Experimental Verification of Acoustic Waveform and VSP Seismic Tube Wave Measurements of Fracture Permeability

Abstract

A variety of established and experimental geophysical techniques was used to measure the vertical distribution of fracture permeability in a 229-meter deep borehole penetrating schist and quartz monzonite near Mirror Lake, New Hampshire. The distribution of fractures in the borehole was determined by acoustic borehole televiewer and other geophysical logs. Fracture permeability was estimated by application of two experimental methods: (1) Analysis of tube-wave-amplitude attenuation in acoustic full-waveform logs; and (2) interpretation of tube waves generated in vertical seismic profiles. Independent information on fracture permeability was obtained by means of packer-isolation flow tests and flowmeter measurement of vertical velocity distributions during pumping in the same borehole. Both experimental methods and packer-isolation-flow tests and flowmeter data indicated a single, near horizontal zone of permeability intersecting the borehole at a depth of about 45 meters. Smaller values of transmissivity were indicated for other fractures at deeper depths, with details of fracture response related to the apparent volume of rock represented by the individual measurements. Tube-wave amplitude attenuation in full-waveform acoustic logs, packer-isolation flow tests, and flowmeter measurements during pumping indicated transmissivity values for the upper permeability zone within the range of 0.6 to 10.0 centimeters squared per second. Vertical seismic-profile data indicated a relative distribution of fracture permeability in agreement with the other methods; however, the calculated values of transmissivity appeared to be too small. This disagreement is attributed to oversimplification of the model for fracture-zone compressibility used in the analysis of vertical seismic-profile data.