Stability analyses for deviated wellbores in unconsolidated cross-anisotropic formations

Abstract

Production of oil from shallow reservoirs typically involves drilling highly deviated wells through unconsolidated (or poorly lithified) rocks or clays. This paper describes numerical analyses of the deformations and stability of deviated wellbores within a K0-consolidated clay. The analyses consider planar deformations in the plane orthogonal to the wellbore using a quasi three-dimensional (3D) finite element model that represents coupled flow and deformations within the soil mass. Cross-anisotropic mechanical properties of the clay are described by a generalized effective stress model, MIT-E3, with parameters previously calibrated from laboratory thick-walled cylinder tests. The analyses compute the relationship between the drilling mud pressure and wellbore stability associated with either the onset of localized failure mechanisms or large plastic deformations around the cavity. The results show that short-term, undrained stability requires mud pressures in excess of the in situ formation pore pressures for more highly deviated wellbores at inclinations greater than 45°. The analyses examine the mechanisms for further destabilization, due to consolidation within the formation, and how they are affected by drainage conditions at the wellbore wall. The results provide qualitative information for the design and control of drilling operations for deviated wellbores in unconsolidated formations.

Impact Characteristics of a Bidirectional Pneumatic DTH Hammer for Unconsolidated Formations

“…In drilling projects, unconsolidated formations like the sand layer, gravel bed, and cobbles formation are regarded as one of the most complex and challenging formations to deal with [1][2][3], since it is prone to causing drilling accidents such as drilling tools sticking, drilling tools burying, and borehole collapse. Once drilling tools are stuck or buried in unconsolidated formations, retrieving them from boreholes is almost impossible, which will incur additional time and cost consumption [4][5][6].…”