Abstract

The high cost of offshore drilling and the safety aspects of penetrating depleted reservoirs bring borehole stability issues to the forefront of resource development. Drilling in deepwater fields, depleted reservoirs and/or low stress environments requires careful assessment of mud weights. Higher collapse pressures combined with lower fracture gradients limit or eliminate mud weight windows and lead to tight holes or lost circulation. Extended-reach wells require minimization of the number of casing shoes to reach the deeper targets. The need for long, open sections while drilling imposes restrictions on mud windows. Borehole strengthening has become the most effective method to address borehole stability in such circumstances.

In this paper, we provide geomechanics insights from the large knowledge base on waterflooding and injector performance to improve the practice of wellbore strengthening. The petroleum industry has morphed over its long history through fewer inventions but a long list of innovations derived from lessons learned from one sector to benefit another. Injector plugging, a nuisance to production/operation engineers, illustrates such an example. The vast amount of knowledge provides the fundamental reasoning and rationale for optimizing wellbore strengthening. Fractured injector geomechanics are extended to improve wellbore strengthening procedures. Strengthening is implemented through creation of plugged minifractures to raise loss circulation pressures. The mud contains particles to mitigate unstable fracture extension created by using higher mud weights. The particles deposit and plug initiated fractures and prevent further propagation. The propagation pressure is raised significantly and lost circulation stops. In the presented model, the stable fracture length and width are related to a given mud weight and rock characteristics. Accordingly, the plugging media (particle) size is optimized based on the fracture geometry and leakage scenarios. The model results are field-verified and demonstrate how the borehole stability problem could be optimized with proper design.

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