A new line of auto-fill float equipment has a filter system that prevents cuttings from clogging valves.

When running casing into the wellbore, certain restraints must be observed to prevent damaging the formations through which the casing is run.
Drilling fluids must be formulated with certain weight and viscosity properties to counteract the pore pressure without exceeding the fracture pressure. Once a formation is fractured, it will never fully recover its basic strength. In addition, fractures can result in lost circulation, in which case large amounts of expensive mud enter the formation, often damaging future production. Therefore, great care should be taken to avoid fracturing while running casing.
When running casing into the hole, the mud must be displaced at a speed that is directly proportional to how fast the casing is run in. Running casing with a closed end (normal float equipment situation) causes the mud to flow faster up the annulus, whereas in a situation where the pipe is run in open-ended, the fluid velocity in the annulus is reduced. The increase in mud velocity in the annulus while running casing results in surge pressure, and when combined with increases in equivalent circulating density (ECD), the total fluid pressure can result in fracturing or wellbore damage.
Surge pressure is dependent upon several factors:
• mud weight;
• mud viscosity;
• mud gel strengths;
• mud velocity (as a function of pipe speed);
• annular clearance;
• hydrostatic pressure;
• pipe length; and
• pipe inner diameter.
Mud weight cannot be reduced, as it is needed to control the well. Viscosity must exceed the cuttings slip velocity to clean the wellbore. Gel strength is a necessary evil for cleaning the wellbore and often is related to viscosity characteristics. So the only factors remaining to reduce surge pressure while maximizing run-in speed are annular clearance or fluid velocity. Both are related to some degree.
Minimizing the cost of tubulars requires minimizing the number of casing strings and reducing the overall tonnage of steel by selecting small-diameter pipe. This must be balanced against the goal of having a large production tubing for rapid production and ease of downhole re-entry. The end result is that tubulars in the wellbore have a minimal clearance between them, such as 113/4-in. pipe inside 135/8-in. casing, with a 95/8-in. string through that combination. These narrow annuli cause the displaced fluid to travel at even higher velocities unless the pipe speed is slowed to unacceptable rates.
Therefore, the only acceptable way to reduce surge pressure is to take the mud inside the casing up a path of less resistance, thereby lowering the fluid velocity up the annulus and reducing frictional pressure loss. This way, any cuttings that may add to the overall hydrostatic pressure are kept inside the casing where they cannot damage the formation.
The Weatherford MudMaster System was designed for use in deepwater cementing applications where surge pressures present a problem while running in casing. Standard auto-fill equipment had tried to reduce surge pressures by simply opening up the inner diameter of the float equipment. Unfortunately, this caused cuttings and debris remaining in the mud to clog the valves inside the float equipment. In one operator's deepwater well, excessive cuttings were left in the riser due to the difference between fluid velocity and slip velocity while drilling. Standard auto-fill float equipment, which acted like a vacuum cleaner while running into the wellbore, became clogged with cuttings and eventually blocked. Cuttings and other debris also can settle inside the casing, preventing any effective face seal when the casing wiper plugs land.
The challenge for the deepwater Gulf of Mexico well was to come up with a system that could resolve the problem when the operator could not guarantee a clean mud system for running auto-fill float equipment. Weatherford devised the basic concept of a downhole mud cleaner system known as the MudMaster and then applied for a patent.
The MudMaster System has several advantages over casing strings equipped with conventional float equipment, starting with an extremely large auto-fill flow area, equal to a 4-in. equivalent inside diameter, or 12.5 sq in. This reduces the surge pressure placed on the wellbore during run-in, thereby increasing the allowable casing running speed. The filter shoe keeps wellbore debris from entering the casing string during run-in, keeping all the internal components contaminant-free. The self-cleaning landing collar allows the stabilizing plug to latch in, creating a barrier to separate the cement from the displacing fluid. The stabilizing plug and landing collar are capable of withstanding high bump pressures and high back pressures. In addition, the drill pipe dart has a small outer diameter, which allows the same dart to be used in a range of drill pipe and casing running components without any adjustments to the plug or dart. And the whole system is easily drilled with a PDC bit.
This fairly simple concept offers a new way to solve an old problem with minimal cost. The product can be purchased as a complete system including the filter, float collar and plug, or the filter shoe component can be used with other vendors' float equipment.

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