Wellbore Stability: The Effect of Strength Criteria on Mud Weight Recommendations

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 linear-elastic analysis is frequently proposed for the prediction of the onset of failure, and consequently the mud weights required to prevent hole instability. However, there is no clear choice of which compressive failure criterion to use in the analysis. This paper assesses the influence of two commonly proposed paper assesses the influence of two commonly proposed failure criteria on mud weight selection. The failure criteria selected is shown to have an extremely significant effect on the computed 'safe' mud weights. Examples related to field situations are presented which highlight the anomalies and contrary evidence relating to the suitability of failure criteria. 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 la) where tensile failure has occurred, and spalling and/or hole closure (Figure lb) 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 mechanical effects are considered here. Due to the large costs associated with wellbore instability this subject has received considerable attention in the literature over the past two decades ([l] to [161). The majority of the literature focuses on analytical aspects of wellbore stability. In a previous publication [151 the authors reviewed strength criteria used in many of the models published and showed that certain criteria do not stand up to close scrutiny when compared with laboratory strength test data. In this paper the effect of strength criteria on mud weight selection is highlighted to show some of the anomalies and contrary evidence which can make the choice of a strength criterion difficult and confusing when performing wellbore stability analyses. 2.0 BACKGROUND TO WELLBORE STABILITY MODELLING Before a well is drilled, compressive stresses exist within the rock formations (Figure 2). With the exception of structurally complex areas (e.g. near salt diapirs), the in-situ stresses can be resolved into a vertical or overburden stress, , and two horizontal stresses, (the maximum horizontal stress), and (the minimum horizontal stress), which are generally unequal. When the well is drilled, the rock stresses in the vicinity of the wellbore are redistributed as the support originally offered by the drilled out rock is replaced by the hydraulic pressure of the mud. pressure of the mud. P. 9