HYDROMECHANICAL SCRAPER HELPS PLUG PERMEABLE ZONES

Jan. 2, 1995
Boris M. Kurochkin Scientific Research Institute of Drilling Techniques Moscow A simple downhole tool, which resembles a scraper with side jets, uses mechanical action and hydraulics to plug permeable, water-bearing formations that may cause cementing problems. The tool has also shown promise in curing lost circulation zones when used with fine, dispersed fillers.
Boris M. Kurochkin
Scientific Research Institute of Drilling Techniques
Moscow

A simple downhole tool, which resembles a scraper with side jets, uses mechanical action and hydraulics to plug permeable, water-bearing formations that may cause cementing problems. The tool has also shown promise in curing lost circulation zones when used with fine, dispersed fillers.

The producing life of an oil well often depends on casing cement quality. The casing and cement life are a function of the strength of the cement behind the casing and the presence of a good cement bond between the pipe and formation.

In some wells, a high-strength cement sleeve does not form against water-bearing formations, resulting in poor contact between the cement column and formation. If this poor cement sheath occurs across the production strata where there is an oil/water contact, the well can produce water instead of oil. The absence of a cement sleeve in other intervals of water-bearing formations results in casing corrosion and premature well repair.

The Scientific Research Institute of Drilling Techniques in Moscow has developed a special downhole tool to improve both cementing operations and help plug lost circulation zones.

This hydromechanical tool, designated OGMB-215.9, is made with the dimensions of a standard reamer so it can fit easily into a typical bottom hole assembly without requiring special connections. The tool is mounted above the bit or between drill collars. The reamer blades have built-in scrapers, and the housing contains one or two nozzles.

CEMENTING

During drilling, the fluid flow rate through the bit and the tool's side nozzles are correlated for maximum hydraulics. The tool's high-pressure jets act on the surface of already-drilled water-bearing formations. The scrapers and the reamer blades make the annular fluid flow turbulent in this intern,al. The coarse cuttings and particles are thrown aside from the pore orifices. The smaller cuttings and particles, however, can enter the pores to form the isolating layer near the borehole wall.

Because of the scraping and jetting, the mudcake on the borehole wall is minimum or absent altogether.

From laboratory investigations and side-wall core samples, the isolating layer is not more than 8-10 mm during mud circulation. Laboratory studies on water breakthrough along the contact surface of the permeable formation with isolating layer show that this hydromechanical isolating layer increases the breakthrough pressure by 1.5 times.

The tool has been successfully used to improve the cementing operations in wells in several oil fields in Russia and in the Uzen field in Kazakhstan.

LOST CIRCULATION

This tool has also shown promise as a preventive measure in drilling through permeable formations where lost circulation is a problem. For lost circulation zones, typically 3-5% of fine, dispersed filler ("slomel") is added to the drilling fluid. This slomel is a finely ground plastic with particles of paper covered with tar. The largest particles are less than 0.25 mm.

Field experience in Kazakhstan has shown that this tool helps effectively plug lost circulation zones. A combination of running this tool and adding slomel lost circulation material has reduced lost circulation in some of the most problematic formations. This combination of filler and tool has controlled zones with severe lost circulation of 0.5-15 cu m/hr.

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