Well kill
Encyclopedia
A well kill is the operation of placing a column of heavy fluid into a well bore
in order to prevent the flow of reservoir
fluids without the need for pressure control equipment at 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 formation fluids. Well kills may be planned in the case of advanced interventions
such as workover
s, or be contingency operations. The situation calling for a well kill will dictate the method taken.
Not all well kills are deliberate. Sometimes, the unintended build up of fluids, either from injection of chemicals like methanol
from surface, or from liquids produced from the reservoir, can be enough to kill the well, particularly gas
wells, which are notoriously easy to kill.
Where P is the pressure at depth h in the column, g is the acceleration of gravity and ρ is the density
of the fluid. It is common in the oil industry to use weight density
, which is the product of mass density and the acceleration of gravity. This reduces the equation to:
Where γ is the weight density. Weight density may also be described as the pressure gradient because it directly determines how much extra pressure will be added by increasing depth of the column of fluid.
The objective in a well kill, is to make the pressure at the bottom of the kill fluid equal (or slightly greater) than the pressure of the reservoir fluids.
. We have a kill fluid with a weight density of 16kN.m-3. What will need to be the height of the hydrostatic head in order to kill the well?
From the equation:
Therefore, a column of 2375m of this fluid is needed. It must be remembered that this refers to the true vertical depth of the column, not the measured depth, which is always larger than true vertical depth due to deviations from vertical.
is far from being used. Weight densities are commonly either given as specific gravity
or in pounds
per gallon
. Simple conversion factors (0.433 for specific gravity and 0.052 for ppg) convert these values to a pressure gradient in psi
per foot. Multiplying by the depth in feet gives the pressure at the bottom of the column.
of the well, through a point of communication between it and the production tubing
just above the production packer
and up the tubing, displacing the lighter well bore fluids, which are allowed to flow to production.
The point of communication was traditionally a device called a sliding sleeve, or sliding side door, which is a hydraulically operated device, built into the production tubing
. During normal operation, it would remain closed sealing off the tubing and the annulus, but for events such as this, it would be opened to allow the free flow of fluids between the two regions. These components have fallen out of favour as they were prone to leaking. Instead, it is now more common to punch a hole in the tubing for circulation kills. Although this permanently damages the tubing, given that most pre-planned well kills are for workover
s, this is not an issue, since the tubing is being pulled for replacement anyway.
. This can be effective at achieving the central aim of a well kill; building up a sufficient hydrostatic head in the well bore. However, it can risk damaging the reservoir, by forcing undesired materials into it. The principal advantage is that it can be done with little advanced planning.
and circulated out through the annulus
. Though effective, it is not as desirable since it is preferred that the well bore fluids be displaced out to production, rather than the annulus.
, which balances out the pressure at the bottom of the hole. In the event of suddenly encountering a high pressure pocket of, say gas
(called a "kick"), it can become necessary to kill the well. This is done by pumping kill mud down the drill pipe
, where it circulates out the bottom and into the well bore.
since it is inert and cheap. A gas can be put under sufficient pressure to allow it to push heavy kill fluid, but will then expand and become light once pressure is removed. This means that having displaced the kill fluid, it will not itself kill the well. The reservoir fluids should be able to flow to surface, displacing the gas.
The cheapest way to do it is similar to bullheading, where the nitrogen is pumped in under high pressure to force the kill fluid into the reservoir. This, of course, runs a high risk of causing well damage. The most effective way is to use coiled tubing
, pumping the gas down the coil and circulating out the bottom into the well bore, where it will displace the kill mud to production.
Oil well
An oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...
in order to prevent the flow of reservoir
Oil reservoir
A petroleum reservoir, or oil and gas reservoir, is a subsurface pool of hydrocarbons contained in porous or fractured rock formations. The naturally occurring hydrocarbons, such as crude oil or natural gas, are trapped by overlying rock formations with lower permeability...
fluids without the need for pressure control equipment at the surface. It works on the principle that the weight of the "kill fluid" or "kill mud" will be enough to suppress the pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
of the formation fluids. Well kills may be planned in the case of advanced interventions
Well intervention
A well intervention, or 'well work', is any operation carried out on an oil or gas well during, or at the end of its productive life, that alters the state of the well and or well geometry, provides well diagnostics or manages the production of the well....
such as workover
Workover
The term workover is used to refer to any kind of oil well intervention involving invasive techniques, such as wireline, coiled tubing or snubbing. More specifically though, it will refer to the expensive process of pulling and replacing a completion....
s, or be contingency operations. The situation calling for a well kill will dictate the method taken.
Not all well kills are deliberate. Sometimes, the unintended build up of fluids, either from injection of chemicals like methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
from surface, or from liquids produced from the reservoir, can be enough to kill the well, particularly gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
wells, which are notoriously easy to kill.
Principles
The principle of a well kill revolves around the weight of a column of fluid and hence the pressure exerted at the bottom.Where P is the pressure at depth h in the column, g is the acceleration of gravity and ρ is the density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
of the fluid. It is common in the oil industry to use weight density
Specific weight
The specific weight is the weight per unit volume of a material. The symbol of specific weight is γ ....
, which is the product of mass density and the acceleration of gravity. This reduces the equation to:
Where γ is the weight density. Weight density may also be described as the pressure gradient because it directly determines how much extra pressure will be added by increasing depth of the column of fluid.
The objective in a well kill, is to make the pressure at the bottom of the kill fluid equal (or slightly greater) than the pressure of the reservoir fluids.
Example
The pressure of the reservoir fluids at the bottom of the hole is 38MPaPascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
. We have a kill fluid with a weight density of 16kN.m-3. What will need to be the height of the hydrostatic head in order to kill the well?
From the equation:
Therefore, a column of 2375m of this fluid is needed. It must be remembered that this refers to the true vertical depth of the column, not the measured depth, which is always larger than true vertical depth due to deviations from vertical.
Maths in the oil field
In the oil industry, a pure SI systemInternational System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
is far from being used. Weight densities are commonly either given as specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
or in pounds
Pound (mass)
The pound or pound-mass is a unit of mass used in the Imperial, United States customary and other systems of measurement...
per gallon
Gallon
The gallon is a measure of volume. Historically it has had many different definitions, but there are three definitions in current use: the imperial gallon which is used in the United Kingdom and semi-officially within Canada, the United States liquid gallon and the lesser used United States dry...
. Simple conversion factors (0.433 for specific gravity and 0.052 for ppg) convert these values to a pressure gradient in psi
Pounds per square inch
The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units...
per foot. Multiplying by the depth in feet gives the pressure at the bottom of the column.
Reverse circulation
This is often tidiest way of making a pre-planned well kill. It involves pumping kill fluid down the 'A' annulusAnnulus (oil well)
The annulus of an oil well refers to any void between any piping, tubing or casing and the piping, tubing, or casing immediately surrounding it. It is named after the corresponding geometric concept...
of the well, through a point of communication between it and the production tubing
Production tubing
Production tubing is a tube used in a wellbore through which production fluids are produced .Production tubing is run into the drilled well after the casing is run and cemented in place. Production tubing protects wellbore casing from wear, tear, corrosion, and deposition of by-products, such as...
just above the production packer
Production packer
A production packer is a standard component of the completion hardware of oil or gas wells used to provide a seal between the outside of the production tubing and the inside of the casing, liner, or wellbore wall....
and up the tubing, displacing the lighter well bore fluids, which are allowed to flow to production.
The point of communication was traditionally a device called a sliding sleeve, or sliding side door, which is a hydraulically operated device, built into the production tubing
Production tubing
Production tubing is a tube used in a wellbore through which production fluids are produced .Production tubing is run into the drilled well after the casing is run and cemented in place. Production tubing protects wellbore casing from wear, tear, corrosion, and deposition of by-products, such as...
. During normal operation, it would remain closed sealing off the tubing and the annulus, but for events such as this, it would be opened to allow the free flow of fluids between the two regions. These components have fallen out of favour as they were prone to leaking. Instead, it is now more common to punch a hole in the tubing for circulation kills. Although this permanently damages the tubing, given that most pre-planned well kills are for workover
Workover
The term workover is used to refer to any kind of oil well intervention involving invasive techniques, such as wireline, coiled tubing or snubbing. More specifically though, it will refer to the expensive process of pulling and replacing a completion....
s, this is not an issue, since the tubing is being pulled for replacement anyway.
Bullheading
This is the most common method of a contingency well kill. If there is a sudden need to kill a well quickly, without the time for rigging up for circulation, the more blunt instrument of bullheading may be used. This involves simply pumping the kill fluid directly down the well bore, forcing the well bore fluids back into the reservoirOil reservoir
A petroleum reservoir, or oil and gas reservoir, is a subsurface pool of hydrocarbons contained in porous or fractured rock formations. The naturally occurring hydrocarbons, such as crude oil or natural gas, are trapped by overlying rock formations with lower permeability...
. This can be effective at achieving the central aim of a well kill; building up a sufficient hydrostatic head in the well bore. However, it can risk damaging the reservoir, by forcing undesired materials into it. The principal advantage is that it can be done with little advanced planning.
Forward circulation
This is similar to reverse circulation, except the kill mud is pumped into the production tubingProduction tubing
Production tubing is a tube used in a wellbore through which production fluids are produced .Production tubing is run into the drilled well after the casing is run and cemented in place. Production tubing protects wellbore casing from wear, tear, corrosion, and deposition of by-products, such as...
and circulated out through the annulus
Annulus (oil well)
The annulus of an oil well refers to any void between any piping, tubing or casing and the piping, tubing, or casing immediately surrounding it. It is named after the corresponding geometric concept...
. Though effective, it is not as desirable since it is preferred that the well bore fluids be displaced out to production, rather than the annulus.
Lubricate and bleed
This is the most time consuming form of well kill. It involves repeatedly pumping in small quantities of kill mud into the well bore and then bleeding off excess pressure. It works on the principle that the heavier kill mud will sink below the lighter well bore fluids and so bleeding off the pressure will remove the latter leaving an increasing quantity of kill mud in the well bore with successive steps.Well kills during drilling operations
During drilling, pressure control is maintained through the use of precisely concocted drilling fluidDrilling fluid
In geotechnical engineering, drilling fluid is a fluid used to aid the drilling of boreholes into the earth. Often used while drilling oil and natural gas wells and on exploration drilling rigs, drilling fluids are also used for much simpler boreholes, such as water wells. Liquid drilling fluid...
, which balances out the pressure at the bottom of the hole. In the event of suddenly encountering a high pressure pocket of, say gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
(called a "kick"), it can become necessary to kill the well. This is done by pumping kill mud down the drill pipe
Drill string
A drill string on a drilling rig is a column, or string, of drill pipe that transmits drilling fluid and torque to the drill bit. The term is loosely applied as the assembled collection of the drill pipe, drill collars, tools and drill bit...
, where it circulates out the bottom and into the well bore.
Reversing a well kill
The intention of a well kill (or the reality of an unintentional well kill) is to stop reservoir fluids flowing to surface. This of course creates problems when it is desirable to get the well flowing again. In order to reverse the well kill, the kill fluid must be displaced from the well bore. This involves injecting a gas at high pressure, usually nitrogenNitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
since it is inert and cheap. A gas can be put under sufficient pressure to allow it to push heavy kill fluid, but will then expand and become light once pressure is removed. This means that having displaced the kill fluid, it will not itself kill the well. The reservoir fluids should be able to flow to surface, displacing the gas.
The cheapest way to do it is similar to bullheading, where the nitrogen is pumped in under high pressure to force the kill fluid into the reservoir. This, of course, runs a high risk of causing well damage. The most effective way is to use coiled tubing
Coiled tubing
In the oil and gas industries, coiled tubing refers to metal piping, normally 1" to 3.25" in diameter, used for interventions in oil and gas wells and sometimes as production tubing in depleted gas wells, which comes spooled on a large reel. Coiled tubing is often used to carry out operations...
, pumping the gas down the coil and circulating out the bottom into the well bore, where it will displace the kill mud to production.
See also
- Blowout preventerBlowout preventerA blowout preventer is a large, specialized valve used to seal, control and monitor oil and gas wells. Blowout preventers were developed to cope with extreme erratic pressures and uncontrolled flow emanating from a well reservoir during drilling. Kicks can lead to a potentially catastrophic...
- Coiled tubingCoiled tubingIn the oil and gas industries, coiled tubing refers to metal piping, normally 1" to 3.25" in diameter, used for interventions in oil and gas wells and sometimes as production tubing in depleted gas wells, which comes spooled on a large reel. Coiled tubing is often used to carry out operations...
- Oil wellOil wellAn oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...
- Top killTop killA top kill is a procedure used as a means of regaining control over an oil well that is experiencing an uncontrolled eruption of crude oil or natural gas . The process involves pumping heavyweight drilling mud into the well...
- Well drillingWell drillingWell drilling is the process of drilling a hole in the ground for the extraction of a natural resource such as ground water, natural gas, or petroleum...
- Well interventionWell interventionA well intervention, or 'well work', is any operation carried out on an oil or gas well during, or at the end of its productive life, that alters the state of the well and or well geometry, provides well diagnostics or manages the production of the well....