Incidents that occur because of destabilization of coal layers during drilling are in particular set. The reasons and solutions for coals stabilization are widely discussed but the problem remains actual issue. The article presents a field experience of wells drilling on Jurassic formations, which contain coal layers as a part of target formation. The stabilization of coal layers with conservative solutions after they were firstly destabilized is almost impossible according to field experience that is why the preventive actions to minimize risks of borehole destabilization are in priority. The best practices that were highlighted during comparison of different technological solutions demonstrate near 100% effectiveness in certain geological conditions. The destabilization of borehole first occurs as a result of hydrodynamic and mechanical impact because coal layers have low solidity and highly fractured. The main solution for reduction of destabilization risks is the right choice of sealants that could form solid filter screen. Because of several factors, the perfect material for bridging the coal layers is gilsonite. Its hydrophobic features will make the bridging more effective. The gilsonite is insoluble in water and there are some occasional issues with its addition to water-based drilling fluids, for this reason the materials such as the suspension of gilsonite in water-based organic solutions (polyatomic alcohols, surfactants etc.) are preferable. The designed solutions for reduction of coal layers destabilization risks (complex sealants additives, reduction of mechanical and hydrodynamic impact) helped to drill horizontal sections 1550 m long. In tough geological conditions 50 horizontal wells were drilled at present. Two wells penetrated the coal lenses of 80 and 110 m, as a result the liner was run to the bottom without any issues.
References
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