As drilling techniques advance and wellbores become more complex, placing successful cement plugs downhole on the first attempt becomes more challenging. Phenomena and well conditions can have a negative influence on the performance of the cement plug, significantly impacting drilling time and costs. When plug-cementing operations are required in an updip hole section, the following concerns become even more difficult to address:
Boycott or extrusion effects, especially in highly deviated wells.
Lost circulation problems caused by the narrow mud-weight window.
High concentrations of lost-circulation materials impede pumping at the desired rates for optimum displacement efficiency.
Shorter plug-length requirements, because reentries are becoming more popular in mature fields as an alternate way to look for different production zones.
Complex wellbore geometry.
Difficulty of effectively placing small volumes of slurry under deep, high-temperature conditions and washed-hole sections.
This paper documents a Middle East field application where an abandonment plug was required to be set in an uphole-downdip section of the wellbore. After several failed attempts using a conventional plug-setting aid, an application-specific plug-setting aid was designed that incorporated torque-capability to help enable the workstring to be run in hole (RIH) in an updip wellbore and an annular isolation element to prevent slurry fallback in the uphole-downdip hole section. Detailed customer expectations and performance criteria will be provided, resulting in a conventional plug-setting aid that was successfully retooled to enable slurry placement in an uphole-downdip wellbore.
There are a number of challenges associated with setting cement plugs in an openhole well. Most importantly, a drillpipe can become differentially stuck across a lost-circulation zone and the plug can 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 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. 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 continue other operations as required.
Although various types of plug-setting aids have been used successfully in many wellbore configurations, including vertical, deviated, and horizontal, the use of plug-setting aids in downhole-updip wellbores posed unique challenges to tool designers.