The industry is routinely tasked with setting plugs to redirect the drill bit away from the pilot hole to intersect reservoir targets through deviated to horizontal well paths. Well-bore data such as actual bore-hole size throughout length of plug to be set, accurate bottom-hole circulating temperature (BHCT), and drilling fluid properties must be modeled within a short turn-around along with correct bottom-hole assembly (BHA) for successful sidetrack operations. Formation characteristics also play an important role in the cement slurry makeup for placement stability, anchoring, and compressive strength development. Detrimental slippage effects or downward plug movement resulting from inclination angle, mud and cement density differential and rat-hole volume can lead to cement slurry contamination. These key factors are many times overlooked and lead to major slurry contamination of plug placement along with many hours of repetitive operations and costly rig time. Setting successful sidetrack plugs in Synthetic Based Mud (SBM) environments has also traditionally been more challenging. Special considerations for spacer/surfactant/mud testing are required to effectively “water-wet” the formation to provide a bondable surface. Technological improvements in the testing of spacer and surfactant package formulations provide a more qualitative method for optimum surfactant design tomaximize mud removal and provide a bonding surface to formation. Shale sensitivity to fresh water fluids must also be considered in both spacer and cement designs to mitigate well-bore instability after drilling operations resume. Deepwater drilling with synthetic based mud projects are increasing globally along with associated rig costs. An unsuccessful “first attempt” to sidetrack can result in significant nonproductive time. Specific deepwater case histories will be presented to provide a qualitative feedback on these recognized key processes and how they are performing in deepwater GOM operations.
Down-hole drilling progress and directional target depths may be changed drastically as a result of any of the following reasons: stuck BHA and/or a drill-string failure, mud losses due to weak zones or wellbore instability, and key geologic objectives not encountered. As a result, rapid planning, design, and execution involving sidetrack plug operation must be communicated in order to minimize down-time and lead to effective sidetrack. Past experience and numerous field studies have identified some key factors which can each lead to sidetrack / kickoff plug failures. Some of the primary causes are: ineffective mud removal combined with residual oil films, cement slurry design properties, insufficient cement volume used based on accurate bore-hole predictions, impatience regarding the WOC time of cement compressive strength development, and the BHA and placement technique for spotting the cement plug. An optimum plug setting interval for sidetrack many times may require that significant open-hole length or rat-hole exist below the base of the sidetrack plug. A number of documented studies have investigated the wellbore conditions that can lead to downward plug movement. Prior to setting the main sidetrack plug located at the optimum wellbore deviation point from the old bore-hole, attention must be devoted to planning for a barrier to support the cement and resist the plug’s movement downward. Without a well developed barrier plug or base, contamination will ensue with mud leaving an unsuccessful plug for sidetracking purposes. Documented industry studies have concluded that successful cement plugs that remain inplace utilizing a proper barrier plug or base depend on the interrelationship between the wellbore angle and bore-hoe diameter, mud properties such as yield point, static gel strength of SBM, and density difference between the cement and mud left in the rathole. Synthetic based (SBM) mud systems lead the deepwater drilling industry in environmental acceptance, recognition for drilling performance, borehole stability and typical near gauge bore-hole diameters. With all the successes associated with drilling performance of SBM’s, the transition from the drilling operation with a synthetic oil mud(oil-wet phase) and wellbore preparation to spot a sidetrack plug hinges on the design of a suitable spacer formulation for cementing and involves optimum testing of and the proper selection of a surfactant package to effectively “water-wet” the pipe/cement interface and the cement/formation but not destabilize any sensitive zones nor adversely affect the mud properties at the spacer/mud interface as it travels uphole. Laboratory methods used for determination of surfactants have been recommended by API such as the glass rod test. Results obtained from this test can be somewhat subjective and quality control sensitive. The final interpretation of these results alone without further assessment of phase changes from oil-wet to water-wet surface and how rapidly this phase change occurs has lead the way for more instrument precision to qualify the type and quantity of surfactants. As a result, the apparent wettability apparatus was introduced in November of 1998 and has gained acceptance for identifying an optimum surfactant package concentration eliminating much of the subjectivity associated with the glass rod test. Phase inversion from oil-wet to water-wet surface can be quantified more accurately and consistently with consideration to down-hole temperature effects.