James N. Albright

Acoustic Emissions as a Tool for Hydraulic Fracture Location: Experience at the Fenton Hill Hot Dry Rock Site

Microearthquakes with magnitudes between -6 and -2 have been observed in three successive massive injections of water at the hot dry rock geothermal energy development projects demonstration site at Fenton Hill, NM. The injections were part of a program to increase the heat transfer area of hydraulic fractures and to decrease the flow-through impedance between wells in the energy extraction system under construction. The microearthquakes were used in mapping the location of the extended hydraulic fractures. The large shear- to compressional-wave-amplitude ratio observed in the signals suggests that the microearthquakes result from shear failure, probably on preexisting planes of weakness that intersect or make up the main hydraulic system. Failure resulting from increased pore pressure is a likely cause for the microearthquakes. Knowledge of the failure criterion for the reservoir rocks permits calculation of the pore pressure increases necessary for failure. At Fenton Hill, it appears that pressures 2 Pa (0.00029 psi) above hydrostatic are necessary. For this hypothesis of microearthquake occurrence to hold, the effective reservoir permeability must be four orders of magnitude above that for the bulk rock, as would be the case if there were permeable joints or fractures in the reservoir before hydraulic stimulation.

Current status of seismic and borehole measurements for HDR/HWR development

Seismic and borehole measurements provide significant information about HDR/HWR reservoirs that is useful for reservoir development, reservoir characterization, and performance evaluation. Both techniques have been widely used during all HDR/HWR development projects. Seismic measurements have advanced from making passive surface measurements during hydraulic fracturing to making passive observations from multiple boreholes during all phases of HDR/HWR development, as well as active seismic measurements to probe regions of the reservoir deemed to be of interest. Seismic data provide information about reservoir extent, locations and orientations of significant fractures, and areas of thermal drawdown. Recent advances include the ability to examine structures within the seismically active zone using statistics-based techniques and methods such as seismic tomography. Seismic method is the only means to obtain direct information about reservoir characteristics away from boreholes. Borehole measurements provide high-resolution information about reservoir characteristics in the vicinity of the borehole. The ability to make borehole measurements has grown during the course of HDR/HWR development as high temperature tools have been developed. Temperature logging, televiewer logs, and electrical property measurements have been made and shown to provide useful information about locations of fractures intersecting wellbores, and regions where water leaves and enters injection and production wellbores, respectively.

Borehole testing of a micromachined silicon accelerometer

Side-by-side field tests of borehole seismic packages containing either micromachined analog servo accelerometers or conventional piezoelectric accelerometers currently used in borehole seismic applications, show important differences in their relative performance. Laboratory testing of the micromachine shows dramatically improved distortion and low frequency response compared to conventional geophones (Gannon et al., this volume).

Reservoir microseismicity at the Ekofisk oil field

A triaxial, downhole geophone was deployed within the Ekofisk oil reservoir for monitoring ambient microseismicity as a test to determine if microearthquake signals generated from discrete shear failure of the reservoir rock could be detected. The results of the test were positive. During 104 hours of monitoring, 572 discrete events were recorded which have been identified as shear-failure microearthquakes. Reservoir microseismicity was detected at large distances (1000 m) from the monitor borehole and at rates (> 5 events per hour) which may allow practical characterization of the reservoir rock and overburden deformation induced by reservoir pressure changes.

A hypervelocity projectile launcher for well perforation

Abstract Current oil well perforation techniques use low- to medium-velocity gun launchers for completing wells in soft rock. Shaped-charge jets are normally used in harder, more competent rock. A launcher for a hypervelocity projectile to be used in well perforation applications has been designed. This launcher will provide an alternative technique to be used when the conventional devices do not yield the maximum well perforation. It is an adaptation of the axial cavity in a high explosive (HE) annulus design, with the axial cavity being filled with a low density foam material. Two configurations were tested; both had an HE annulus filled with organic foam, one had a projectile. Comparison of the two shots was made. A time sequence of Image Intensifier Camera photographs and sequential, orthogonal flash x-ray radiographs provided information on the propagation of the foam fragments, the first shock wave disturbance, the projectile motion and deformation, and the direct shock wave transmission from the main HE charge. Perforation tests of both device configurations (with and without the pellet) into steel-jacketed sandstone cylinders were made. Static radiographs of the cavities in the sandstone showed similar cavities, however, the perforation of the steel cap was larger in response to the pellet. DYNA2D calculations were made to assist in the interpretation of the experimental records. The preliminary results show promise that a useful perforating tool can be developed. Plans for an extended experimental program are outlined.

Development and testing of a 0.5-inch microhole geophone package

Summary Tests of prototype horizontal and vertical microgeophones suitable for inclusion in 0.5-inch OD borehole geophone packages show sensitivities exceeding 20% of full-size borehole geophones. Borehole testing of conventional borehole geophones, microgeophones, and cemented-in geophones indicates qualitatively comparable performances for early arriving seismic energy from explosive shots in a neighboring well. Installation of access tubing for insertion of microgeophone packages in the annulus between production strings and casing in a pumped well was shown to be feasible. In the only case studied, microgeophones inserted in the access tubing appear to be well coupled and in a low noise environment.

Conversion of borehole Stoneley waves to channel waves in coal

Evidence for the mode conversion of borehole Stoneley waves to stratigraphically guided channel waves was discovered in data from a crosswell acoustic experiment conducted between wells penetrating thin coal strata located near Rifle, Colorado. Traveltime moveout observations show that borehole Stoneley waves, excited by a transmitter positioned at substantial distances in one well above and below a coal stratum at 2025 m depth, underwent partial conversion to a channel wave propagating away from the well through the coal. In an adjacent well the channel wave was detected at receiver locations within the coal, and borehole Stoneley waves, arising from a second partial conversion of channel waves, were detected at locations above and below the coal. The observed channel wave is inferred to be the third-higher Rayleigh mode based on comparison of the measured group velocity with theoretically derived dispersion curves. The identification of the mode conversion between borehole and stratigraphically guided waves is significant because coal penetrated by multiple wells may be detected without placing an acoustic transmitter or receiver within the waveguide. 13 refs., 6 figs., 1 tab.

Retrievable 1/2=inch OD borehole seismic package

Initial laboratory and field tests show that a miniaturized borehole seismic package can approach the performance of its larger-sized predecessors. The package is designed so that it can either be used singly or as a unit in a multi-level array deployed in a production tubing annulus or small diameter well. Interchangeable subassemblies of vertical geophones and 3-component accelerometers enable quick field substitution. A 0.3-inch diameter geophone was developed to meet the size limitations imposed by space requirements for feed through of conductors and pressure protection. The package is deployed using light weight, small-diameter multi-conductor cable.