Drillable Tailpipe Disconnect: Used Successfully in More Than 120 Wells Worldwide

Abstract

There are a number of challenges associated with setting cement plugs in an openhole well. Most importantly, drillpipe can become differentially stuck across a lost-circulation zone, and the plug may become contaminated with the intermixing of the mud resulting in inadequate isolation or insufficient strength. Cement plugs are used for various reasons including healing losses, abandonment, and directional drilling. It is essential to these operations that a competent cement plug is placed the first time. The value of placing the designed cement plug properly is measured by nonproductive rig time, wasted material, and additional cementing services. An innovative tool and a special process1 were designed to meet the challenges associated with setting cement plugs. The tool connects sacrificial/drillable tubing to the drillpipe and allows an operator to trip into the well and spot the cement plug across the problematic zone. Once cement is placed, the tool is disengaged and the operator trips the drillpipe out of the hole, leaving the cement plug and tubing undisturbed. The sacrificial tubing can be drillable; therefore, the operator can drill through the plug or commence other operations as required. This paper discusses the challenges operators face when setting cement plugs and how the risk and nonproductive time are reduced with this innovative plug-setting process and tool. Well examples are documented from case histories to illustrate the success and lessons learned. Introduction Drilling for natural resources all over the world is not getting easier. To maintain current production levels in a demanding market, operators are forced to either drill into mature fields or into unconventional reservoirs. Production from a mature formation will reduce the pore pressure and this reduced pore pressure can lead to a lower fracture gradient. The chances of lost circulation while drilling into these lower fracture-gradient zones continue to increase. As the reservoir depletes, an operator might drill the next well deeper to capture the resources from a deeper zone. This too exposes the operator to the risk of drilling through this shallower depleted zone. Unique challenges are likely to occur when an operator decides to drill an unconventional reservoir, for example, lost circulation of drilling fluids to a cleated or vugular formation may occur. A properly designed cement slurry is the best choice to heal these lost-circulation events.2,3 Cement can be squeezed into the lost-circulation zones and when set, will strengthen the wellbore. The cement plug is drilled through, leaving the residual cement inside these problem zones. Cement plugs can be placed by bullheading down drillpipe into the loss zone or balanced across the loss zone. There are challenges with both of these operations. When bullheading cement, the zone does not get full coverage. This can lead to cement not deep enough or dehydration of the cement below the plug. When drilling through this plug, losses are usually experienced again. For the greatest chances of success, an operator must place the drillstring down into the lost-circulation zone. This however is a risky operation. The loss of drilling fluid may create a differential pressure across this problem zone and could cause the pipe to stick to the wellbore. Even if the plug is balanced successfully and no pipe sticking occurs, the plug could be compromised as the drillpipe is pulled out. The swabbing effect introduced by pulling out of the thick cement slurry could cause intermixing of the cement and mud. This contamination can lead to an insufficient seal or a soft top. A disconnect assembly (Figs. 1–3), bottomhole kickoff assembly disconnect (BHKA disconnect), was successfully introduced in 1999 in Australia to set a plugback plug using a predrilled appraisal section of the well. A second disconnect assembly Figs. 4–5 was introduced in the fall of 2002 in the Monument field near Hobbs, New Mexico to solve loss circulation problems. A tubing-release tool (TRT) provides a method of placing a sacrificial tubing into the loss circulation zone and balancing the cement plug. The sacrificial tubing is released from the drillstring and remains in the wellbore. Cement covers the full loss zone; pipe sticking is not a concern; and the plug is not disturbed once the cement is placed. A fluid retention mechanism in the TRT prevents energized fluid from the workstring from dumping onto the cement plug.

Plug-Setting Aid Retooled for Uphole-Downdip Plugback Application Enables Pinpoint Slurry Placement in Complex Updip Wellbore

“…When approached with the operational problems with spotting cement plugs in an updip-downhole wellbore, the authors reviewed conventional plug-setting aids available and tried to identify the components that could be applied in an updipdownhole wellbore while also redesigning those components that could not be used successfully. Specifically, tail-pipe disconnects introduced by Rogers et al (2006) and the bottomhole kickoff assembly (BHKA) (Fig. 2) introduced by Araiza et al (2007) were reviewed for application in updip-downhole wellbores.…”

“…The value of placing the designed cement plug properly is measured by nonproductive rig time, wasted material, and additional cementing services. An innovative tool and a special process (Rogers et al 2006) were designed to meet the challenges associated with setting cement plugs. The tool connects sacrificial/drillable tubing to the drillpipe and allows an operator to trip into the well and spot the cement plug across the problematic zone.…”