys that give detailed information along the borehole. More specialized methods include shear shadowing, coring or mineback experiments and anisotropy from surface seismics. While less detailed than borehole surveys, less convenient than surface measurements such as tiltmeter or seismic, and less directly interpretable than coring studies, the microseismic technique provides a combination of resolution, coverage and economy that is difficult to surpass with other methods.

Downhole microseismic monitoring has been applied successfully to hydraulic-stimulation experiments in hot-dry-rock geothermal reservoirs at Fenton Hill, NM, the U.K., Japan and France. Tomography has been performed using these data indicating low-velocity process zones in the seismic region. Additional data processing defined planar features that represent individual joints that slipped. These experiments took place in hard, crystalline rock, through which elastic waves propagate efficiently.

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