Composite Bridge Plug Technique for Multizone Commingled Gas Wells

Abstract

Abstract Composite bridge plugs save operator time and money by enabling quick and easy installation and removal. While easily deployed on either standard electric line or coiled tubing equipment, composite bridge plugs (CBPs) can be removed in one-fourth the time required to mill traditional cast iron bridge plugs and with reduced formation damage. CBPs can be installed under pressure in multi-zone, commingled gas wells without expensive formation kill-weight fluids. After remedial operations, CBPs are removed with coiled-tubing-conveyed milling equipment using low viscosity milling fluids that minimize formation damage and easily remove composite cuttings from the wellbore. This is especially important because of the low annular velocities characteristic of coiled tubing operations. The wellbore is left cleaner than those using cast iron bridge plugs in similar operations. CBPs can withstand high pressures for up to 10 days without compromising their pressure integrity and without the need for a cement barrier on top of the CBP. Initial field runs have demonstrated that CBPs can be milled as quickly as 30 minutes compared to 2 hours for milling conventional cast iron bridge plugs. Multi-zone, commingled gas wells using as many as 7 CBPs have been brought back on production after taking only 12 hours to remove all CBPs, including rig-up and rig-down of the coiled tubing unit. Field runs also indicate CBPs can be milled quickly when set at depths exceeding 19,000 feet. Introduction The CBP was developed primarily to give the operator the option of setting several cased hole bridge plugs in a single wellbore to isolate multiple zones, and quickly remove them from the wellbore using conventional milling tools after the remedial operation has been completed. Composite bridge plugs enable the operator to independently treat and test several zones, selectively isolate each zone, and then remove multiple bridge plugs in an under-balanced environment. Typically, the under-balanced removal of composite bridge plugs is accomplished with coiled-tubing-deployed downhole motors and milling tools. Application An operator's main objectives in completing a multi-zone, commingled gas well are to independently perforate, treat, and isolate each zone of interest without damaging the formation, and to leave a clean wellbore that will produce the zones when the treatment of each zone is complete. Cast iron bridge plugs have been used in the past to isolate multiple zones in a wellbore but require heavy weight fluids to remove cuttings from the wellbore. These heavyweight fluids have the potential to damage lower pressure zones. For example, using a higher mud weight to remove a cast iron plug at the bottom of the well in proximity to Zone 1 may result in formation damage to Zones 2 and 3 (Fig. 1). In a well with multiple zones, using multiple cast iron plugs in a single wellbore can make removal of the plugs very difficult due to the heavy weight of the cast iron cuttings under the mill and the difficulty in circulating the cast iron cuttings from the wellbore. Fig. 1 shows a typical scenario for a multi-zone, commingled gas well. The well contains three distinct zones of interest with varying bottom hole pressures where each zone needs to be perforated, treated, and isolated. In Figure 1, Zone 1 has a bottom hole pressure equivalent of 11.4 lb/gal fluid weight, Zone 2 has a bottom pressure equivalent of 8.8 lb/gal fluid weight, and Zone 3 has a bottom hole equivalent of 10.2 lb/gal fluid weight. The varying bottom hole pressures can make remedial operations difficult, especially when a zone of higher bottom hole pressure equivalent is below a target zone. Any attempt to use kill-weight fluids or frac fluids on Zone 1 could result in formation damage to Zones 2 and 3 after the zones have been perforated.