Torque and Drag
The drag represents the force needed to push or pull the pipe in and out of the hole. Torque represents the required moment to rotate the pipe. The friction forces are generated in the opposite direction of the drill pipe movement and this is due to the drill string contact with the well bore walls.
Torque and drag are affected by many factors: well trajectory design, drilling fluid type, hole size, drill string design, hole cleaning. Torque and drag are important and serious issues for any extended reach well (ERW), because they can bring serious limitations for drilling operations. Excessive torque and drag can generate situations where casing cannot reach the planned depth. Also, generating the appropriate weight on bit is a real problem in long sections, for example, Cuttings bed increases the frictional drag and limits the ability to get weight to the bit.
Modeling torque and drag is required to plan for drilling operations because it can help to prevent drilling problems.
Torque and drag can be good signs of hole cleaning quality. It is common to record torque and drag data, especially during the last trip after achieving the total depth of a section. This provides a check for casing running.
Experience from ERD operations has led to develop many products and techniques in order to reduce torque and drag. For example: novel fibrous lost circulation material was used to drill the world record ERW at Wytch Farm.
Rotary Steerable System (RSS) can lead to Better ROP due to reduced effect of torque and drag. It was practically impossible to drill the ERD well with sliding mode. Orienting and maintaining with slide drilling can be achieved by rotating the drill string many revolutions and making the pipe to turn to the desired orientation. For example: at 8km or more the pipe may need up to 20 turns at the surface to turn the tool one revolution down hole. Low dog legs angles help in reducing torque and drag. Smoother well bore and better hole cleaning can be achieved by drilling with rotary steerable system.
Casing Running
Running long casing in extended reach well (ERW) is considered as one the most critical well operations. Well deliverability can failed to be achieved due to casing running problems. Friction factors can be higher due the poor hole cleaning, thus introducing running problems and more complexity.
The development in drilling industry has proved a reduction in torque and drag effects, thereby extending drilling limits. The successes in drilling operations have to be associated by a success of casing running to total depth. These have pushed to significant advances in casing design, running procedures and techniques.
The use of glass beads has reduced the torque while running casing. These beads act as bearing between the open hole and casing or drill pipe and casing. They can resist to a temperature up to 1400 °F and a pressure up to 48000 psi. they are chemically inert and has no effect on the base drilling fluid.
Many lessons were learnt while using PWD tools. They were involved in running long casings. More engineered are focused on casing running speed, because the level of hydrodynamic loads which are applied on formations can lead to bore hole instability. Surge pressure modeling was used to develop the running speed schedule to do not exceed the limits while running the casing string and ensure that the casing reaches the bottom. This is a result gained with use of PWD tools.
Conditioning the hole before running the casing is a common practice by circulating several bottoms up, in the same time checking the shale shakers. If the shakers are clean, the hole is considered suitable to run the casing, but this is not always true because it depends on the density and viscosity of the fluid pumped down hole. Using the PWD, the circulation is stopped when the real time ECD stabilized on the predicted value.
Many improvements have been achieved in running casing: low weight casing, invert casing strings, but even with these improvements, the casing running in very long and complex ERW was a problem. At Wytch farm running casing by partial or full floatation using an air chamber was a solution to overcome this problem, the production casing was run successfully with these techniques.
Casing floatation procedures consists to do not fill the casing in every joint run in the hole, this method makes casing virtually weightless while running in the mud. Even more, in some cases the casing is filled with different fluids: for example, mud in the upper or vertical part, and air in the lower part. The part which is filled with mud provides weight to run and push the casing down hole.
Using the same principle, a new method called New Improved Casing Floatation Methodology (NICF) is used to avoid some disadvantages of casing floatation methods. Casing collapse becomes a real risk while running casing with air chamber. The principle is running the casing with many separated fluids to improve casing collapse resistance. The fluids are separated by Buoyancy Assisted Casing Equipment (BACE). Different fluids can be used: air, water, lighter or heavier mud, when the casing reaches the planned depth, the pressure is increased to shear the BACE and circulate all the fluids to the surface.
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Fig 01- NICF Casing Running Methodology |
Barite Sag
Barite sag is a complex phenomenon and it is probably impossible to eliminate this process but it can be managed by a combination of good operational procedures and drilling fluid design. In deviated well, barite settlement can lead to down hole losses, mud weight fluctuations, stuck pipe and bore hole instability. Barite sag appears when mud is not circulated for long period of time.
Also, there are many factors which can accelerate this process: slow circulating rates, casing running, wire line logging. Increasing shear rate viscosity of drilling fluid can help to reduce the barite sag, rotating the drill pipe can also help to minimize the settling.
Barite sag is not always a static phenomenon, it is proved that barite beds can be formed while circulating and be thicker the flow is static.
Many benefits can be achieved by using particles with high density and very low mass. Micronized weight materials (MWM) have shown very successful behavior in drilling ERW.
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