ECD / Equivalent Circulating Density is an important parameter to monitor in the drilling process of a well, especially in horizontal & HPHT wells. In this article, our objective is as following: ECD Definition, the calculations & formula of theoretical ECD in Drilling & to explain the parameters that influence ECD. you can also download the ECD calculation spreadsheet at the end of the article.
Why do we need to control the ECD In Drilling?
- Most importantly it is to reduce the risk of fracturing/losses to the formation. In some cases this has lead to the loss of weeks of operational time and the loss of substantial volumes of mud (upto 10,000 bbls). Mud Losses can also lead to well control problems.
- To prevent well ballooning/supercharging of the formation and thus elimination of the loss/gain scenario.
- Minimization of hole instability due to pressure fluctuations.
- Minimize the risk of differential sticking.
- The increased costs of a well (check also oil well drilling well cost per foot) with problems may make the well uneconomic leading to the loss of future opportunities.
In Which Drilling Wells Is The Control of ECD Most Important?
- Drilling ERD/horizontal wells in which the ECD increases with measured depth yet the fracture gradient does not increase significantly due to little change in the TVD.
- HPHT wells where the drilling margin (the pressure difference between the pore pressure band the fracture pressure) is low.
- Depleted reservoirs where the fracture pressure has decreased along with the pore pressure yet the mud weight can not be decreased due to either a) the presence of isolated virgin/higher pressured reservoir or b) the need to stabilise inter-bedded of overlying shales.
What Is ECD In Drilling? (EQUIVALENT CIRCULATING DENSITY)
ECD is the term given to the total pressure exerted on the wellbore. It is generally used to indicate the increase in pressure exerted when circulating but it also indicates the increase or decrease in pressure caused by pipe movement (surge and swab pressures). It is expressed as a density in the same units as the mud weight.
Major Components That Affect ECD Value In Drilling
In summary, ECD is the combination of raw mud density plus other influences on wellbore annular pressure.
There are two components of ECD :
- Static pressure component
- mud hydrostatic pressure and drilled solids.
- Dynamic pressure component which is the
- annular pressure loss,
- pipe velocity,
- inertial pressures from string acceleration and deceleration,
- pressure to break mud gels (break circulation).
Mud Hydrostatic Pressure
Static pressure is the pressure exerted by a stationary fluid of a given density at a given vertical depth. It also includes any drilled solids which may be suspended in the mud column. It is measured in pounds force per square inch (psi).
This pressure is calculated from the mud density as shown below:
Pressure (psi) = mud gradient (psi/ft) * depth
– psi/ft = pounds-force per square inch per foot
Mud density can also be expressed in terms of psi/1000ft. This is denoted as pptf = pounds-force per thousand feet.
pptf is a way of showing the pressure generated by the mud for each interval of 1000ft TVD (Note: TVD not MD). To convert a pressure gradient of psi/ft to pptf just multiply by 1000.
i.e. 0.5 psi/ft * 1000 = 500 pptf
ECD is expressed in the same units as mud weight, i.e. pptf, ppg, psi/ft, SG.
For an example of the calculation of static mud pressure (hydrostatic pressure) see Fig 1.
A 500 pptf mud exerts the following pressure at 6000 ft TVD:
Pressure (psi) = 500 (pptf) * 6 (thousand ft) = 3000 psi
Similarly a 0.5 psi/ft mud exerts the following at 6000 ft TVD:
Pressure (psi) = 0.5 (psi/ft) * 6000 (ft) = 3000 psi
Annular Pressure Loss
Pressure is required to circulate mud around a wellbore. This pressure is required to overcome the following:
- Internal Friction of the fluid moving past itself.
- Back-pressure as fluid flows through a restriction
- To turn PDM stator, Turbine and MWD tool impellor.
The effort required to overcome the resistance to flow is called pressure loss. This pressure loss is measured by the standpipe pressure gauge to have more info please visit the pressure loss calculation.
Circulating Pressure loss is dependent upon:
- Flow rate
- Drilling fluid properties – Viscosity (PV and YP)
- Geometry of flow path – Pipe/annular dimensions
Increases in either the flow rate or the viscosity cause an increase in pressure loss. An increase in the annular clearance due to a larger hole ID or a smaller pipe OD results in a reduction in pressure loss.
The Circulating Pressure Loss can be split into two main components,
It is the annular pressure loss that we are normally concerned with as this affects the ECD. (NB. If reverse circulating it is the internal pressure loss that acts upon the bottom of the hole, not the annular pressure loss, and affects the Drilling ECD. The internal pressure loss is considerably greater than the annular pressure loss and so great care must be taken in these circumstances.)
The Annular Pressure Loss can be calculated using various equations. One such equation is:
- D = Depth in ft,
- PV/YP = plastic viscosity in cp and yield point (check also Yield Point In Drilling Mud Formula) in lb/100ft2
- d1 = Hole ID, inch
- d2 = String OD, inch
- v = annular velocity, ft/sec
- L= length
The Annular Pressure Loss can be calculated with moderate accuracy but this often ignores some important factors which have an effect on the Annular Pressure Loss:
- Solids loading in the annulus
- Speed of drill string movement
- PV and YP variations (sag and temp)
- Unknown changes in annular dimensions i.e. washouts or restrictions
- Increases in drill string component ODs e.g. Enhanced performance drill pipe (check API drill pipe Specifications), drill string protectors
Bottom Hole Pressure (BHP)
Why is it important to know:
a) the static bottom hole pressure and
b) the dynamic bottom hole pressures.
- The static BHP must be known in order to avoid kicks in Drilling, differential sticking, well- bore instability and Mud Circulation Losses when the Mud Circulation System is not being circulated.
- Dynamic BHP must be known in order to avoid kicks, differential sticking, wellbore instability, ballooning and losses when the mud system is being circulated
- Spikes (short duration increases) in either pressure can cause fracturing of the formation and losses. It is the spikes in the dynamic pressure that are usually the cause behind the initiation of losses.
Calculating Bottom Hole Pressure
Static bottom hole pressure (SBHP)
This is the mud weight gradient multiplied by the total vertical depth.
pptf x depth in 1000’s ft (TVD)
Static pressure can be calculated easily from the mud weight (if the mud balance test procedure is correct).
Dynamic or Circulating bottom hole pressure (CBHP)
This is the Static Pressure + Annular Pressure Loss
The circulating bottom hole pressure can be presented in two ways, as a pressure (in psi) or as an equivalent mud gradient in pptf.
Pressure = psi For CBHP
Pressure / TVD = psi / 1000ft (pptf) For ECD
Representing the BHP as a pressure gradient in pptf gives a value that can be compared with mud density and formation fracture gradient.
ECD Formula In Drilling
- MW = mud gradient in pptf,
- APL = annular pressure loss in psi,
- TVD = True Vertical depth in ft
Measuring Bottom Hole Pressure
The most accurate means of knowing what the bottom hole pressure is is to measure it. Pressure gauges (eg. Amerada and Quartz) run on wireline or gauge carriers used to be a means of measuring the BHP, however a more flexible and practical alternative is to run a pressure while drilling (PWD) sub.
Parameters Influencing Equivalent Circulating Density ECD In Drilling
Increasing the annular velocity increases the fluid frictional pressure losses and hence the annular pressure loss, consequently the ECD increases and vice-versa.
Depth AH (along hole) and TVD
The relationship between AHD and TVD affects the ECD. For example, in a horizontal hole, the MD increases but the TVD stays the same. This results in the ECD increasing with measured depth although the TVD does not increase.
Rotation and Axial String Speed
As rotation increases so do the annular pressure loss and consequently the ECD. This is due to Taylor vortices being set up which extract energy from the mean axial-flow and yield a higher pressure drop.
String speed in to and out of the hole affects the fluid movement in the annulus causing a change in the AV. This results in swab and surge pressures that decrease and increase the ECD respectively. They have the same effect on the static mud gradient.
Cross-Sectional Area of Annulus – CSA
A reduction in the CSA of the annulus increases the annular pressure loss by restricting fluid flow in the annulus and thus increases the ECD. Examples of equipment causing restrictions in the annulus cross-sectional area are drill pipe tool joints, drill pipe protectors, EHP-DP, Stabilisers, etc.,
The viscosity of the mud is a measure of the frictional pressure losses the mud experiences when it is being pumped. The frictional pressure losses in the annulus are measured as the annular pressure loss. Thus changes in mud viscosity affect the Drilling Equivalent Circulating Density ECD.
Breaking circulation (gels)
When the mud is stationary it develops a static viscosity referred to as a gel strength. Pressure is required to overcome the gel strength (break the gels) in order to start the mud moving. This pressure is exerted on the wellbore. The higher the gel strength the higher the pressure required to initiate fluid movement. Once in motion, the pressure required to keep the fluid moving is generally less than that required to break circulation, consequently, the annular pressure loss will be higher when breaking circulation resulting in a higher ECD.
Drilled solids in the mud increase the mud density which can be considered as either an increase in mud weight or an ECD effect. It is usually considered as an ECD effect as the solids are removed at the surface (thus are temporary) and the original weight mud is returned to the wellbore.
If the drilled solids are re-ground due to poor hole cleaning they may become so small that they can not be removed from the mud (measured as LGS – low gravity solids) in which case they will have a permanent increase in the mud weight.
Wellbore stability problems can result in large quantities of rock caving into the wellbore resulting in a similar increase in ECD as above.
Solids can settle to the low side of the hole in deviated wells to be stirred up by the later movement of the drill string (rotation after sliding or back reaming). The increase in Drilling Equivalent Circulating Density ECD due to this action can be significant.
If hole cleaning is not sufficient the quantity of drilled solids in the mud continues to increase and consequently so does the ECD.
The build-up of a cuttings bed reduces the cross-sectional area of the annulus which increases the APL and therefore the ECD.
Barite Sag is a specific case of solids in the wellbore, this time not drilled solids but mud weighting solids. The barite falls out of suspension and forms dense layers of mud. When the depleted portion of mud is circulated out of the hole the denser layers are left which results in an increase in the hydrostatic pressure and consequently the ECD.
Drilled Solids Loading and ECD
All of the following can significantly affect the Equivalent Circulating Density. However, they may be masked by other concurrent operations or actions and so may not be highlighted on the PWD log or considered by the drilling team at the time of drilling.
- Inadequate hole cleaning
- Efficient solids (cuttings and cavings) transport causes an increase in the ECD
- The ROP affects the amount of drilled solids in the annulus.
- Solids stirred up by string movement after motor drilling.
- Hole Cleaning procedures chosen i.e. pills, backreaming
- Barite sag.
It is better to ensure that good hole cleaning is achieved by a combination of high flow rate and drilling optimization for rheology.
Inadequate Hole Cleaning
A typical shale weight is 1000 pptf ( 2.3 SG ), almost double that of a typical mud weight of 550 pptf (1.26 sg). Consequently, solids loading results in an increase in the effective mud weight.
Inadequate hole cleaning results in cuttings accumulating in the annulus which leads to a continual loading up of the mud weight. The formation of cuttings beds leads to a decrease in the annular cross-section and hence increases the ECD.
Hole washouts can cause an accumulation of solids that slough downwards and bridge off where the hole is in gauge and cause pressure increases and pack-offs. Cuttings beds may avalanche, packing off and pressurizing the formation.
Efficient Drilled solids transport For Lower ECD
Mudflow rate is the most important parameter in determining effective hole cleaning.
Good hole cleaning via high flow rates leads to an increase in the suspended solids. The increase in suspension results in both a higher mud weight as well as an increase in the annular pressure loss due to the high flow rates. Both of these contribute to an increase in the Equivalent Circulating Density.
Fig 3 illustrates the effect of flow rate on hole cleaning/solids transport and how it affects the ECD.
At low flow rates, the cuttings have fallen to the low side of the hole forming a cuttings bed which has increased the Drilling ECD due to the annular restriction.
As the flow rate increases further the majority of cuttings are transported along the low side with some suspended in fluid flow just above the bed, increasing the Equivalent Circulating Density.
At the higher flow rates, frictional pressure losses are significant and cuttings are transported completely suspended.
Drilling ROP & ECD
If the ROP exceeds the ability of the operations to clean the hole then there will be a gradual increase in the ECD. This is demonstrated by the annulus pressure and equivalent mud weight curves in the PWD log shown in Fig 4
NB. In the case of long ERD wells indications of cuttings accumulations during a drilling break can take several hours to appear in the ECD because of the long travel time in beds and not in suspension.
As can be seen in Fig 5, rotation following a period of sliding has caused an increase in the ECD of approx. 15 pptf.
There was a decrease in the Drilling ECD while sliding due to cuttings falling to the low side and settling out of the mud stream. Rotation stirs these cuttings back up into the flow increasing the effective mud density.
Long periods of steering will load the annulus once rotation recommences so it is wise to break up the sliding with rotary drilling or with circulation, rotation and reciprocation (“TimeOut” for hole cleaning)
The PWD data can help determine the minimum RPM to effectively stir up the cuttings and clean the wellbore.
ECD Controls for the Drilling Team
|Action||Prevent ECD Increases by:|
|String Movement||Start Movement Slowly and smoothly.|
|Pump speed changes||Start and Stop pumps slowly, Starting the pumps slowly is most important|
|Breaking Gels||Rotate the string prior to starting pumps to break gels|
|Backreaming||Should be avoided if possible.|
|Reaming in||This is the most likely time to cause a spike in ECD|
|Mud Condition||Fluctuations in mud weight and viscosity have a direct effect on ECD. Control these carefully within the specified limits|
Tripping In (any downward pipe movement)
- Surge pressures are still significant in the casing and tripping speeds may have to be reduced well below a ‘normal’ speed. Make sure the appropriate calculations have been made on the available software.
- Mud properties, Bottom Hole Assembly Types, BHA configurations, filter cake thickness and Bit TFA Total Flow Area all have a dramatic effect on the well pressures.
- Be aware of the max. allowable pipe speeds both with and without the pumps on.
- Accelerate the pipe slowly to max speed allowed avoiding sudden movements which can create significant surge pressures.
- If tripping in with a new assy be aware of the hydraulic bypass; was a rock bit run previously?
Reaming to bottom
- The effects on the Equivalent Circulating Density increase significantly in smaller holes.
- Limit the speed, pump rate, and rpm
- The speed of pipe movement when reaming is generally much greater than when drilling and can create large surge pressures.
- The ROP while sliding is limited whilst in rotary mode it can be high. Try to maintain a steady ROP and cuttings loading to limit the Drilling ECD.
- Break up long periods of sliding with either rotary drilling or hole cleaning opera- tions i.e. circulation, rotation and reciprocation.
Connections and circulating clean
- Control the string speed on the downstroke.
- Don’t run down and kick the pumps and rotary in together to avoid pressure surges
- If the gel strengths are producing high pressures to break circulation then try starting rotation first however beware of disturbing unstable cuttings beds which may fall down on top of the BHA.
- Slowly build up circulation rates eg. 100 GPM stages
- Emphasis should be on cleaning the hole with the designed mud system, by maintaining high flow rates and optimizing the rheology, and not leaving cuttings behind.
- Pills can pick up significant amounts of cuttings which can dramatically increase the Drilling ECD.
- Keep the mud in good condition – fluid loss, rheology (PV/YP/gels), mud weight
- Avoid back reaming by cleaning the hole sufficiently prior to tripping.
- Back reaming knocks off the filter cake which may allow losses to start at a lower pressure.
- It also causes increases in the LGS which may cause a PV increase and therefore an increased Equivalent Circulating Density.
- With backreaming there is little or no warning sign before a packoff occurs.
Casing and Liners
- For all types of casing & Casing liner Swab and surge can be significant, the inertia in the mud system caused by cooling and gelling exacerbate the situation. Returns only start after joint run.
- Good hole cleaning gives angular cuttings.
- More rounded and mushy cuttings indicate regrinding and possibly cuttings beds.
- Essential to stir up a cuttings bed. The PWD data should be used to determine the best rotary speed. 80 – 100 rpm o.k.
- Increases the ECD
Ref: ECD Management Book – Fearnley Procter Group