Wireline Deployed Metal Sealing Bridge Plug System: Operational Learning Curve and Subsequent Redevelopment

Abstract

This paper will focus on a project to redevelop a wireline deployable metal-to-metal seal, ISO 14310 VO qualified, retrievable bridge plug system to allow it to be conveyed successfully in a high rate gas well for acid stimulation zonal isolation operations. Functionality of the original system including the plug and hydrostatic setting assembly had previously been tested in a fluid environment yet envisioned for a gas well application. Initial well installations gave results that led to a reconsideration of the forces being generated by the running and pulling tools due to the differences between a gas and a fluid environment and their impact on system reliability and functionality. The paper will discuss some of the problems discovered with the system for this particular application and the system redesign over a one-year period, which included a number of qualification tests performed at near to real well conditions. This discussion will include information on the complexities of performing such tests and the solutions used to develop a successfully qualified "fit-for-purpose" redesign. The paper will conclude with a high-level overview of the system's first deployment in an actual field well application. Introduction In mid 2006, the operator ran a specifically developed high expansion, retrievable, metal-to-metal (MTM) bridge plug on three 7-in. monobore wells to provide mechanical zonal isolation for high rate, multi-stage acid treatments. The acid stimulation objectives in these three wells were fully met and the MTM seal performed to expectations; however, there were significant operational problems related to the plug system. The MTM bridge plug had been selected due to previous operational problems with conventional elastomeric element type retrievable bridge plugs as well as a number of advantages that the technology presented over these conventional types of plugs for the operator's specific application. Over the course of a year, the system was redesigned to address specific findings from those initial operations. The redesign involved modifications to the plug and the inclusion of additional components and functionality in the running and pulling tools to ensure repeatable system reliability and operability. Final system qualification was extensive and more comprehensive than normally performed for such a system. The redevelopment program and qualification testing demonstrated the importance of testing complete systems at conditions representative of those that would be encountered in actual well operations. Repeat testing was important to establishing system reliability. It also highlighted the need for a project-based approach with clear communication on a regular basis between multi-disciplinary, cross-functional teams. Completion and Reservoir Overview The operator has implemented two different types of big bore wells to deliver high flow rates. The well types are a 7-in. tubing monobore and a 9" x 7"-in. tubing optimized big bore (OBB). They are shown in Fig. 1. The reservoir formations are cased and cemented behind a 7-in. production liner in both types of well. Typical well deviations are approximately 55º. The reservoir formations have significant vertical extent and are initially over-pressured but at moderate temperature. Reservoir pressures may vary between zones and this causes cross flow between completed reservoirs. To produce at the design rates for an extended period, high volume matrix-rate-multi-stage (MRMS) acid stimulations are required. The substantial reservoir heterogeneities combined with the significant vertical extent of the productive zones present extreme challenges for acid treatments for all zones.

Effective Stimulation of Very Thick, Layered Carbonate Reservoirs Without the Use of Mechanical Isolation

“…As a result, both metal-to-metal sealing and elastomer sealing plugs were re-designed and have been fully qualified for North Field stimulations. 4,5 The elastomer sealing plug has been successfully utilized in nine recent applications in three vertical wells and one application in a deviated well. Recent deployment and retrieval of the re-designed metal-to-metal sealing plug in a North Field well was operationally successful and confirmed its suitability for matrix acid stimulation.…”