Rig Processes
1. . Drilling
Normal drilling operations are what the rig and crew are hired to do. Simply stated normal drilling operations are:-
- Keeping a sharp bit on bottom, drilling as efficiently as possible.
- Adding a new joint of pipe as the hole deepens.
- Tripping the drill string out of the hole to put on a new bit and running it back to bottom, i.e. making a round trip.
- Running and cementing casing large diameter steel pipe that is put into the hole at various predetermined intervals. Often however special casing crews
- are hired to run the casing and usually a cementing company is called on to place cement around the casing to bond it in place. The rig crew usually assists in casing and cementing operations.
To understand the sequence of the operation and to get the ball rolling, assume that the crew is ready to begin drilling the first part of the well. Firstly about 20 to 100 feet has to be drilled and lined with conductor pipe. The diameter of the conductor pipe varies but in this example assume it is 20 inches. Conductor or surface pipe can be hammered driven or a borehole made to lower the conductor into and cement the pipe in place. A casing head is fixed to the top of the conductor at the surface. The next bit used will have to be smaller than 20 inches. Here, a 17½ inch bit is chosen. This fairly large bit is made up on the end of the first drill collar and both bit and drill collars are lowered into the conductor hole. Enough collars and drill pipe are made up and lowered in until the bit is almost to bottom.
Over the years a standard selection of bit sizes has evolved usually starting with a maximum of 36“ bit followed by:- 26”- 17½ - “ 12 ¼” - 8 ½” - to- 6” . Less common, but available, are a number of bit sizes below 6”. Similarly, standard casing are used. These naturally, are of slightly smaller diameter from the bore drilled in that the casing will be run. Namely:- 30”, 20” 13 3/8” 9 5/8” to 7”. Again smaller sizes are available but these tend to be more common in production operations. The Kelly is picked up out of the rat hole where it has been stored and is made up on the topmost joint of drill pipe sticking up out of the rotary table. This joint of pipe is suspended in the rotary table by the slips. With the Kelly made up, the pump started to begin circulating drilling mud and the Kelly bushing in the rotary table and rotation begins. Next, the driller gradually releases the drawworks brake and the rotating bit touches bottom and begins “making hole”. Using an instrument called the weight indicator, the driller monitors the amount on weight put on the bit, Since the Kelly is about 40 feet long, after 40 feet of hole is made the driller stops the rotary, stops the pumps and raises the Kelly exposing the top of the previously connected joint. The drilling crew prepares to make the first connection. They set the slips around the joint of pipe and latch a big set of wrenches - called tongs – around the base of the Kelly. A tong pull line a length of a strong wire rope - runs from the end of the tongs over to the breakout cat-head on the drawworks. The driller engages the cat-head and it starts pulling on the line with tremendous force. The pulling force on the tongs breaks out or loosens the threaded joint between the Kelly and drill pipe. Once the joint is loosened the crew removes the tongs - and the driller engages the Kelly spinner (an air-actuated device mounted permanently near the top of the Kelly).The Kelly spinner turns or spins the Kelly so that it unscrews rapidly from the drill pipe. The crew moves the Kelly over to the mouse-hole, which is just a hole in the rig floor lined with pipe into that a joint of drill pipe is placed prior to it’s begin made up in the string. The crew stabs the Kelly into the box of the drill pipe and spins the Kelly. The crew grabs the tongs, latches them onto the Kelly and pipe and bucks up (tightens) the joint to final tightness.(each pipe size and grade has its own tightening torque range that must never be exceeded by the driller).Next, the driller uses the drawworks to raise the Kelly and attached joint out of the mouse-hole. The crew stabs the end of the new joint hanging in the rotary and the two are connected together, that is, the joint is spun up and tongs are used to make them up to final tightness. Finally, the driller lifts up the Kelly and attached strings a little, the crew removes the slips and the newly added joint
and Kelly are lowered until the Kelly bushing engages the rotary. What has just been described is called “making connection” and can actually be carried out almost in less time than it takes to tell about it.The pump is started, the bit is set back on bottom and another thirty or so feet are drilled, A connection is made each time the Kelly is drilled down i.e. each time about thirty feet of hole is made. The Kelly is normally fifteen feet longer than a joint giving room for manoeuvre. Near the surface, where the drilling is usually easy, the crew will probably make many connections whilst they are on tour. At some predetermined depth, perhaps as shallow as a few hundred feet to as deep as several thousand feet, drilling stops,. The first part of the hole the surface hole- is only drilled deep enough to get past soft sticky formations, gravel beds, freshwater - beating formations and so forth that lie relatively near the surface. At this point the drill string and bit are tripped out of the hole to run the casing.
2. . Tripping
There are a number of occasions when all the drill pipe would have to be removed from the hole and then re-run again. Imagine a well having reached a depth of say 10,000’. Then bit has become worn and needs replacing. This means that 10,000’ of drill pipe must be pulled from the hole, then the old bit is replaced with a new one. Once this is done, everything must then be lowered back into the hole until the bit is on the bottom and drilling can recommence.
The pipe would have to come out of the hole to change a worn bit, or to insert casing, or if any part of the bottom hole assembly had to be changed, etc. This whole process is called a round trip and would take a very long time however, and time means money in the drilling industry. To save time, the pipe is removed in lengths of three joints of pipe. Each length of three joints, which measures approximately 100’, is called a stand.
This process is done in the steps below:-
Step 1
The drillpipe is suspended in the hole and the Kelly is disconnected. (Using slips and tongs as before).
Step 2
The Kelly is swung across the rig floor and lowered into the rat hole, then the swivel is unlatched from the travelling block hook. The rat hole is a tube rather like the mouse-hole. It provides a storage receptacle for the Kelly, Kelly bushings and swivel when they are not in use during the round trip.
Step 3
One of the rig crew - the derrickman, climbs to the monkey board high in the derrick. He secures himself at this working platform using a safety harness. It is his job to handle the top of the stand during the round trip.
Step 4
Elevators are latched around the drill pipe just below the tool joint. The elevators are a set of hinged clamps, which are part of the hook and travelling block assembly. They are connected to links which themselves are attached to the eyes of the hook.
Step 5
The driller can now start to pull the drill string out of the hole. As he starts to raise the string, the slips are removed by the roughnecks on the rig floor. The string is then lifted until the third tool joint is clear of the rotary table and the slips are re-set. We now have a stand of drill pipe up in the derrick being held by the elevators, while the rest of the string is in the hole suspended from the slips.
Step 6
The next job for the roughnecks is to disconnect the stand from the drill string. This is done using the tongs and pipe spinner. The lower end of the stand is then swung to one side of the rig floor and stood down.
Step 7
The derrickman job now is to unlatch the elevators having first secured the top of the stand with a rope. With the stand now clear, he can pull the top of the stand into the fingers of the monkey board. The stand is now racked (stored) in the derrick.
Step 8
The driller now lowers the travelling assembly, allowing the roughnecks to latch
the elevators round the next tool joint ready to pull another stand. The procedure which was just described is repeated until all the pipe is out of the hole. Depending on the depth of the hole, this could take an entire day to complete. The drill collars and bit are the last items to come out of the hole. To unscrew the bit from the bit sub, a device called a bit breaker is placed in the rotary table. This piece of equipment holds the bit while the tongs are used to break the connection. We have now seen one half of a round trip, tripping out. The second half of a round trip is called tripping in and is just the reverse procedure. See figure 27
A sketch showing connection procedureTRIP
MONITORING
During trips the drilling crew and the mud logger should keep a close watch on the amount of mud used to fill the hole as pipe is removed , and the amount of mud received from the hole as pipe is added. This is made easy by using of the trip tank that is a small tank showing sensitive changes in loss/gain. The detailed discussion of trip monitoring procedure will be covered later on as a separate important subject within the section of duties and responsibilities of the Mudlogger...
3. . Casing
Casing is steel pipe placed in an oil or gas well at the end of every drilled phase, and then cemented in place prior to striating drilling the lower smaller hole section. See figure 28
Cross Section shows Casing Profile
Figure 28
FUNCTIONS OF CASING:
- Prevents the hole from caving or collapsing.
- Prevents loss of drilling fluids into weak formations.
- Isolate troublesome formations.
- Prevents communication between formations.
- Provides means of extracting hydrocarbons if the well is productive.
- To effect a method of control and safety as depth increases.
- Provides a means of support for the well head equipment.
Once the drill pipe is out, the casing crew moves in to do their work, the first string of casing they run is called surface casing. Other strings of casing include intermediate (or protective) and production casing. A short string of casing called a “ liner” may be hung-off from another string instead of extending up to the surface. Running casing into the hole is very similar to running drill pipe, except that the casing diameter is much larger and thus requires special elevators, tongs and slips to fit it.
Casing Accessories
To lower an open ended pipe in an open hole for great depths could be a difficult process, the pipe would tend to dig into the formation as it is lowered. Also casing may tend to lie to one side of the hole,etc. In order to overcome such difficulties and to assist in the placement of the cement a number of items or equipment are used; these are called “Accessories”. A number of centralizers and scratchers are often installed on the outside of the casing before it is lowered into the hole.
The centralise are attached to the casing and since they have a bowed spring arrangement keep the casing centred in the hole after it is lowered in. Centralised casing can make for a better cement job later.The scratchers also came into play when the casing is cemented. The idea is that if the casing is moved up and down or rotated (depending on scratchier design) the scratchers will remove the wall cake formed by the drilling mud and the cement will thus be able to bond better to the hole.
The scratchers also came into play when the casing is cemented. The idea is that if the casing is moved up and down or rotated (depending on scratchier design) the scratchers will remove the wall cake formed by the drilling mud and the cement will thus be able to bond better to the hole.
Other casing accessories include:
A float collar; a device with a valve, installed in the casing string two or three joints from bottom. A float collar is designed to serve as a receptacle for cement plugs and to keep drilling mud in the hole from entering the casing. Just as ship floats in water, casing floats in a hole full of mud )if mud is kept out of the casing). This buoyant effect helps relieve some of the weight carries on the mast or derrick as the long string of heaving casing hangs suspended in the hole. Alternatively the casing may be allowed to fill up to avoid the possibility of collapsing the casing at greater depths. This is affected by a surface or automatic filling via differential fill - up float shoe.
A guide shoe; a heavy steel - concrete piece attached to the bottoms joint of casing that helps guide the casing past small ledges or debris in the hole .
4. Cement
After the casing string is run, the next task is cementing the case in the place. An oil well cementing Service Company is usually called-in for this job, although, as when casing is run, the rig crew is available to lend assistance. Cementing service companies stock various types of cement and have special
transport equipment to handle this material in bulk. Bulk cement storage and handling equipment is moved out to the rig. Making it possible to mix large quantities of cement on site. The cementing crew mixes the dry cement with water, using a device called a job mixing Hopper. The dry cement is gradually added to the hopper and a jet of water thoroughly mixes with the cement to make a slurry (very thin, watery cement). Weighted slurries are often used to insure a control of the formation pressure. Special (cement) pumps pick up thecement slurry and send it up to a valve called a cementing head (also called a plug container) mounted on the topmost joint of casing that is hanging in the mast
or derrick a little above the rig floor. Just before the cement slurry arrives a rubber plug (called the bottom plug) (Figure 31) is released from the cementing head and precedes the slurry down the inside of the casing. A pre-calculated volume of cement that is equal to the annular volume between casing pipe and hole is then pumped-in, then a top plug (Figure 31) is released from the circulation head and soon mud is pumped behind the top plug -usually with the rig pump to drive the cement to the annulus; This process is called “Cement Displacement”.
The bottom plug stops or “seats” in the float collar, but continued pressure of pumps ruptures a passageway through the bottom plug, thus the cement slurry passes through it and continues on down the casing. The slurry then flows out through the opening in the guide shoe and starts up the annular space between the outside of the casing and walls of the hole. By the time the top plug seals on or “bumps” the bottom plug in the float collar; pump pressure increased sharply which signals the pump operator to shut-off the pumps, the cement is only in the casing below the float collar and in the annular space and the rest of the casing is full of displacing mud.
After the cement is run, a waiting time is allowed to allow the slurry to harden.
This period of time is referred to as Waiting On Cement or simply “WOC”. After the cement hardens, tests may be run to ensure a good cement job, for cement is very important. Cement supports the casing so the cement should completely surround the casing; this is where centralizers on the casing help. If the casing is centred in the hole, a cement sheath should completely envelop the casing. Also cement seals-off formations to prevent fluids from one formation migrating up or down the hole and polluting the fluids in another formation. For example, cement can protect a freshwater formation (that perhaps a nearby town is using as a drinking water supply) from saltwater contamination. Further, cement protects the casing from the corrosive effects that formation fluids (as salt water) may have on it. After the cement hardens and tests indicate that the job is good, the rig crew attaches or nipples up the blow-out prevented stack on the top of the casing. The BOP slack is then pressure tested and drilling is resumed. After nippling up the BOP stack a smaller bit on a slick bottom hole assembly is run in hole to drill out cement. The slick assembly is an assembly without stabilizers to avoid hitting the casing inside. This bit drills out the float collars and the drillable casing shoe along with the cement in between.
(Figure 31)
5. Coring
Besides the above mentioned tests, formation core samples are sometimes taken. Two methods of obtaining cores are frequently used.
Conventional coring:
An assembly called a "core barrel" is made up on the drill string with a special type of bits called”Core Head” and run to the bottom of the hole. As the core barrel is rotated, it cuts a cylindrical core a few inches in diameter that is received in a tube above the core cutting bit. There are many types of core barrel in use as the conventional, the rubber sleeved, The fibreglass, ..Etc. A complete round trip is required for each core taken.
Sidewall coring
In a sidewall sampler a small explosive charge is fired to ram a small cylinder into the wall of the hole. When the tool is pulled out of the hole, the small core samples come out with the tool. Either type of core can be examined in a laboratory and may reveal much about the nature of the reservoir.
6. Drilling to total depth (TD)
The final part of the hole is what the operating company hopes will be the production hole. To drill it the crew picks out the smallest bit, say one of 8 inches. This bit is tripped in, drills out the intermediate casing shoe and heads toward what everyone hopes is “the pay zone”, a formation capable of producing enough oil and/or gas to make it economically feasible for the operating company to complete the well. Once again, several bits will be dulled and many round trips made and soon the “formation of interest” (the pay zone; the oil sand or the formation that is supposed to contain hydrocarbons) will be penetrated by the hole. It is then time to consider whether the well contains enough oil or gas (or strong enough suggestions of ) to make it worthwhile running the final production string of casing and completing the well.
7. Wireline logging (electric) logging
A valuable technique for evaluating a borehole is the wireline well logging. An electric logging company is called to the well while the crew trips out all the drill string. Using a laboratory, truck-mounted for land rigs and permanently mounted on offshore rigs the loggers lower devices called logging tools (or sonde's) into the well on wireline. The tools are lowered all the way to bottand then reeled slowly back upwards. As the tools are coming up the hole they are able to measure the properties of the formations they pass. Some logs measure and record natural and induced electricity in formations. Other log pings formations with sound and measure as well as record sound reactions. Radioactivity logs measure and record the effects of natural and induced radiation in the formations. These are only a few of many types of E logs available.
Since all the logging tools make a record, which resembles a graph or an electrocardiogram (EKG). The records or logs can be studied and interpreted by an experienced geologist to indicate the existence of oil or gas and how much may exist. Computers have made the interpretation of logs much easier.
8. Completing the well
After the operating company carefully considers all the data obtained from the various tests it has ordered to be run on the formation or formations of interest, a decision is made on whether to set production casing and complete the well - or to plug and abandon it. If the decision is to abandon it, the hole is considered to be dry, that is, not capable of producing oil or gas in commercial quantities. Some oil or gas may be present but not sufficient to justify the expense of completing the well. Several cement plugs are set in the well to seal it off more or less permanently. Sometimes wells that were plugged and abandoned as dry at one time in the past may, however, be reopened and produced if the price of oil or gas has become more favourable. The cost of plugging and abandoning a well may only be a few thousand dollars. Contrast that cost with the price of setting a production string of casing. The operator's decision, therefore, is not always ways easy.
9. Setting production casing
If the operating company decides to set casing, this will be brought to the well and for one final time the casing and crew run and cement a string of casing. Usually, the production casing is set and cemented through the pay zone, that is, the hole is drilled to a depth beyond the producing formation, and the casing is set at a point near the bottom of the hole. As a result, the casing and cement actually seal off the producing zone - but only temporarily. After the production string is cemented, the drilling contractor has almost finished his job except for a few final touches.
10. Installing the Christmas tree
Even though the oil or gas can flow into the casing after it is perforated, the well is not usually produced through the casing Instead, a small diameter pipe called “tubing” is placed in the well to serve as a way for the oil or gas to flow to the surface. The tubing is run into the well with a packer. The packer goes on the outside of the tubing and is placed at a depth just above the producing zone. Then the packer is expanded, it grips the walls of the production casing and forms a seal in the annular space between the outside of the tubing and the inside of the casing. As the produced fluids flow out of the formation through the perforations, they are forced to enter the tubing to get to the surface. When casing is set, cemented and perforated, and when the tubing string is run, then a collection of valves called a “Christmas Tree” is installed on the surface at the top of the casing. Like so many terms in the oil industry, no one knows why this device on top of the well is called a Christmas tree. Perhaps all the valves and piping reminded someone of the traditional Christmas tree. The tubing in the well is connected to the Christmas tree, so as the well's production flows up the tubing, it enters the Christmas tree. As a result, the production from the well can be controlled by operating or closing valves on the Christmas tree. Usually, once the Christmas tree is installed the well is complete. The drilling contractor has done his job according to the drilling contract and he can move the rig to another location to start another well drilling process all over again.
11. Perforating production casing
Since the pay zone is sealed-off by the production string and cement, perforation must be made in order for the oil or gas to flow into the wellbore. Perforations are simply holes that are made through the casing and cement and extend some distance into the formation. The best common method of perforation incorporates shaped-charge explosives (similar to those used in armour-piercing shells). Shaped charges accomplish penetration by creating a jet of high pressure, high-velocity gas. The charges are arranged in a tool called a gun that is lowered into the well opposite the producing zone. Usually the gun is lowered in on wireline. When the gun is in position, the charges are fired by electronic means from the surface. After the perforations are made, the tool is retrieved. Perforating is usually performed by a service company that specialises in this technique.
12. Drill Stem Test (DST)
Another helpful technique is the drill stem test (DST) tool, which is made up on the drill string (the drill stem) and set at the depth required. A packer, which is an expandable hard-rubber sealing element, seals-off the hole above it by expanding when weight is set down on it. A valve is opened to allow any formation pressure and fluids present to enter the tool. A recorder in the tool makes a graph of the formation pressures. Then the packer is released and the tool retrieved back to the surface. By looking at a record of the downhole pressures and surface flows a good measure of the characteristics and contents of the reservoir can be obtained. Other valves or points are opened to expose the formation to atmospheric pressure, allowing the well to flow.
13. Acidizing
Sometimes, petroleum exists in a formation but is unable flow readily into the well because the formation has very low permeability. If the formation is composed of rocks that dissolve upon being contacted by acid, such as lime stone or dolomite, then a technique known as acidizing may be required. Acidizing is usually performed by an acidizing service company and may be done before the rig is moved off the well or after the rig is moved away. In any case, the acidizing operation basically consists of pumping anywhere a quantity of acid down the well. The acid travels down the tubing, enters the perforations and contacts the formation. Continued pumping forces the acid into the formation where it etches channels; these provide a way for the formation's oil or gas to enter the well through the perforations.
14. Fracturing
When sandstone rocks contain oil or gas in commercial quantities but the permeability is too low to permit good recovery, a process called fracturing may be used to increase permeability to a practical level. To fracture a formation, a company providing this service pumps a specially blended fluid down the well and into the formation under great pressure. Pumping ing continues until the formation literally cracks open. Sand, walnut hulls or aluminium pellets are mixed into the fracturing fluid. These materials are called prop pants. The prop pant enters the fractures in the formation. When pumping is stopped and the pressure is allowed to dissipate, the prop pant remains in the fractures. Since the fractures try to close back together after the pressure on the well is released, the prop pant is needed to hold or prop the fractures open. These propped-open fractures provide passages for oil or gas to flow into the well.
15. Additional aids
Directional Drilling A
Usually but not always, the crew tries to drill the hole as straight as possible, but at times it is desirable to deflect the hole from vertical. The most dramatic example of this is the offshore platform. Many wells may be drilled from a single platform without having to move the rig. The technique used is called “ directional drilling”. Only the first hole drilled into the reservoir tray be vertical; each subsequent well may be drilled vertically to a certain depth then kickedoff (deflected) directionally so that the bottoms of the hole ends up away from its starting point on the surface. By using directional drilling, as many as twenty or more wells may be drilled into the reservoir from one platform. Thus directional drilling has become a routine development operation throughout the world. Controlled directional drilling has many applications. It is used for inaccessible locations, offshore drilling from shore or sea platforms, geological corrections, relief wells, side-tracking tools lost in the hole, redrills to save surface cased hole and a portion of the open hole and bottom hole re-completion.
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