ABSTRACT
Results of a highly-instrumented multiple-well field test to measure pressure responses during hydraulic stimulation of a naturally-fractured gas reservoir are presented. The field test consisted of a 70,000 gallon (265 m3) "foam frac" in one of four test wells while monitoring bottomhole pressures in each of the four wells and monitoring the site with an array of tiltmeters. A 75-quality sand-laden foam was injected into the naturally fractured formation at a rate of 20 bbl/min (3.2 m3/min). The injection rate was held nearly constant except for two brief interruptions. Sand concentration during the treatment ranged from zero to two and one-half pounds per gallon while average one pound per gallon. Bottomhole pressures were monitored in the fraced well and in three observation wells. Tiltmeters were installed at 12 surface locations encircling the stimulated well at distances of approximately 600 to 1000 feet (180 to 300 meters) to aid in determining the geometry of the induced fracture.
The tiltmeter and pressure responses were analyzed in conjunction with pre- and post-stimulation interference tests to obtain fracture geometry. Eight of the twelve tiltmeters gave clear and analyzable responses. Application of computer codes to the field data provided estimates of fracture geometry caused by the treatment. Bottomhole pressure responses at the three offset wells ranged from 15 to 40 psi (100 to 275 kPa), and occurred gradually over a period of several hours with occasional rapid increases to 1 to 2 psi (7 to 14 kPa) within a 15-second interval.
