Well control
The aim of oil operations is to complete all tasks in a safe and efficient manner without detrimental environmental effects.
Its magnitude varies with the concentration of dissolved salts, type of fluid, gases present and temperature gradient.
In areas where faulting is present, salt layers or domes are predicted, or excessive geothermal gradients are known, drilling operations may encounter abnormal pressure.
An abnormally pressured formation can often be predicted using well history, surface geology, downhole logs or geophysical surveys.
Naturally occurring subnormal pressure can develop when the overburden has been stripped away, leaving the formation exposed at the surface.
Depletion of original pore fluids through evaporation, capillary action, and dilution produce hydrostatic gradients below 0.433 psi/ft (0.0979 bar/m).
Loosely compacted formations, such as those found offshore in deep water, can fracture at low gradients (a situation exacerbated by the fact that some of total "overburden" up the surface is sea water rather than the heavier rock that would be present in an otherwise-comparable land well).
Bottom hole pressure can be estimated during the following activities If no fluid is moving, the well is static.
An accurate evaluation of a casing cement job as well as of the formation is important during the drilling and subsequent phases.
Casing depths, well control options, formation fracture pressures and limiting fluid weights may be based on this information.
The LOT determines the pressure and/or fluid weight the test zone below the casing can sustain.
It is a good idea to make a graph of the pressure versus time or volume for all leak-off tests.
Similarly, while moving up, there is a low-pressure zone created below the drill string, which sucks the formation fluid into the wellbore, which is called swab.
The net effect of the upward and downward movements creates a decrease in bottom hole pressure.
Finally, the downhole tools such as packers and scrapers, which have small annular clearance, also increase surge and swab effects.
Determination of actual surge and swab pressures can be accomplished with the use of WORKPRO and DRILPRO calculator programs or hydraulics manuals.
Cuttings are rock fragments chipped, scraped or crushed away from a formation by the action of the drill bit.
If left unchecked, a kick can develop into a blowout (an uncontrolled influx of formation fluid into the wellbore).
This can happen when one or a combination of the following occurs: When tripping out of the hole, the volume of the removed pipe results in a corresponding decrease in the wellbore fluid.
The API7G[clarification needed] illustrates the methodology for calculating accurate pipe displacement and gives correct charts and tables.
In some fields, wells have low reservoir pressures and will not support a full column of fluid.
The most common reason for insufficient fluid density is drilling into unexpected abnormally pressured formations.
In some cases, the bottom hole pressure reduction can be large enough to cause the well to go underbalanced and allow formation fluids to enter the wellbore.
The initial swabbing action compounded by the reduction in hydrostatic pressure (from formation fluids entering the well) can lead to a significant reduction in bottom hole pressure and a larger influx of formation fluids.
Poor fluid properties, such as high viscosity and gel strengths, also increase the chances of swabbing a well in.
Special attention should be paid to hole fill-up volumes since statistics indicate that most kicks occur on trips.
Loss of circulation leads to a drop of both the fluid level and hydrostatic pressure in a well.
When the gas is circulated to the surface, it expands and reduces the hydrostatic pressure sufficient to allow a kick.
These programs must be flexible enough to allow progressively deeper casing strings to be set; otherwise a situation may arise where it is not possible to control kicks or lost circulation.
During drilling, kicks are usually killed using the Driller's, Engineer's or a hybrid method called Concurrent, while forward circulating.
