Impacts of Uncertainty in Minimum Horizontal Stress Determination on Wellbore Stability

The purpose of this study is to highlight sources of uncertainty in the determination of minimum horizontal stress estimates from leak-off tests (LOTs) and their effect on wellbore stability modelling. To achieve this, we use petroleum datasets from the Moomba gas field in the Cooper-Eromanga basin, South Australia, Australia. We analysed 151 LOTs that had few data points on the pressure-time plots (low data density) from the Winton Formation. We use two techniques for selecting points on pressure-time curves that are meant to reflect fracture initiation, an interpolation technique, and a point of linear deviation technique (`eye technique'). Our study compares (1) The technique used to pick the point of leak-off. (2) The range of estimates of Shmin magnitude at 900 m depth in the Eromanga Basin, particularly between the lower bound and average values. (3) Filtering parameters to ensure FIT (Formation Integrity Tests) are removed and only tests with clear leak-off pressures are visible for consistency (4) How engineering complications affect LOT reliability, i.e., cement channels, non-linear build-up, and plastic-elastic leak-off. (5) Shmin calculation based on tensile failure or shear failure. Of 151 leak-off tests 23 reliable tests are identified that have a range of 3 MPa between them at 900m. The lower bound well selected was 2.09 MPa lower than the average of all wells at 900m. The technique used to pick the leak-off point varies based on the LOT data point density but was up to 0.86 MPa at 900m. Traditional Shmin calculation methods based on the assumption of tensile failure produced estimates 1 MPa lower (at 900 m) than calculations allowing for the possibility of shear failure. Finally, wellbore stability models extrapolated to 2.8 km depth in producing formations show notable differences in mud weights required to contain breakouts when drilling vertically and horizontally, depending on how Shmin is estimated by leak-off test analysis and calculated through traditional and tensile failure methods.