Interface Test Series: An in Situ Study of Factors Affecting the Containment of Hydraulic Fractures

摘要

In situ experiments, which are accessible for direct observation by mineback, have been conducted to determine the effect that material-property interfaces and in situ stress differences have on hydraulic fracture propagation and the resultant overall geometry. These experiments show conclusively that a difference in elastic modulus at a geologic interface has little or no effect on crack growth and, therefore, is not a feature which would promote containment of fractures within a specified reservoir zone. However, differences in the in situ stress between adjacent layers is shown to have a considerable influence on fracture propagation. Experiments were conducted in a low modulus ash-fall tuff which contained two layers of high minimum principal in situ stress and which was overlain by a formation with at least a factor of 5 increase in elastic modulus. Fractures were observed to terminate in regions of high minimum principal in situ stress in nearly every case. Fracture growth into a higher (by a factor of 5 to 15) modulus region was preferred to propagation into a region of higher (by a factor of 2 to 3) stress. Thus, determination of the in situ stresses in a field environment is important for proper design of hydraulic fracture treatments, particularly for determining the height of the fracture. This would provide an estimate of whether stress magnitudes in the reservoir rock and adjacent layers would result in containment of the fracture or enhance out-of-zone propagation and failure of the treatment.