Development and Use of a Through-Tubing Aluminum Cement Basket

Abstract

An ingenious and inexpensive removable through-tubing cement basket has been constructed for use in open or cased hole and highly corrosive environments. While the basket was designed specifically for use in deep sour gas wells with sulphur deposition problems, it can be used in any zonal isolation application. A conventional inflatable through-tubing bridge plug can be used in sour service, but its elastomeric components make the plug vulnerable in the presence of sulphur solvents and amine based corrosion inhibitors. The cement basket is constructed from 100% aluminum, which eliminates the problems associated with the elastomeric materials and assures complete removal with acid, thus leaving no debris in the well. In June 2000, the cement basket was successfully deployed in a deep, sour, horizontal gas well. The 54mm OD basket was run through the tapered 89/73mm OD tubing string before being expanded in the 127mm OD production liner and capped with 4 metres of calcium carbonate chips and 10 metres of cement to provide the pressure integrity. Once cemented, the basket became a plug providing well control for workover operations and a barrier preventing fluid and debris loss to the formation. Subsequently, the permanent packer was milled and retrieved, enabling a new packer and larger tubing to be installed. The aluminum basket with the carbonate and cement cap was successfully milled out using coiled tubing in less than three hours. A small quantity of acid was used to remove any debris from the cement basket and cement sheath from the liner wall. While the cement basket is a new development, it is deployed using conventional, field proven equipment. It is also simpler and offers the potential for cheaper solutions than a conventional expandable bridge plug. With the experience gained, the next challenge is to build a 42 mm O.D. plug to be set inside 178mm OD casing, which will be more universally applicable for wells in Western Canada and around the World. Introduction Downhole operations require the use of plugs for a variety of reasons. For example, downhole plugs may be used to isolate a section of a wellbore or to abandon a zone in the reservoir. They are also used to control production from a well or to permit selective zonal work such as stimulating part of the reservoir. Downhole plugs can also be used as a platform in conjunction with cementing operations or to prevent debris from falling deeper into the wellbore. Debris can be naturally occurring or can be generated by operations such as the milling of a packer. Plugs are also commonly used to prevent fluid loss to the formation during workover operations, which might otherwise result in formation damage. In the production of highly sour reservoirs, downhole conditions can be highly corrosive, due to the presence of aggressive chemicals such as downhole solvents like dimethyl disulfide (DMDS), corrosion inhibitors, H2S, CO2, and elemental sulfur. To be useful in the production of such reservoirs, a downhole plug must remain functional in such a highly corrosive environment. It should be possible to remove the plug without leaving debris in the wellbore, which could interfere with subsequent operations or production of reservoir fluids. This is especially true for wells that are prone to scaling and sulfur deposition and that have no sump to hold the debris below the location of the downhole plug, such as in the case of a wellbore that is highly deviated or horizontal through the producing zone.