ARMA 12-315 Use of Multiple Lithology and Rock Property Indicators to Evaluate Wellbore Failure in Layered Formations

摘要

To identify suitable correlations between UCS and sonic velocity for a variety of lithologies, this paper analyzes the variations of formation lithology (e. g., shale, sand, or limestone) and the transitional lithology (e. g., sandy shale, shaly sand, tight limestone, and porous limestone) and examines their relations to the mechanical properties of the respective formation (e. g., UCS). Interpreting the characteristic responses from various wireline logs, such as gamma ray (GR), spontaneous potential (SP), resistivity (RES), sonic (DT), neutron porosity (NPHI), and formation density (RHOB), more accurate formation identification is sensibly obtained than the conventional method of using GR as the sole lithology indicator. The proposed methodology is illustrated using an actual wellbore stability case study in an oil field of the Gulf of Mexico (GOM). After the lithologies are defined, the correlated mechanical properties are determined (e. g., UCS, friction angle, cohesion, Young's modulus, and Poisson's ratio). The proposed method is used to evaluate the lower bound of safe mud weight windows in assessing the wellbore stability in drilling, invoking the shear failure gradient. The evaluation is based on each formation layer where the desired correlations between layer lithology and mechanical properties are provided. The calculated shear failure gradient will be used with the determined fracture gradient (or closure pressure) to define the safe mud weight window. With the reasonable sensitivity study, the match between the calculation and the observation can be ensured.