Lost circulation is the loss of bore hole mud to the exposed formations in the well and is one of the most common problem of oil well drilling. Mud flowing into the formation implies that there is less mud return at the flow line than is being pumped or that there is no return at all. In brief, there are 9 simple tips to help in mitigation of lost circulation problem which are as following:
- Casing Seat To Prevent Circulation Lost Problems
- Minimize downhole pressures
- Drill String Movement
- Minimize Circulation Loss Problem Severity By Decreasing ROP
- Mud Pumps Effect On Circulation Loss
- Drill String Whipping
- Casing & Drill String Design
- Wellbore Kicks
- Control Mud Properties
As this article is one of five articles to cover the lost of circulation problem, I recommend visiting this links for better understanding:
- Important Introduction For Lost Circulation Problem
- Causes Of Lost Circulation Problem
- Remedial Of Lost circulation Problem,
- Lost Of Circulation Material Used In drilling
Preventive Methods & Mitigation Of Circulation Lost Problems
Good planning and proper drilling practices are the keys to preventing lost circulation by minimizing excessive pressures on the formation. Several measures can be taken to prevent or minimize lost circulation:
1 Casing Seat To Prevent Circulation Lost Problems
Set the casing in the appropriate zone so the fracture gradient of the formation at the casing shoe will be sufficient to support the hydrostatic head of heavier muds required to balance pressures in the formations below.
2 Minimize downhole pressures.
Pipe movement should not exceed critical speeds when tripping pipe. When the drill string is run in the hole, there is a surging pressure from the piston effect of the bit and collars increasing the pressure exerted on the bottom of the hole. Good drilling practices will keep these pressure surges within the fracture and formation pressure.
Many wells experience lost circulation while running pipe or casing into the hole. The length of pipe in the hole affects the magnitude of the surge. Tests show that the flow of mud along the pipe creates most of the pressure surge. The longer the pipe, the greater the surge. Therefore, the deeper the well, the slower the pipe should be run into the hole as the depth of the bit increases.
Smaller annular clearances also increase surge pressures much in the same way annular pressure losses are increased as annular clearances decrease .
3 Drill String Movement
Rapid movement of pipe while circulating also causes even greater pressure surges. Rapid “spudding” of the pipe or fast reaming while circulating can create large surges. Wash and ream cautiously through bridges.
4 Minimize Circulation Loss Problem Severity By Decreasing ROP
Very high ROP loads the annulus with cuttings, thus increases the Drilling ECD, making any further surging on connections more likely to cause fracturing. It is important to control the ROP and circulate prior to making connections when the ECD is near the fracture pressure.
Maintain the cuttings concentration in the annulus below 4% to minimize the effect of cuttings on ECD.
5 Mud Pumps Effect On Circulation Loss
Rapid starting or stopping of the mud pumps can cause pressure surges. Starting the pumps too rapidly will create a pressure that can cause lost circulation, especially when breaking circulation on bottom after a trip. Part of the surge is caused by pressure required to break the gel structure of the mud.
Rotating the pipe when starting circulation will aid breaking the gel strengths and greatly reduce the surge pressure. The other part of the surge is the pressure required to accelerate the mud column to the normal circulating rate. Maintaining low gel structure and gradually increasing the pump rate will reduce this type of surge pressure. Breaking circulation at several intervals when tripping in hole is another way to minimize these pressures.
6 Drill String Whipping
Use enough drill collars to keep the neutral point in the Bottom Hole Assembly (BHA) to minimize drill string whipping.
7 Casing & Drill String Design
During the planning phase of the well, casings design and drill string design should be engineered for proper and safe operation, and also to optimize hydraulics for good hole cleaning and minimum ECD, especially in sensitive areas.
8 Wellbore Kicks
Avoid kicks if possible. Shut-in pressure at the surface is transmitted down the wellbore, often breaking the formation down at the weakest point. This not only results in loss of circulation, but losing control of the well.
Proper research, well planning and execution will minimize the possibility and severity of a kick. Those responsible for the operation at the wellsite should always be aware of the maximum shut-in casing pressure and volume. The volume of the intruding fluid is directly related to the shut-in pressures and should be minimized. If a well has to be shut-in, proper kill procedures should be used to maintain the right constant bottom-hole pressure required to kill the well.
9 Control Mud Properties
Viscosity and Gel Strengths
High Viscosity and Gel Strengths increase surge pressures each time circulation is interrupted and restored. They also increase the ECD while drilling. These values should be optimized to ensure good hole cleaning and solids suspension, and minimize ECD, surge and swab pressures. Many times mud properties can not be kept at a level which will provide adequate hole cleaning due to other operational considerations.
Higher flow rates and aggressive drill pipe rotation are the best methods to improve hole cleaning. High viscosity sweeps are recommended in such cases where good hole cleaning is questionable. These sweeps are usually made of mud from the active system that has been viscosified by additions of bentonite, polymers or Lost-Circulation Material (LCM).
The use of LCM in these sweeps is preferable in many cases since they are screened out at the surface and have no permanent effect on the viscosity of the mud. Controlling the ROP may be necessary if efficient hole cleaning can not be achieved. Although this may lengthen the rotating hours, it will generally be less expensive than the costs incurred by losing returns.
Drill Solids & Fluid Loss
Control drill solids at the minimum practical level and add proper treatment to minimize filter-cake build-up. Anything that reduces the annular clearance causes a pressure increase. Balling of the bit, collars, stabilizers or tool joints decreases the annular clearance.
In the case of extensive bit and/or stabilizer balling, a significant pressure will be exerted on the formation. An increase in drag or swabbing on connections are possible indicators of balling. Sometimes a ball can be pumped off a bit, but if that fails, the common practice of spudding the bit should be avoided. The combination of the reduced annular clearance and the pipe surge can cause the pressure to exceed the fracture pressure.
High fluid-loss muds deposit a thick filter cake that can reduce the annular clearance. The smaller annular space increases the ECD. Therefore, fluid loss and filter-cake thickness should always be controlled in the proper range. Mud that develops a thin, strong filter cake is more effective in preventing lost circulation to small fractures or pores.
Drill with minimum mud density. This not only enhances the ROP but also diminishes other mud-related effects.
Selection Of Bridging Material
A good selection of the proper size of bridging materials helps reduce and eliminate whole mud losses into porous formations. The choice of such bridging agents will depend on the formation characteristics. Generally, particles that are one-third to one-half the square root of the permeability in millidarcies (md) should be able to bridge such formations.