Borehole Instability in the Western Canadian Overthrust Belt

Abstract

Abstract Borehole instability is primarily a rock mechanical problem. Regional earth stresses and typical rock strengths are used to explain the widespread nature of irregularly shaped, fragile boreholes in the western Canadian overthrust belt (foothills). Drilling fluids and practices are investigated using a rock mechanical model of borehole failure. Although drilling practices can significantly affect problems associated with borehole wall failure, they are not the primary cause. Introduction Borehole instability is a major drilling problem in the western Canadian basin. A recent operator survey places the total industry dollars lost due to instability related drilling problems at $60 million per year (Can). Thirteen major operators estimate their average problem cost at $1.7 million per year. Individual wells in the foothills often experience instability problems that cost in excess of $250,000. Drilling problems associated with borehole wall failure are:Stuck pipe requiring fishing, washing over or plugging back and redrilling;Excessive time (days to weeks) reaming back to bottom after trips;Overpull on trips, connections and surveys; andSevere doglegs as a result of poor drill string stabilization in enlarged hole sections. Drilling personnel are well aware that shale formations cause the most severe drilling problems. However, the mechanism of failure has been the subject of considerable debate. Any valid model of borehole instability must explain the following shale sloughing characteristics:Exceptionally weak coals and shales can stick the bit immediately upon penetration;Most thick Cretaceous shales are stable during drilling but start to spall days or weeks after penetration;Considerable shale may be removed before the hole stabilizes - after a period of days or weeks, the sloughing may reoccur;Most boreholes in shales are out of gauge to some degree when drilled with water-based muds;Most sandstones and carbonates (limestones, dolomites) are close to bit gauge;Many boreholes are elliptical in shape with the long axis aligned parallel to the mountains - the short axis may be bit size or larger;The sloughing problems become more severe with time; andShale sloughing occurs over a wide geographic area and depth range in western Canada.