Drilling a surface hole with total losses through shallow aquifers is normal practice in Oman and most of the Gulf region. If losses cannot be cured while drilling, and the casing cement does not return to surface, required zonal isolation is most likely being compromised. During the life of the well, flow behind the casing from the shallow aquifers results in accelerated casing corrosion that compromises the integrity of the well and results in relatively costly workover operations to restore the well and allow it to be operated. Squeeze cement that repairs each corroded section of casing is a relatively high-risk and expensive operation with a low chance of success. During a workover on one of the water-injection wells in North Oman, severely corroded 9 5/8-in. production casing was confirmed. The initial investigation confirmed multiple leaks in the 9 5/8-in. casing, of which the exact location was difficult to establish; however, it was expected to have resulted from a failed 13 3/8-in. surface casing caused by corrosion from the shallow aquifers. Communication between the 9 5/8-in. production casing and the 13 3/8-in. surface casing was confirmed in multiple zones. Different options were evaluated to restore the integrity of this well. The options evaluated included the use of mechanical casing clads (expandables) to fix each corroded section in the 9 5/8-in. production casing before running a 7-in. cemented tieback string and the option to squeeze cement across each corroded section in the 9 5/8-in. production casing before running a 7-in. cemented tieback string. Both of these options were abandoned because of economic concerns and low chances of success. It was decided to approach this integrity repair differently by running a 7-in. tieback casing to surface and using foamed cement to cement the 7-in. tieback casing and existing corroded 9 5/8-in. casing to surface. To maximize the chance of success, the 7-in. tieback casing incorporated a multistage cementing collar to help ensure foamed cement returned to surface. The two-stage, foamed-cement job with controlled returns from two annuli might be considered to be the first of its kind, globally. The foamed-cement job was executed successfully as per program, with good foamed cement returns from both the 7-in. × 9 5/8-in. annulus and the 9 5/8-in. × 13 3/8-in. annulus. The overall job met all the objectives and this technique has been repeated successfully on subsequent wells with similar integrity issues.
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
In a north Oman field, it is common practice to cement vertical pilot holes up to the kickoff point. A cement plug is then used to initiate the sidetrack through extremely hard formations. Previously, it has taken several attempts to sidetrack these holes and, in some cases, a mechanical whipstock had to be used.There are challenges when trying to initiate the sidetrack because the formation hardness far exceeds that of the kickoff cement plug. As a result, the sidetracking assembly is repeatedly directed back into the relatively softer cement plug (i.e., vertically). Gravity also plays a role to some extent in bringing the sidetracking assembly back into the vertical hole, increasing the challenge.This paper documents successful implementation of modifications to the slurry design and cementing procedure, as well as the successful use of a side-tracking method/technique in North Oman.The compressive strength of cement has been increased from 5,000 to around 9,000 psi for standard, 17-lbm/gal, Class G cement.The effect of oil-based mud (OBM) contamination has been examined and spacer designs have been improved to help ensure good mud displacement while performing cementing operations.The sidetracking method and tools have also been improved with impressive results. This includes proper selection of the kickoff point based on the drill-speed log of the vertical pilot hole, selecting the correct combination and configuration (e.g., outside diameters of components and angle setting) for the bottomhole assembly (BHA), and using the correct application parameters and techniques while initiating the sidetrack, which is also called "time-drilling." This paper investigates the new techniques applied in detail and clearly illustrates their development. Some case histories are also presented.
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