SECCIÓN III- MARCO APLICATIVO
CAPÍTULO 7: PROYECTO DE INVESTIGACIÓN Y PLAN DE MEJORA
7.2 El plan de trabajo
The ability of a drilling fluid to lift cuttings is affected by many factors including fluid density and rheology, annulus size and eccentricity, annular velocity and flow regime, pipe rotation and cuttings density and particle size and shape.
The relationship between the various parameters is complicated and no one theory or set of equations can satisfactorily combine all of the observed phenomena. Nevertheless, the monitoring of cuttings generation and transport rates is imperative for a successful drilling operation.
6.1 General Factors Affecting Hole Cleaning
• Inclination
Vertical and Near Vertical Wells - in holes with an inclination less than 30°, cuttings are effectively suspended by fluid shear and cuttings beds do not form. Hole cleaning is general not problematic providing that mud rheology is adequate.
Deviated wells (inclination greater than 30°) - cuttings tend to settle on the low side of the hole forming cuttings beds. These may either migrate up hole or slide down hole resulting in the annulus packing-off.
• Rheology
Laminar flow conditions - increasing mud viscosity improves hole cleaning. Particularly effective if low shear rheology and YP/PV ratio are high.
Turbulent flow conditions - reducing viscosity will help remove cuttings.
• Yield Stress
A measure of the low shear properties of a mud system, yield stress governs the size of cuttings which can be dynamically suspended.
• Mud Density
Mud density affects the buoyancy of drilled cuttings. A heavier mud system enables cuttings to 'float' more easily.
• Flow Rate
In highly deviated holes, flow rate combined with mechanical agitation are important factors for effective hole cleaning. In vertical wells, increasing annular velocity and/or increased rheological properties will improve hole cleaning.
6.2 Cuttings Slip Velocity
The cuttings slip velocity is the velocity at which a drilled cutting will fall through the mud column under the influence of gravity. In the simplest case, in order to effectively remove drilled cuttings, the velocity of the fluid in the annulus must exceed the cuttings slip velocity. The situation is made more complicated by flow conditions and friction along with the many other factors mentioned above. For instance, under laminar flow conditions, particles in the centre of the mud column will be moving at a velocity greater than the average annular velocity and will be recovered at surface more quickly than expected. However, frictional forces between the borehole / casing wall and the fluid means that drilled cuttings in that region will be moving upwards at a rate less than the average annular velocity. In contrast, during turbulent flow conditions, providing that the fluid velocity exceeds the particle slip velocity, then solids will be removed continuously in all parts of the annulus. Turbulent flow therefore provides better hole cleaning than laminar flow but is less desirable because of the increased chances of erosion.
As a general guide, it is recommended that slip velocity should be less than half of the annular velocity (averaged over the cross-section).
As already noted, the relationship between the many factors affecting the rate of cuttings slip is complicated and researchers have developed a number of different methods of estimating it's value none of which are considered to be definitive. The most comprehensive methods are based upon particle Reynolds numbers, drag coefficients, particle density, shape and size and mud density and rheology. The following example, based upon a correlation devised by Walker and Mayes, 1975, is a simplified method.
DRILLING PRACTICES COURSE
Step 1: Find the shear stress developed by the particle
( 20.8 - W )
The table below provides an approximation for the thickness and diameters of disk shaped particles:
Step 2: Determine the boundary shear rate
The boundary shear rate is like a critical shear rate. Particle shear rates above this value are treated with calculations for the turbulent condition. Shear rates below use the laminar calculations.
The turbulent or laminar condition of the particle is not related to the turbulent or laminar flow condition of the fluid in the annulus.
W
Step 3: Find the shear rate developed by the particle using the laminar power law constants (na and Ka) for the mud.
Step 4: determine the slip velocity for the laminar or turbulent condition.
Laminar Condition
DRILLING PRACTICES COURSE
Where: Vs = slip velocity, ft/min γp = particle shear rate, sec-1
τp = particle shear stress from, lb/100 ft2 dp = particle diameter, inches
W = mud density, ppg
Turbulent Condition
If γp > γb, the slip velocity is determined by:
W 16.62
V
s× τ
p=
Where: Vs = slip velocity, ft/min
τp = particle shear stress from, lb/100 ft2
W = mud density, ppg
6.3 Cuttings Transport Velocity
The cuttings transport velocity for each different hole geometry is obtained by subtracting the slip velocity of the cuttings from the annular velocity in that particular section.
s a t
V - V V =
Where: Vt = cuttings transport, ft/min Va = annular velocity, ft/min
Vs = slip velocity, ft/min
6.4 Cuttings Transport Efficiency
Perhaps more important than the actual cuttings transport velocity is the cuttings transport efficiency. This is simply the ratio of cuttings transport to annular velocity. Please note that the equation shown here is valid for hole angles less than 30°. Evaluation of hole cleaning for wells of higher inclination is much more complicated as the drilled cuttings may form a cuttings bed on the low side of the hole.
100 V x E V
a t
t
=
Where: Et = transport efficiency, % Vt = cuttings transport, ft/min Va = annular velocity, ft/min
6.5 Cuttings Concentration
Note: valid for hole angles less than 30°.
The concentration of cuttings in the annulus depends upon the transport efficiency as well as the volumetric flow rate and the rate at which cuttings are generated at the bit (ROP and hole size dependent). Experience has shown that cuttings concentration in excess of four or five volume % can lead to pack-off, tight hole or stuck pipe incidents. When drilling soft formations, the cuttings concentration may easily exceed 5% by volume if penetration rates are not controlled. Some operators recommend a maximum cuttings concentration of 4% by volume.
Cuttings concentration is given by:
DRILLING PRACTICES COURSE
ROP = rate of penetration, ft/hr IDHOLE = hole diameter, inches Et = transport efficiency, % Q = flow rate, gpmWhen cuttings concentration exceeds 4 or 5 % by volume, the effect upon hydrostatic pressure and equivalent circulating density can by substantial. The change in hydrostatic pressure depends upon the density of the cuttings as well as their concentration in that particular hole section.
The effective static mud density due to the cuttings concentration in that section of hole is given by:
( )
The effect is most pronounced when drilling top hole sections. The following conditions cause an increase in cuttings concentration:
• large diameter holes drilled at high ROP
• pumps unable to provide sufficient annular velocities
• rapid mud system building rate may yield insufficient viscosity
It is clear that an increase in cuttings concentration in the annulus results in a corresponding increase in effective mud density.
6.6 Equivalent Circulating Density (ECD)
When the drilling fluid is circulating through the wellbore, the circulating pressure must be sufficient to overcome not only the friction losses through the drillstring and bit, but also the hydrostatic pressure of the fluid in the annulus and the friction losses through the annulus. The pressure to required to overcome the total friction losses in the annulus, when added to the hydrostatic pressure of the fluid, gives the equivalent circulating density as follows:
TVD
Where: ECD = equivalent circulating density, ppg
∑ Pa = sum of friction pressure losses in the annulus (corresponds the pump pressure minus the pressure losses through the surface equipment, drillstring and bit), psi
TVD = true vertical depth of hole, ft
W = mud density, ppg
The majority of drilling situations may not be limited by frictional ECD. Exceptions are in the case of drilling slimhole wells. ECD is particularly aggravated by deep, slim holes using heavy mud weights close to the formation fracture pressure. The flow rates selected may be lowered to prevent loss of circulation.