Method for estimating confined compressive strength for rock formations utilizing Skempton theory

摘要

A method for estimating the CCS for a rock in the depth of cut zone of a subterraneanformation which is to be drilled using a drilling fluid is disclosed. An UCS isdetermined for a rock in the depth of cut zone. A change in the strength of the rockdue to applied stresses imposed on the rock during drilling id calculated whichincludes estimating the ΔPP. The CCS for the rock in the depth of cut zoneis calculated by adding the estimated change in strength to the UCS. The presentinvention calculates the ΔPP in accordance with Skempton theory whereimpermeable rock or soil has a change in pore volume due to applied loads or stresseswhile fluid flow into and out of the rock or soil is substantially non-existent.CCS may be calculated for deviated wellbores and to account for factors such aswellbore profile, stress raisers, bore diameter, and mud weight utilizing correctionfactors derived using computer modeling and using a baseline formula for determiningan uncorrected value for CCS.