Wellbore Stability Assessment for Highly Inclined Wells Using Limited Rock-Mechanics Data

Abstract

Abstract Assessment of wellbore stability involves several parameters for which data that may not be readily available. The rock mechanics input data required for analysis are complicated and costly to acquire. Nevertheless, wellbore stability assessment plays an important role in the design of drilling and production of oil and gas wells; therefore, a methodology for bridging the data gap is needed. This paper presents a methodology for wellbore stability assessment with limited data, using common reservoir data. The methodology provides correlations for estimating in-situ stress regimes based on the regional data and rock mechanics parameters based on its lithology. Afterwards, in-situ stress and rock mechanics parameters are used to investigate the effect of stress anisotropy on the mechanical stability of the borehole for various inclination angles. Three failure criteria are reviewed to assess borehole stability, namely Mohr-Coulomb, Drucker-Prager, and modified Lade criteria. These failure criteria are combined with linear and isotropic rock mechanics behavior. Evaluation of stress behavior of a variety of rock lithologies was performed, using failure criteria. Results indicate that the modified Lade criteria tend to be more realistic than the Mohr-Coulomb and Drucker-Prager criteria for these evaluations. Furthermore, this study confirms that, for any type of rock, an increase in borehole inclination angle increases the risk of borehole instability. Sensitivity analysis, based on various reservoir parameters, is performed and confirms the reliability of the correlations reviewed. The methodology presented here can be used as an indicator of borehole instability risk before drilling. Subsequently, the results can be used to optimize drilling design and enhance safety in drilling operations. Introduction Accurate planning before commencement of a drilling operation is necessary to achieve a cost-saving and stable borehole. One of the most crucial aspects of a drilling operation is wellbore stability; wellbore instability can jeopardize the process and achievement of drilling goals. Depending on the source of the problem, wellbore instability is classified as either mechanical or chemical. Chemical wellbore instability, often called shale instability, is most commonly associated with water adsorption in shaly formations, where the water phase is present and can cause borehole collapse. In contrast, mechanical wellbore instability is caused by applying mud of insufficient weight, which will create higher hoop stresses around the hole-wall. Hoop stresses around the hole-wall are often excessively high and result in rock failure. The most rapid remedy for this instability is to increase mud weight and/or adjust the well trajectory for high-angle wells. The mechanical instability occurs as soon as the new formation is drilled, but chemical instability is time dependent because shales are subject to strength alteration once exposed to different drilling fluids. A series of experimental studies led to the conclusion that shale strength decreases with time when the shale is exposed to most drilling fluids.1 Despite the tendency of shale to experience chemical instability, it can also experience mechanical instability simultaneously, which can lead to a more complex problem. In this paper, only mechanical wellbore instability is considered. This study has three objectives. The first objective is to present the extensively used correlations for estimating rock strength and in-situ acting stresses, based on the limited input data and conservative assumptions to proceed with the wellbore stability analysis. The second objective is to demonstrate the use of risk analysis theory in wellbore stability, which incorporates the uncertainties of the rock strength and in-situ stress parameters. The risk analysis theory allows for stochastic modeling in assessing the outcome of wellbore stability analysis. The third objective is to extend the stochastic wellbore stability analysis in evaluating the borehole collapse risk in high-angle or horizontal wells in underbalanced drilling operations.