Refracture Reorientation Enhances Gas Production in Barnett Shale Tight Gas Wells

Abstract

Abstract Refracturing can be used to increase production in poorly fractured wells. A different application of this technology is to refracture wells with strong initial fractures. In this paper, we provide evidence of increased production due to refracturing two tight gas wells having deeply penetrating initial fractures. Surface tiltmeter measurements show refracture orientations at oblique angles to the azimuth of the initial fractures. Introduction Refracture reorientation has previously been postulated [1, 2] and directly observed in soft, shallow formations [3, 4, 5]. We present the results of two refracture treatments to test the concept of orthogonal refracture reorientation in a tight gas formation. Previous work, based on theoretical considerations in tight gas reservoirs [6], shows that a refracture can orient at 90 degrees to an initial hydraulic fracture under certain conditions. In such cases, the refracture can penetrate untapped sections of the reservoir, significantly increasing production rate and reserves. Candidates for the field tests were those that exhibited production behavior indicative of a deeply penetrating highly conductive initial fracture. It is important to point out that such wells are not usually considered for refracturing. The field tests were carried out in the Barnett Shale, north of Fort Worth. The refracture treatments were monitored with an array of surface tiltmeters. Results indicate significant reorientation of the refracture treatment in Well A, and oblique reorientation in Well B. Production data indicates a substantial increase in production after both refracture treatments. Other wells in the area, not part of this study, have shown similar production increases after refracturing. In this paper, we summarize the field candidate selection process, results of production forecasts, and provide details and discussion on the refracture designs and treatments. Refracture Reorientation Concept Fig. 1 is a schematic representation of the concept of refracture reorientation. The figure shows a horizontal section through a vertical well containing an initial fracture, oriented west-east. Production, after placement of the initial fracture, will cause a local redistribution of pore pressure in an expanding elliptical region around the wellbore and initial fracture [6]. The pore pressure depletion changes the stress distribution in the reservoir. Numerical simulations [6] show that the total horizontal stress component parallel to the initial fracture reduces quicker than the orthogonal one as a function of time, at locations along the line of the proposed refracture direction. If the induced stress changes are large enough to overcome the effect of the initial horizontal deviatoric stress, then the direction of the minimum horizontal stress becomes the maximum within an elliptical region around the wellbore and initial fracture (see Fig. 1). Under these conditions, a refracture will propagate at 90 degrees to the initial fracture azimuth, until it reaches the limit of the elliptical stress reversal region. The boundary of this region along the proposed refracture propagation direction is defined by isotropic points - points with equal horizontal stress. We can expect the refracture to start to reorient itself at some distance Lxf" beyond the isotropic point (at distance Lxf'), as shown in Fig. 1. The isotropic point will typically locate at a distance less than half the initial fracture penetration. However, fracture toughness extends the orthogonal penetration of the refracture beyond this point. The distance to the isotropic point depends on the magnitude of the initial horizontal stress contrast, initial fracture penetration, production rate, reservoir permeability, and the elastic moduli contrast between the pay and barrier zones [6]. These are parameters that should be considered in the selection of a candidate well.