GUIDELINES FOR HYDRAULICS OPTIMIZATION

The following guidelines, recommendations, and rules-of-thumb are intended to provide a means for monitoring conditions at the rig and to get a feel for how well things are going.

They are not "the answer" but flags only, indicating whether further scrutiny is needed or as a starting point for hydraulic program planning

Hole Cleaning

The main symptoms of poor hole cleaning depends largely on hole angle. At low angles (< 20°) the cuttings tend to fall downhole as soon as the pumps are stopped. The best sign of poor cleaning is fill on bottom, either on connections or after tripping. In extreme cases it may be difficult to pull off bottom with the pumps off. At high angles (>50°) the cuttings fall to the low side of the hole forming a stationary cuttings bed. There is typically no fill on bottom and no trouble making connections. The main evidence of poor hole cleaning is seen on trips. The string may pull tight or get stuck off bottom while attempting to pull through this cuttings bed. At intermediate angles (40°-60°) the cuttings fall to the low side of the hole forming a cuttings bed. This bed is not stationary;

consequently, when circulation is stopped the cuttings bed may begin to slide (avalanche) downhole. Symptoms of poor hole cleaning for the intermediate angle case, will range between those seen for the low angle and high angle wells. In any event, if the drag gets high, RIH 2-3 stands, put the top drive on and circulate and rotate at maximum allowable rates until the hole is cleaned up; don't try to pull through tight spots. It may be necessary to pump out or back ream out of the hole in the higher angle wells. Backreaming out of the hole requires Operations Superintendent approval. Utilizing a bit with a cross sectional area as low a possible, or an open area as high as possible, will provide benefits when tripping through intermediate and high angle hole cuttings beds.

Carrying Capacity Index (CCI)

For low angle and intermediate holes up to 35°, the CCI still appears to be the best indicator of hole cleaning. There is no mathematical derivation for CCI; field observations indicate that the numerical product of K, annular velocity, and mud weight should equal or exceed 400,000 for good hole cleaning. The carrying capacity of a mud depends upon the difference in density between the cuttings and the drilling fluid, the annular velocity, and the viscosity of the fluid in the annulus. As any one of these numbers increases, the carrying capacity of the mud increases.

NOTE: The CCI is only meaningful when circulating. A suspension capacity of the drilling fluid is also needed for making concoctions and immobilizing cuttings in washouts during trips.

Adequate gel strengths are needed for trips.

CCI = (MW)(K)(AV) Good hole cleaning occurs when CCI > 1 400,000

K = (511)1-n (PV+YP) Where: MW = Mud Weight (ppg)

AV = Annular Velocity (fpm)

n = 3.322 log 2PV+YP PV = Plastic Viscosity (cp) PV+YP YP = Yield Point (lb/100 ft2)

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The K is the consistency index which corresponds to the viscosity of the mud at a shear rate of one reciprocal second, and n is the measure of the non-Newtonian flow behavior in the power law rheological model, SS = K (SR)n.

The following graph provides a graphical solution for the K value utilizing PV and YP of the mud.

Graphical Solution for Low Shear Rate Viscosity - K

Hole Cleaning Ratio (HCR)

For intermediate and high angle holes which develop cuttings beds, EMURC has developed a parameter called the Hole Cleaning Ratio (HCR) that is highly correlative with hole cleaning problems. Because of the many drilling variables and the complicated physical system involved, the simple "Recommended Annular Velocity" table which appeared in past EPR literature is no longer endorsed. In its place, EMDRC has developed a new tool from fluid mechanics theory, published laboratory data, new experimental data, and field data that provides an optimal combinations of drilling variables for efficient hole cleaning. It has been used for planning or well design to predict the likelihood of encountering hole cleaning problems based on drill string design (bit design, hole size, collars, drill pipe), drill pipe rotating speed, drilling fluid rheology, flow rates, and well profile.

EMURC is currently developing a PC program for surveillance in the field.

HCR = H/H_crit. Good hole cleaning occurs when HCR > 1.1

Where: H = the equilibrium height of the free region over the cuttings bed and is a function of the variables listed in figure 1. below.

H_{crit =} the critical height is a primarily a function of bit geometry.

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Hole Cleaning Ratio (HCR)

(continued)

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Hole Cleaning Operations (Intermediate and High Angle Holes)

Based on this work, the following pump out procedure is recommended for the deviated portion of the wellbore where problems due to cuttings bed are suspected.

• Monitor torque and drag using the Torque & Drag Surveillance spreadsheet.

• Circulate and rotate drillpipe at the maximum allowable flow/recommended rate prior to starting the trip. Experience has shown that 2 to 3 bottoms up volumes may be needed to clean the hole enough for tripping. If sidetracking is possible, move the bit slowly over a short interval

• Rotate will help stir up and remove cuttings beds especially if lots of sliding is done. Refer to EMDC Technology Group for detailed guidelines.

• In the deviated section, POH slowly as detailed in the drilling procedure (~2-1/2 to 3-1/2 minutes per stand).

• If excess drag is indicated, stop pulling, slack off 1 joint, then circulate and rotate at least one bottoms up at the maximum allowable flow rate. Rotating aids significantly to hole cleaning in high angle holes (normal practice is 100-120 rpm).

• Then, if a top drive is available, pump out of the hole at the maximum allowable/recommended flow rates while pulling at 2-1/2-3-1/2 minutes per stand or longer, continue until hole frees-up.

• Once in the lower angle section of the wellbore (preferably inside casing), circulate at least two bottoms up at the maximum allowable flow rate until cuttings returns decrease.

• Once the hole is clean, finish POH without pumping.

• For drilling operations with extended hole sections above 45°, backreaming may be necessary. Operational details will be provided in the applicable drilling procedure. Ensure that the dangers of backreaming in high-angle holes are thoroughly discussed prior to beginning the well so that everyone is clear on the strategy to be used.

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Rules-of-Thumb

1. Flow rate: Normally offshore drilling flow rates fall between 50 to 70 GPM per inch of bit diameter. However, flow rates greater than 70 GPM per inch of bit diameter are not unheard of in high angle wells.

• Do not sacrifice flow rate to get more horsepower, jet velocity, or bit pressure drop.

• Too low a flow rate will ball the bit and reduce effective hole cleaning.

• The annulus flow rate is too low to cause erosion. However, nozzle velocities which are typically 200-400 ft/sec may cause enlargement in low strength rock (<1,500 psi).

Limit nozzle velocity to <400 fps in soft rock.

• Fast drilling with low mud weights requires a minimum of 50 GPM per inch of bit diameter for holes < 20°; higher angle holes may require more.

2. Hydraulic Horsepower: Maintain 2 to 7 hydraulic horsepower per square inch of borehole area (HHP/in²).

• PDC bits with OBM require less HHP/in2 than with WBM. Total flow rate is more important when drilling with PDC bits and OBM than HHP/in2 .

• Fast drilling generally requires high HHP/in² ; however, some PDC bits in OBM can get by with as little as 2 HHP/in2.

• Larger bits require more HHP. However, many times in larger hole sizes high HHP is not possible. In these cases, pump the maximum volume possible.

• Maximum HHP/in² should be considered only when excess pump horsepower is available.

3. Bit Pressure Drop: When operating below Q_Crit, design hydraulics for 48% to 65%

pressure drop across the bit; this is usually the case below surface casing.

• Optimum Hydraulic Impact occurs when 48% of the system pressure loss is at the bit while optimum Hydraulic Horsepower occurs with 65% of the loss at the bit.

• If the total of drill string and annulus pressure loss is greater than 52% of the available pump pressure, smaller nozzles are required. However, do not operate below 30 GPM per inch of bit diameter. Consider using larger drill pipe.

• When running a PDM, it is recommended that the differential pressure across the bit not exceed 1000 psi to prevent accelerated wear of the rotor / stator assembly.

4. Jet Velocity: Good jet velocities are typically between 350 and 450 feet per second (use less than 400 fps in very soft rock to avoid washout).

• Jet velocity will influence chip hold down and ROP.

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