Geomechanical Model and Wellbore Stability Analysis of Brazil's Pre-Salt Carbonates, a Case Study in Block BMS-8

Abstract

Abstract In this study, a wellbore stability model was built of the pre-salt area using analytical correlations to obtain an optimum mud window. The operational window assists in optimizing the fluid weight to be used while drilling and is defined by a lower limit (pore or collapse pressure) and an upper limit (fracture gradient). To build a geomechanical model, it is necessary to estimate different parameters. Log data from offset wells, pressure measurements, formation integrity tests, and data from the World Stress Map were used to estimate overburden, pore pressure, rock strength, the magnitude of the minimum horizontal stress, and the magnitude and direction of the maximum horizontal stress. These parameters made it possible to estimate the collapse gradient and define the mud window. The sensitivity analysis of the parameters was performed on the rock strength (UCS) and maximum horizontal stress due to the lack of available data, especially borehole images, 4 to 6 arm caliper, and triaxial tests. The geomechanical model indicates that the overburden in the carbonates of the studied area ranges from 14.0 to 14.6 lb/gal. Pore pressure indicates two different behaviors, presenting slightly over-pressurized (approximately 9.4lb/gal) and over-pressurized in another area (13.2lb/gal). The results of the formation integrity test (leak-off tests) were used to estimate the minimum effective stress ratio (ESRmin), a constant value around 0.65 with a minimum horizontal stress values between 12.4 to 12.6lb/gal in the slightly over-pressured zone and in the over-pressured zone the values are between 13.8 to 13.9lb/gal. The average value of uniaxial compressive strength (UCS) was estimated at approximately 43MPa. The azimuth of maximum horizontal stress was estimated to be approximately 95° and was obtained from the World Stress Map. The effective maximum horizontal stress ratio (ESRmax) was between 0.95 and 1.45, according to the presented scenarios, and the conclusion was that the faulting stress regime was between normal and strike-slip. For the mud window calculation, the effects of slightly over-pressured pore pressure (Pp) and the selected failure criterion, elastic properties and allowable breakouts provided a lower bound limit to the mud window in the carbonates of between 10.0 to 10.1lb/gal and an upper bound limit between 12.4 to 12.6lb/gal for the study area. In the zone with the high-pore pressure gradient, the geomechanical model provided a lower bound limit between 13.3 to 13.4lb/gal and an upper bound limit between 13.8 to 13.9lb/gal.