Oil Well Drilling
So, How Are Oil Wells Drilled?
Oil well drilling has been the main means of producing oil ever since Colonel Drake drilled that first well in 1859, which signaled the start of the American petroleum industry.
Drilling techniques and equipment have changed throughout the decades from cable tools to rotary-based ones, from straight holes to sidetrack and GPS-based directional drilling, and from “guess-timates” and “feel” to computer-based accuracy.
The biggest improvement in oil well drilling, however, has been in the preparations prior to ever breaking ground.
The drilling of a well, especially a “wildcat”
(see oil exploration discussion),
is a milestone event, involving practically every sub-discipline of the oil business, and signifies the start of direct field investigation.
For the oil exploration and production company, the drilling of the well represents final exploration sunk costs prior to the possibility of recovering those costs through well production revenues. For the petroleum geologist and the reservoir engineer, the drilling of the well represents the final confirmation of the interpretation of numerous strands of indirect evidence of oil’s presence. For the production and facilities engineers, it represents the soon to be realized asset requiring sub-surface and surface management and equipment to maximize production
(see oil production discussion).
And, for the drilling engineer, well, it is time to earn their pay!
Through experience and communications with geologists, reservoir engineers, production engineers, and facilities engineers - the technical team - the drilling engineer develops a plan for reaching the targeted formation at the bottomhole location identified, from the surface location specified – at the cost authorized.
Before ever setting up on the drilling location, the drilling engineer has gained all of the necessary approvals to drill from company and regulatory authorities
(click here for regulatory contact information).
The appropriate hole dimensions, the wireline testing procedures, the well casing program, and the cement volumes are all known upfront
(see well capacity tables).
The drilling engineer has already scheduled an oil well drilling rig, alerted a wireline and cementing service company, and ordered necessary drilling fluids, tanks, pipe and safety equipment (including blowout prevention equipment;
Click here to view oil field service company contacts
Operations normally proceed on a 24 hours per day basis and depending on methods, depths, and rock types encountered, can last anywhere from a few days to several months.
History has shown that rarely do operations proceed in a “normal” fashion. Each well is its own story. It is quite normal to encounter hard rock zones, and experience sand control problems, as well as for minor equipment breakdowns to occur - right next to a well which didn't experience half of the problems! Drill bits wear out, wrong auxiliary equipment is delivered, and various other events happen that slow progress, raise corporate anxieties, and compromise schedules.
Due to all of the problems, which can and do happen on site, oil companies have increasingly focused on safe operations. This is something everyone can control.
Most oil well drilling operations are actually completed by drilling service companies, with oil company drilling engineers supervising. Oil companies are using their natural leverage by insisting on safe operations by contractors, which minimize employee “accidents” and environmental impacts, and maximize accountability. Drilling service companies with poor safety records are not kept for long.
Operations proceed in accordance with terms of a permit issued by the regulatory agency with jurisdiction. Normally, a drilling location is graded, a conductor pipe is set to support subsequent casing strings, blowout control equipment is installed and tested for well safety, the drilling rig and auxiliary equipment is moved in and set up, and drilling operations are underway.
Contemporary drilling operations consist of downhole tools (drill bits, reamers, shock absorbers, etc.), drill string components (drill collars, drillpipe, kelly, etc.), suspension equipment (rotary swivel, hook, blocks, and wire rope), supporting structures (derrick), rotary drive mechanism (rotary table, turbodrill, dynadrill), hoisting equipment (drawworks, auxiliary brakes, cathead, etc.), transmission systems (mechanical transmission, clutch, belts, and chains), prime movers (diesel, turbo-electric), hydraulic circulating system (slush pump, high pressure surface equipment, drill string, shale shaker, desander, degasser, mud tanks/mixers), and rig floor and wellhead accessories/tools (cat lines, elevators, rotary slips, power slip, safety clamps, power tongs, rig instrumentation, blowout prevention equipment, etc.)
(look at a picture of a drilling rig).
Although each area is vitally important to safe and efficient drilling operations, the drill string including the downhole tools is the most important area; being at the point of impact, transmitting surface derived energy into bottom-hole torque and hole digging.
The drill string/subsurface assembly is composed primarily of a swivel, a kelly, drillpipe, a drill collar, and a bit. The swivel connects the rotating drill string to the drilling rig support system. It suspends the drill string, permits free rotation and serves as the means for drilling fluid circulation. Drilling fluid is circulated through the drill pipe and bit to cool the bit and assist in cuttings removal. The drilling fluid also serves to coat the open-hole to prevent cave-ins and prevent any reservoir fluids (oil, gas and water) encountered from rushing in.
The swivel connects to the kelly, which is usually either a square or hexagonal-sided pipe of about 43 feet long, that transmits the torque from the rotary table on the rig floor to the drill string causing the bit to turn and make hole. Drill pipe sections are connected to the kelly one at a time allowing the bit to work deeper and deeper in the hole.
The drill collar is a heavy-walled pipe which connects the drill bit to the drillpipe. Its weight puts pressure on the bit to keep it working at the bottom of the hole.
The drill bit is the primary downhole tool, cutting up formation as it rotates. Diamond bits are used for hard formations. However, tri-coned steel-teethed bits are most commonly used today.
Sometimes, geologists inspect the cuttings that are circulated to surface to identify and confirm the formation that is currently being drilled.
At various and defined intervals, the well may be logged by wireline service companies. Why is this done? Well logging tells the industry experts the formations they are in, the fluids present within the formation (including oil!) and the quality of the cement job. In some drilling operations, wells are logged as they are being drilled using sophisticated tools which continuously transmit vital downhole information to surface without having to cease hole-making.
Metal pipe called surface casing is inserted into the well once the drillpipe is removed, and is cemented to the earth by cementing service companies. Cement is pumped and circulated within the well to permanently affix the pipe to the earth. This provides support, and limits communication between the surface and the subsurface to just that space inside of the pipe.
Subsequent drilling punctures the bottom of the recently placed cement sheath and continues down to the objective depth. To drill deeper, the rig crew performs the seemingly routine act of ceasing rotary table rotation and mud circulation, lifting the drill string, setting it on slips at the rig floor, breaking the joint between the kelly and the topmost drillpipe with tongs, screwing on an additional length of drillpipe at the kelly, lifting the string again, removing the slips, lowering the string downhole, and reestablishing mud circulation and rotary table rotation.
Intermediate casing might be run in hole and cemented, too, depending on well design criteria and formation characteristics. When the total depth is reached a final cement job is conducted to either plug the well back up because no significant hydrocarbon was found, or to secure the production casing string in place for future completion and production.
The drilling contractor rigs down and moves off of the drilling location and heads on to the next assignment.
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