Drilling the 12 1/4" sections for the Kharyaga oilfield in the Timan-Pechora region of Russia has always been considerably complicated by wellbore instability and hole cleaning issues. These issues have been most serious when drilling through Triassic and Permian shales, sandstones, argillites and silts, followed by drilling Carboniferous limestones and dolomites. Unstable formations sloughing and packing off, wash outs and severe caving has resulted in many problems including drill pipe pack-offs, high torque and drag, the need for additional reaming operations, and difficulties in reaching bottom with casing.

Fluid treatments of swelling shales with chemical inhibitors helped considerably with shales hydration, but the problem of caving and packing off still persisted. High angles (up to 52.8°) and long section lengths (up to 2,707 m) aggravated the problems. The key remedy was determined to be effective hole cleaning. Cleaning efficiency of different types of sweeps was studied during drilling of 10 Kharyaga wells. Sweeps were pumped on a regular basis in drilling intervals of 100, 150, 200 and 300 meters, prior to pulling out of hole and when indications of packing off had been observed.

Pumped cleaning sweeps included high-viscosity or high-density single sweeps; tandem sweeps (low-viscosity following by high-viscosity or high-density sweeps); and sweeps with special additives (carbon-based LCM material or innovative monofilament fiber sweeping agent). Also, special attention was given to combined tandem sweeps, which are low-viscosity sweeps treated with a monofilament fiber sweeping agent followed by a high-density sweep (sometimes treated with carbon-based LCM material).

Investigation of different sweeps performance showed that the best hole cleaning results for Kharyaga field wells were achieved by circulating combined tandem sweeps, which are low-viscosity sweeps treated with a monofilament fiber sweeping agent followed by a treated or untreated high-density sweep.

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