Optimizing a Deepwater Subsalt Drilling Program by Evaluating Anisotropic Rock Strength Effects on Wellbore Stability and Near Wellbore Stress Effects on the Fracture Gradient

Abstract

Abstract An understanding of geomechanics can save millions of dollars in drilling costs by reducing the likelihood of poor borehole conditions, stuck pipe and lost circulation. Basic wellbore stability modeling has been successful in the past, but may prove inadequate in some cases because the mechanism of borehole failure and characterization of fracturing is not sufficiently addressed. In these cases, a more complex geomechanical model is required. We describe an example where a complex geomechanical model was applied to address anisotropic wellbore failure due to weakly bedded rocks, and lost circulation due to equivalent annular mud weights in excess of near and far field stresses. This same model was then used to predict mud weights that should mitigate problems in future wellbores. Introduction Drilling operations in the Gulf of Mexico often experience a challenging drilling environment. The combination of high overpressures and weak rocks can quickly narrow the range of downhole operating mud weights. In many cases, basic geomechanical modeling can help to mitigate these challenges by helping to understand pressures at which boreholes fail and/or pressures at which fractures propagate away from the wellbore. Unfortunately in some cases, these basic models are not adequate in fully describing the earth model and therefore have shortcomings when determining operating mud weight ranges. The Shenzi Field in Green Canyon blocks 653 and 654 (figure 1) is an instance where understanding parameters outside of the general geomechanical failure model has allowed for improved drilling operations. Early exploration and appraisal drilling in the deepwater (+3000 ft water depth) subsalt field resulted in significant wellbore instabilities and lost circulation in the subsalt formations. After the first wellbores were drilled, it was believed the instabilities could be accounted for by the uncertainty in the pre-drill models. Adjustments were made to key parameters such as pore pressure, rock strength and stress magnitudes, however additional drilling problems continued to occur in subsequent wells. It was quickly realize the problem extended beyond the uncertainty in the current geomechanical model and that there were other considerations that needed to be made before future wells were drilled. While drilling the first wells, it was observed that the borehole instabilities were more severe when drilling down dip at low angles-of-attack to bedding, but almost non-existent when drilling up dip at angles nearly perpendicular to the bedding planes. Operations also experienced significant lost circulation when raising the mud weight to compensate for the instabilities in the down dip wellbores. The high mud weights required to limit borehole collapse only increased the risk of losses in naturally depleted sands. The narrow mud window was also constraining the casing design, which resulted in severe cost overruns. A greater understanding of the mechanism for instabilities and lost circulation needed to be gained to prevent these cost overruns in future wells. Theory and Geomechanical Modeling Despite sometimes being under-utilized, basic geomechanical modeling has been a part of the industry for quite some time. It is the modeling of the earth's mechanical properties coupled with the regional in-situ earth effective stresses[1,2]. The complexity of this model is determined by how many of the mechanical properties can accurately be quantified, how robust the calibration of the stress magnitudes and directions has been, and how these properties vary across the reservoir. When applied correctly, the model can be used to understand how the earth will react when subjected to a drilling scenario[3]. These "reactions" are generally things we would like to avoid when drilling wells. They include, but are not limited to, wellbore cavings, lost circulation, wellbore ballooning events[4], and reaction with the mud[5]. To mitigate all of these potential problems, the geomechanical model should be as robust as possible. Unfortunately, often the lack of proper data available does not allow for robust models.