Abstract:

The Milk River formation, a tight-gas reservoir, is a weak mudstone within a shallow, low-energy lake environment, with thin sand lenses of upward coarsely grading sequences. Stimulation of wells in the Milk River formation are compared for a) conventional perforations in a cased hole and b) expanded dilating casings in nearby offset wells. All wells were stimulated with an energized fracturing fluid carrying 20/40-mesh sand proppant and completed at seven depth horizons. Wellhead treatment pressures for the conventionally perforated stimulations are 20 MPa at depth, rising to 27.5 MPa at the shallowest horizon. Surface tiltmeter data show fracture planes are vertical at depth and horizontal at shallower depths, with an apparent stress crossover at ~400 m depth. Wellhead treatment pressures for the split dilating casing are constant at 17.5 MPa for all depths, with all fractures vertical and no stress crossover observed. Therefore, the stress crossover is attributed as an artifact of stimulating through perforations and is not formation stress related. It is concluded that, within formations acting in the non-brittle regime, the well stimulation procedure dictates the outcome. Stimulations through perforations do not excite the least energy dissipating mechanism in non-brittle weak formations.

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