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The design of oil and gas wells will vary from case to case and have a broad variety in respect of detailed solutions based on local conditions, rules and regulations as well as operators experience and preferences.
The main principles of drilling are more or less the same for all oil and gas drilling (with the exeption of some very specialised or experimental concepts). However when it comes to degree of automation, environmental and hazardous exposure there are large differences through the global oil and gas industry.
The sequence of drilling operations involves drilling large diameter holes first and running large diameter conductors then drilling progressively smaller hole sizes, installing smaller and stronger casings and liners as downhole pressures increase.
A simplified sequence of a typical oil and ga drilling operation would be:
- Structural Casing (offshore drilling from a floating units) is set
- Conductor casing is set
- Surface Casing is set
- Intermediate casing is set
- Production casing is set
- Production String with the production tubing is set
The drilling starts by setting the large diameter casings and then progressively drilling smaller diameter holes and setting smaller and stronger casing strings and liners, as the formation pressure increases.
- The sequence of drilling operations involves drilling a large diameter hole, using a jet bit, jetting away the sediments by high-pressure seawater (or drilled with a drill bit if the it is hard rock) to set the structural casing. This section might also be driven into the ground. When the conductor has reached a depth of around 75 metres (250 ft) the jet bit is pulled out of the hole and replaced with a drill bit and drilling continued, still using seawater to circulate the drill cuttings.
- After continued drilling to a suitable depth that might vary pending on ground conditions the conductor casing is set. Close to the bottom of the conductor casing the float collar has been installed allowing the cementing to take place. The cementing is done by means of a cementing tool attached to the top of the conducter. The high pressure cement is pushing the seawater out and the cement is then flowing out of the bottom of the conductor, through the float collar and up the annulus space between the well bore and the conductor.
- When the sement is cured the drilling continus through the float collar and the cement into the formation rock. At the desired depth, the drill string is pulled out and the surface conductor is set. At this stage the wellhead, BOP and the marine riser (if offshore drilling) is attached.
- For the continued drilling, with the riser in place the seawater is replaced by water based or oil based drilling mud. As drilling proceeds new strings of casing are lowered and cemented and samples of the drilling mud, core samples and geophysical logging (when formation rock is reached) are being made to determine if there are traces of oil and gas present.
- Based on the result of the analysis it is decided if the well shall be plugged and aboandoned (no potential for oil and gas production), temporarily suspend(seal the well) for later production or more investigation or prepare the well for production. If it is considered that the well shall be produced the production casing is set at the preferred depth.
- Production String with the production tubing is installed and the production casing is perforated to allow flow of oil and gas to the surface.
The drill bit is rotated either from a surface-located mechanical motor (rotation table or top drive) or by a downhole mud motor. The hole is drilled into subsurface formations as high-pressure drilling fluid is pumped down the inside of the drill string to circulate downward and lift the drilling cuttings upward through the casing annulus.
Well completion converts a borehole into an operational system for controlled recovery of underground il and gas. This includes installation of the final well casings that isolate the fluid.
Casing that is cemented in place serves various purposes such as:
- Prevent contamination of fresh water well zones.
- Prevent unstable upper formations from caving in.
- Provides a strong upper foundation.
- Isolates different zones that may have different pressures or fluids (Zonal isolation).
- Seals off high-pressure zones from the surface, reducing potential for a blowout.
- Prevents fluid loss into or contamination of production zones.
- Provides a smooth internal bore.
The number and type of casing strings and the depth for each string is determined by evaluating the subsurface rock stress and pore pressure, the strength of the casing, anticipated hole problems, required hole sizes, and the type of completion to be used.
Basic types of casing strings that will vary in length and diameterbased on local conditions:
Structural casing (OD:16” – 60 “, typically 30”)
Conductor Casing (OD: 16” – 48”, typically 20”)
Surface Casing (OD: 8 5/8” – 20”, typically 13 3/8”)
Intermediate Casing (OD: 7 5/8” – 13 3/8”, typically 9 5/8”)
Production Casing ( OD: 4 ½” – 9 5/8”, typically 7”)
Structural Casing (offshore drilling from a floating unit) – The outer string of large-diameter, heavy-wall pipe installed in wells drilled from floating installations to resist the bending moments imposed by the marine riser and to help support the wellhead installed on the conductor casing.
Conductor casing – The first string set below the structural casing with the pupose provide structural support for the well and to enable circulation of drilling fluid. This casing string is not normally designed for pressure containment, but upon completion of the well, it may have a casing head; therefore, it may be capable of containing low annular pressuresi.
Surface casing – This casing run inside the conductor casing and is set to isolate water sands, and prevent lost circulation. It also often provides adequate shoe strength to drill into high-pressure transition zones. This is usually the string onto which the casing head is installed and normally where the diverter or the BOP (blow out prevention) stack is installed. When cemented, this this is typically to the surface or to the seabed in offshore wells.
Intermediate casing – The intermediate casing run from the wellhead, between the surface casing and the production casing or the production liner and is cemented in place. Intermediate casing is normally, only used in deeper wells to isolate abnormal pressured formations, lost circulation zones, salt sections, and unstable shale sections such that it is possible to drill deeper without significantly oncreasing the mud weights.
Production casing – Is the innermost string of casing, running from the wellhead. Production fluids enter the casing below the production packer and continue to the surface through the production string.
Production String – Normally consist of the production tubing but as relevant may also include additional components such as the subsurface safety valve (SCSSV), gas lift mandrels, chemical injection and instrument ports, landing nipples, and packer or packer seal assemblies. The production string is run inside the production casing and used to conduct production fluids to the surface.
Production Tubing – Run inside the production casing and is used to convey produced fluids from the hydrocarbon-bearing formation to the surface. Tubing may also be used for injection.
Directional drilling means drilling wells that divert from a straight vertical line by controlling the direction and deviation of a wellbore to a predetermined target or location.
Wells are drilled directionally for several purposes:
- Increasing the exposed section length through the reservoir by drilling through the reservoir at an angle
- Drilling into the reservoir where vertical access is difficult or not possible.
- Allowing more wellheads to be grouped together on one surface location can allow fewer rig moves, less surface area disturbance, and make it easier and cheaper to complete and produce the wells.
- To relieve the pressure of a well producing without restraint (a “blowout”) to enable pumping in heavy fluid (kill fluid) to suppress the high pressure in the original wellbore causing the blowout.
Tools utilized in achieving directional drills include whipstocks, bottomhole assembly (BHA) configurations, mud motors and specialized drillbits.
One way of directional drilling utilizes some sort of mechanical device (ie. a bend) near the bit, as well as a downhole steerable mud motor. The mechanical device directs the bit in the desired direction (at the kick off point) without the entire drillstring rotating, which is achieved by pumping drilling fluid through the mud motor. When the desired angle is reached, the complete drillstring is rotated, including the bend drilling the desired direction.
Coiled Tubing Drilling
This method use coiled tubing instead of conventional drill pipe. The advantage being that less effort to trip in and out of the well is needed, the tubing is continuously being pulled of the coil and run into the well as drilling proceed.
An additional advantage is that the tubing enters the hole via a stripper, mounted on an injector, which provides a hydraulic seal around the coil. This the ability to drill underbalanced (the pressure in the wellbore is kept lower than the fluid pressure in the formation being drilled) with increased speed and less formation damage.
The drill bit it is turned by a downhole ud motor, powered by the drilling fluid pumped from surface (slide drilling).
The mud motor will is one component of the coiled tubing drilling bottom hole assembly (BHA). In addition the BHA provides various logs during drilling.