Innovative Cement Plug Setting Process Reduces Risk and Lowers NPT (2006)

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 maybecome 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. Thevalue of placing the designed cement plug properly is measured by non-productive 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 is 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 non-productive 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 in Canada 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 will 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 to acleated 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 lostcirculationzones and when set, will strengthen the wellbore. The cement plug is drilled through, leaving the residual cement settinginside 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 thecement below the plug. When drilling through this plug, losses areusually experienced once again.For the greatest chances of success, an operator must place thedrillstring down into the lost-circulation zone. This however, is a risky operation.