Abstract

It is a challenge to drill a highly deviated or horizontal hole in high permeable formations. High differential pressures may lead to several problems like tight holes, wellbore instability, differential sticking and mud loss while drilling across these permeable or fractured formations. It was always preferred to drill these wells with Oil base muds which showed some success. While operators always prefer the standard solution, which is casing isolation for problematic sections, challenges have increased due to continuously drilling in depleted reservoirs which leads to considerable nonproductive time.

The other solution to overcome such problematic sections was to re-design a fluid system that would target drilling through serious of highly permeable sand and shale formations. The fluid system would primarily address shale inhibition along with effective bridging, minimizing pore pressure transmission and wellbore strengthen with increased hoop stress in the wellbore. Software modelling and permeability plugging tests were performed to evaluate the fluid behavior under downhole conditions and to predict the characteristics of induced micro fractures based on rock mechanics. Porosity, permeability and induced micro fractures were considered to optimize the bridging mechanism. It was identified that normal bridging solutions involving calcium carbonates and graphite material were not enough to address the pore pressure transmission problem. It was essential to include a micronized sealing deformable polymer along with normal bridging material was effective in plugging pore throats and minimizing fluid invasion. The deformable polymer component is able to re-shape itself to fit a broad range of pore throat sizes which was previously unattainable with conventional bridging technology which was confirmed by particle plugging tests.

A one well was identified to be drilled in highly depleted reservoir at an inclination of almost 45 degrees. The section involving the highly depleted and permeable sand involved drilling highly stressed shale formations which requires high mud weight for their stability. This was the first attempt on a high-angle well with development drilling operations in Kuwait and was performed to facilitate the successful drilling of the reservoir. Drilling and logging were successfully performed along with logging and LWD runs with no recordable differential sticking or losses incidents.

This paper also presents 2 successful applications in the same field with the application of proper bridging and utilization of deformable sealing polymer to address drilling problems through highly depleted and permeable formations while managing over balance of 3500 psi across them.

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