Pressure Control –Well Control Procedures cont

Well Control Procedures2

•The Wait-and-Weight Method
–After the well is shut in, the rig crew “waits”while the drilling fluid in the pits is “weighted”up to the kill-mud weight.
–In order to use this method successfully, sufficient weight material must be on location and the mixing capacity must be sufficient to maintain the kill-mud weight while circulating at the slow pump rate.
–This procedure is more complicated than the Driller’s Method.
–In the Driller’s Method, weighted mud is not pumped into the well until the kick has been circulated out of the well.
–The gas expansion is compensated for by maintaining a constant drill pipe pressure while circulating the kick out.
–When weighted mud is pumped into the well, the casing pressure is held constant until the weighted mud reaches the bit.
–This compensates for the changing hydrostatic pressure in the drill pipe.
•The Wait-and-Weight Method
–In the Wait-and-Weight Method, gas is expanding in the annulus while the hydrostatic pressure is increasing in the drill pipe.
–This requires that the pump pressure needed for maintaining a constant bottom-hole pressure must change as the fluid is circulated.
–A chart of the scheduled pump or drill pipe pressure changes simplifies the kill procedure and reduces the chance of error.
–The pressure schedule or graph determines the pump pressure while the kill mud is being pumped down the drill pipe.
–As the hydrostatic pressure in the drill pipe increases, the pump pressure necessary to maintain the correct bottom-hole pressure is reduced.
–Well-control worksheets for the Wait-and-Weight Method contain a pressure schedule graph.
–The schedule is drawn on standard rectangular coordinates.
–The vertical axis is for the pump pressure and the horizontal axis is for the pump strokes.
–At zero (0) pump strokes, plot the ICP on the pressure scale.
–Plot the surface-to-bit strokes and plot the FCP on the graph.
–Draw a straight line between the two points.
–It is not practical to try to maintain too fine a control on the drill pipe pressure while killing the well.
–Instead, make a chart that shows the pump pressure from the schedule at a selected stroke interval (i.e. 100, 150, 200 etc.).
–The pump pressure is maintained according to this pressure until the selected number of strokes is pumped.
–The pump pressure is then reduced to the next pressure until the stroke interval is pumped.
–This stair step fashion is continued until the kill mud reaches the bit.
–At that time, the pump pressure is held constant until the kill mud is observed at the surface.
•NOTE: The pump pressure will decrease on its own as the kill-mud weight is pumped down the drill pipe.
•This is due to the increase in hydrostatic pressure in the drill pipe.
•As a result, few, if any, choke adjustments are required while pumping kill mud down the drill pipe.
•Some adjustments will be required to account for the changing hydrostatic pressure in the annulus due to the intruding fluid moving up the annulus.
•The Wait-and-Weight Method–Once the kill-weight mud reaches the bit, the pump pressure is held constant at the Final Circulating Pressure (FCP) until the kill mud reaches the surface. –This FCP is calculated with the following equation.
FCP =( RCP x kill-mud weight)/(original mud weight)
–This equation calculates the reduced circulating pressure using the kill-mud weight as the circulating fluid.
–The calculations for pressures through these two sections of the circulating system are based on turbulent pressure losses and energy changes.
–Since the only significant change to the drilling fluid properties used to calculate these pressure losses is the mud density, the circulating pressure is increased by the ratio of the kill-mud weight to the original mud weight.
–The Initial Circulating Pressure (ICP) is calculated the same way as in the Driller’s Method:
ICP = RCP + SIDPP
–The pressure schedule is drawn using the ICP, FCP and the surface to-bit strokes.
–Shut the well in and record the pertinent kick information.
–Calculate the kill-weight mud.
–Begin increasing the mud weight in the surface pits to the kill-weight mud.
–Calculate the ICP.
–Calculate the FCP.
–Calculate the surface-to-bit strokes.
–Construct a pressure schedule.
–Open the adjustable choke and start pumping at the pre-selected slow pump rate.
–Adjust the choke to obtain a pump pressure equal to the ICP.
–Circulate out the kick following the pressure schedule using the adjustable choke.
–Maintain a constant pump rate throughout the
–circulating process.
–Maintain the mud weight in the surface system at the kill-mud weight.
–Once the kill mud reaches the bit, maintain the FCP until the kill mud is observed at the surface.
–Stop pumping and shut the well in to check for pressures.
–If shut-in pressures exist, additional mud weight and circulation will be required.
–If no shut-in pressures exist, the well is under control.
–At this time, one or two circulations can be made to condition the mud and increase the mud weight to provide a trip margin.
•The Circulate-and-Weight Method
–The Circulate-and-Weight (Concurrent) Method is used to circulate the kick out of the hole while increasing the density of the drilling fluid gradually to the kill-mud weight.
–The well is shut in only long enough to obtain the pertinent information about the kick situation.
–The calculations and techniques used in the Wait-and-Weight Method are used in the Circulate-and-Weight (Concurrent) Method.
–Start circulating with the initial circulating pressure and begin adding barite to the system until you reach the kill-weight mud.
–This method uses a gradual increase in mud weight as the kick is circulated out.
–The Circulate-and-Weight (Concurrent) Method is more complex than either the Driller’s Method or the Wait-and-Weight Method due to the various densities of drilling fluid in the drill pipe.
–The number of different densities and the volumes of each depends upon the mixing capability and circulating rate of the drilling rig used.
–A complicated pressure schedule is necessary, as is a precise knowledge of when a mud density was achieved and pumped down the drill pipe.
–Excellent communications between the choke operator and the mud pits is required.
–A pressure schedule similar to that of the Wait-and-Weight Method must be developed.
–The difference between the schedules is that the circulating pressure will be plotted vs. the mud weight.
–Use the Y-axis for the pressure and the X-axis for the mud weight.
–Three calculations will be required to complete the schedule:
•Kill-mud weight, ICP and FCP.
–The equations for these are the same as for the Wait-and-Weight Method.
–To construct the schedule, plot the ICP at the original mud weight.
–Then, plot the FCP at the kill-mud weight.
–Use a straight edge to connect the two points.
–Start circulating at the reduced circulating rate.
–Adjust the choke to reach the ICP.
–While circulating, begin increasing the density of the mud in the pits.
–When an increase of 0.1 lb/gal is achieved in the pits, determine the time it will take to reach the bit.
–When this density reaches the bit, decrease the circulating pressure to the value associated with the density on the pressure schedule.
–Maintain this pressure until a new density reaches the bit.
–At this time, reduce the pressure according to the pressure schedule.
–Continue this process until the mud weight at the bit has been increased to the kill-mud weight.
–Maintain the FCP until the kill-mud weight has been observed at the surface.
•The methods outlined in this topic have advantages and disadvantages.
•Knowing the specifics about the well will determine the appropriate method to be successful in circulating the intruding fluid out of the well and circulating the kill-mud into it.
•A brief list of advantages and disadvantages for each method is listed below.
•Wait-and-Weight Method
–Advantages
•Kills the well in one circulation.
•Subjects the casing shoe to the minimum amount of pressure due to additional hydrostatic pressure from the mud weight increase.
–Disadvantages
•The well is shut in for a long period of time with no circulation.
–A gas kick will migrate up the hole, increasing the pressure, unless pressures are monitored constantly.
–Fluids such as saltwater will contaminate the fluid, causing increases in fluid loss. This, in turn, increases the possibility of sticking the drillstring.
–A gas kick in oil-or synthetic-base fluid can strip the barite from the fluid due to the solubility of gas in the base fluid.
–Gas changes phases and acts as a liquid when it solubilizesin the oil-base mud.
–This dilutes the fluid and may reduce the viscosity enough to allow weight material to settle and plug the annulus.
•Requires more calculations than the Driller’s Method.
•Requires sufficient supplies of weight material and a good mixing system to maintain the density as the fluid is circulated.
•Concurrent Method
–Advantages
•Removes the intruding fluid in a minimum amount of time.
•Subjects the casing shoe to a reduced pressure due to increasing hydrostatic pressure.
•Weight-up can be adjusted as weight material supplies allow.
•Concurrent Method
–Advantages
•Removes the intruding fluid in a minimum amount of time.
•Subjects the casing shoe to a reduced pressure due to increasing hydrostatic pressure.
•Weight-up can be adjusted as weight material supplies allow.

Pressure Control –Well Control Procedures

this will be into 2 topics
Well Control Procedures 1

•Kick Detection
–Early detection of a kick is important.
–It can reduce the size of the kick, lower the quantity of pressure exerted on the casing shoe and simplify regaining control of the well.
•Indications that a kick has entered the well are:
–Increases in flow at the flow line.
–Increases in pit volume.
–Flow with the pump off.
–Hole taking insufficient mud volume on trip.
•Shut in the well
–When the warning signs of a kick are recognized, steps should be taken immediately to determine if the well is flowing and to shut the well in as quickly and safely as possible, to prevent any further influx into the wellbore.
–Reducing the size of the influx is a high-priority objective.
–A kick can occur while drilling or while tripping.
•Well Control Methods
–Once the well has been shut in, steps should be taken to circulate the intruding fluid out of the well.
–Also, the density of the drilling fluid should be increased to provide sufficient hydrostatic pressure to control the formation pressure.
–Over the years, several methods have been developed to circulate the kick out and weight up the drilling fluid.
–All recognized well-control methods use a constant, but slow, pump rate when circulating a kick out of the hole and replacing the light mud with kill mud.
–Additional formation fluids must be kept from entering the wellbore while the kick is being circulated out of the well and the weighted kill mud is being circulated.
–If the kick has not been allowed to flow back through the drill pipe and the bit is on bottom, the shut-in drill pipe pressure plus the hydrostatic pressure (PHYD) of the mud in the drill pipe is equal to the formation pressure.
–The information that should be recorded after taking a kick are:
•Measured depth.
•Total vertical depth.
•Mud weight.
•Shut-In Drill Pipe Pressure (SIDPP).
•Shut-In Casing Pressure (SICP).
•Kick volume.
•Fractured gradient.
•Casing TVD.
•Reduced Circulating Pressure (RCP).
•Reduced Circulating Rate (RCR).
•Reduced Pump Output (RPO).
–If off-bottom:
•Measured depth of bit.
•TVD of bit
–The first 6 items are taken at the time the kick is taken and the well is shut in.
–The next five items should be known or measured prior to taking a kick.
–They must be used to make the necessary calculations to circulate the kick out of the hole and to kill the well.
–The last 2 items apply if the kick occurs while tripping.
•The Driller’s Method
–The simplest of the approved well-control methods.
–It was developed to circulate the kick out of the well and circulate the kill mud into the well (in two circulations) with a minimum number of calculations.
–The method’s original purpose was to control wells with minimal supervision, poor mixing capabilities or insufficient weighting material on location.
•The Driller’s Method Procedure
–Shut the well in and record the pertinent kick information.
–Calculate the Initial Circulation Pressure (ICP):
ICP = RCP + SIDPP
–Open the adjustable choke and start pumping at the pre-selected slow pump rate.
–Adjust the choke to obtain a pump pressure equal to the ICP.
–Circulate the kick out by maintaining the ICP using the adjustable choke.
–Maintain a constant pump rate throughout the circulating process.
–Once the kick has been circulated out of the well, the well can be shut in.
–The SIDPP and the SICP should be equal, since the intruding fluid has been circulated out of the well.
–Calculate the kill-mud weight and weight up the fluid in the surface system.
–Open the adjustable choke and start pumping at the pre-selected slow pump rate.
–Adjust the choke to maintain the casing pressure at the SICP.
–Maintain the mud weight in the surface system at the kill-mud weight.
–Once the kill mud reaches the bit, record the pump pressure.
–Maintain this pump pressure by adjusting the choke until the kill mud is observed at the surface.
–Stop pumping and shut the well in to check for pressures.
–If shut in pressure exists, additional mud weight and circulation will be required.
–If no shut-in pressures exists, the well is under control.
–At this time, one or two circulations may be made to condition the mud and increase the mud weight to provide a trip margin.
•Driller’s Method
–Advantages
•Involves a minimal number of calculations (3).
•A simple procedure that can be understood by most rig crews.
•Removes the intruding fluid from the well in a minimum amount of time.
–Disadvantages
•Requires two circulations to kill the well.
•Subjects the casing shoe to the maximum amount of pressure due to no additional hydrostatic pressure from additional mud weight.

Geological Indicators of Increasing Pressure

Geological Indicators of Increasing Pressure

•Size and shape of cuttings cont
–These cuttings should not be confused with even larger, block shaped cuttings, which are rectangular.
–These block-shaped cuttings do not originate from the bottom of the well.
–They are formed by improper drillstring and bottom-hole assembly mechanics or existing fracturing.
•Sloughing shale and abnormal hole fill-up
–Sloughing shale and abnormal hole fill-up are indications of increasing formation pressure.
–As the transition zone is penetrated, the pore pressure within the shale will increase.
–Shales have relatively low permeability, but in a transition zone, shale porosity will increase.
•Sloughing shale and abnormal hole fill-up Cont.
–If this overpressure in the shale is not offset by increasing the hydrostatic pressure of the mud, the shale will collapse or slough into the annulus.
–This can cause enlarged holes through transition zones and fill on bottom during connections and trips.
•Bulk Density
–During normal shale compaction, water is squeezed out of the shale as the overburden pressure increases.
–Shale porosity decreases and density increases with depth.
–If normal compaction is interrupted by the formation of a seal, the formation water cannot be squeezed out of the shale.
•Bulk Density Cont.
–When this occurs, the fluid supports part of the overburden load and will have higher-than-normal pressure.
–Since fluids remain in the shale, the shales have a higher-than-normal porosity and lower-than-normal density.
•Bulk Density Cont.
–If shale densities are checked and plotted at regular intervals during drilling, a normal compaction trend is established for the predominant formation being drilled.
–When a seal is penetrated, the formation density will increase rapidly, followed by decreased density as the over compacted pressure seal and transition zone are drilled.
•Gas
–Gas is an indication of underbalanced formation pressure.
–When drilling is underway, most well-logging companies measure and record the gases entrained in the circulating fluid. It is helpful to classify this gas into one of three different categories:
•Background Gas
•Connection Gas
•Trip Gas
•Background gas
–This is the total gas entrained in the mud.
–The background gas which comes from the cuttings as the hole is being drilled is not an indication of increasing pressure and should not be compensated for with higher mud weight.
–Background gas from cuttings should always be circulated bottoms-up.
•Background gas cont
–A continued increase in background gas indicates a higher formation porosity and/or a higher hydrocarbon saturation in the available pore space.
–If lithology and ROP are given due consideration, an increase in background gas would indicate drilling into a transition zone.
•Connection gas
–the amount of gas in excess of the background gas.
–This is the increase in gas readings caused by the swabbing action of drillstring movement while a pipe connection is made.
–Pulling of the drillstring causes the effective bottom-hole pressure to be less than the hydrostatic pressure of the mud column.
–Such a reduction in hydrostatic pressure could lead to formation fluids feeding into the hole.
–A small but constant amount of connection gas is an indicator that the formation pressure is slightly less than the hydrostatic pressure, whereas a continuous increase of gas at each connection would indicate an increase in formation pressure.
–This is an excellent tool for detection of abnormal pressures when used in conjunction with background gas.
•Trip gas.
–This is the increase in gas associated with pulling the drillstring out of the hole.
–Trip gas is recorded when bottoms-up is being circulated out after a trip.
–The time period during which trip gas is being recorded gives some idea about the amount and the migration of gases in the
–annulus.
–This parameter is used in the same manner as connection gas, but is not as useful due to the long interval between trips.
–In some instances, a short trip will be made (10 to 20 stands) for the purpose of determining changes in pore pressure and changes in bottom-hole conditions.
•Gas-cut mud
–Gas-cut mud is the reduction in mud weight due to gas entrainment.
–Gas-cut mud is checked at the flow line, where the fluid will contain the maximum amount of gas.
–The use of gas-removal equipment, as well as surface retention time, will normally remove most or all of the gas from the mud.
–A continued reduction in mud weight due to gas is an indication of increasing gas content in the formations and the potential of increasing pore pressures
•Chloride ion
–Dissolved solids in the formation water are often correlated to total chloride concentration —or salinity, as it is commonly called.
–The salinity of water found in shale is known to increase with depth in a normally compacted sedimentary basin, but shows a decrease in a transition zone.
–In normally compacted formations, the salinity of water found in sandstone is known to follow the same trend, but at much higher concentrations than those found in shale.
–In a transition zone, the salinity of water in sands approaches that of water in the shales.
–The change in the salinity of the mud filtrate is not used for detecting abnormal pressures because it is affected by numerous variables and could give an erroneous indication of a transition zone.
•Flow-line temperature
–Increasing flow-line temperature is an excellent indicator of a transition zone.
–Since certain other variables affect flowlinetemperature, it is necessary to usean end-to-end plot.
•An end-to-end plot is constructed by identifying changes in flow-line temperature caused by a change in the variables, rather than a change in formation pressure

Flow-line temperature cont.–A normal trend can be established and departures from the normal trend can be readily recognized.–An end-to-end plot will produce a curve as shown in the Figure
–At about 150 to 300 ft above the seal, a marked decrease in flow-line temperature will be noted (Point A in the Figure).
–Usually, this decrease is 18 to 20°.
–After the seal is drilled, a very rapid increase in temperature will occur —to perhaps as much as 30 to 35°from the time the seal is drilled until a porous zone is encountered.
–Changes in flow-line temperature cannot be used to estimate formation pressures directly, due to flow-line temperature variables and because each geographic area has a different temperature gradient.
–Changes in flow-line temperature are a qualitative indication that a change in pressure may be occurring.

Pressure Control – Indicators of Increasing Pressure

•Pressure indicators are divided into two groups:
–I. Engineering.
–II. Geological.

Engineering Indicators of Increasing Pressure

•Changes in Rate of Penetration (ROP)
–ROP increases while drilling the transition zone.
–While drilling normally pressured shale sections, the ROP will decrease with depth if drilling parameters such as weight-on-bit, RPM, bit types, hydraulics and mud weight remain fairly constant.
–There will be a marked reduction in ROP as the pressure seal is penetrated.
–After penetrating the seal in sur-normally pressured formations, there will be an increase in ROP.
–This is due to the higher porosity of the sur-normal pressured zone.
•Decreases in dcsexponent trend
–Calculations for “d exponent”and “dcs exponent”can be made to normalize ROP data and predict the magnitude of increasing formation pressure.
–Trends can be graphically established using complicated formula
•Changes in rotary torque
–Rotary torque may increase rapidly in the transition zone.
–Torque increases gradually with depth because the contact friction between the drillstring and the wellbore increases with depth.
–Torque will increase in the transition zone because a larger volume of shale cuttings will enter the wellbore. Shale tends to close in the hole, causing additional contact with the drillstring and impeding bit rotation.
•Changes in drag
–An increase in drag may be experienced while making connections in the transition zone.
–After the kellyis drilled down, the recommended practice is to pick up 5 to 10 ft (to allow for working the drill pipe if it sticks), turn the pumps off and pull the kellyfrom the hole.
•Changes in rotary torque
–Extra cuttings may enter the wellbore when the transition zone is penetrated.
–The hole may also tend to close-in around the drill collars and bit.
–Some transition zone shales tend to flow under differential pressure.
–There have been instances where it was necessary to backreamand circulate to trip out of the hole.
•Kicks
–An actual kick is the most obvious indication of an increase in pressure.
–Any pit gain, if not accounted for, is an indication of an influx of formation fluid (kick).
–When this happens, the amount of fluid returning increases, and the flow sensor records the increase.
•Kicks Cont.
–When approaching a transition zone if an increase in pit volume or flow is detected, drilling should be stopped and the well checked for flow.
–If the well continues to flow, it should be shut in.
•Filling the hole on trips
–When pulling the drillstring out of the hole, the amount of pipe in the hole is reduced, and the mud level drops.
–The volume can be calculated from the size and weight of the pipe and the length of the pipe removed, so that an appropriate amount of mud can be pumped into the hole to fill it up.
•Filling the hole on trips
–If the drillstring volume is not replaced and the mud column drops, then the hydrostatic pressure is reduced and may result in a kick.
–If the hydrostatic pressure is reduced to less than formation pressure, formation fluids will flow into the well.
–If the hole takes less mud than the calculated displacement volume for the number of stands pulled, fluid is entering the wellbore.
–This signals an impending kick.

Pressure Control – Subsurface Pressures

•Many different pressures are involved in drilling and controlling oil and gas wells.
•It is important to understand these pressures and how they are used to detect and control formation pressures.
•Pressure is defined as force per unit area:
Pressure (psi) = force (lb) /area (in.2)
•Hydrostatic pressure (PHYD) is the pressure caused by the density or Mud Weight (MW) and True Vertical Depth (TVD) of a column of fluid.
–The hole size and shape of the fluid column have no effect on hydrostatic pressure since, at a given depth, pressure is equal in all directions.
•PHYDis calculated by:
–PHYD(psi) =0.052 x MW (lb/gal) x TVD (ft)
–0.052 = The units conversion factor equal to:
12 in./ft/231 in.3/gal
or 0.052 gal/(in.2x ft)
•What is the hydrostatic pressure of a fluid column for the following conditions?
–MW = 12.8 lb/gal
–MD (Measured Depth) = 14,300 ft
–TVD = 13,200 ft
•The hydrostatic pressure is always calculated using the TVD.
PHYD= 0.052 x 12.8 x 13,200
= 8,786 psi at TVD
•Hydrostatic pressure gradient is the pressure increase per unit of vertical depth.
–PHYDG(psi/ft) = 0.052 x MW (lb/gal)
•What is the pressure gradient of a 12.0 lb/gal mud?
PHYDG(psi) = 0.052 x 12.0
=0.624 psi/ft
•Typical pressure gradients are:
–Freshwater 0.433 psi/ft
–Seawater 0.444 psi/ft
–Marine formation water
–(100,000 mg/l salt) 0.465 psi/ft
–Saturated saltwater
–(10 lb/gal) 0.520 psi/ft
–16-lb/gal mud 0.832 psi/ft
–19.2-lb/gal mud 1.0 psi/ft
Formation pressure (Pform) is the fluid pressure exerted within the pore spaces of any oil, water or gas formation, and is commonly called pore pressure.
•Normal pressure is the hydrostatic pressure exerted by a column of fluid equal to the density of the native fluid that existed in the geological environment when the solids were deposited.
–Since more wells are drilled in sediments characterized by marine formation water with about 100,000 mg/l salt, a gradient of 0.465 psi/ft will be used as the normal gradient for purposes of this discussion.
–Deviations from normal hydrostatic pressures are referred to as being abnormal —sur-pressures (high) and sub-pressures (low).
Indicators of Increasing Pressure

Pressure Control

•Despite efforts to understand and control formation pressures, blowouts still occur.
•A blowout is an uncontrolled flow of formation fluids as the result of failure to control subsurface pressures.
•Blowouts can occur at the surface or into an underground formation.
•Nearly every well drilled has the potential to blow out.
•Experience has shown that blowouts occur as the result of human error and/or mechanical failures.
•However, a carefully planned, continuously supervised pressure-control program will lessen the possibility of a blowout considerably.
•It is important to identify high formation pressures before drilling, to detect pressure changes while drilling, and to control them safely during drilling and completion operations.
•Pressure control can be divided into three categories:
•Primary control.
–The proper use of hydrostatic pressure to overbalance the formation and prevent unwanted formation fluids from entering the wellbore.
–The advantages of control at this level are self-evident.
•Secondary control.
–The use of equipment to control the well in the event primary control is lost.
–Formation fluids that have entered the annulus can cause a blowout quickly if not properly controlled.
•Tertiary control.
–The use of equipment and hydrostatic pressure to regain control once a blowout has occurred.
–This could involve the drilling of a relief well.
–Although tertiary control is normally handled by experts, many things can be done during the planning and drilling of a relief well to simplify the final kill procedure and regain control of the well.
A kick is an influx of formation fluid into the well.
A blowout is an uncontrolled kick.
•Failure of primary control.
–Any event or chain of events that create a negative differential pressure between the hydrostatic pressure of the drilling fluid and the formation pressure can cause a “kick.”
•The most common causes of a kick are:
–Failure to keep the hole full of mud during trips.
–Insufficient mud weight.
–Lost circulation causing the hydrostatic pressure to be reduced.
–Swabbing in when pulling out of the hole.
–Improper casing design and pore pressure prediction.
•Failure of secondary control.
–It has been estimated that 95% of the wells in which secondary control is lost arrive at that condition as the result of either poor maintenance and inadequate testing programs, which result in leaks that erode pressure-control equipment, or inadequate crew training, which results in miss-use or no use at all of pressure control equipment.

Controlling Formation Pressures

•Controlling the pressure of the formations drilled is a very important function of the drilling fluid

•As the formation pressure increases, the density of the drilling fluid is increased to balance or slightly overbalance the well and keep it in control
•Pressure exerted by the drilling fluid while static is called hydrostatic pressure •Drilling fluids are typically weighted up with heavy material such as barite, calcium carbonate, hematite and in extreme situations galena
•The well is considered under control when no formation fluids or gasses are allowed into the well bore
–In some situations small amounts of background gas are allowed into the well bore the well bore may be considered in control if the flow is controllable •Hydrostatic pressure is also used to control unstable wellbores
–Formations may be tectonically stressed especially in deviated wells
–Drilling fluid density can be increased to balance the tectonic stress and help provide a stable wellbore
•Density of drilling fluid ranges from air (0 psi/ft) to 20 lb/gal (2400 kg/m3)