Abstract

Simulators have been developed to study fracture and collapse behavior around boreholes. To take into account the directional properties of real rocks, an anisotropic stress model is used. In addition to take into account anisotropic elastic properties, directional shear and tensile strengths are also included. The orientation of the borehole, the in-situ stresses and the bedding plane can all be arbitrarily oriented relative to each other to model field situations. This paper presents some of the results of this model. It is shown that by neglecting the anisotropic effects, an error is introduced. Also, by studying the anisotropic model, further insight into the behavior of the borehole is gained.

Introduction

The integrity of the borehole plays an important role in many well operations. During drilling, lost circulation and borehole collapse causes economic losses, and in production operations controlled fracturing and sand control is of utmost importance for the economy of an oil field. Therefore, a better understanding of the rock mechanics is necessary to improve the economy of the well operations. In the past, there have not been accurate methods to predict critical fracturing or collapse pressures. Simple isotropic stress equations have been used to sane extent, but these fail to take into account real rock properties, which are clearly anisotropic. Sedimentary rocks have a laminated structure, with directional elastic properties as well as directional shear and tensile strengths. To better understand field situations, a complete mathematical model was developed, which takes all directional properties into account.

Figure (1) gives an overview over typical borehole problems. Only two cases are covered in this paper, namely borehole collapse at low borehole pressures and fractures initiating along the borehole axis at high pressures. Our analysis is limited to wells deeper than 2000 feet (Aadnoy and Chenevert). Plastic or time-dependent effects are neglected. Figure (1) shows the versatility of the simulators. The borehole, the in-situ stresses and the rock property reference frame may assume any orientation. This gives us a tool to model any field situation.

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