To optimize hydraulics and achieve sufficent hole cleaning the following guidelines should be adhered to:

Pressure Losses Through System

P₁= Frictional losses in surface lines

P₂= Frictional losses in Drillpipe

P₃= Frictional losses in Drill collars

P_B= Pressure loss across bit

P₄= Annular pressure loss across Drill collars

P₅= Annular pressure loss across Drillpipe

Surface Connection Losses

Pressure losses in surface connections are losses in standpipe, rotary hose, swivel and kelly. These losses are very difficult to calculate and the general equation is

P_S= Er^0.8Q^1.8(PV) ^0.2 psi

Where, r = mud density, ppg

Q = flow rate, gal/min

PV = plastic viscosity, cP

E = Constant depending type of surface equipment

 

Surface Equip.	Stand pipe		Rotary Hose		Swivel		Kelly
Type	Length ft	ID in	Length ft	ID in	Length ft	ID in	Length ft	ID in
1	40	3.0	40	2.0	4	2.0	40	2.25
2	40	3.5	55	2.5	5	2.5	40	3.25
3	45	4.0	55	3.0	5	2.5	40	3.25
4	45	4.0	55	3.0	6	3.0	40	4.00

Pipe and Annular Losses

Pipe losses take place inside drill pipe (P₂) and drill collars  (P₃). Annular losses takes place across the drill collars  (P₄)  and drill pipe annulus  (P₅). These pipe and annular losses depend on

• dimension of Drillpipe/Drill collars
• mud rheological properties
• type of flow

The two major models for calculation of these losses are the “Bingham plastic model” and the “Power law model”.

Pressure Drop Across Bit, P_B

This is the most important element in the hydraulics equation. The objective is to optimise pressure drop across the bit such that maximum bottom hole cleaning is achieved. For a given length of drill string and mud properties pressure losses P₁, P₂, P₃, P₄ and P₅ remain constant. The pressure loss across the bit is influenced by the size of nozzles used and also determine the amount of hydraulic horsepower available at the bit. The smaller the nozzle area the greater the pressure drop and velocity. In situations where the rock is soft to medium hard, hole cleaning is the major objective and thus bigger nozzles are used to maximise nozzle areas.

P_B = P_S - (P₁+ P₂+ P₃+ P₄+ P₅)

P_S = Surface pressure

Circulation Rates

In optimisation of circulation rates, the emphasis must be placed upon supplying sufficient drilling fluid, at a high enough rate, to ensure that drilled cuttings are removed from the annulus in the shortest possible time. Once hole cleaning requirements are fulfilled, bit hydraulics may be optimised.

The recommended flow rate for roller cone bit is 30 to 50 GPM/in of bit diameter.

The minimum flow rate required for PDC bit = 12.72 (D)^1.47

D = Diameter of Bit

Note: If a mudmotor is used, flow rates are depended on the mudmotor requirements.

Types of Flow

There are generally two types of flow

• Laminar
• Turbulent

In Laminar flow the pattern is smooth, with fluid layers travelling in parallel lines parallel to the conduit axis. The velocity of each layer increases towards the centre until a maximum velocity is reached. In laminar flow shear resistance is caused by the sliding action and is independent of the roughness of pipe. There is only one component of fluid velocity in laminar flow, a longitudinal component. A special type of laminar flow with a flat centre is called plug flow. In the flat centre there is no shear of fluid layers.

In turbulent flow the pattern is random in both time and space. The disordered motion of fluid properties in turbulent flow results in two components of velocity, longitudinal and transverse components. The longitudinal velocity attempts to make a fluid flow parallel to the conduit axis, while the transverse component attempts to move the fluid in a direction normal to the pipe axis.

Prevention of Hole Erosion

If unconsolidated formations are to be penetrated, in an effort to minimise the potential for hole erosion, the flow regime shall be maintained in the laminar range, and nozzle velocities shall be limited to a maximum of 1 000 ft/sec. Pressure losses also increase with turbulence.

Annular Velocities

The low limit on annular velocities is the capability to lift the cuttings to surface. For most ordinary mud 100 ft/min can be taken as a lower limit, however in large surface holes with limited pump capacity, high viscosity muds can be seen to work at half this rate as long as penetration rates are limited.

With higher penetration rates (50ft/hr) the annulus tends to overload with cuttings and the velocities therefore have to be increased substantially. The upper limit can in most cases be taken at the point where the fluid goes into turbulent flow around the drill collars, however in certain situations like PDC or turbo drilling this parameter may be exceeded.

Annular Velocity   =   24.51 * GPM

                               (Dh^2 - Dp^2)

D_h = Diameter of Hole

D_p = Outside Diameter of Pipe

Slip Velocity

The rate at which solid particles settle out of the well fluids is called the slip velocity.

Turbulent Flow

Laminar Flow

Where,

m = PV + 399 YP (D_H - D_P) / AV

Check, V_S = > 50%

Optimisation of Wellbore Hydraulics

For optimising hydraulics, since the flow rate has been fixed for providing adequate annular velocity, the only variable to optimise is the pressure drop across the bit.

Two recognised methods exist to optimise bit hydraulics.

• hydraulic horsepower method
• hydraulic impact force method.

Hydraulic Horsepower Method

In this method the pressure drop across the bit is

P_B = P_S {n / (n+1)}

where, n = slope of  P_S vs Q curve. The value of n is usually between 1.8 and 1.86.

When n=1.86, the hydraulic horsepower method optimised conditions exist and 65 % of available pump horsepower (or standpipe pressure) is applied at the bit.

Hydraulic Impact Force Method

n this method the pressure drop across the bit is

P_B = P_S {n / (n+2)}

With the hydraulic impact force method optimised conditions exist when 48 % of available pump horsepower (or standpipe pressure) is applied at the bit.

In general the hydraulic impact method should be applied in soft, fast top hole drilling and the hydraulic horsepower method used deeper down. Whichever method is used the result will be within 95 % of the optimum of the other.

TFA (Total Flow Area) and Nozzle Selection

Smaller nozzle sizes are obtained with BHHP method as this method gives larger values of pressure drop across the bit.

Where:

A_T = Total flow area

Where:

d_N = Nozzle size in multiples of 32

Jet Velocities

The nozzle jet velocity is the speed at which the drilling fluid travels through the nozzle. Abnormally high nozzle velocities can cause nozzle wear.

J₁, J₂, J₃ = Nozzle sizes

Annular Velocities

The low limit on annular velocities is the capability to lift the cuttings to surface. For most ordinary mud 100 ft/min can be taken as a lower limit, however in large surface holes with limited pump capacity, high viscosity muds can be seen to work at half this rate as long as penetration rates are limited.

With higher penetration rates (50ft/hr) the annulus tends to overload with cuttings and the velocities therefore have to be increased substantially. The upper limit can in most cases be taken at the point where the fluid goes into turbulent flow around the drill collars, however in certain situations like PDC or turbo drilling this parameter may be exceeded

D_h = Diameter of Hole

D_p = Outside Diameter of Pipe

Bottom Hole Cleaning

To ensure sufficient cleaning at the bit 3 to 7 HHP/in² are normally required.
7 HHP/in² in soft, fast drilled claystone where bit balling is a problem and down to
3 HHP/in² in harder formations like pure sandstone or granite.

Vertical and Horizontal Wells

For effective hole cleaning in vertical and horizontal wells the flow rate must exceed the minimum values listed below.

HOLE SIZE	RECOMMENDED FLOW RATE (gpm)
17.1/2in/16in	900-1000
12.1/4in	800-900
8.1/2in	400-450
6.1/8in / 6in	250-300

 

The table assumes average drilling conditions, i.e. that average penetration rates are achieved and that drilled cuttings, not cavings, are to be removed from the hole. In deviated wells and if abnormally high ROP is achieved or if a significant amount of cavings are being produced, then the values listed may not clean the hole effectively.

The tabulated values are intended as a guide. Reference must be made to the drilling programme for recommendations for specific wells.

If the minimum flow rate proves impossible to achieve, use of larger nozzles will permit circulation at higher rates. Increase the bit nozzle size in increments until the minimum flow rate is achieved. If maximum bit nozzle size is reached before this point, then contact the Operations Engineer.

As a guide, for onshore wells through the reservoir section, the minimum size of nozzles is 14/32in in case LCM has to be pumped.

Formation of Cutting Beds

Well Inclination Greater Than 30deg

In deviated wells, with hole angles of greater than 30°, cuttings will tend to form a bed on the low side of the hole, as shown in Figure 7.5. In such cases, the mechanism of cuttings transport is one of inducing movement of the cuttings bed itself and of saltation, or “jumping” of individual particles along the bed surface. The figure illustrates how increasing annular velocity improves hole cleaning. With reference to the figure, the best hole cleaning is achieved in zones 1 and 2, whereas zone 5 is virtually a guarantee of tight hole problems.

Well Inclination Between 45deg and 70deg

Hole angles between 45° and 70° are termed critical angle holes since, in practice, such holes are the most difficult to clean. Moreover, when the pumps are shut off for any reason the bed tends to slide back down the hole. Consequently, circulation before trips etc. is of critical importance.

Removal of Cuttings Beds

Mud Rheology

Mud rheology should not be adjusted in an attempt to improve hole cleaning in deviated wells.

An increase in rheology, specifically the true yield point, decreases the settling velocity of the particles, which can often improve hole cleaning efficiency, in vertical wells, by reducing the time that cuttings spend in the annulus. The true yield point is the intercept on the Shear Stress axis of the plot of Shear Stress vs Shear Rate, as measured on the Fann viscometer. The conventional YP often over predicts the true yield point, consequently, it has become fashionable to quote the 600 rpm readings as the important rheological control parameter, since this often gives a better estimate of the true yield point.

In deviated wells, decreasing the settling velocity does little to improve hole cleaning since cuttings only have to fall through a very short distance before they are incorporated into the cuttings bed. Rheology must be maintained within the range detailed in the Drilling Programme.

Annular Velocity

The annular velocity is the single most important parameter in the hole cleaning process. It must be optimised, at all times, remaining within any pressure constraints imposed by surface equipment or ECD. In general, higher pump rates are required to clean deviated wells than comparable vertical wells.

Circulation Before Trips

Simply pumping bottoms up before trips, may be sufficient for vertical wells but is insufficient to ensure that a deviated hole is clean. In all cases, the hole should be circulated until the shakers are running clear of cuttings. In horizontal wells low/high vis pills shall be used prior to POOH. The Drilling Supervisor shall determine when the hole is clean and the pumps may be shut off to begin the trip.

The following table provides a general guideline to calculate circulation volume.

Inclination of Well	Section Length Factor
(Degrees)	17 ½”	12 ¼”	8 ½”	6”
0 - 10	1.5	1.3	1.3	1.3
10 - 30	1.7	1.4	1.4	1.4
30 - 60	2.5	1.8	1.6	1.5
60 - 90	3.0	2.0	1.7	1.6

 No. of Circulation’s = Total Effective Length / Measure Depth

Cavings

In many instances, particularly in deviated wells, poor hole cleaning is diagnosed when, in fact, the problem is one of mechanical hole instability.

Drilling Fluids and Hydraulics Programmes are designed to transport cuttings of average size. Cavings which are significantly larger are much more difficult to transport and may present significant cuttings transport difficulties. In this case, efforts must be made to cure the instability by increasing gradient etc. if possible. If the hole instability is cured, then so will be the hole cleaning problem.

Clean up Pills

Vertical Wells

In conventional vertical wells, high viscosity pills are often pumped to boost hole cleaning at critical points in the operation.

Deviated Well with Wells Inclination greater than 45o

High Viscosity pills are largely ineffective in removing cuttings beds in deviated wells. The following procedure should be used:

1. Pump a thin, turbulent flow low vis 20bbl pill composed solely of the mud base fluid or brine.

2. Pump a second, high gradient / viscosity 40bbl mud pill immediately behind the initial pill. The gradient of the second pill shall be high enough such that when it is mixed with the unweighted pill the resultant gradient will be the same as the gradient of the circulating system.

3. Monitor returns at the shakers.

4. Return both pills to the circulating system.

Use the following total volumes i.e. total volume of low viscosity + high viscosity pills:

HOLE SECTION	TOTAL VOLUME OF PILLS
17.1/2in / 16in	50-100 bbls
12.1/4in	30-50 bbls
8.1/2in	30-50 bbls
6.1/8in	20-30 bbls

 

 Use pills judiciously. Monitor shakers carefully for increased returns. If no increase is seen do not continue to pump pills. This will result in unnecessary building of mud volume and make mud properties variable around the system.

Analysis of Hydraulics

ROP

The ROP determines directly the amount of cuttings to be removed from the annulus. Consequently, in some critical wells, ROP may have to be controlled for reasons of hole cleaning efficiency. High instantaneous ROP should be avoided.

Pipe Rotation

Pipe rotation helps stir up the cuttings bed in deviated wells. Consequently, the incidence of sliding of pipe should be reduced to a minimum in critical holes.

Backreaming

Backreaming (where top drive available) on wiper trips and trips may be necessary to remove cuttings beds in critical holes.

Barites Sag

It is of the utmost importance to ensure that True Yield Point, and Gel Strength are sufficiently high to suspend Barite. Barite sag can be a particular problem in deviated wells since the Barite particles only have a very short distance to fall before they form a bed on the low side of the hole. This bed tends to slump down the hole thus promoting further Barite sag. Evidence of Barite sag will be seen as uneven mud gradient measured at the shakers.

Jet Selection

To run one jet blanked off can at times give a slight improvement in penetration rates. In general it should be tried in medium to hard formations where bit balling is no problem. The concentration of all hydraulic horsepower on only two nozzles gives an erosive pulsating effect which in addition to the bit cutting increases the rate of penetration.

In addition where lost circulation is a problem the chances of plugging two big nozzles is less than the chances for plugging three small nozzles of equivalent total area.

Slug Pipe

A slug shall be pumped down the drill pipe before every trip.

Data Required

The following input data are required to carry out complete analysis of hydraulics:

• Lithology
• Cuttings gradient
• Bit type
• Cuttings size
• ROP
• Total depth of hole section
• Formation fracture gradient
• Hole geometry
• Hole angle
• String geometry (BHA)
• Mud gradient
• Mud rheology
• Type of drilling (rotary, slide)
• Equipment available

The following procedure has been designed with the aim of maximising annular velocity to ensure hole cleaning efficiency is maximised at all times. It is assumed that system pressures and bit optimisation hydraulics will be calculated using either a computer or a programmable calculator.

1. If unconsolidated formations are to be penetrated, in an effort to minimise the potential for hole erosion, Maintain the flow regime in the Laminar range, and limit nozzle velocities to a maximum of 100 m/s.

2. Estimate required bit nozzles (TFA) based upon previous experience and recommendations of the bit supplier.

3. Calculate the system pressure drop using the maximum pump output available with the current pump/liner configuration.

4. If the calculated pressure drop is within the constraints imposed by the pressure rating of surface equipment, and does not produce an ECD higher than the Fracture Gradient, continue with Step 5.

5. If the pressure drop is too high, reduce the pump rate in increments until both constraints are satisfied.

6. Check the flow regime. If unconsolidated formations are to be penetrated, maintain the circulating fluid in the Laminar Flow regime. In this case, if the flow regime is transitional or turbulent, then reduce pump rate in increments until Laminar Flow is predicted.

7. Check that the hole is being cleaned effectively. For vertical wells, the flow rate must exceed the minimum values listed below.

Hole Size	Recommended Flow Rate (gpm)
17 1/2”	900 -1000
12 1/4”	800 - 900
8 1/2”	400 - 450
6”	250 - 300

The table assumes average drilling conditions, i.e. that average penetration rates are achieved and that drilled cuttings, not cavings, are to be removed from the hole. If abnormally high ROP is achieved or if a significant amount of cavings are being produced, then the values listed may not clean the hole effectively.

The tabulated values are intended as a guide. Reference must be made to the Drilling Programme for recommendations for specific wells.

If the minimum flow rate proves impossible to achieve, use of larger nozzles will permit circulation at higher rates. INCREASE the bit nozzle size in increments until the minimum

8. Check optimisation of bit hydraulics. Take into account minimum flow requirements recommended by the bit supplier. The values below shall be used as a guide for optimisation flow rate is achieved. If maximum bit nozzle size is reached before this point, contact the Operations Engineer.

Optimisation Method	% Of Total System Pressure Loss Expended At Bit
Maximum Hydraulic Horsepower at Bit	65%
Maximum Hydraulic Impact Force	49%

 If optimisation requires an increase in nozzle size, due to too much pressure being expended across the bit, recalculate system pressure drop. increade flow rate if possible.

If smaller nozzles are required, ensure that the increased pressure drop does not result in the flow rate having to be reduced below the minimum for good hole cleaning. If this is the case, do not optimise the bit hydraulics further.

9. Check that the nozzle velocity does not exceed 300 ft/sec if unconsolidated formations are to be penetrated. If nozzle velocity is too high, select larger nozzles.