1. Field of the Disclosure
The present invention relates in general to drill strings and, in particular, to a system, method and apparatus for regulating fluid flow through a drill string.
2. Description of the Related Art
Conventional oil and gas drilling typically includes pumping a quantity of fluid through a pipe or drill string to a drill bit for cutting the hole in the rock. The fluid is then circulated back up though the wellbore in the annular or outer section of the hole. Drilling fluid is beneficial to the drilling process since it clears away pieces of rock that have been cut from the bottom of the wellbore. Without this cleaning action the cut pieces of rock would accumulate near the drill bit and interfere with further drilling.
In general, the higher level of fluid flow that a drilling operation can achieve, the better that cut pieces of rock or “cuttings” are cleared from the bottom of the wellbore. However, there are several factors that limit the fluid flow level. One of these factors is the amount of pressure that it takes to pump a large amount of fluid. As the drill string becomes longer or narrower, the resistance to pumping a given amount of fluid increases, which increases the need for higher pressure. With any fluid pump set up there is a limit to the amount of pressure that can be overcome in order to make the fluid flow. Accordingly, the size or type of pump can limit the available flow rate.
Another limiting factor is the capability of the downhole mud motor. Mud motors are used to make the rock cutting drill bit rotate faster than the drill pipe that it is connected to. For example, a drilling operator may desire to drill while holding the drill string stationary, or may want to rotate the drill bit faster to achieve a higher rate of rock penetration. The mud motor works in a manner similar to a turbine in that the mud that flows through the motor turns a rotor that is connected to the drill bit. Energy from the pressure of the fluid flow is converted into rotational work by the drill bit. Mud motors are usually designed such that there is a maximum amount of flow that the motors are designed to handle. Forcing excess fluid through a mud motor can damage the motor and inhibit the drilling process.
The desire to flow higher volumes of drilling fluid through the well and the need to limit the volume flow rate due to the constraints of the motor can be conflicting. It would be desirable to flow as much fluid as is desired while ensuring that the motor did not experience a rate of flow higher than its design criteria.
A conventional solution to this problem is to form annular ports in the drill string above the mud motor. By choosing the size of the ports, the amount of flow that exits through the ports and the amount of flow that continues on through the drill string into the mud motor can be approximated.
A problem with this technique is that the amount of fluid that exits through the ports varies depending on the back pressure from the mud motor. The back pressure from the mud motor is a factor of the torque that it delivers. Thus, the more torque that is needed or generated by the motor, the higher the back pressure from the motor, which diverts more fluid through the ports in the sides of the drill string. More diverted flow means less fluid is transferred down through the motor. Less fluid to the motor reduces its torque and power, which can induce a situation where the motor stalls and needs more torque to overcome its bound condition. Conversely, an off-bottom situation where there is relatively low amounts of back pressure generated by the motor because there is no drilling torque resistance can result in a higher amount of fluid passing through the motor and a lower amount of fluid exiting the drill string. This too is problematic since a low torque situation causes the motor to spin faster at a given flow rate. Increased amounts of flow will only exacerbate this situation.
Some motor manufacturers attempt to solve this problem by drilling a hole through the rotor of the mud motor so that some fluid may pass through the tool without generating torque or causing damage to the motor. Unfortunately, since the drilled hole is static and does not change its shape to account for differing flow or pressure conditions, it is subject to the same limitations as the previously described method. Thus, improvements in controlling drill string fluid flow continue to be of interest.