Wellbore Stability Analysis: A Review of Current Methods of Analysis and Their Field Application

Abstract

Abstract Hole problems during the drilling phase of operations are often the consequence of mechanical wellbore instability. This leads to higher than necessary drilling costs. A number of analytical and numerical models are available for the diagnosis and prediction of wellbore instability; this paper reviews the merits and pitfalls of applying these models to field situations. Attention is focused on the peak-strength criterion and constitutive behaviour model. Anomalies from the incorporation of the intermediate principal stress into peak-strength criteria are highlighted. To illustrate the reliability of a number of models, their predictions are compared with laboratory results, and with a case history of a horizontal well drilled in the Cyrus Field in the North Sea. Introduction The increasing demand for wellbore stability analyses during the planning stage of a field arises from economic considerations and the escalating use of deviated, extended reach and horizontal wells. Wellbore instability can result in lost circulation (Figure 1a) where tensile failure has occurred, and spalling and/or hole closure (Figure 1b) in the case of compressive failure. In severe cases the hole instability can lead to stuck pipe and eventually loss of the open hole section. The causes of instability are often classified into either chemical or mechanical effects. Often, field instances of instability are a result of a combination of both chemical and mechanical effects. However, only the mechanical effects are considered here. Time dependent effects resulting from pore fluid migration (e.g. Detournay and Cheng [1]) are not considered either. A number of publications on the subject of mechanical wellbore stability can be found in the literature; however, only a few actually attempt to predict the stability of a field case. Most workers concentrate on specific aspects of an analysis, e.g. in-situ stress determination, stress concentrations around a borehole, rock mechanical properties etc.. This paper reviews some of the elements which go into the development of a wellbore stability model, and applies the model to a field case. The two main elements required in a wellbore stability model are the failure criterion and the constitutive behaviour model. A number of previously used criteria and behaviour models are reviewed, and their suitability assessed. Results are presented from a pre-spud analysis of a horizontal well drilled in 1988 in the Cyrus Field, North Sea. Comparisons are made between a linear-elastic analysis and a Finite Element Method (FEM) analysis using a constitutive model considered more representative of the reservoir rock. The well response during drilling indicated that the predictions of the FEM were significantly more accurate than the linear-elastic analysis. 2.0 BACKGROUND TO WELLBORE STABILITY MODELLING Before a well is drilled, compressive stresses exist within the rock formations (Figure 2). P. 261