FRACTURE DETECTION USING CROSSWELL AND SINGLE WELL SURVEYS

We recorded high-resolution (1 to 10 kHz), crosswell and single well s eismic data in a shallow (15 to 35 m), water-saturated, fractured lime stone sequence at Conoco's borehole test facility near Newkirk, Oklaho ma. Our objective was to develop seismic methodologies for imaging gas -filled fractures in naturally fractured gas reservoirs. The crosswell (1/4 m receiver spacing, 50 to 100 m well separation) surveys used a piezoelectric source and hydrophones before, during, and after an air injection that we designed to displace water from a fracture zone. Our intent was to increase the visibility of the fracture zone to seismic imaging and to confirm previous hydrologic data that indicated a pref erred pathway. For the single well seismic imaging (a piezoelectric so urce and an eight-element hydrophone array at 1/4 m spacing), we also recorded data before and after the air injection. The crosswell result s indicate that the air did follow a preferred pathway that was predic ted by hydrologic modeling. In addition, the single well seismic imagi ng using vertical common depth-point (CDP) gathers indicated an anomal y consistent with the anomaly location of crosswell and hydrologic inv ersion results. Following the field tests, a slant well was drilled an d cored to confirm the existence and nature of the rock associated wit h the seismic anomalies. A vertical fracture was intersected within le ss than 1 m of where the seismic results had predicted.