Drilling problems include any difficulties encountered while drilling a well. The most common drilling problems are the creation of doglegs and key seats, hole instability, lost circulation, and excessive bottom hole temperatures.
A dogleg is usually defined as any deviation greater than 3° per 100 ft, and it occurs when a sharp change of direction is taken in the wellbore. Typically, a dogleg is caused by a change in the dip of the formation or by a change in the weight applied to the bit. Severe doglegs can result in stuck casing, drill pipe failure, and inability to run casing to total depth. If casing is successfully run through the dogleg, excessive wear on production equipment can occur. The use of properly placed stabilizers, large diameter drill collars, and the proper weight applied to the bit will minimize the formation of doglegs.
Key seats usually form as a result of dog legs. A key seat is formed when a channel or groove is cut in the side of the hole, parallel to the axis of the hole. The drill pipe dragging action through the sharp bend in a doglog creates the groove in the side of the wellbore. Key seats can be prevented by not creating doglegs.
Instability can result from the following phenomena:
- Overburden pressure
- Earth movement forces
- Pore Pressure
- Water absorption, swelling, or dispersion
Instability occurs when the relief of overburden pressure exceeds the yield strength of the drilled formation, resulting in flow of the formation (plastic flow). Abnormally high pore pressures can cause blowouts when present in highly permeable formations. If the pressure differential between the wall of the hole and the fluid in the hole is large, the formation may slough off. Structural stresses can also cause hole instability.
Problems associated with hole instability include the following:
- Ineffective hole cleaning
- Stuck pipe
- Bridges and fill-up
- Wellbore enlargement
- Increased mud volume
- Increased cost
- Poor cement jobs
- Logging difficulties
Hole instability can usually be controlled by use of the proper drilling fluid.
Lost circulation is the complete or partial loss of drilling mud into a formation. The loss occurs when the total pressure exerted against the formation exceeds the total pressure of the formation. Formations succeptible to lost circulation have the following characteristics:
- Cavernous and open fissured
- Very coarse, permeable, and shallow, such as loose gravel
- Naturally or intrinsically fractured
- Easily fractured
- Underpressured or depleted
Lost circulation results in increased mud expense and may cause subsurface blowouts. The proper drilling fluids and use of lost circulation material will minimize lost circulation.
Bottom hole temperatures
Extremely high bottom hole temperatures can occur in deep wellbores or in areas of abnormally high geothermal gradient. These excessively high bottom hole temperatures (greater than 250°C) can cause drilling problems because of the accelerated thickening of water-based drilling fluids. The increase in viscosity and density of the drilling can may result in the following:
- Reduction in penetration rates
- Lost circulation
- The well being swabbed when drill pipe is pulled
- Stuck tools
This problem can be mitigated by using oil-based muds.