Effect of Production-Induced Stress Field on Refracture Propagation and Pressure Response

Abstract

Abstract In this work, the propagation of an orthogonal fracture and reopening along the initial fracture during a refracture treatment is studied by taking into account the production induced stress field surrounding the initial fracture. It is shown that the propagation pressure of the orthogonal fracture quickly increases to above the closure stress on the initial fracture due to the fracture penetrating into the higher stress region, which leads to fracture reopening along the initial fracture plane (called in-plane frac hereafter). A dual-frac PKN model is developed to predict the growth of the two intersecting fractures in a variable stress field and the associated pressure response in order to obtain an insight into the refracture process. The modeling results show that a refracture treatment can undergo three distinct periods of fracture growth: Period I: Dominant orthogonal fracture propagation. It exhibits a rapid pressure increase due to the stress increase at the tip of the orthogonal fracture. Period II: Reopening of the initial fracture. This period shows a relatively flat pressure since the stress at the fracture tip is nearly constant. Period III: Extension of the initial frac and the orthogonal frac. It exhibits steeper pressure increase (tip screen-out like pressure response) as the stress at the tip increases when the in-plane fracture propagates past the initial fracture tip. The in-plane fracture length growth is significantly slowed. Two field cases from the Barnett Shale are evaluated by comparing the model prediction to the pressure response. The model prediction agrees well with the observed pressure response and surface tiltmeter observations. The production-induced stress has significant implications on refracture design and candidate selection and they are discussed for various reservoir scenarios. Introduction Previous work by Mack and Elbel1–2 showed that production from a fractured well causes the stresses surrounding the fracture to decrease over time as the reservoir depletes, and this decrease is greater in the direction parallel to the fracture than normal to the fracture. For a low permeability formation having a small contrast between the two horizontal stresses, this production-induced stress change could cause a rotation of the local minimum stress direction by 90° (i.e., parallel to the initial frac instead of normal to it as in the virgin formation). As a result, a refracture treatment may initiate a fracture orthogonal to the initial propped fracture. As this orthogonal fracture propagates away from the initial fracture, the production induced stress change diminishes and the minimum stress at the fracture tip reverts back to the original direction, causing the fracture to turn towards the original fracture orientation, as illustrated in Fig. 1. Siebrits et al.3 further extended this work by studying the stress distribution around a producing fracture using a 3D numerical simulator and investigated the size of the stress reversal region as a function of various reservoir properties. Fracture turning was also studied using a 2D hydraulic fracture model.