Borehole Stability Analysis for Underbalanced Drilling
Authors: McLellan P. , Hawkes C. 
Published: March 2010

Abstract
Underbalanced drilling techniques are often considered to avoid or mitigate formation damage, reduce lost circulation risk, and increase drilling rate of penetration. However, drilling with a bottomhole pressure less than the formation pore pressure will usually increase the risk of borehole instability due to yielding or failure of the rock adjacent to the borehole. Numerous theoretical models for assessing borehole collapse and fracture breakdown risks exist. However, until recently, it has been difficult for non-specialists to use many of these models because they are not easily implemented, or because they required input parameters that are unfamiliar or difficult to obtain. A user friendly PC WindowsTM-based software package called STABViewTM has been developed to help the well designer determine the optimal range of bottomhole pressure for underbalanced drilling, i.e., the bottomhole pressures that are high enough to avoid severe hole collapse, yet low enough to avoid initiating hydraulic fractures. The software has been designed to perform rapid parametric analyses for all types of ells in most geological settings. Guidance in the selection of rock properties and in situ stresses is provided to the user with an online database of typical values and a comprehensive help utility. Applications of the software to underbalanced drilling of orizontal wells in a number of sandstone reservoirs are demonstrated.

Introduction
Underbalanced drilling technology is often considered for naturally fractured formations, partially depleted reservoirs that are susceptible to formation damage, heavy oil reservoirs that have been geomechanically disturbed by sand production, and in settings where improved drilling rates of penetration are required. Underbalanced drilling can have some positive effects on borehole stability. For example, shale formations containing reactive clays often suffer from hydration-related mechanical degradation, swelling, and pore pressure penetration when infiltrated by drilling muds that flow into the formation at overbalanced conditions. owever, in many cases, borehole instability can be made worse when bottomhole pressures are low. For example, low bottomhole pressures lead to an increase in shear stresses acting around the circumference of a well, hence leading to an increased risk of shear failure (Figure 1). Furthermore, the presence of steep inflow pressure gradients around a well can lead to tensile failure and spalling of the borehole wall (Figure 2).
FIGURE 1: Shear yielding occurs for underbalanced conditions due to the absence of a support pressure on the borehole wall.
There is usually an optimal window of bottomhole pressure that is high enough to avoid catastrophic hole collapse, yet low enough to avoid fracturing, differential sticking or unacceptable levels of formation damage. This paper describes the use of a commercial software package to identify optimal mud densities or circulating bottomhole pressures. The theory behind the stability model is briefly summarized, and its application is demonstrated with a number of field examples.
FIGURE 2: Radial tensile fracturing occurs due to steep inflow gradient.

Borehole Stability Modelling

Background
A wide range of modelling approaches are available for assessing borehole instability risks.