PERMEABILITY REDUCTION BY PYROBITUMEN, MINERALIZATION, AND STRESS ALONG LARGE NATURAL FRACTURES IN SANDSTONES AT 18,300-FT DEPTH - DESTRUCTION OF A RESERVOIR

Gas production from the Frontier formation at 18,300-ft depth in the F rewen Deep #4 well, eastern Green River basin (Wyoming), was uneconomi c despite the presence of three sets of numerous, partially open, vert ical natural fractures. Production dropped from 360 Mcf/D to 140 Mcf/D during a 10-day production test, and the well was abandoned. Examinat ion of the fractures in the core suggests several possible reasons for this poor production. One factor is the presence of mineralization in the fractures. Another more important factor is that the remnant poro sity left in the fractures by partial mineralization is commonly plugg ed with an overmature hydrocarbon residue (pyrobitumen). Reorientation of the in-situ horizontal compressive stress to a trend normal to the main fractures, which now acts to close fracture apertures during res ervoir drawdown, is also an important factor.