According to many company policies, unless circumstances and conditions apply, as stated in the Drilling Policy and Guidelines Manual, the surface hole should be drilled riserless. By doing this, it’s possible to eliminate the most common cause of blowouts in a shallow and pressured gas reservoir (most importantly, the loss of hydrostatic head). Of course, there will still be a risk of penetrating an overpressured reservoir so there must always be a contingency plan in place. Prior to stud, the operator and the drilling contractor must also together on the plan and it needs to include;
Common procedure when winching the rig off location
Common procedure when a shallow gas flow occurs
Normally, the pre-spud meeting will be the ideal time and place to discuss contingency plans. A 10-degree cone of low-density water is normally produced after a gas blowout in open water and there will also be a discharge of highly-flammable gas. The current and water depth will decide the intensity of the blowout with greater water depth leading to more dispersed water from the plume. When a current is active, the result would be a plume away from the rig.
This is the introduction to shallow gas well control which will briefly describe the overview of shallow gas and some related information. We have few articles regarding this topics and we will separate into small parts for better understanding. Let’s get started.
Whenever offshore shallow gas accumulations are seen, they’re normally linked with down sand lenses enveloped by mudstones. Typically, lenses will be permeable, unconsolidated, and highly-porous when found in shallow depths. Although normally flat, thin, and normally pressured, many have previously encountered over-pressured lenses. When at this depth, one cause of over pressure is inclination of the lens; this can therefore increase both the lens height and pore pressure gradient (top of the lens).
Although rare, shallow gas can also be linked with vuggy limestone or buried reefs; these have the risk of being infinitely permeable and incredibly porous.
Shallow gas kick
When drilling in the top-hole section, resulting kicks from shallow sands can be dangerous with short casing strings; there are many case histories to show this. Charged formations can also cause kicks from shallow sands and this itself can be a result of improper abandonments, previous underground blowouts, casing leaks, injection operations, and poor cement jobs.
The example of the shallow gas blow out is below.
Sedco 700 Shallow Gas Blow Out 6 June 2009
When it comes to shallow gas kicks, the most common cause is a loss of hydrostatic head and this can be a result of two common problems;
Expanding drilled gas unloading the annulus
Poor hole fill while tripping
Losses through the overloading of the annulus with cuttings
In order to minimize the risk of inducing a shallow gas flow, we recommend some general precautions including restricting the rate of penetration, drilling a pilot hole, drilling riserless, and always monitoring the hole.
High flow rates of gas are often produced by shallow gas flows; high quantities of rocks from the formation are also possible. This is particularly true after long sections of sand have been exposed. When a shallow gas flow occurs, the representative responsible should contact a senior contract representative; all non-essential individuals should be evacuated from the rig. This eventuality should always be addressed, and there should be an implementation of the contractor’s emergency evacuation. Continue reading →
In the previous article, Common Problems and Complications During Well Kill Operation, it is about commons complications that can be possibly seen while performing well control operation. For this article, it will discuss other wellbore problems which are stuck pipe, surface pressure reaching to MAASP, lost of control and hydrate.
During a well control operation, a stuck pipe can occur and this has the potential to lead to serious issues. Whenever the pipe is off bottom, the chances of the pipe getting stuck increases. Therefore, rotating the pipe should reduce the risk of this problem occurring. However, with the well shut it, it is impossible to rotate to minimize stuck pipe so the stuck pipe should be dealt after the well is properly secured.
Throughout well control operation, wellbore pressures will be high and this means the most common cause of a stuck pipe comes from differential sticking. However, this isn’t to say mechanical sticking can’t occur if the hole sloughs and packs-off after coming into contact with the influx fluids.
Operation can normally continue when the pipe is differentially stuck (with the bit on bottom) because the well can still be killed with circulation. Once the well is killed, then the pipe can be free safely later.
When the bit is off bottom and the pipe becomes differentially stuck, this is a more complicated scenario since it’s more difficult to reduce wellbore pressure; at that depth, it’s normally impossible to achieve a reduction by circulation. Although there may be opportunities to spot a freeing agent and free the pipe, volumetric control is the chosen method if the influx was swabbed in.
When the pipe is mechanically stuck, the pipe can be freed by spotting a freeing agent and working the pipe (by combining the two, the desired result is achievable!).
Figure 1 – Stuck Pipe due to Differential Sticking
Poor Boy Degasser or Mud Gas Seperator, located downstream of the choke manifold, is a vertical vessel used to separate any gas from drilling fluid during well control situation. Once the gas has been separated, it can pass through the vent line in the derrick. Alternatively, as long as it’s a safe distance from the rig, it could even be vented.
Figure 1 – Poor Boy Degasser (Courtesy of H-Screening)
With mud’s separators, there are two main types. Also known as a ‘poor-boy’ and a ‘gas buster’, the more common of the two is called an atmospheric mud/gas separator. However, some mud/gas separators are designed to operate at moderate back pressure. Although these will mostly operate under 100 psig, it’s possible to come across those that work at the atmospheric gas vent line pressure plus the vent line friction drop. The simple diagram of poor boy degasser is show in figure 2.
A degasser is equipment used to remove entrained gas in drilling fluid so it prevent or minimize reduction of hydrostatic pressure due to gas cut mud. When drilling mud passing over the shale shakers while drilling, gas will normally be released. However, the wellbore could receive additional volumes of gas and these need to be removed from the mud. If not removed from the circulating system properly, recirculation of mud containing gas will reduce the well’s hydrostatic head. With a degasser, this can eliminate or minimize loss of hydrostatic pressure.
Figure 1 – Degasser (Courtesy of NOV)
Mounted over the active pit, degassers are essentially a one-stage liquid/gas separator. With a maximum lift to the inlet of around ten feet, mud vacates the submerged pipework in the mud pit and enters the degasser. From here, a three hp electric motor will power a vacuum pump and this should be mounted atop the degasser itself. By the pump, the vacuum is then applied to the vapor space.
Ultimately, the range applied by the vacuum will depend on the density of the mud passing through. In most cases, it will offer between 2-5 pounds per square inch (between 8 and 15 inches of mercury). In terms of extracting gas from mud flows, 900 gallons per minute is likely to be the maximum rate. Continue reading →