As part of the field development plans for the South Oman oil field, four trial wells have been drilled to assess the impact of steam injection on productivity and recovery. At the outset of the trial, it had been determined that lost circulation was a major drilling problem in the UeR formation in this area. Lost circulation (LC) is a phenomenon wherein circulating drilling fluid is lost to fractures and pores in the rock formation rather than returning to the surface through the wellbore annulus. If not corrected beforehand, the lost circulation that occurs during the cement job can lead to incomplete sealing of the annulus, which may require remedial cementing to attempt to fill the annulus. Moreover, voids in the annular space are potentially disastrous, especially in steam injection wells, because of the expansion of the pipes when exposed to elevated temperature during the steam injection. Typical reported LC-based problems such as buckling of casing, bi-axial collapse, and wellhead growth are the results of poor cement coverage behind casing strings. The normal practice is to stop the losses either with loss circulation material (LCM) pills or with cement plugs of a recipe different from that used to mix the original slurry. These methods were tried in the first well in this campaign; however, they did not give satisfactory results. As an alternative approach, an improved fluids train was programmed that would maximize the benefits of the cement slurries. This new technique was successfully applied and subsequently used in the next three wells in the campaign. The same technique was also used successfully in subsequent wells drilled in the field. As a result of the successful curing of losses, the production casing could then be cemented to surface. The successful curing of losses with the new technique has also led to rig-time savings, and bond logs also indicate good cement bonds. Introduction Lost circulation is the phenomenon wherein circulating drilling fluid is lost to fractures or pores in the rock formation rather than returning to the surface through the wellbore annulus. LC can cause serious problems during drilling and while cementing casing across the LC zone. If not corrected beforehand, the LC that occurs during the cement job can lead to incomplete sealing of the annulus, which may require remedial cementing to attempt to fill the annulus. Moreover, voids in the annular space are potentially disastrous, especially in steam injection wells, because of the expansion of the pipes when exposed to elevated temperature during the steam injection. Bridging materials used as drilling mud additives for LC control in drilling are ineffective in plugging large fracture apertures. The standard LC treatment in such a situation is to fill the loss zone around the wellbore with cement. In the late 1980s and early 1990s, a total of eight vertical producing wells were drilled in the South Oman field. Oil from this field is quite heavy and viscous (4,000-20,000 cp). Given the high viscosity, production rates were extremely low. Steam-soak stimulation was carried out and proved a technical success. Subsequently, four trial wells were planned to be drilled in the same field to assess the impact of steam injection on productivity and recovery. LC, attributed to a severely fractured formation, is a significant drilling problem in the UeR formation in this area. For the steam injection wells, it was apparent that these losses had to be cured during the drilling phase to protect the subsequent cementing activities. If the loss zones were not treated, cement could be lost to the open formation during the casing cementation and result in a poor bond between the casing and rock formation, ultimately leading to:Buckling of casingBi-axial collapseWellhead growth
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