Borehole instability when drilling across a fault in a terrigenous sand-clay formation is manifested by the effect of overburden pressure and depends on the inclination angle. When a hole penetrates a fault with an inclination angle of more than 45° the drilling tool may become stuck at the initial stage, expressed by a peak-shaped increase in the torque on the top drive and the surface pump pressure. At the same time, after the cuttings are lifted to the surface, their number on the shakers will increase; caving with polished surface typical for a fault will appear. The development of this process over time can lead to annulus blockage, loss of circulation and mechanical sticking of the drilling tool. Prevention of complications and accidents associated with the collapse of fractured rocks during drilling lies in controlling the well trajectory, the mud density and equivalent circulation density. Therefore, such cases at the design stage require the development of additional measures, as well as calculations of the borehole stability in the fault zone. This can be a detailed pre-drill mechanical earth model that estimates the safe drilling window in advance. During drilling, it is necessary to carefully monitor mud density and drilling parameters, the amount and size of cuttings. When the first signs of borehole instability appear, take prompt measures to contain the problem.
References
1. Basarygin Yu.M., Bulatov A.I., Proselkov Yu.M., Oslozhneniya i avarii pri burenii neftyanykh i gazovykh skvazhin (Complications and failures in drilling oil and gas wells), Moscow: Nedra Publ., 2000, 680 p.
2. Voytenko V.S., Upravlenie gornym davleniem pri burenii skvazhin (Rock pressure control when drilling wells), Moscow: Nedra Publ., 1985, 185 p.
3. Hossain M.E., Islam M.R., Drilling engineering problems and solutions : A field guide for engineers and students, Hoboken, NJ, USA : Wiley-Scrivener, 2018, 627 p.
