Induced Stresses Due to Propped Hydraulic Fracture in Coalbed Methane Wells

Abstract

Abstract Many coalbed methane wells are hydraulically fractured to stimulate the flow of gas. An inflated hydraulic fracture, e.g., during pumping, or after shut-in when it is held open by proppant, induces extra stresses in the formation surrounding the fracture. Because of high net pressures in coalbed fractures, stresses induced in the formation by the propped fracture opening can be substantial, and these stresses can extend to significant distances into the formation because of the large area of the fracture face. Because cleated coal is a stress-sensitive formation, the induced stresses surrounding a propped fracture will damage the permeability. A second implication is in the area of refracturing. If the original propped fracture has altered the in situ stresses sufficiently, the new fracture (refracture) may initiate orthogonal to the original fracture. To achieve this, the stress increase induced by the original propped fracture would have to be large enough to switch (i.e., reverse the directions of) the maximum and minimum horizontal stresses (in the vicinity of the original vertical fracture). The assumption is that the original minimum stress is increased to a value substantially greater than the original maximum stress. Principal conclusions of the study are: The permeability damage is maximum at the center of the fracture face, decreasing toward the fracture tip, and at a distance from the fracture face of about 1.5 times the minimum fracture areal dimension (normally fracture height). The permeability damage can be serious, especially for high pressure, confined-height fractures. For example, a typical fracture of height 30 ft and net pressure 700 psi reduces the permeability to 26% of original (maximum reduction). However, in many field cases the net pressure is even higher, and could reduce the permeability to less than 10%. The damage may extend 50 ft into the formation. The permeability damage will be exacerbated by fracture treatments which screen out. In such cases, it is possible that the permeability is reduced to under 2% of original. High pressure, confined-height fractures (e.g., T-fractures) are likely to switch the horizontal stresses in both San Juan and Black Warrior basins. This could mean that a vertical refracture will initially propagate orthogonal to the original vertical fracture. On the other hand, if the original fracture were horizontal, the refracture might be vertical, with the same advantages. These phenomena might contribute to the success of water refracture treatments. If the original fracture screened out, stress reversal and orthogonal refracture propagation would be even more likely. This concept of altered-stress fracturing has several applications, which are described in this paper. In three of these cases, the original fracture alters the in situ stress, and is followed by a subsequent fracture that propagates orthogonal to the original fracture. Possible benefits are:circumventing the gel damage caused by the original fracture,propagating a fracture across the dominant natural fracture system (face cleat in coal), to enhance the production of gas. The latter may have application in other naturally fractured formations. In the fourth case, a sacrificial fracture can change an unfavorable stress contrast into a favorable one. A possible application is in preventing a vertical coalbed fracture from breaking out into the Pictured Cliffs sandstone in the San Juan basin. Since fracturing pressures in water fracture treatments are similar to those in gel fracture treatments, the discussions of induced stress, permeability damage, and refracture orientation should apply equally well to both types of treatment. Finally, the permeability damage is likely to be part of the reason why, in the San Juan basin fairway, openhole cavity completions outperform fracture stimulations in gas production, often by 5-10 times. P. 221^