Going the Distance Cement Wiper Plug Technology

Abstract

The 13 3/8 to 14-in. cement wiper plug system is a critical component for successfully separating fluids during displacement of cement for long strings of casing in deepwater applications. With casing strings becoming longer, there was a need to validate the endurance capabilities of these plugs. This paper will discuss the methods used to test the endurance of a cement wiper plug system.

The test consisted of pumping the same plugs through 130 ft of casing repeatedly, until a maximum length of 40,000 ft was achieved. These plugs were displaced horizontally through the casing using water. This created the worst case scenario because no fluid would be in front of the plug: The elastomer is more susceptible to abrasion in dry pipe than in lubricated pipe. Each plug was dimensionally analyzed every 5,000 ft and then was validated by measuring the bypass pressure from the downhole side. The bypass pressure is directly related to the amount of surface contact between the plug fins and the casing wall and can be used to validate wiping efficiency. Theoretically, the farther the plug travels, the more likely the surface contact is reduced between the plug fins and the casing wall, and that is validated by measuring the bypass pressure.

The results of the endurance test are based on the relative wiping-efficiency test in the same inside diameter as the casing through which the plug is pumped. These results are compared to minimum allowable bypass pressure determined from a test using the largest published inside diameter. Results show that the bypass pressure decreases until a certain length and that at this length, contact force becomes significantly less, inadvertently causing the plug to wear at a slower rate. This relates to the reduction of the bypass pressure until a point where the pressure equalizes at approximately 20 percent less than the control.

Plug bypass can result in displacement errors, which further result in either underdisplacement leaving excess cement in the casing or over displacement, potentially contaminating the shoe tracks. Testing the ability of the plug to mechanically separate fluids and wipe the pipe eliminates the potential for bypass around the plug fins in the forward or reverse direction in long strings of casing.