9 DRILLING PROBLEMS
9.1 FORMATION PROBLEMS AND HOLE STABILITY
While fractures can occur naturally in any formation, they are more common in harder, more consolidated formations, as well as in and around faulted areas or other areas subjected to natural forces and stress.
Fractures range from microscopic sizes to widths exceeding 1/8 inch (3mm) and can be well-ordered to random orientations. Older, harder formations at deeper depths tend to be more highly fractured than younger, softer formations at shallower depths.
9.1.1.1 Associated Problems
Lost Circulation—Lost circulation below the surface casing in normally pressured formations may be caused by naturally occurring fractures in formations with subnormal pore fluid pressure. If, as drilling proceeds, no drilling fluid or cuttings are returned to the surface, they are most likely being lost to the fractured zone.
Sloughing, Increased Cuttings—Formation particles in fractured formations have a tendency to fall into the hole, thereby increasing the volume of cuttings. The volume and size of formation particles that fall into the hole depends upon the hole size, hole inclination, angle of formation dip and extent of fracturing. Typically, they can be recognized as being larger than normal drilled cuttings. Hole fill (that is, cuttings accumulating at the bottom of the hole), may be seen after connections.
Inhibited Rotation, Sticking Pipe—As the hole fills with an excessive volume of cuttings, rotation of the drillpipe is inhibited. If these cuttings are not removed from the hole and carried to the surface, the drillpipe can become stuck, thus stopping further rotation and blocking circulation (pack off).
Enlarged Hole, Reduced Annular Velocity, and Hole Cleaning—Drilling through fractured, unstable formations invariably results in enlarged holes which, in turn, causes reduced annular velocity and requires additional hole cleaning.
Keyseating, Ledges, Deviation—Fractured formations can create problem-causing ledges and, depending upon the hole inclination and deviation, the formation of keyseats. These can lead to subsequent problems of higher drag and pick-up weight, and sticking pipe.
Erratic Torque—Fractured cavings falling into the hole will act against the rotation of the drillpipe, leading to higher and erratic torque. In extreme cases, rotation may be completely stalled with the built-up torque in the drillstring, presenting the danger of twisting off, or breaking, the pipe.
9.1.1.2 Drilling Fractured Formations
Control Rate of Penetration—The rate of penetration must be controlled when drilling fractured formations to minimize the volume of sloughing material.
Work and Clean Hole—adequate time must be allowed for complete hole cleaning to remove the cuttings from the bottom of the hole. Good mud properties and viscous mud sweeps are important to keep the hole clean. Careful reaming during trips will help clean the fractured zone.
Increase Mud Weight—a good quality filter cake can provide support to some fractured zones, but in highly fractured formations where continuous, extra-heavy sloughing is encountered; increasing the mud weight can be effective in holding back sloughing and stabilizing the fractured formation.
Avoid Pressure Surges—Pressure surges can add to, or increase fracturing. Therefore, it is important to slow tripping speeds when the bottomhole assembly passes through a fractured zone, and start and stop the pump slowly.
Dump Cement—Typically, fractured zones are likely to stabilize after a period of time. If, after taking all of the above measures, the hole still does not stabilize, a final recourse is the use of cement.
Dumping cement can seal and stabilize the fractured formation thereby preventing further problems.
9.1.2 Shales
9.1.2.1 Reactive Shales
Swelling (that is, absorbing filtrate from drilling fluid) is typically a tendency of younger, shallower shales.
As shales swell, they separate into small particles that may fall into the wellbore. This results in heaving (that is, the partial or complete collapse of the wellbore walls) and causes tight hole conditions, increased pipe drag on connections, sticking pipe and the formation of ledges.
Selecting the appropriate drilling fluid will minimize shale reactivity and swelling. Mud inhibitors (such as salt or lime) and oil-based muds are the most effective drilling fluids for controlling swelling.
By increasing the rate of penetration, it is sometimes possible to drill through a sensitive shale section and complete operations before swelling and heaving occurs. However, fast drilling through a thick interval without good hole cleaning can result in severe hole stability problems.
Tight hole sections caused by swelling shales should be reamed and cleaned. Depending upon the sensitivity of the shales, it may be necessary to ream and clean more than once as drilling proceeds deeper. To prevent pipe from sticking, the upper hole should be clean and free of heaving before the bit is worked deeper into the problem shale section.
More sensitive shale sections require periodic wiper trips to ensure the hole is not closing around the drillpipe above the drill collars.
If severe shale problems persist, the hole may have to be cased off to prevent losing the hole. The standard practice is to condition the hole, pull out, run logs and then run the bit back to the bottom of the hole for a clean-out trip, ensuring that the hole is in a condition to allow casing to be run.
9.1.2.2 Overpressured Shales
Overpressured shales possess a higher than normal pore fluid pressure for the depth of their occurrence.
Although there are many different mechanisms that can lead to this, it typically results from incomplete compaction and de-watering when formation fluids are squeezed from the formation due to the overlying overburden as the shale sediments are buried. The shales, therefore, retain an abnormally large amount of formation fluid. The increased volume of fluid will support part of the overburden weight, normally supported by the rock matrix, resulting in a higher pore pressure. If this pressure exceeds the mud hydrostatic pressure, the fluid will try to escape from the shale. Since this is prevented by the shale’s impermeability, the higher pressure will cause fracturing of shale, allowing fragments (or cavings) to break away and fall into the borehole. These cavings will lead to hole fill (that is, accumulated cavings at the bottom of the hole) after trips and connections.
Tight hole problems, due to pressure exerted by the shale and due to cavings falling in and around the drillstring, lead to increased rotary torque while drilling and increased overpull required to lift the pipe for connections and trips.
As the shale fractures and breaks away, gas will be released. An increase in the gas level, the presence of connection gas or the presence of gas-cut mud may therefore be an indication of over-pressured shale and the requirement to increase the mud weight.
Increasing the mud weight is the most effective method of controlling under-compacted and over-pressured shale sections.
N
OTE For more information on occurrences, causes, and detection of over-pressured shales, see the Weatherford Well Control manual.9.1.3 Surface Formations
Drilling formations at shallower depths can result in a number of different problems and operational considerations. Surface formations are often loose and unconsolidated, and are therefore highly susceptible to caving and collapse. Gravel and boulders in conglomerate-type formations present hard obstacles against drilling and can often deflect the bit, creating deviation problems.
Even without any associated problems, the large size of surface holes results in a large volume of cuttings that require very efficient hydraulics and surface equipment to lift and remove cuttings from the wellbore.
As mentioned previously, shales at a shallow depth, especially in offshore basins, are particularly prone to swelling, which creates an additional problem.
Shallow gas-bearing formations are a further drilling hazard. Upon encountering shallow, pressured gas, there is very little warning before the gas reaches the surface. With deeper gas kicks, there is normally some delay between the time when mud is seen being displaced at the surface (that is, flow and pit
volume increase) and when the gas reaches the surface, allowing the well to be safely closed in and controlled.
With shallow wells, typically light water-based systems are used, providing very little balance against pressured gas, which expands and reaches the surface very rapidly. This situation requires extreme vigilance from the drilling crew and mud loggers to avoid a very dangerous situation.
Freshwater reservoirs present a different kind of problem. With over-pressured aquifers, there is the associated problem of kicks. However, reservoirs may also be under pressured with the associated problems of lost circulation. Equally important is the fact that these aquifers may be the water supply for a particular community; so they must not be contaminated by the drilling operations.
To prevent the drilling mud from invading the aquifer, the aquifer should be quickly cased off for protection against all subsequent drilling operations.
9.1.4 Salt Sections
If salt sections are drilled using an incorrect drilling fluid (for example, fresh-water mud), the salt will dissolve in the mud. This will result in washed-out sections where cuttings can accumulate and cause hole problems. Thus, a salt-saturated or oil-based mud must always be used to drill salt sections.
Salt can be very mobile or plastic (that is, behave much like a fluid) and build up pressure against the borehole and drillpipe. This, in turn, can cause stuck and damaged pipe. To prevent such problems, it is important to regularly work the drillpipe and circulate often when drilling salt sections. Higher mud weight will help in holding the salt back, but should the pipe become completely stuck, a common recourse is to spot fresh water in order to dissolve the salt and free the pipe.
9.1.5 Coal Beds
Coal beds are generally fractured formations. As a result, sloughing and its associated problems are typically encountered when a coal bed is penetrated. The procedures for drilling a coal bed are the same as drilling a fractured formation; they require thorough hole cleaning and maintenance.
9.1.6 Anhydrite / Gypsum Formations
Anhydrite especially, and gypsum create a major challenge for the mud engineer.
Both anhydrite and gypsum increase the viscosity and the gel strength of the mud. This alters the flow properties and hydraulics of the mud, leading to increased circulating swab and surge pressures, and creates a handling problem at the surface in that the mud will gum up surface equipment.