Allan R. Sattler

Results of the multiwell experiment in situ stresses, natural fractures, and other geological controls on reservoirs

Hundreds of millions of cubic meters of natural gas are locked up in low-permeability, natural gas reservoirs. The Multiwell Experiment (MWX) was designed to characterize such reservoirs, typical of much of the western United States, and to assess and develop a technology for the production of this unconventional resource. Flow-rate tests of the MWX reservoirs indicate a system permeability that is several orders of magnitude higher than laboratory permeability measurements made on matrix-rock sandstones. This enhanced permeability is caused by natural fractures. The single set of fractures present in the reservoirs provides a significant permeability anisotropy that is aligned with the maximum in situ horizontal stress. Hydraulic fractures therefore form parallel to the natural fractures and are consequently an inefficient mechanism for stimulation. Successful stimulation may be possible by perturbing the local stress field with a large hydraulic fracture in one well so that a second hydraulic fracture in an offset well propagates transverse to the natural fracture permeability trend.

Case study of a stimulation experiment in a fluvial, tight-sandstone gas reservoir

This paper reports that a successful stimulation experiment was conducted in a fluvial sandstone of the Mesaverde formation at the U.S. DOEs Multiwell Experiment (MWX) Site in the Piceance basin of Colorado. The stimulation experiment consisted of stress tests, a three-well prefracture interference test, step-rate/flowback tests, a minifracture, a full stimulation treatment borehole geophone diagnostics during fracturing, and a postfracture interference test.