Abstract

Re-fracturing is a necessary and efficient way to improve the stimulated reservoir volume, enlarge the drainage area and enhance production again. The initiation and propagation mechanism of re-fracture, whereas, still maintain unclear due to complex in-situ stresses distribution, engineering factor and so on. Several of large-scale true tri-axial hydraulic fracturing simulation experiment were conducted to study that mechanism in cased and perforated horizontal well. Meanwhile, the effect of pump rate on re-fracture initiation and propagation was discussed in this experiments. The experimental results revealed that: re-fracture initiated from the same perforation as initial fracture did or different perforation from it by two initiation forms: tensile failure and shear failure. The re-fracture had an angle from 70° to 90° with the initial fracture, instead of orthogonally. Moreover, injection rate had an impact on re-fracture propagation behavior. Low injection rate reduced re-fracture length and caused initial fracture deflection. High injection rate, whereas, enhanced re-fracture propagate to a long distance. In addition, re-fracture required higher energy to initiate resulting in higher fracture pressure than initial fracture did.

1.
Introduction

Hydraulic fracturing was commonly applied in most unconventional reservoirs. The purpose of that was to induce fractures with high flow conductivity in such tight formation with high density, low permeability and low porosity. Long term production, however, leaded to reservoir depletion, which could not ensure high production. Therefore, re-fracturing operation was proposed and studied by many scholars to stimulate formation for high production.

Warpinsk and Branagan (1989) indicated that in-situ stresses near wellbore were changed after hydraulic fracturing. Elbel and Mack (1993) claimed that induced stress was created near initial fracture by initial fracturing and subsequent production. They also though that in-situ stress decreased faster in parallel to the fracture direction than perpendicular to the fracture direction. Wright (1994) found that re-fracture had an angle about 30° to 40° with initial fracture by inclinometer. Siebrits et al. (2000) confirmed that re-fracture was perpendicular to initial cracks based on re-fracturing field tests on two old tight gas wells. Roussel and Sharma (2010) established numerical model to study stress diversion area and the optimal re-fracturing time based on fluid-solid coupling. Bu et al. (2015) conducted simulation experiments to investigate the effect of in-situ stresses alteration on fracture propagation in open-hole well.

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