SS: Deepwater Cementing: Key Factors to Consider for Sidetrack Plug Success in Deepwater Cementing Operations Using Synthetic Based Muds
Authors: Fuller George Allen , Edwards Joseph 
Published: April 2012

The industry is routinely tasked with setting plugs to redirect the drill bitaway from the pilot hole to intersect reservoir targets through deviated tohorizontal well paths. Well-bore data such as actual bore-hole size throughoutlength of plug to be set, accurate bottom-hole circulating temperature (BHCT), and drilling fluid properties must be modeled within a short turn-around alongwith correct bottom-hole assembly (BHA) for successful sidetrack operations. Formation characteristics also play an important role in the cement slurrymakeup for placement stability, anchoring, and compressive strengthdevelopment. Detrimental slippage effects or downward plug movement resultingfrom inclination angle, mud and cement density differential and rat-hole volumecan lead to cement slurry contamination. These key factors are many timesoverlooked and lead to major slurry contamination of plug placement along withmany hours of repetitive operations and costly rig time. Setting successfulsidetrack plugs in Synthetic Based Mud (SBM) environments has alsotraditionally been more challenging. Special considerations forspacer/surfactant/mud testing are required to effectively " water-wet" theformation to provide a bondable surface. Technological improvements in thetesting of spacer and surfactant package formulations provide a morequalitative method for optimum surfactant design to maximize mud removal andprovide a bonding surface to formation. Shale sensitivity to fresh water fluidsmust also be considered in both spacer and cement designs to mitigate well-boreinstability after drilling operations resume. Deepwater drilling with syntheticbased mud projects are increasing globally along with associated rig costs. Anunsuccessful " first attempt" to sidetrack can result in significantnon-productive time. Specific deepwater case histories will be presented toprovide a qualitative feedback on these recognized key processes and how theyare performing in deepwater GOM operations.

Introduction

Down-hole drilling progress and directional target depths may be changeddrastically as a result of any of the following reasons: stuck BHA and/or adrill-string failure, mud losses due to weak zones or wellbore instability, andkey geologic objectives not encountered. As a result, rapid planning, design, and execution involving sidetrack plug operation must be communicated in orderto minimize down-time and lead to effective sidetrack. Past experience andnumerous field studies have identified some key factors which can each lead tosidetrack / kickoff plug failures. Some of the primary causes are: ineffectivemud 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 optimumplug setting interval for sidetrack many times may require that significantopen-hole length or rat-hole exist below the base of the sidetrack plug. Anumber of documented studies have investigated the wellbore conditions that canlead to downward plug movement. Prior to setting the main sidetrack pluglocated at the optimum wellbore deviation point from the old bore-hole, attention must be devoted to planning for a barrier to support the cement andresist the plug's movement downward. Without a well developed barrier plug orbase, contamination will ensue with mud leaving an unsuccessful plug forsidetracking purposes. Documented industry studies have concluded thatsuccessful cement plugs that remain inplace utilizing a proper barrier plug orbase depend on the interrelationship between the wellbore angle and bore-hoediameter, mud properties such as yield point, static gel strength of SBM, anddensity difference between the cement and mud left in the rathole. 1,2

Synthetic based (SBM) mud systems lead the deepwater drilling industry inenvironmental acceptance, recognition for drilling performance, boreholestability and typical near gauge bore-hole diameters. With all the successesassociated with drilling performance of SBM's, the transition from the drillingoperation with a synthetic oil mud(oil-wet phase) and wellbore preparation tospot a sidetrack plug hinges on the design of a suitable spacer formulation forcementing and involves optimum testing of and the proper selection of asurfactant package to effectively " water-wet" the pipe/cement interface and thecement/formation but not destabilize any sensitive zones nor adversely affectthe mud properties at the spacer/mud interface as it travels uphole. Laboratorymethods used for determination of surfactants have been recommended by API suchas the glass rod test. Results obtained from this test can be somewhatsubjective and quality control sensitive. The final interpretation of theseresults alone without further assessment of phase changes from oil-wet towater-wet surface and how rapidly this phase change occurs has lead the way formore instrument precision to qualify the type and quantity of surfactants. As aresult, the apparent wettability apparatus was introduced in November of 1998and has gained acceptance for identifying an optimum surfactant packageconcentration eliminating much of the subjectivity associated with the glassrod test. Phase inversion from oil-wet to water-wet surface can be quantifiedmore accurately and consistently with consideration to down-hole temperatureeffects. 3