A Statistical Approach to Estimate the Drilling Mud Window Based on Field Data and Ten Failure Criteria in Depleted Reservoirs - New Modified Failure Criterion
Authors: Alsubaih Ahmed , Sepehrnoori Kamy , Lopez Manriquez Alberto , Espinoza D. Nicolas , Allawi H. Raed , Hayfaa Swadi 
Published: August 2024

ABSTRACT:
Predicting the appropriate mud density in drilling operations is crucial in the construction of oil and gas wells, in order to mitigate the drilling problems and improve the drilling efficiency. This paper investigates ten failure criteria in directional wells drilled through different rock formations in the southern Iraq region. These rock formations experience sever drilling problems and colossal nonproductive time likely due to inappropriate selection of the mud densities with respect to directional drilling parameters. The geomechanical model proposed in this work was constructed from the available leak-off tests data, core analysis, and wireline geophysical logs. We calculated optimal mud densities from field data and show that the drilling densities in the field were inapropriate in many cases with respect to well inclination and azimuth. The Mogi Coulomb, Circumscribed Drucker-Prager, Modified Lade, and Modified Wiebols-Cook criteria predict optimal mud densities that are close enough to field observations. Also, we conducted a statistical analysis of the field data and core data and propose a new criterion to improve the prediction accuracy for shale rocks. This approach can be used as guideline to robust estimation of the mud windows in these problematic rock formations

1. INTRODUCTION
Despite modern technologies in the gas and oil industry, wellbore instability is the most crucial challenge in terms of cost and time to complete the wells. Every year, drilling operations cost more than eight million dollars due to wellbore instability problems (Peng & Zhang, 2007), which wastes 10 percent of the wells budget (Aadnoy & Ong, 2003). Therefore, the main stages of well planning depend on wellbore stability (Bell & Gough, 1979). Thus, to avoid wellbore collapses, the mud weight should be equivalent to control the stress concentration, and the drilling direction should be according to the direction of the stresses. The drilling mud pressure provides wellbore stability when the mud pressure is within the limits of shear failure and fracture gradients (Bourgoyne, Millheim, Chenevert, & Young, 1986). Far-field stresses exist inherently in the earth, which will be changed due to drilling operations when replacing the mud weight instead of the drilled rocks. In contrast, borehole stresses occur on the wall that can be controlled using drilling mud weight. The wellbore stability model necessitates the inclusion of rock strength properties that govern how the rock behaves under in-situ stresses. A rock failure criterion defines the stress conditions leading to failure. Thus, to ascertain wellbore failure stresses, it is essential to determine the rock strength, select an appropriate constitutive model, and choose a precise rock failure criterion. The prevalent method for examining mechanical wellbore failure is through linear-poroelastic modeling. For the secure construction of wells, in-situ stresses must be leveraged to predict the minimum mud weight required to prevent wellbore wall breakouts. Several models have been developed to estimate the optimal mud density required to avert shear or tensile failure, aiming to ensure drilling operations are both safe and cost-efficient. Nevertheless, certain models may not accurately predict the necessary mud density, leading to significant non-productive drilling time or, in the worst-case scenario, the loss of the well, necessitating a sidetrack operation.