Abstract
The aim of this abstract is to present examples whereby a decrease in the bypass area between the frac plug and the casing wall caused the wireline BHA to be unintentionally pumped off from the wireline weak point during plug and perf (P&P) operations. This abstract helps engineers involved in P&P operations better understand the dynamics, forces, and details connected with pump down operations.
In any cased hole wireline bottom hole assembly (BHA), the plug will have the largest outside diameter (OD). Plugs are typically being pumped down with less than 0.25 in clearance on each side of the plug. Considering the dynamics of the fluid velocity around the plug, they can experience some extreme forces. Most plug providers provide guidance on the balance of pump rate and line rate, their goal is to manage the flow bypassing the plug. Because the bypass around the plug is so much smaller than that of the casing, the velocity of the fluid around the plug is going to accelerate. When calculating the speed of the fluid passing the plug during pump down, it must be taken into consideration, the velocity of the plug, the pump rate and the casing and plug OD.
In the first example, a section of deformed casing was suspected to cause a decrease in fluid bypass area across the plug during pump down. The decrease in fluid bypass area increased the fluid bypass velocity which created a low pressure around the plug. With lower pressures around the OD of the plug, the rubber element responded to the lower pressure causing it to swell. This swelling of the element can cause it to contact the casing and cause an instantaneous spike in line tension which triggered the BHA pump off.
In the second example, continuous hard tagging of the plug against a restriction caused the plug OD to expand due to mechanical deformation, thus decreasing the fluid bypass area between the plug and casing ID. This did not allow the plug to pass through an up-hole restriction when it was decided to POOH. A suspected surface valve manipulation caused downhole fluid movement into the well which acted on the increased plug OD which caused the BHA pump off.
These findings help in better understanding of the P&A operations. This is achieved through the in depth understanding of the variables that contribute to the mathematical equations which calculate the fluid velocity across the plug and understanding its limitations. Whenever there are signs of casing deformation, downhole anomaly or restriction, poor cement behind casing or vacuum in the well, these are all warning signs of a potential issue that could affect the fluid bypass velocity across the plug. Running procedures in future well designs will be performed to mitigate those risks when there is a possibility of a critical decrease in fluid bypass area across the plug.