Abstract
Drilling fluid loss in depleted reservoir has been an import issue faced by further tapping the potential of old oil fields. Accurate evaluation of the fracture pressure is the foundation to avoid mud loss. Traditional views suggest that tensile failure is the only fracture mode and the fracture pressure should be determined by a tensile failure criterion, which are not suitable for wells drilled in the depleted reservoir. In this paper, the analysis focuses on the fracture mode and fracture pressure in depleted reservoir, and case studies show that three fracture modes may first occur, and the fracture mode will be changed with reservoir depletion which highly depends on reservoir depletion degree, well azimuth and deviation angle, and the in situ stress state; different failure criteria at different stages of reservoir depletion should be selected to accurately evaluate the fracture pressure. For the vertical well, fracture pressure is no longer a single linear reduction with reservoir depletion; instead, a three-step and two-step reduction may appear, and for the directional well, the fracture pressure is not always decreased; the other patterns such as increase and first increase then decrease may also appear for the wells drilled in reverse and strike fault stress regimes.
Highlights
IntroductionMore than 70% hydrocarbon resources are obtained from secondary and tertiary recovery [1]
At present, more than 70% hydrocarbon resources are obtained from secondary and tertiary recovery [1]
The results show several differences from conventional views: (1) Tension failure is not the only fracture mode; two types of shear fracture modes occur; for example, in reverse fault stress regime (Figure 3), Mode 3 always first occurs when the vertical well is fractured with reservoir depletion, and the fracture pressure should always be determined by the Mohr-Coulomb shear failure criterion, and the tensile failure criterion used in previous calculations may make a great overestimate for the fracture pressure
Summary
More than 70% hydrocarbon resources are obtained from secondary and tertiary recovery [1]. With reservoir depletion, the original tensile failure mode may be transformed into shear failure mode, and shear fracture may first occur at the wellbore when the drilling fluid pressure inside the wellbore is high enough. In this case, fracture pressure should not be determined by a tensile failure criterion, and the reduction of fracture pressure of vertical well will present a more complex law. The problem becomes more complicated in directional wells due to changes in well inclination and azimuth Aiming at this problem, the analytical method for wellbore fracture mode and fracture pressure in depleted reservoir is established in this paper, and the fracture mode and fracture pressure in different reservoir and wellbore conditions are analyzed in depth
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