Wellbore Strengthening in Shale

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 110713, "A New Treatment for Wellbore Strengthening in Shale," by Mark S. Aston, SPE, Mark W. Alberty, SPE, and Simon Duncum, BP plc, and James E. Friedheim, SPE, and Mark W. Sanders, SPE, M-I Swaco, prepared for the 2007 SPE Annual Technical Conference and Exhibition, Anaheim, California, 11–14 November. The full-length paper demonstrates that wellbore strengthening in shale is feasible. A treatment pill was developed in the laboratory and field tested successfully at a US land-based location. The treatment consisted of a blend of particulates (known as stress-cage solids) and proprietary crosslinked gelling polymers that set with time. Properties of the system such as compressive strength, adhesion to shale, and sensitivity to temperature and pressure were evaluated. Modeling work was conducted to engineer the size and concentration of bridging solids required. Introduction Drilling in depleted zones, or intervals where high-pressured formations are interbedded with normally and abnormally pressured layers, has created the need for these strengthening technologies. The goal is to increase the fracture resistance of weaker formations and thereby avoid mud losses, wellbore instabilities, and the potential loss of the drilled interval. The primary consequences of these undesired events are an increase in nonproductive time and associated costs. In addition to avoiding these problems, the economic benefits that wellbore strengthening can provide are the possible elimination of a casing string or, more importantly, the ability to reach deeper reservoir targets. One of the most popular wellbore-strengthening technologies is inducing an increase in wellbore stresses by use of sized-particulate additions to the drilling fluid. The common name for this approach is "stress caging." The principle is to increase the hoop stress around the wellbore by using fractures to cause stress changes in the rock. These fractures would be held open with bridging material, thereby creating the stress-cage effect, or strengthening of the wellbore. This bridge of particles across the mouth of the fracture must have low permeability to provide pressure isolation of the fluid in the wellbore from that in the fracture, thus preventing any further fracture elongation. The degree of stress caging relative to the native stresses is a function of both fracture width and radius and the rock properties (i.e., Young's modulus and Poisson's ratio). To achieve this effect, the proper type, size, and amount of loss-prevention material (LPM) must be used. Using the stress-cage concept and associated factors, drilling fluids can be designed to maximize this strengthening effect and minimize losses and other problems encountered when drilling significantly above the fracture gradient. In fact, use of these LPMs for wellbore strengthening has led to several complementary technologies.