Best Practice for Cementing and Zonal Isolation Using the Jet-Type Perforate, Wash and Cement Technique
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Résumé
Abstract In 2014 ConocoPhillips (CoP) decided to focus on the Jet-type perforate, wash and cement (P/W/C) technique as its primary permanent well abandonment method for setting a full cross-sectional cement/steel barrier. Following this decision, a quality improvement project was launched to prepare for a transition from Cup-type to Jet-type P/W/C to improve barrier plug quality. The initiative was presented during the Stavanger P&A Forum (PAF) conference October 2014 for information and to indicate for specific vendors that there were opportunities for participation. The perforate, wash and cement method is complex and as with any cementing operation there is operational risk involved. This paper is written to share ConocoPhillips experience and learning from the improvement project which is still ongoing. It is specifically written for drilling engineers planning Jet-type P/W/C operation to help identifying key parameters to achieve a top-quality operation as well as managing the operational risk inherent to the operation. Quality requirement in P&A operations? It can be a hard sell to argue for costly fluids, BHAs (bottom hole assembly) and TCP (tubing conveyed perforation) guns to plug off a depleted reservoir where the test of time is hundreds of years from now as the reservoir gets pressurized again. The latter is not the case in the Greater Ekofisk Area (GEA) where active water injection increases the average reservoir pressure quickly and the established production strategy is dependent on good isolation of abandoned wellbores. To get a ‘’good’’ P&A plug you need to understand how to prioritize and control the variables such as cement properties, drilling fluid properties, drill pipe and BHA design, hole cleaning, ECD, displacement, TCP performance, operational sequence and many more. ConocoPhillips have made a structured approach to build a process which captures the basics of getting a high quality well abandonment plug. The displacement process itself has been modelled with computational fluids dynamics (CFD) software to understand key drivers for the placement of a cross-sectional P&A plug. This understanding has been mated with the practical limitations from a standard drilling unit to get a robust operational plan. To ensure the quality is maintained from operation to operation, the process have been accurately described in a Best Practice document. The operation itself is checked against a QA form to verify that execution was performed according to plan. The QA form is then filed as part of the P&A documentation, similar as a casing test during well construction. In the GEA experience a good P/W/C P&A plug is one set as per the Best Practice document where the operation is quality assured with an "all green" QA (quality assurance) sheet. A more tangible description is offered: After the operation the offshore cement sample which is contaminated with 10% mud will set up in the UCA (ultrasonic cement analyser) cell per programmed setting time plus no more than 3 hrs. The cement plug itself will be tagged at planned TOC (top of cement) +/- 20 ft. It will hold the positive pressure test to fracture gradient + 1000 psi. From time to time the plug will be drilled out. If that is the case, we expect to see an average weight on bit larger than 15,000 lbs for a 9 5/8’’ casing plug. The external well barrier element may be logged using a CBL/USIT (cement bond log) string or similar. In that event we expect to see "bond quality", which is a weighted interpretation of CBL, VDL (variable density log), flexural attenuation and pulse echo measurements as "good" to "very good" throughout the perforated section. This paper is one of many discussing the P/W/C technique (1) and is written to help you prioritize focus areas and optimize those. We think a high focus on QA will be your fundament, not only for a "good" P&A plug but also for a flawless execution of the complex offshore operation. Our results can be reviewed in Appendix B. Performance approach technical limit.
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