Fracture Initiation and Propagation From Deviated Wellbores

Abstract

SPE Member Abstract This paper presents the results of an analytical study of hydraulic fracture initiation and propagation from deviated wellbores. The interaction and link-up of the starter fractures initiated from perforations and the turning of the linked fracture are investigated. A criterion that correlates fracture link-up to stresses and wellbore parameters is established. The characters of the near-wellbore multiple fractures and their effects on fracture width reduction and friction loss are also studied. The model-predicted pressure response, which takes into account the near-wellbore friction loss, is compared to the measured bottom hole pressure data during a fracture treatment. Introduction High near-wellbore friction loss has been frequently observed in fracture treatments of deviated wells. This friction loss is indicative of fracture width restriction in the near-wellbore region which could be detrimental to the success of fracturing treatments. Indeed, more screen-outs have been experienced in deviated wells than vertical wells. The production data in Prudhoe Bay field in Alaska also indicated poorer performance of high angle, poorly oriented (parallel to the minimum in-situ stress) wells. To improve fracture treatments in deviated wells, it is essential to identify and evaluate the sources causing the near wellbore friction loss and to obtain a better understanding of what takes place downhole during a fracture treatment of a deviated well. To achieve this goal, a study has been carried out using analytical and numerical approaches to evaluate the width restriction and friction loss associated with fracture turning and multiple fractures. The main components of this paper are:evaluation of the width restriction and friction loss associated with fracture turning;study of the interaction and link-up of starter fractures initiated from perforations; study of multiple fractures and their effects on width restriction and friction loss;comparison with field data. FRACTURE TURNING AND TWISTING Basic Concept It had been a common view that during a fracture treatment of a deviated well, a single fracture will initiate along the wellbore. Only until recently, did many laboratory tests reveal much more complicated structures in the region adjacent to a deviated or horizontal wellbore. Tortuous fracture paths associated with fracture reorientation and multiple fractures have been observed. In this section, the fracture turning process will be investigated assuming a single fracture does initiate along a deviated wellbore. In later sections, we will further investigate the likelihood of fracture linkage and the possibility of multiple fractures. Yew et al. carried out a theoretical study of fracture initiation, in which the concept of starter fractures initiated from perforations and their linkage to form a single "zipper" fracture was proposed. The study indicated that there exists a point of maximum tensile stress on the wall of a deviated wellbore, which should be the location of fracture initiation and also the most favorable spot to place perforations. Based on the study, the technique of oriented perforation was originated and had been implemented in Kuparuk River field with success. Since a deviated wellbore usually does not lie in a plane normal to the minimum horizontal stress, the fracture formed at the wellbore has to reorient itself such that the fracture face is perpendicular to the minimum stress. Initially, the formed fracture is very elongated along the wellbore, as depicted by a contour of fracture front at time t, in Fig. 1. Based on the theory of fracture mechanics, an elongated fracture would preferably propagate in the direction of the shorter dimension until lengths in both directions are approximately equal. P. 849^