The Multiwell Experiment in the Piceance Basin, Colorado: Reprise from 30 Years Ago
A review of the tight-gas experiments from thirty years ago illustrates the effort required to arrive at today's understandings and successful technologies. In the late 1970's the US government decided that given the recent oil embargo, America might someday need the natural-gas resources of the low-permeability Rocky Mountain sandstone reservoirs. However, it was unclear why, even when stimulated with Massive Hydraulic Fractures and nuclear blasts, gas did not flow readily from these formations. After several failed engineering-only tests, the government decided on a radical approach: characterize and understand the reservoirs before applying technology. The Multiwell Experiment (MWX), funded by the US DOE, was an applied-research field test located in the Rulison field. At the time there were 12 wells in the field, with 640 acre spacing. Borehole-image logs did not exist, few cores were taken, and the available fracture-identification logs turned out to be remarkably unreliable. The reservoirs were not thought of as being fractured, and the few cored fractures were considered to be anomalies. One revelation of the MWX project, derived from characterization of 4500 ft of core from three vertical and a deviated well, was that fractures are pervasive, with average spacings of a few feet. Engineering tests could then document the previously unrecognized contribution of fractures in converting microdarcy matrix rock into reservoir systems with highly anisotropic, milidarcy-scale permeabilities. Pre- and post-stimulation production tests showed that the standard completions of the day were actually damaging the fracture-permeability system, which was also stress-sensitive. MWX data showed that pore pressures and the in situ stresses had to be accommodated during drilling, completion, and production. $40 million were spent to reach these conclusions over a span of 10 years in this multidisciplinary, multi-organizational effort. Core from the MWX program is now stored in the USGS CRC in Denver, and much of the data set is accessible on the Internet.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012