This paper proposes a practical and cost-effective procedure to perform a wellbore stability analysis, including the elaboration of a safety mud window and the evaluation of the wellbore trajectory using drilling data (drilling reports, drilling parameters, mud properties, and mud-logging samples) that are routinely gathered in every well. The workflow includes the estimation of the rock geomechanical properties through a back analysis from the drilling parameters and the geomechanical events. These steps comprise the calculation of the pore pressure and stress state using the classical D-exponent method and Eaton's equation, along with the generation of a safety mud window and the evaluation of the wellbore trajectory using the Mohr-Coulomb (MC) failure criteria and the Kirsch's equations. Since the output of the proposed method depends on parameters that are not exactly known, significant engineering criteria and geomechanical expertise are crucial to achieve a quantitative analysis. The proposed approach was evaluated in a study case in a well drilled in the south-west of Colombia. The results were compared to those obtained from a wellbore stability analysis using geomechanical properties estimated from offset well logs, obtaining general agreement in magnitude and trend of the values. These findings suggest that drilling data, when properly used, can provide a reliable wellbore stability analysis for assisting post-drill studies, pre-drill forecasting, and decision making in real-time during the drilling operation. Moreover, the systematic application of the proposed methodology in several wells might allow updating the 3D geomechanical parameters at the reservoir scale.
Wellbore stability analysis has demonstrated to be an important aid for reducing drilling risks and costs related to unwanted drilling events such as loss of circulation, kicks, tight hole, stuck pipe, and hole collapse. The core of a wellbore stability analysis is an accurate one-dimensional mechanical earth model (MEM), whose accuracy is highly affected by the amount of information available for determining the rock geomechanical properties; which are the input parameters of the MEM. Oftentimes, the scarce available information is associated with the high costs of logging, coring, and lab testing programs that in some cases correspond to a large fraction of the total cost of the well; while in other cases, the lack of information is attributed to unsuccessful logging and coring runs in wells with high inclination or instability issues. The use of drilling data for estimating the geomechanical parameters of the drilled formations have been recognized since the 1960s (Cunningham and Eenink, 1959; Combs et al., 1968). A handful of models for estimating the pore pressure from drilling parameters have been proposed (Bourgoyne and Young 1974; Majidi et al. 2017), among these models the more traditional is the D-exponent method (Jorden and Shirley, 1966; Rehm and McClendon, 1971). Despite the theoretical assumptions needed to normalize the rate of penetration (ROP) and the neglect of drilling hydraulics, the D-exponent method is the most widespread in the industry, mainly due to its practicality and the low amount of information required for its implementation.
