Abstract
Wet shoe tracks are a common problem in oilwell cementing. The problem can range from a simple nuisance to many hours of lost rig time costing thousands of dollars. In most cases over retardation or bad cement plugs are blamed for this problem, but a new mechanism has been identified, which may be far more prevalent.
A "Wet" shoe track is defined as the occurrence of unset, contaminated or no cement in the casing section between float collar and shoe after a primary cement job. Wet shoe tracks are a common occurrence in oilwell cementing and many hours of lost rig time can be attributed to the problem. The most common explanation is; over retardation of cement slurry due to over estimation of BHCT. However, the authors have found another mechanism, which, to date, has not been addressed.
In JVPC Rang Dong oil field located offshore of Vietnam, horizontal wells are used to fully access the Miocene sand and Basement granite reservoirs. Productive zones are drilled with a thixotropic mud designed to enhance drilling. Ten-minute Gel Strengths of over 140 cp have been reported for these muds. While running production casing to depth, mud in the shoe track remains static for periods of many hours. Due to thixotropy, static mud gel strengths get very high and mobility is greatly reduced. Then, when the cement job is performed, cement slurry channels a small path through the viscous mud for some distance. The mechanism for this is two fold; 1) viscosity differential and 2) inside diameter (ID) change at the collar. Diameter change refers to the valve assembly O.D. versus the casing I.D. This profile forces fluid directly ahead. The analogy is – pinching a garden hose to make water travel further. These mechanisms can leave the shoe track with large amounts of mud and/or contaminated, unset cement. Further, thixotropic mud combined with solids settling in a horizontal wells makes the problem even more severe. "Low-side" mud/solids removal then becomes nearly impossible with conventional float equipment.
To address this issue, the authors made a simple design change to the standard float collar, which affects the velocity profile at the fluid exit point. A fluid diversion plate is inserted just below the collar check valve to create a 360-degree, turbulent, spray of fluid similar to the common showerhead (hereafter referred to as the Showerhead Float Collar or SFC). Using a taper profile on the holes in the diversion plate enhances turbulence. The result is complete mud removal within the shoe track even with highly viscous muds. This paper describes the new design and also reviews successful case studies from Vietnam.
