UNDERBALANCED DRILLING.pdf

8/18/2019 UNDERBALANCED DRILLING.pdf

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When the effective circulating downhole pressure of the drilling fluid -

which is equal to the hydrostatic pressure of the fluid column, plus pump

pressure, plus associated friction pressures - is less than the effective nearbore formation pore pressure.

Conventionally, wells are drilled overbalanced, which provides the primary

well control mechanism. Imposed wellbore pressure arises from three

different mechanisms:

Hydrostatic pressure of materials in the wellbore due to the density

of the fluid used (mud) and the density contribution of any drilled

cuttings (passive).

Dynamic pressure from fluid movement due to circulating friction of

the fluid used and the relative fluid motion caused by surge/swab of

the drillpipe (dynamic).

Imposed pressure, with occurs due to the pipe being sealed at

surface resulting in an area with pressure differential (e.g., a rotating

head or stripper element) (confining or active).


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Underbalanced drilling, or UBD, is a procedure used to drill oil and gas

wells where the pressure in the wellbore is kept lower than the fluid

pressure in the formation being drilled. As the well is being drilled,

formation fluid flows into the wellbore and up to the surface. This is theopposite of the usual situation, where the wellbore is kept at a pressure

above the formation to prevent formation fluid entering the well. In such a

conventional "overbalanced" well, the invasion of fluid is considered a kick,

and if the well is not shut-in it can lead to a blowout, a dangerous situation.

In underbalanced drilling, however, there is a "rotating head" at the surface

- essentially a seal that diverts produced fluids to a separator while allowing

the drill string to continue rotating.

If the formation pressure is relatively high, using a lower density mud will

reduce the well bore pressure below the pore pressure of the formation.Sometimes an inert gas is injected into the drilling mud to reduce its

equivalent density and hence its hydrostatic pressure throughout the well

depth. This gas is commonly nitrogen, as it is non-combustible and readily

available, but air, reduced oxygen air, processed flue gas and natural gas

have all been used in this fashion.

Objectives of Underbalanced Drilling:

The objectives of underbalanced drilling can be broken down into two maincategories:

• Maximizing hydrocarbon recovery

• Minimizing drilling problems

Maximizing Hydrocarbon Recovery:

The two main objectives of underbalanced drilling can be subdivided as

follows:

Reduced formation damage:

No invasion of solids or mud filtrate into the reservoir formation

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Early production:

Well is producing as soon as the reservoir is penetrated with a bit. This

could be a disadvantage if hydrocarbon production cannot be handled or

stored on site or if the required export lines are not available.

Reduced Stimulation:

As there is no filtrate or solids invasion in an underbalanced drilled

reservoir, the need for reservoir stimulation is eliminated. It has been

noted in wells drilled underbalanced that stimulation with fluids

significantly reduces the productivity of the reservoir. An acid wash carried

out on an underbalanced drilled well, reduced productivity from

02MMscft/day to 0 MMscft/day. The full benefits of underbalanced drillingwere never regained.

Enhanced recovery

Due to the increased productivity of an underbalanced drilled well

combined with the ability to drill infill wells in depleted fields, the recovery

of bypassed hydrocarbons is possible. This can significantly

extend the life of a field. The improved productivity of the wells also leads

to a lower drawdown, which can, in turn, reduce water coning.

Technique Selection of Underbalanced Drilling

A basic four-step process can be applied to determine the options and

requirements for a well to be drilled underbalanced:

1. Determine BHP requirements.

0. Identify the drilling fluid options.

3. Establish the well design and perform flow modelling.

4. Select the surface equipment.Initial fluid selection for underbalanced drilling operations is classified into

five major fluid types based primarily equivalent circulating density (gas,

mist, foam, gasified liquid, liquid).


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Further delineation within these groups is dependent upon considerations

outlined in the

IADC-UBO Classification System for Underbalanced Wells (Level 2,1,0,3,4,)

that further delineates selection based on well flow characteristics, well

fluid type, surface operating pressure, fluid containment, well control, andapplicable health, safety, and environmental issues.

UNDERBALANCED DRILLING TECHNIQUES:

Underbalanced drilling techniques are classified according to density of the

fluids used in the process.

Typical fluid densities range from near 2 to 7 pounds per gallon.

In fresh-water applications, the density of the circulating fluid can be

reduced by nitrogen gas injection.This reduced density helps to achieve a bottom hole circulating pressure

that is less than that of the formation pressure.

Even conventional liquids can provide underbalanced conditions with

proper density control of the drilling fluid. On the other hand, it is also

possible for a low-density fluid to cause overbalance due to the frictional

pressure drop.

Underbalanced drilling has proved to be an economical method for drilling

in depleted/low pressure reservoirs. Since it is possible to recordproduction during drilling, operators can easily and accurately identify

inflow mechanisms and pay intervals, and cease drilling operation as soon

as the target zones are identified.

One method of controlling the bottomhole pressure (BHP) is to use a choke

at the surface. BHP is controlled by opening or closing the choke to lower or

raise the standpipe pressure. Since the speed of a pressure wave through a

static fluid column is equal to the speed of sound in the same medium, a lag

time is experienced until the choking action at the surfaces reachesbottomhole. Estimating the lag time in a single-phase system is relatively

easy, whereas the same calculation in multi-phase systems can be

quite complicated. Instead of using a choke, the BHP can also be controlled

by adjusting the Equivalent Circulating Density (ECD).


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This technique essentially creates an increasing fluid density gradient

between the surface and the bottomhole. If the casing is set at a shallower

depth, ECD is preferred over choke pressure control.

Since ECD is a function of flow, underbalanced conditions should be

preserved by controlling the hydrostatic head when flow stops duringconnections.

The greater the flow resistance, the higher ECD will be. On the other hand,

it can also create an opposing condition when pipe is pulled out of the hole,

causing a swabbing effect.

Underbalanced Drilling Fluids:

o Gaseous (Compressible)

o Two-phaseo Liquid (Incompressible)

The fluid type is dictated by the boundary conditions of the drilling system.

Typically, the boundary conditions are defined by bottomhole flowing

pressure, formation fracture pressure, borehole collapse pressure and

formation pore pressure.

The density range of various drilling fluids is summarized in the following

diagram.


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Two different measures are used to define the type of fluid system:

Ratio the gas-to-liquid volume at standard conditions Quality,

the ratio of gas volume to liquid volume at hole conditions.Gas-to-liquid ratios of various drilling fluid systems are shown in

the following figure.

Gaseous Drilling Fluids:

The oldest and most basic technique is dry air drilling, which involves

pumping air down the drill string and up through the annulus. A rotating

wellhead between the blowout preventer and rotary table is used to divert

the returns. The cuttings are sent away from the rig via a discharge pipe,

and a water spray is used to kill dust at the outlet. A flame is used to burn

any returning hydrocarbons.

Nitrogen is another common drilling fluid. Other inert gases are too

expensive to be used in this process. A typical method to generate N0 is to

use membrane type filters that extract the N0 from the air stream


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before it is pumped into the wellbore. Natural gas is also a drilling fluid

option, since it is easily available from pipelines. It can directly be used

without the help of compressors. More information on nitrogen and

natural gas drilling is provided under the "Underbalanced Drilling Methods"

subtopic. Circulating pressure and hole cleanup are dependent on eachother. More cuttings in the wellbore cause higher downhole pressures.

Angel’s method provides some guidelines regarding air flow rates required

for hole cleaning. His charts are still widely used. According to these charts,

3222 ft/min is the minimum velocity for effective cutting transport.

Gaseous Fluids:

These are basically the gas systems. In initial underbalanced drilling

operations, air was used to drill. Today air drilling or dusting is still applied

in hard rock drilling and in the drilling of water wells. The use of air in

hydrocarbon bearing formations is not recommended as the combinationof oxygen and natural gas may cause an explosive mixture. There have been

a number of reported cases where downhole fires have destroyed

drillstring with the obvious potential consequences of the rig burning down

if the mixture gets to surface.

To avoid the use of air, nitrogen was introduced. The experience with

nitrogen in well servicing operations made it a first choice for

underbalanced drilling operations. The use of so-called cryogenic nitrogen

or tanks of liquid nitrogen in drilling operations can be restricted. This

depends on the logistical issues involved due to the large amount ofnitrogen required for a drilling operation.

Another option is to use natural gas, which, if available, has sometimes

proved a worthy alternative in drilling operations. If a gas reservoir is being

drilled underbalanced, a producing well or the export pipeline may well

produce sufficient gas at the right pressure to drill. This avoids the

introduction of oxygen into the well and, if available, may provide a cheap

drilling system.

Characteristics of air-drilling:

• Fast penetration rates.• Longer bit life.

• Greater footage per bit.

• Good cement jobs.

• Better production.

• Requires minimal water influx.


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• Slugging can occur.

• Mud rings can occur in the presence of fluid ingress.

• Relies on annular velocity to remove cuttings from the well.

Mist Drilling:

If gas systems are not suitable the addition of a small amount of liquid will

initially form a mist system. The fluid added to the gas environment will

disperse into fine droplets and form a mist system to drill with. In general

this technique needs to be used in areas where some formation water

exists which prevents the use of complete 'dry air' drilling.

Characteristics of mist-drilling:

• Similar to air drilling but with addition of liquid

• Relies on annular velocity to remove cuttings from the well

• Reduces formation of mud rings

• High volumes required (32-42 more than dry air drilling)

• Pressures generally higher than dry air drilling

• Incorrect air/gas-liquid ratio leads to slugging, with attendant pressure

Increase

Foam drilling:

If more liquid and a surfactant is added to the fluid, stable foam is

generated. Stable foam used for drilling has a texture not unlike shaving

foam. It is a particularly good drilling fluid with a high carrying capacity and

a low density. One of the problems encountered with the conventional

foam systems is that a stable foam is as it sounds. The foam remains stable

even when it returns to the surface and this can cause problems on a rig if

the foam cannot be broken down fast enough. In the old foam systems, the

amount of defoamer had to be tested carefully so that the foam wasbroken down before any fluid left the separators. In closed circulation

drilling systems stable foam could cause particular problems with carry

over. The recently developed stable foam systems are simpler to break and

the liquid can also be re-foamed so that less foaming agent is required and

a closed circulation system can be used. These systems, in general, rely on


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either a chemical method, of breaking and making the foam or the

utilization of an increase and decrease of pH, to make and break the foam.

The foam quality at surface used for drilling is normally between82 and

9. The same amount, 82 to 9, is gas with the remainder being liquid.

Downhole, due to the hydrostatic pressure of the annular column, this ratiochanges as the volume of gas is reduced. An average acceptable bottom-

hole foam quality (FQ) is in the region of 2-62. Characteristics of foam-

drilling:

• Extra fluid in the system reduces the influence of formation water

• Very high carrying capacity

• Reduced pump rates due to improved cuttings transport

• Stable foam reduces slugging tendencies of the wellbore

• The stable foam can withstand limited circulation stoppages without

affecting the cuttings removal or ECD to any significant degree• Improved surface control and more stable downhole environment

• The breaking down of the foam at surface needs to be addressed at the

design stage

• More increased surface equipment required

Gasified Systems:

If a foam system is too light for the well, a gasified system can be used. In

these systems, liquid is gasified to reduce the density. There are a number

of methods that can be used to gasify a liquid system and these methodsare discussed within the injection systems section. The use of gas and

liquid, as a circulation system in a well, complicates the hydraulics

programme. The ratio of gas and liquid must be carefully calculated to

ensure that a stable circulation system is used. If too much gas is used,

slugging will occur. If not enough gas is used, the required bottom hole

pressure will be exceeded and the well will become overbalanced.

Characteristics of gasified-mud systems:

• Extra fluid in the system will almost eliminate the influence of formationfluid unless incompatibilities occur

• The mud properties can easily be identified prior to commencing the

operation

• Generally, less gas is required

• Slugging of the gas and fluid must be managed correctly


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Kinds of Underbalanced Drilling

There are several kinds of underbalanced drilling. The most common are

listed below.

Dry Air: This is also known as dusting. Here air compressors combinedwith a booster (which takes the head from the compressors and increases

the pressure of the air, but does not increase the volume of air going down

hole) are used and the only fluid injected into the well is a small amount of

oil to reduce corrosion.

Mist: A small amount of foaming agent (soap) is added into the flow of air.

Fine particles of water and foam in an atmosphere of air bring cuttings back

to the surface.

Foam: A larger amount of foaming agent is added into the flow. Bubblesand slugs of bubbles in an atmosphere of mist bring cuttings back to the

surface.

Stable foam: An even larger amount of foaming agent is added into theflow. This is the consistency of a shaving cream.

Airlift: Slugs and bubbles of air in a matrix of water, soap can or can not be

added into the fluid flow of air.

Aerated Mud: Air or another gas is injected into the flow of drilling mud.

Degassing units are required to remove air before it can be recirculated.

Two-Phase Drilling Fluids:

Two-phase drilling fluids, or lightened drilling fluids, consist of either foam-type fluids or aerated drilling mud. Liquids are mixed with gas to achieve a

required circulating fluid density. The equation of state method is used to

predict fluid properties at downhole conditions.

A pump is used to inject liquid into a gas stream before it enters the well.

The small liquid droplets affect


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the behavior of the circulating gas. If more liquids (05 -0) are

introduced, then a foam phase is generated in which the liquid forms a

continuous structure, entrapping the gas bubbles inside.

Once the liquid volume exceeds 0, we no longer have a foam structure.

This next level comprises aerated drilling muds (fresh water, brine, diesel orcrude oil). Parasite strings are typically used to introduce gas into the

circulating liquid stream. A parasite string is an external flow path (possibly

coiled tubing), which is run and cemented outside of the casing.

Since gas and liquid compressibility values differ significantly from each

other as pressure and temperature changes, the liquid fraction changes as

well. Frictional pressure drops are controlled mainly by the flow regime,

flow rate, fluid properties and flow geometry. Therefore, phase behavior is

a very important component in underbalanced drilling models. Many

investigators, including a special institute atthe University of Tulsa, have extensively analyzed two-phase flow patterns

and regimes. Some of the most common two-phase flow regimes are:

Bubble flow

Annular dispersed flow

Stratified or laminar flow

Plug or churn flow

Slug flow

Liquid Drilling Fluids:

Since formation pressure is usually larger than the hydrostatic pressure of

fresh water or saline water, conventional drilling fluids might also provide

underbalanced conditions. Even if the drilling fluid density exceeds the

formation pore gradient, fluid loss into a formation can cause reduced

pressure regions within the wellbore, thus allowing formation fluids to flow

in.

Selection of drilling fluid used in underbalanced drilling :

A very important part in the design of an underbalanced drilling operation

is the selection of the drilling fluid. Drilling fluid selection will effect

equivalent circulating density (ECD), borehole stability, formation damage,


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under balance pressure, surface equipment configuration, and waste

management considerations. Given these and other important operational

and economical considerations, a Selection Fluid Guide for Underbalanced

Drilling Operations has been developed. This Selection Guide is presented

in three main “stages” for planning, based primarily on information and/orcalculations needed for evaluation, and on the most likely options resulting

from the evaluation5 The stages are further divided into “blocks“ so that the

evaluation and selection processes can be conducted in a logical and

organized manner.

The three main stages of the Selection Guide are:

STAGE 1: What are The Objectives?

Is the purpose for drilling underbalanced related to:(a) Reservoir Considerations, or

(b)Drilling Operation Considerations?

General Considerations must be evaluated to determine the purpose for

drilling under balanced. These include mainly skin damage/ productivity

index (reservoir) and borehole stability/rate of penetration (drilling).

! STAGE 2: What types of fluids can be considered, near or

underbalanced?

Data must be collected, calculations made, and results evaluated in order

to determine the level of under balance pressure.What is the well classification? (BLOCK 0)

What are the general considerations? (BLOCK 3)

What is the ECD? (BLOCK 4)

What classifications of fluids should be considered? (BLOCK )

! STAGE 3: What is the optimum drilling fluid selection?

Final fluid selection must be based on fluids compatibility, borehole

cleaning and stability, temperature, corrosion effects, and environmental

issues. Primary well design considerations must be reviewed before making

a final fluid selection. (BLOCK 6)


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Injection methods are available to reduce the hydrostatic pressure :

1-Drillpipe injection

0-Annular injection

3-Parasite string injection


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Advantages:

Underbalanced wells have several advantages over conventional drilling

including:

Eliminated formation damage. In a conventional well, drilling mud is

forced into the formation in a process called invasion, which

frequently causes formation damage - a decrease in the ability of the

formation to transmit oil into the wellbore at a given pressure and

flow rate. It may or may not be repairable. In underbalanced drilling,

if the underbalanced state is maintained until the well becomesproductive, invasion does not occur and formation damage can be

completely avoided.

Increased Rate of Penetration (ROP). With less pressure at the

bottom of the wellbore, it is easier for the drill bit to cut and remove

rock.

http://en.wikipedia.org/w/index.php?title=Formation_damage&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Formation_damage&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Formation_damage&action=edit&redlink=1http://en.wikipedia.org/wiki/Drilling_mudhttp://en.wikipedia.org/wiki/Drilling_mudhttp://en.wikipedia.org/wiki/Drilling_mudhttp://en.wikipedia.org/wiki/Drilling_fluid_invasionhttp://en.wikipedia.org/wiki/Drilling_fluid_invasionhttp://en.wikipedia.org/wiki/Drilling_fluid_invasionhttp://en.wikipedia.org/wiki/Drilling_fluid_invasionhttp://en.wikipedia.org/wiki/Drilling_mudhttp://en.wikipedia.org/w/index.php?title=Formation_damage&action=edit&redlink=1


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Reduction of lost circulation. Lost circulation is when drilling mud

flows into the formation uncontrollably. Large amounts of mud can

be lost before a proper mud cake forms, or the loss can continue

indefinitely. If the well is drilled underbalanced, mud will not enter

the formation and the problem can be avoided.1. Differential sticking is eliminated. Differential sticking is when

the drill pipe is pressed against the wellbore wall so that part

of its circumference will see only reservoir pressure, while the

rest will continue to be pushed by wellbore pressure. As a

result the pipe becomes stuck to the wall, and can require

thousands of pounds of force to remove, which may prove

impossible. Because the reservoir pressure is greater than the

wellbore pressure in UBD, the pipe is pushed away from the

walls, eliminating differential sticking.

Disadvantages:

Underbalanced drilling is usually more expensive than conventional drilling

(when drilling a deviated well which requires directional drilling tools), and

has safety issues of its own. Technically the well is always in a blowout

condition unless a heavier fluid is displaced into the well. Air drilling

requires a faster up hole volume as the cuttings will fall faster down the

annulus when the compressors are taken off the hole compared to having ahigher viscosity fluid in the hole. Because air is compressible mud pulse

telemetry measurement while drilling (MWD) tools which require an

incompressible fluid can not work. Common technologies used to eliminate

this problem are either electromagnetic MWD tools or wireline MWD tools.

Downhole mechanics are usually more violent also because the volume of

fluid going through a downhole motor or downhole hammer is greater than

an equivalent fluid when drilling balanced or over balanced because of the

need of higher up hole velocities. Corrosion is also a problem, but can be

largely avoided using a coating oil or rust inhibitors

Hole Cleaning Considerations:

Decreased bottom hole pressure typically causes higher penetration rates.

However, higher penetration rates can increase the circulating bottom hole

http://en.wikipedia.org/wiki/Lost_circulationhttp://en.wikipedia.org/wiki/Lost_circulationhttp://en.wikipedia.org/wiki/Lost_circulationhttp://en.wikipedia.org/w/index.php?title=Mud_cake_(oil_and_gas)&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Mud_cake_(oil_and_gas)&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Mud_cake_(oil_and_gas)&action=edit&redlink=1http://en.wikipedia.org/wiki/Differential_stickinghttp://en.wikipedia.org/wiki/Differential_stickinghttp://en.wikipedia.org/wiki/Directional_drillinghttp://en.wikipedia.org/wiki/Directional_drillinghttp://en.wikipedia.org/wiki/Directional_drillinghttp://en.wikipedia.org/wiki/Directional_drillinghttp://en.wikipedia.org/wiki/Differential_stickinghttp://en.wikipedia.org/w/index.php?title=Mud_cake_(oil_and_gas)&action=edit&redlink=1http://en.wikipedia.org/wiki/Lost_circulation


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pressure and bring the well back to overbalanced conditions. Moreover,

due to the annular fluid segregation, there is an increased risk that the

wellbore will pack-off, resulting in stuck pipe. In this situation, gas tends to

rise while the liquid settles to the bottom of the hole. This is a major cause

of increased bottom hole pressures because of the increased fluid densityat the sand face.

Large cutting volumes generated by high penetration rates are also difficult

to remove. Therefore, penetration rates should be carefully adjusted to

ensure sufficient hole cleaning and slug removal.

Inadequate liquid flow rates can cause sticky-hole conditions that result in

differential sticking. A decrease in ROP would therefore be needed for the

cuttings to be transferred to the surface. A viscosified aqueous phase is animportant factor in achieving better ROP When drilling with foam and mist,

hole cleaning efficiency reaches a limit after a certain level of underbalance,

and the drilling rate starts to decrease as illustrated in the following figure.

In this situation, an increase in fluid rate is needed to increase the cleaning

action and allow a higher rate of penetration.

Limiting Technical Factors:

Major technical factors that restrict the application of underbalanced

drilling techniques are listed as follows:


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Reduced wellbore pressure gradients can cause hole stability

problems

Formation of mud rings can block air flow, leading to downhole

fires.

Water causes cuttings to accumulate, possibly causing the drill

string to stick. If aerated mud is used rather than air, differential

underbalance can be reduced.

HC’s and air often mix to achieve a flammable range5 With a small

spark, this can be generated by the contact between the drill

string and hard minerals, the risk of fire increases.

Stable foam isn’t easy to achieve 5

Depending on the drilling site location, logistical and economical

constraints can be substantial. Similarly, the need for specialized drilling

equipment can also render underbalanced operations uneconomical. "Even

though the cost of drilling underbalanced could be more expensive than

conventional overbalanced drilling, due to the increased ROP and reduced

formation damage, it often turns out to be the more cost-effective drilling

technique.

References: Leading Edge Advantage International Ltd 00.

Underbalanced Drilling Manual.

‘Fluid Selection for Underbalanced Drilling Operations” presented at the UB Technology

Conference 002.

SUCCESSFUL APPLICATION OF UNDERBALANCED DRILLING WELLS USING PARASITE STRING

INJECTION CONTINUES IN ROCKIES Deepak M. Gala, Weatherford International Ltd., SPE, AADE

Jose Danilo Morales, Weatherford International Ltd., SPE, AADE Jeff Cutler, Occidental Petroleum

Corporation, SPE, AADE.

Offshore Applications of Underbalanced Drilling Technology -Well Control Considerations

When Applied to Overbalanced Drilling Don M. Hannegan, P.E., Weatherford Drilling &Intervention Services, UBS.

Nas, Steve, Chapter 2 Underbalanced Drilling, from Petroleum Engineering Handbook,

Volume II, Editor Robert Mitchell, 00, pages II-925 to 965. Handbook available from

Society of Petroleum Engineers.

http://en.wikipedia.org/wiki/Underbalanced_drilling.

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UNDERBALANCED DRILLING.pdf