Deviated-Wellbore Cementing: Part 1 - Problems

Abstract

Summary Field experience suggests that in deviated wells, solids settling from the drilling fluid to the low side of the hole may adversely affect mud displacement during cementing. Large-scale laboratory cementing experiments confirm that in a deviated wellbore, solids settling from the drilling fluid can cause a continuous mud channel to remain along the low side of the cemented annulus. Tests also suggest that excess water in the cement slurry can result in a water channel along the high side of the cemented annulus in a highly deviated well. Introduction Over the past 50 years, substantial research has been done on cementing vertical wells. While much of this work also applies to deviated wells, there has been little research specifically directed at cementing deviated wells. Yet, as drilling has expanded into offshore and arctic areas, the number of deviated wells has increased. This has suggested the need to examine more closely how wellbore inclination may affect primary cementing. Past investigations have identified a number of critical factors that are conducive to successful primary cementing in vertical wells,1–4 including pipe centralization, pipe movement, low mud gel strength, high cement flow rates, and the use of preflushes. Most of these factors should also be relevant to cementing deviated wells. While a number of techniques for effective primary cementing are known, their application in a deviated well is often more difficult than in a vertical well. For example, it is generally more difficult to achieve good pipe centralization in a deviated well because the loads acting on the casing tend to force it toward the wellbore wall.5 These loads also tend to create high drag, torque, and bending stress that often limit pipe movement. The purpose of this paper is to demonstrate two problems that can significantly affect primary cementing in a deviated well:solids settling from the drilling fluid to the low side of the hole andfree-water breakout from the cement slurry to the high side of the hole. The tendency for drilled cuttings to settle to the low side of the hole has been cited as a formidable problem for directional drilling.6 Research has shown that in deviated wellbores, drilled cuttings Concentrate on the low side of the annulus and are difficult to remove.7 There does not appear to have been any previous investigation into the possible effects of solids settling from the drilling fluid to the low side of the hole on primary cementing. Free-water breakout from a cement slurry to the high side of the annulus, however, has been suggested as a cause of primary cement failure in directional wells. Webster and Eikerts8 observed that water channels occur along the high side of inclined plastic tubes that contain a cement slurry. They imply that a number of field cases of annular flow after cementing of deviated wells were caused by excess free water breaking out to the high side of the hole. Our investigation examines this phenomenon under more realistic laboratory conditions. Hole-Cleaning Problems Many authors have discussed problems encountered during cementing deviated wells.8–10 Operators have confirmed unwanted communication in some deviated wells behind pipe through perforations separated by hundreds of feet. While success or failure of a primary cement job depends on many factors, it is possible that in some of these wells, settled solids caused the mud on the low side of the hole to be difficult to displace. If the material on the low side of the hole is not displaced by the cement, a continuous mud channel will remain within the cement sheath. This reduces the integrity of the sheath and could lead to interzonal flow. Field experience has shown that it can be difficult to achieve good hole cleaning in a high-angle well. For example, Nance et al.11 reported severe hole-cleaning problems on a 52° [0.9-rad] well using the same mud system that previously provided good hole cleaning on a 28° [O.5-rad] well. In spite of steps taken to improve hole cleaning on the 52° [0.9-rad] well, the amount and size of the cuttings that reached the surface were significantly less than those of the 28° [0.5-rad] well, even though the penetration rates were the same.