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Critical RPM to Avoid Excessive Vibration

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When you operate top drive, you may need to know critical RPM that you can go. If you rotate pipe more than the critical RPM, it will create a lot of vibration that can cause failure in your drilling equipment such as drill pipe, TDS, etc.

In order to find out how much critical RPM, you may need high-tech simulation but sometimes you don’t really have that information supplied from town. So you really need to be able to roughly estimate how much critical RPM is ( at least you get a idea for this limitation). This formula below shows you how to estimate the critical RPM and it has accuracy of 15% roughly.

Critical RPM = 33,055 x (OD2 + ID2) 1/2 ÷ (L)2

Where;
OD = drill pipe outside diameter in inch
ID = drill pipe inside diameter in inch
L = length of one joint of drill pipe in feet

Example: Determine critical RPM from these following information

L = length of one joint of drill pipe = 32 ft
OD = drill pipe outside diameter = 4.0 in.
ID = drill pipe inside diameter = 3.5 in.

Critical RPM = 33,055 x (42+ 3.52)1/2 ÷ (32)2

Critical RPM = 172 RPM

Please remember this is ONLY estimation of the critical RPM. If you have your service companies or you have specific programs to determine it, please use the value from those programs because it should consider many parameters than this simple formula. USE IT IN CASE OF YOU HAVE NOTHING AVAILABLE TO CALCULATE THE CRITICAL RPM.

Ref books: Lapeyrouse, N.J., 2002. Formulas and calculations for drilling, production and workover, Boston: Gulf Professional publishing.

Bourgoyne, A.J.T., Chenevert , M.E. & Millheim, K.K., 1986. SPE Textbook Series, Volume 2: Applied Drilling Engineering, Society of Petroleum Engineers.

Mitchell, R.F., Miska, S. & Aadny, B.S., 2011. Fundamentals of drilling engineering, Richardson, TX: Society of Petroleum Engineers.

Bulk Density of Cuttings Using Mud Balance

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Wet cutting coming over shale shakers can be used to determine cutting bulk density. This post demonstrates you how to find bulk density (specific gravity) in specific gravity of cutting using mud balance so please read and understand procedure to determine resulting weith with cutting plus water (RW) and then use RW to determine specific gravity of cutting bulk density.

Bulk Density of Cuttings by Using Mud Balance - (OilfieldPix.com, 2017)

Bulk Density of Cuttings by Using Mud Balance
– (OilfieldPix.com, 2017)

Procedure to obtain resulting weight with cuttings plus water (RW):

1. Cuttings must be washed free of mud. In oil base mud, diesel oil can be used instead of water.
2. Set mud balance at 8.33 ppg.
3. Fill the mud balance with cuttings until a balance is obtained with the lid in place.
4. Remove lid, fill cup with water (cuttings included), replace lid and dry outside of mud balance.
5. Move counterweight to obtain new balance. This value is “Rw” = resulting weight with cuttings plus water in ppg.

The specific gravity of cutting is calculated as follows:

SG = 1 ÷ (2 – (0.12 x Rw))

Where;
SG = specific gravity of cuttings – bulk density
Rw = resulting weight with cuttings plus water in ppg

Example:
Determine the bulk density of cuttings:
Rw = 14.0 ppg
SG= 1 ÷ (2 – (0.12 x 14.0))
SG = 3.13

Moreover, if you can to convert specific gravity to mud weight (ppg and lb/ft3) and pressure gradient (psi/ft). Learn more about how to Convert specific gravity to mud weight (ppg and lb/ft3) and pressure gradient (psi/ft)

Please the Excel sheet used to calculate Bulk Density of Cuttings Using Mud Balance

Ref books: Lapeyrouse, N.J., 2002. Formulas and calculations for drilling, production and workover, Boston: Gulf Professional publishing.

Bourgoyne, A.J.T., Chenevert , M.E. & Millheim, K.K., 1986. SPE Textbook Series, Volume 2: Applied Drilling Engineering, Society of Petroleum Engineers.

Mitchell, R.F., Miska, S. & Aadny, B.S., 2011. Fundamentals of drilling engineering, Richardson, TX: Society of Petroleum Engineers.

Drilling Cost Per Foot

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Drilling cost per foot is the total drilling cost per footage drilled. This value is used for evaluating drilling projects, bit performance, drilling performance, etc.

Drilling Cost Per Foot – (OilfieldPix.com,2017)

The formula for drilling cost per foot as shown below;

CT = (B + CR (t + T)) ÷ F

Where:
CT = drilling cost per foot
B = bit cost
t = drilling time
CR = Rig cost per hour. This number must include all fixed daily cost not just only drilling contractor rate.
T = round trip
F = footage drill

Example: Determine the drilling cost per foot (CT) using the following data:

Bit cost (B) = 27,000 $
Drilling time (t) = 50 hours
Rig cost (CR) = $3500/hour
Round trip time (T) = 12 hours
Footage per bit (F) = 5000 ft

CT = (27,000 + 3,500 (50 + 12)) ÷ 5000
CT = 48.8$ per foot

Please find the excel sheet for calculating drilling cost per foot.

Ref books: Lapeyrouse, N.J., 2002. Formulas and calculations for drilling, production and workover, Boston: Gulf Professional publishing.

Bourgoyne, A.J.T., Chenevert , M.E. & Millheim, K.K., 1986. SPE Textbook Series, Volume 2: Applied Drilling Engineering, Society of Petroleum Engineers.

Mitchell, R.F., Miska, S. & Aadny, B.S., 2011. Fundamentals of drilling engineering, Richardson, TX: Society of Petroleum Engineers.

Pump Pressure and Pump Stroke Relationship

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There are relationships between pump pressure and pump stroke that you really need to understand and be able to determine pump pressure after adjusting new pump stroke.

Mud Pump (OilfieldPix.com, 2017)

There are 2 formulas used to determine pump pressure as shown in the detail below:

1st formula for estimating new circulating pressure (simple and handy for field use)

New circulating pressure in psi = present circulating pressure in psi x (new pump rate in spm ÷ old pump rate in spm) 2

Example: Determine the new circulating pressure, psi using the following data:
Present circulating pressure = 2500 psi
Old pump rate = 40 spm
New pump rate = 25 spm
New circulating pressure in psi = 2500 psi x (25 spm ÷ 40 spm) 2
New circulating pressure = 976.6 psi

2nd formula for estimating new circulating pressure (more complex)

For the 1st formula, the factor “2” is used but it’s just the round up figure. If you want more accurate figure, you need to figure out an exact figure. So the 2nd formula has one additional formula to calculate the factor based on 2 pressure readings at different pump rate.  Please follow these steps to determine new circulating pressure

1. Determine the factor ”n” and  the formula to determine factor “n” is below:

Factor (n) = log (pressure 1 ÷ pressure 2) ÷ log (pump rate 1÷pump rate 2)

2. Determine new circulating pressure with this following formula.

New circulating pressure in psi = present circulating pressure in psi x (new pump rate in spm ÷ old pump rate in spm) n

Note: factor “n” comes from the first step of calculation.

Example: Determine the factor “n” from 2 pump pressure reading
Pressure 1 = 2700 psi at 320 gpm
Pressure 2 = 500 psi at 130 gpm
Factor (n)   = log (2700 psi ÷ 500 psi) ÷ log (320 gpm ÷ 130 gpm)
Factor (n) = 1.872

Example: Determine new circulating pressure by using these following information and the factor “n” from above example:
Present circulating pressure = 2500 psi
Old pump rate = 40 spm
New pump rate = 25 spm
New circulating pressure, psi = 2500 psi x (25 spm ÷ 40 spm) 1.872
New circulating pressure = 1037 psi

Please find the Excel sheet used to calculate new circulating pressure based on pump pressure and pump stroke relationship.

Ref books: Lapeyrouse, N.J., 2002. Formulas and calculations for drilling, production and workover, Boston: Gulf Professional publishing.

Bourgoyne, A.J.T., Chenevert , M.E. & Millheim, K.K., 1986. SPE Textbook Series, Volume 2: Applied Drilling Engineering, Society of Petroleum Engineers.

Mitchell, R.F., Miska, S. & Aadny, B.S., 2011. Fundamentals of drilling engineering, Richardson, TX: Society of Petroleum Engineers.

Drill pipe pulled to lose hydrostatic pressure

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You previously learn about hydrostatic pressure lose due to pulling out of hole . This article will use the same concept but we will determine how many feet of drill pipe pulled to lose certain amount of hydrostatic pressure in well bore.

Oilfield Unit

Pipe pulled, ft = (hydrostatic pressure loss in psi × (casing cap in bbl/ft – pipe displacement in bbl/ft)) ÷ (mud weight in ppg × 0.052 × pipe displacement in bbl/ft)

Example: Determine the FEET of drill pipe that must be pulled to lose 200 psi overbalance using the following data:

Hydrostatic pressure loss = 200 psi
Casing capacity = 0.0873 bbl/ft
Pipe displacement = 0.01876 bbl/ft
Mud weight = 12.0 ppg
Pipe pulled  = 200 psi × (0.0873 – 0.01876) ÷ (12.0 ppg × 0.052 × 0.01876)
Pipe pulled = 1171 ft
You need to pull 1171 ft of dry pipe to lose 200 psi hydrostatic pressure.

Metric Unit

Pipe pulled, m = (hydrostatic pressure loss in KPa× (casing cap in m3/m- pipe displacement in m3/m-)) ÷ (mud weight in kg/m3 × 0.00981× pipe displacement in m3/m)

Example: Determine how many meters of drill pipe that must be pulled to lose 1400 KPa overbalance using the following data:

Hydrostatic pressure loss = 1400 KPa
Casing capacity = 0.04554 m3/m
Pipe displacement = 0.00979 m3/m
Mud weight = 1440 kg/m3
Pipe pulled  = 1400× (0.04554 – 0.00979 ) ÷ (1440 × 0.00981 × 0.00979 )
Pipe pulled = 362 m
You need to pull 362 m of dry pipe to lose 1400 KPa hydrostatic pressure.

Please find the Excel Spreadsheet to calculate how many feet  or meter of drill pipe pulled to lose certain amount of hydrostatic pressure in well bore.

Ref books:

Lapeyrouse, N.J., 2002. Formulas and calculations for drilling, production and workover, Boston: Gulf Professional publishing.

Bourgoyne, A.J.T., Chenevert , M.E. & Millheim, K.K., 1986. SPE Textbook Series, Volume 2: Applied Drilling Engineering, Society of Petroleum Engineers.

Mitchell, R.F., Miska, S. & Aadny, B.S., 2011. Fundamentals of drilling engineering, Richardson, TX: Society of Petroleum Engineers.