Low-permeability gas-bearing sandstones present in basins of the Rocky Mountain area are commonly bounded by lithologies that are not barriers to hydraulically induced fracture migration. Use of standard hydraulic fracture treatments commonly results in most of the induced fracture area developing outside of the pay zone. Modified hydraulic fracturing techniques may be employed to optimize placement and penetration of the induced fracture area when the geologic and in-situ stress properties of boundary and pay zone lithologies are known.
Laboratory and field efforts have been made to improve stimulation treatments in the Mancos B formation within the North Douglas Creek Arch field near Rangely, Colorado. To evaluate these efforts, a combination of core analyses and in-situ stress measurements was used in developing a modified hydraulic fracturing program. Whole core samples were taken from upper and lower boundary lithologies as well as within the Mancos B pay zone for special core tests including gas permeabilities, static elastic moduli, fracture toughness, and fluid compatibility tests measured at simulated in-situ conditions. Reservoir quality within pay zones was determined by a combination of petrography, x-ray diffraction, and scanning electron microscopy. In-situ minimal horizontal stress was determined in the field from instantaneous shut-in pressure (ISIP) data which were measured in all three lithologies.
One modified hydraulic fracturing technique that has been field tested and shows promising results is based on fracture initiation placement (perforation placement). A second technique to be tested in the near future will use lightweight additives in fracturing fluids to help seal vertical propagation of hydraulically induced fractures, thereby insuring a deep penetrating fracture into the pay zone.