In-Situ Stresses: The Predominant Influence on Hydraulic Fracture Containment

Abstract

Summary In-situ experiments, which are accessible by mineback, have been conducted to determine the parameters that control hydraulic fracture containment. These experiments demonstrate that a stress contrast between the pay zone and a bounding layer is the most important factor controlling fracture height. Material property interfaces are shown to have little effect. Introduction Hydraulic fracturing has been used extensively for more than 30 years to stimulate the production of natural gas from many different reservoir rocks. Most treatments were small since their primary, purpose was to link the wellbore to the undamaged reservoir rock, and fracture lengths greater than a few hundred feet seldom were required. With increasing depletion of conventional natural gas reserves, attention has been focused on producing gas from unconventional gas resources such as tight gas sands and Devonian shales. Presently, stimulation of these formations is being attempted by massive hydraulic fracturing, a scale-up of at least an order of magnitude over conventional fracturing treatments. It is proposed that fractures longer than 4,000 ft (1200 m) be propagated in the low-permeability, gas-bearing formations to provide a high-conductivity path for the gas to reach the wellbore. Unlike small conventional treatments in which the fractures are propagated only a short distance, it is imperative that the large and massive hydraulic fractures be contained largely within the pay zone. Obviously, if only a small portion of the fracture surface is in contact with the reservoir rock, the result may well be an uneconomic well in a reservoir that has sufficient resources if stimulated correctly. Detrimental results will occur should the fracture break into a water-bearing zone. When referring to massive hydraulic fracturing in the tight gas sands, containment may referto confinement of the fracture to specified intervals comprising both gas-bearing sandstones lenses and surrounding shale zones orto the usual concept of confinement within a single reservoir zone. In either case, the study of the containment of hydraulic fractures is directed toward determining the parameters and the conditions that will limit the height of a fracture and control its direction of propagation so that the necessary fracture lengths will be obtained. Implicit in these studies is the presumption that sufficient understanding of these conditions will allow operators to alter treatment parameters to help control containment, at least in those formations where there is already some propensity for containing the fractures. If this is not possible, it still may be possible to define a priori those zones that most likely will provide economic production as a result of favorable fracture growth (or containment) conditions. Background The parameters that are considered to have some effect on the containment of hydraulic fractures have been detailed previously in the literature. A difference in elastic modulus between the reservoir rock and the barrier rock usually is singled out as the primary mechanism controlling containment. In their work on composite materials, Cook and Erdogan calculated the stress intensity factor for the two-dimensional crack approaching an interface between two materials with different clastic moduli. Simonson et al. and Rogers et al. applied these results to hydraulic fracturing and observed that since the stress intensity factor K at the tip approaches zero as a fracture in a lower-modulus material propagates toward a higher-modulus material, the fracture will tend to be arrested. JPT P. 653^