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Flow Drilling Surface Equipment
Surface equipment is a critical part of the entire system in underbalanced drilling. It is crucial in terms of success and safety of the drilling operation. A rotating control head is used to direct the returning fluids into the mud/gas separator while drilling. Rotating control heads (integrated with a BOP) are designed to operate at working pressures up to 2,500 psi.
The gas that is separated from the returning stream is forwarded to a flare, while the cuttings and fluids are collected in special separation pits. The most economical method of surface separation is the use of an atmospheric system.
In flow-drilling, the blowout preventer stack is composed of a rotating head or a rotating blowout preventer (RBOP) placed above a conventional blowout preventer (BOP) assembly, as shown in Figure 1.
The pressure rating of the BOP should be selected to withstand the highest expected pressure in order to provide the maximum safety for the rig personnel.
The RBOP allows the drill string to be moved up and down, as well as rotate, while under pressure. It needs to be used with extreme caution since gas can migrate up the annulus with returns, and could lead to very high surface pressures. It also provides a creative solution to the height limitation problem of most land rig structures.
There are two types of rotating BOPs:
- Passive type
- Active type
The passive type rotating BOP is also named as the rotating control head. It has two rubber components that establish the seal around the drill pipe. The operational life of rubber components is relatively short due to mechanical wear. The rotating wellhead provides a low-pressure way of diverting returns away from the rig.
The active type of rotating BOP equipment is designed for high-pressure applications. Different from the rotating control head, the packing elements are hydraulically activated. The internal packer element has two sections. The inner section can be replaced without replacing the outer element. Any wear on the packers is automatically compensated by an increase in the volume of hydraulic oil.
A typical flow drilling surface setup is shown in Figure 2. Safe operating backpressure is maintained by using a choke manifold. The components of this equipment should be visually and radio-graphically inspected after every single flow-drilling job.
Flow rates and pressures at the surface are controlled by gradually closing the chokes during flow drilling.
After passing though the manifold, the returns flow into a vertical mud/gas separator which should be at least 6 feet in diameter and 12 feet tall with a gas flare at its outlet.
After the gas is removed, oil, water, cuttings and the drilling liquid are directed to an open-topped skimmer pit, which consists of an gas-scrubbing device that is used to remove any detained gas. The open-top design enables the cuttings to be removed from the pit by jetting. Next, the drilling fluid is gravity flowed to another 400 barrels settling pit. After additional retention in this pit, the fluid is transferred to a drilling fluid return pit. Chemical injection in these separation pits may be used to break oil-water emulsions and aid in oil recovery and drilling fluid cleaning.
Because the pit has an open-top design, it has to be placed at a safe distance from the wellhead. If hydrogen sulfide is anticipated, oil separation should not be done in an open pit. Cleaned drilling fluid is kept in the drilling fluid pit, which is equipped with at least two centrifugal pumps to maintain the necessary pit level to ensure the proper gravity flow from the oil separation pits. The recovered oil from the primary and secondary oil separation pits is then directed into an oil transfer tank that is located at a safe distance from the drilling rig. The tank should also be equipped with a liquid level alarm.
