Oil & Gas: Drilling The Well
The Drilling the Well lesson consists of the following topics:
- Learning Objectives
- What is a well?
- Land Drilling Preparation
- Offshore Drilling Preparation
- Spudding In
- Rigging Up
- Major Drilling Rig Systems
- Tubulars – Drill Pipe
- Tubulars – Casing & Tubing
- Drill Bits
- Bit Types
- MUD – Drilling Fluids
- MWD – Measurement While Drilling
- Fishing
- Cementing
- Hydro Pressure
- Kick and Detection
- Blowout Preventer (BOP)
- Well Control
- Killing the Well
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Drilling The Well: What is a Well?
An oil or gas well is the term for any perforation through the earth’s surface designed to find and release oil and gas hydrocarbons. This lesson describes the processes and equipment used to drill a well, often called “making hole” by the industry.
In summary, a well is created by drilling a hole into the earth with a drilling rig that rotates a drill string with a bit attached.
After the hole is drilled to a prescribed depth, sections of steel tubing known as casing are set in the hole (slightly smaller than the borehole) to provide an annulus for cementing. The casing provides structural integrity to the newly drilled wellbore and isolates any potentially dangerous high pressure zones from each other and from the surface.
With these zones safely isolated and the formation protected by the casing, the well can then be drilled deeper, often into potentially more unstable formations. Here, a smaller bit and smaller size casing is used. As the chart shows, modern wells often have 2-5 sets of subsequently smaller hole sizes drilled, each inside one another and each cemented with a different size of casing.
Drilling The Well: Land Drilling Preparation
A drilling rig is a machine which creates the holes (usually called boreholes) and/or shafts in the ground. The term “rig,“ therefore, generally refers to the complete complex of equipment that is used to make a well.
Before a land-based well can be drilled:
- The site must be prepared, including leveling the land on which the derrick will be assembled.
- Access roads must be created so workers and equipment can get to/from the rig.
- Reserve pits need to be dug so cuttings, material and used mud can be properly disposed of. In some locations, environmental laws prohibit drillers from digging mud pits. Instead, large metal bins are used at the rig site to catch all cuttings and waste materials which are then hauled offsite for disposal.
The final step in site preparation is to dig what is called a cellar for equipment that will be needed during the well drilling operations. The cellar can be dug and formed in place with concrete, or it can be a pre-fabricated, four-walled steel structure that is driven into the ground to the desired depth. The cellar is set at the exact place where the well will be centered.
Drilling The Well: Offshore Drilling Preparation
Offshore drilling refers to the act of extracting underground resources which lie underwater off the coasts. The term can also apply to drilling in lakes, inland seas, bays or channels.
The actual process of drilling the well is similar to a land-based well, but the offshore location makes it more complex. Often, new wells are drilled while others are being produced from the same offshore production platform.
An offshore rig/platform is a small society or city with many support functions, including: cafeteria, sleeping quarters, entertainment halls and management teams.
All rig personnel are transported in by helicopter or crew boats. Most staff are required to be onsite for rotating two-week shifts. shift rotation can vary depending on location (especially, foreign) the E&P operator, drilling contactor and OFS service provider.
All equipment, supplies and waste must be constantly transported in and out by work boat. Movements need to be delicately choreographed because of the limited floor space on the drilling rig/platform.
Drilling The Well: Spudding In
The start of drilling is very commonly referred to as “spudding in.”
Once the cellar is set, spudding in begins with drilling a large-diameter but shallow hole. This is referred to as a conductor hole and is typically 100-200 ft deep. Once drilled, it is lined with conductor casing or ‘pipe’ and cemented into place. Conductor casing is used to keep the top of the well-bore from collapsing in on itself.
Onshore, conductor casing can be drilled and often set by a third-party mobile casing crew contractor.
Offshore, the drilling rig is used for this process.
Drilling The Well: Rigging Up
Another onshore process known as “rigging-up” begins as the rig is hoisted into position and the equipment substructure is centered over the conductor pipe.
The mast or derrick is raised over the substructure and other equipment such as engines, pumps, and rotating and hoisting equipment are aligned and connected.
Water and fuel tanks are filled. Additives for the drilling fluids (drilling mud) are stored on location. When all these matters have been attended to, the rig is then ready to drill another large diameter hole, approximately 100 to 150 meters deep, to
- seal off surface water aquifers
- stabilize the top of the well
- provide an attachment for the blowout preventers (BOPs)
It is very important to make this hole right because all subsequent drilling operations will be based on this first hole.
Once the BOPs are activated, the true well drilling begins towards the target depth and reservoir.
Drilling The Well: Major Drilling Rig Systems
There are five major components or systems in a typical drilling rig as shown in the picture and described below:
- The Derrick must be tall enough to allow new sections of drill pipe to be added as drilling progresses.
- Mechanical systems include the hoisting system (called the drawworks) and are used for lifting heavy loads and numerous pieces of rotating equipment, such as the:
- Swivel, a large handle that holds the weight of the drill string while rotating and makes a pressure-tight seal on the hole.
- Kelly, a four-six-sided pipe that transfers rotary motion to the turntable and drill string.
- Turntable or rotary table – which drives the drill string.
- Drill string – which consists of drill pipe and drill collars (large diameter, heavier pipe that places weight on the drill bit).
- Drill bit, the end of the drill that actually cuts up the rock.
- A circulation system which pumps drilling fluids (mud – which is a mixture of water, clay, weighting material and chemicals) made up of a:
- Pump, which sucks mud from the mud pits and pumps it to the drilling rig
- Mud-return line, which returns mud to/from from the hole
- Shale shaker, a sieve that separates rock cuttings from the mud
- Reserve pit, which collects rock cuttings separated from the mud
- Mud pit, where drilling mud is mixed and recycled
- Mud-mixing hopper, where new mud is mixed and then sent to the mud pits, ready for use
- Power systems provide electric power to the entire rig, often provided by large diesel engines and electrical generators.
- Blowout preventer, the high-pressure valves that seal the drill string to control pressure.
Drilling The Well: Tubulars
Drill Pipe
In simple terms, tubulars are the steel pipe that is used to drill the well, case the wellbore, and produce the reservoir fluids. They come in many varieties and are engineered for high strength and rigidity.
Tubulars are coupled together by independent couplings or screwed directly into each by virtue of a box (female) end and a pin (male) end.
Drill pipe tubulars are commonly found in 30 ft lengths (called joints) and play a very important role in the overall drilling process.
There is a wide variety of equipment that either attaches to or runs through the drill pipe to aid in drilling. Mud flows through the center of the drill pipe. Special collars are attached to perform other tasks, such as aid in the well deviation proces.
The bit is securely attached at the end of the drill string.
Casing & Tubing
The borehole is lined with steel casing to provide a conduit from the surface to the producing formation.
The casing lines the raw, open hole made by the bit as it travels through the formations. It keeps the borehole from caving in, and once cemented into place, keeps fluids in one formation from migrating to another.
Like drill pipe tubulars, casing is commonly found in 30 ft lengths (also called joints). Casing joints are also coupled together by independent threaded couplings.
Casing is manufactured around the world to API (American Petroleum Institute) specifications, in a wide variety of metallurgies, using either a seamless or ERW (Electric Resistance Welding) manufacturing process. Casing casing size range from 4-1/2″ to13-3/8“.
Standard tubing sizes used to produce the reservoir fluids are 2 3/8”, 2 7/8” and 3 ½”
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Drilling The Well: Drill Bits
A drill bit, seen on the chart, is a device attached to the end of the drill string that breaks apart, cuts or crushes the rock formations when drilling a well.
Everything on a drilling rig directly or indirectly assists the bit in cutting the rock. The bit is on the bottom of the drill string and must be changed when it becomes excessively dull or stops making progress. The term for this activity is “tripping pipe” and can take days on a deep well. Therefore, bits are designed to do their job in severe service for as long as possible.
Most bits work by scraping or crushing (or both) the rock as part of a rotational motion. Some bits, known as hammer bits, are activated by a downhole motor which pounds the rock vertically in much the same fashion as a construction site air hammer.
The drill bit is hollow and has jets to allow for the expulsion of the drilling fluid at high velocity and high pressure. This helps clean the bit and breaks apart the rock (in softer formations).
Bit Types
Each bit type is custom-engineered for the expected formations. Three common types are shown in the chart and described below.
- Roller Bit – Crushes/chews/cuts/gouges rock. The configuration is dependant on hardness of rock. It can be used to drill any formations if the proper cutter, bearing, and nozzle are selected.
- Diamond Bit, often called PDC (poly crystalline diamond cutting) – Grinds rock and is more durable than the roller rock bit. They are also used for core samples and designed to excavate hole by shearing the formation. They drill very quickly in soft to moderately hard formations (Sand, Shale, Clay and Siltstone).
- Directional Bit – Includes a steerable motor mounted behind the drill bit which allows the ability to drill at an angle. They are used especially in offshore sites where a number of wells may be drilled from a single platform/borehole and in horizontal wells.
Drilling The Well: Drilling Mud
From the rig floor, rotating hollow drill pipes continuously circulate drilling fluids, called mud, to:
- lubricate, cool, and clean the drilling bit
- control downhole pressure
- stabilize the wall of the borehole
- remove drillbit cuttings from the bottom of the hole
Each drilling fluid is a highly engineered substance that is tailor-made for each specific well application. The mud chemical make-up is monitored closely and changed throughout the drilling process to meet any new geological and operational conditions that inevitably occur down-hole.
Drilling fluids can be classified by singling out the component that clearly defines the function and performance of the fluid. A common classification is oil or water-based mud, which can be used depending on reservoir formation characteristics.
The Drilling Mud System
The mud circulation system in the rig, pictured in the chart, is as important to a drilling rig as the blood circulation system is to the human body. Any mechanical or technical difficulties (breakdowns) in the mud system can immediately stop the drilling.
Mud pressure must be kept greater than the borehole formation pressure to prevent oil and gas escaping into well while being drilled.
The mud travels back to the surface around the outside of the drill pipe, called the annulus
Examining rock chips extracted from the mud is known as mud logging. Another form of well logging is electronic and is frequently employed to evaluate the existence of possible oil and gas deposits in the borehole. This can take place while the well is being drilled, using Measurement While Drilling (MWD) tools, or after drilling, by lowering measurement tools into the newly-drilled hole.
Drilling The Well: Measurement While Drilling (MWD)
Drilling progress is monitored and managed “on the fly” using Measurement While Drilling (MWD) technology, in which instruments monitor variables in the borehole, such as position, temperature, pressure and porosity.
These data are transmitted back to the surface through pressure changes in the drilling mud.
Mud motors are also used. They are a hydraulically powered device positioned just above the drill bit used to spin the bit independently from the rest of the drill string.
The ability to do mud logging and real time cuttings analysis provides immediate data on the rocks being penetrated and eliminates the need to pull the drill string out of the borehole in order to take these
Drilling The Well: Fishing
Fishing is the process of removing tools, equipment and, debris (all are called fish) that have inadvertently been dropped or (broken) down-hole. The key elements of a fishing operation include:
- an understanding of the dimensions and nature of the fish to be removed,
- the wellbore conditions,
- the tools and techniques employed, and
- the process to handle the recovered fish at surface.
Fishing is simply a methodology of retrieving “fish”downhole by onsite innovations using a host of specialized hooks and grabbing equipment.
Fishing is an art and a specialized process in drilling operations. It has the potential to save drilling companies huge amounts of money by providing a service that retrieves objects down-hole rather than abandoning the well altogether.
Drilling The Well: Cementing
Cementing is the process by which specially formulated cement is pumped down-hole to secure and seal the strings of casing that have been lowered into or set the well-bore.
Cementing operations are undertaken to:
- seal the annulus after a casing string has been set
- seal a lost circulation zone
- set a plug in an existing well from which to push off with directional tools
- plug a well if it must be abandone
Before cementing operations commence, engineers determine the volume and the required physical properties of both the slurry and the set cement.
A cementing crew uses special mixers and pumps at the surface which displace the drilling mud as the cement is placed in the wellbore.
Drilling The Well: Hydro Pressure
Hydrostatic pressure is caused by pressurized fluid that can occur naturally when a channel is opened up in the earth’s crust, e.g., the driller’s borehole.
Abnormally high pressure commonly occurs in areas where the water-filled sediments, such as clay, exist.
If the pressure of the fluid is greater than that of the surrounding rocks or formations, then the formation will crumble under its force.
Early identification of these types of pressure is critical to drilling operations because of the devastating effect it can have on personnel and equipment – either downhole or at the well’s surface.
Surface“blowouts” occur when this hydrostatic pressure reaches the surface; un-noticed and unrestricted.
There are many types of well-head equipment used by the drillers to monitor downhole pressure conditions and either prevent a blowout; or at the very least, minimize the disastrous effect of a blowout.
Drilling The Well: Kick and Detection
Kick is the term for the situation where reservoir fluids flow into the wellbore during drilling operations. The kick occurs when the pressure in the wellbore is less than that of the formation fluids, thus causing flow.
This condition is caused in two ways.
- First, if the mud weight is too low, then the hydrostatic pressure exerted on the formation by the mud column may be insufficient to hold the formation fluid in the formation. This can happen if the mud density is suddenly lightened or is not to specification to begin with, or if a drilled formation has a higher pressure than anticipated. This type of kick is called an underbalanced kick.
- The second way a kick can occur is due to the motion of the drill string or casing, effectively lowering the pressure in the wellbore below that of the formation.
Detection is the process of identifying key indicators of a kick before a kick actually occurs. There are always signs of impending well kick and a number of instruments are used to detect these signs.
If a well kick is left unchecked it can result in a sub-surface or (worse) a surface well blowout.
Drilling The Well: Blowout Preventer (BOP)
A blowout preventer (BOP) is a large valve assembly that encases the oil or gas well at the surface. It includes a series of valves, rams and seals that restrict the pressurized wellbore fluid from breeching the well and getting to the surface. Invention and use of BOPs has resulted in the end of oil gushers, which were dangerous and costly.
BOPs are built by a number of OFS manufacturers and come in a variety of styles, sizes and pressure ratings.
- Blind rams are designed to close over an open wellbore.
- Pipe rams seal around the tubular components in the well (drillpipe, casing, tubing, wireline or coiled tubing).
- Shear-seal BOPs are fitted with hardened steel shearing surfaces that can actually cut through the drill string, if all other barriers fail.
BOPs are critically important to the safety of the crew, the rig, and the wellbore. Therefore, BOPs are inspected, tested and reconditioned at regular intervals.
Any of the above BOPs may be installed underwater at the seabed.
Drilling The Well: Well Control
During drilling operations sub-surface pressures must be continuously controlled as the various layers of the downhole formation are pierced by the wellbore.
Often, “downhole blowouts” can occur when a zone of high pressure is penetrated and uncontrolled pressure starts flowing to another zone of lower pressure. This case is the most common of all well control problems. It occurs:
- during drilling operations or
- in production fields if the casing and tubing starts to corrode and deteriorate.
To control this situation three well control measures are used:
- Primary Well Control – Here, mud pressure is increased above the pressure in the formation. This form of well control is maintained and closely monitored throughout the entire well drilling process.
- Secondary Well Control – A BOP stack is activated which seals off the well annulus to prevent the downhole pressure from reaching the surface.
- Tertiary Well Control – In extreme situations, to contain the downhole pressure, a chemical compound is pumped into the well bore to counteract the flow at the zone of high pressure. One of the most common is a barite (a very heavy solid) and oil mixture. The materials solidifies as it is pumped into the wellbore and is often referred to as abarite plug.
Unfortunately, plugging the flow path with barite is rarely effective in controlling a significant underground flow. If alternative drilling paths are not readily available to get to the reservoir, cement is pumped down the bore hole and the well is abandoned.
A barite plug is tried first then cement is used if situation continues to be unstable.
Drilling The Well: Killing the Well
Killing the well is the process used totally stop a well from flowing.
A well kill refers to the operation of placing a column of heavy fluid into a well bore to prevent the flow of reservoir fluids to the surface. It works on the principle that the weight of the “kill fluid” or “kill mud” will be enough to suppress the pressure of the fluids in the formation.
During drilling, pressure control is maintained through the use of precisely concocted drilling mud, which balances out the pressure at the bottom of the hole.
In the event of sudden kick from a high pressure pocket of, say gas, that cannot be controlled with standard well control techniques, it can become necessary to kill the well. This is done by pumping kill mud (like the barite plug compound) or cement down the drill pipe, where it circulates out the bottom and into the well bore.
Related Resources:
What is the difference between Upstream and Downstream?
Directional Drilling and Horizontal Drilling
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