Enhanced Recovery from a Tight Gas Sand through Hydraulic Refracturing:

Codell Formation, Wattenberg Field, Colorado

By

Thomas J. Birmingham, D. M. Lytle, and R. N. Sencenbaugh

Kerr McGee Rocky Mountain Corporation, Denver, CO

The Upper Cretaceous Codell Sandstone in the Denver Basin is a tight gas sand that produces hydrocarbons at depths from 4000-8200 ft. Analyses from thousands of wells have identified patterns of Codell properties that substantially aid in the trapping and production of oil and gas from a small pore throat network. Properties such as maximum porosity, permeability, grain density, pressure gradient, geothermal gradient, accessory mineral composition, gas-oil ratio (GOR), and estimated ultimate recovery (EUR), all show remarkable, unique overlap in common geographic areas of the Wattenberg Field. Trapping is enhanced by regional basement fault trends to the west and north, erosional pinchout of the Codell Sandstone to the south, southeast and northeast, and an important paleostructure, the “Wattenberg High”, to the south. Codell individual well production and well life are extended by use of hydraulic refracturing of the reservoir. Due to favorable stress contrasts in the overlying Ft. Hays Limestone, refracturing foces applied to pressure-depleted Codell producers become more focused and stay in zone, promoting longer lateral extension hydraulic fractures (“wings”) than the original fracturing treatments. Refracturing wings travel parallel to or within local natural fracture networks, which display both east-west and north-south orientations, and are area-specific. These natural fractures parallel two perpendicular principal horizontal stress directions. Occasionally, refracture wings directly communicate with neighboring wellbores, aided by the pre-existing structural weakness of the natural fracture networks. Although refracturing is practical in many different areas of the Wattenberg Field, the strongest areas for Codell refracture potential are the same as those chosen for the originally drilled wells: in the high GOR/EUR area north of the Wattenberg High. Low-volume sand concentrations in near-neutral or high pH frac fluids provide the best stimulation rheology for refrac optimization. Over 1800 Codell refracs through November, 2001, indicate this process is an economically ffective method to extract production stranded between wells.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming