While drilling, cuttings are generated every footage drilled and this topic will demonstrate how to determine volume of cutting entering into the wellbore.
Figure 1 – Cutting Generated While Drilling
The following formula is used to calculate cutting volume generated while drilling;
Where;
Vc is volume of cutting in bbl/hr.
Ø is formation porosity (%).
D is wellbore diameter in inch.
ROP is rate of penetration in feet per hour.
Vc can be presented in several unit as follows;
Buoyancy Factor is the factor that is used to compensate loss of weight due to immersion in drilling fluid and you can find more information from this article > buoyancy factor calculation . In that article, it demonstrates the buoyancy formula only for one fluid in the wellbore. However, this time, we will have the details about buoyancy factor when inside and outside fluid are different.
Buoyancy factor with different fluid inside and outside of tubular is listed below;
Where;
Ao is an external area of the component.
Ai is an internal area of the component.
ρo is fluid density in the annulus at the component depth in the wellbore.
ρi is fluid density in the component depth in the wellbore.
ρs is steel weight density. Steel density is 65.4 ppg.
If you can the same mud weight inside and outside, the equation 1 will be like this
This is the same relationship as this article buoyancy factor calculation.
Let’s take a look at the following example to get more understanding.
Example
13-3/8” casing shoe was at 2,500’MD/2,000’TVD
9-5/8” casing was run to 6,800’MD/6,000 TVD.
9-5/8” casing weight is 40 ppf and casing ID is 8.835 inch.
Current mud weight is 9.5 ppg oil based mud.
The well bore diagram is show below (Figure 1).
Figure 1 – Wellbore Diagram
The well is planned to cement from shoe to surface and the planned cement weight is 14.0 ppg. The displacement fluid is drilling mud currently used.
Please determine the following items.
Air weight of casing string
Air weight of casing string, lb = length of casing, ft × casing weight, lb/ft
Air weight of casing string, lb = 6,800 × 40 = 272,000 lb
Buoyed weight of casing in drilling mud
Figure 2 – Buoyed Weight When Submersed In Drilling Mud
Buoyed weight = Buoyancy Factor (BF) × Air Weight of Casing
Buoyancy Factor (BF) = 0.855
Buoyed weight = 0.855 × 272,000 = 232,489 lb
Buoyed weight of casing when cement is inside casing and drilling mud is outside casing
Figure 3 – Buoyed weight of casing when cement is inside casing and drilling mud is outside casing
We will apply the Equation-1 for this case.
Ao is an external area of the component.
Ao = π × (Outside Diameter of casing)2 ÷ 4
Ao = π × (9.625)2 ÷ 4 = 72.76 square inch
Ai is an internal area of the component.
Ai = π × (Inside Diameter of casing)2 ÷ 4
Ai = π × (8.835)2 ÷ 4 = 61.31 square inch
ρo = 9.5 ppf (mud in the annulus)
ρi = 14.0 ppg (cement inside casing)
ρs = 65.4 ppg.
Buoyancy Factor (BF) = 1.22
Buoyed weight = 1.22 × 272,000 = 331,840 lb
Buoyed weight of casing when cement is outside casing and drilling mud is inside casing
Figure 4 – Buoyed weight of casing when cement is outside casing and drilling mud is inside casing
We will apply Equation-1 for this case as well.
All the calculation parameters are the same.
Ao = π × (9.625)2 ÷ 4 = 72.76 square inch
Ai = π × (8.835)2 ÷ 4 = 61.31 square inch
ρo = 9.5 ppg (mud in the annulus)
ρi = 14.0 ppg (cement inside casing)
ρs = 65.4 ppg.
Buoyancy Factor (BF) = 0.42
Buoyed weight = 0.42 × 272,000 = 114,240 lb
Conclusion: At different stage of the well, you may have different buoyed weight depending on density of fluid inside and outside of the component and it is not always that buoyed weight is less than air weight.
Reference book => 
Formulas and Calculations for Drilling Operations
Buoyancy factor is the factor that is used to compensate loss of weight due to immersion in drilling fluid and you can find the calculation from here https://www.drillingformulas.com/buoyancy-factor-calculation/.
We have created a simple table to help people determine the buoyancy factor quickly. Let’s take a look at the table. In Figure 1, it shows the main page and you can select the mud weight range from 4.0 ppg to 19.0 ppg.
Figure 1– Main Page BF Table
For instant, we choose 8.0 ppg and the table will show buoyancy from 8.0 – 8.9 ppg (Figure 2)
Figure 2 – Buoyancy Factor for 8 ppg Range
Frictional pressure is pressure loss acting in the opposite direction of fluid flow and today we will look into each component in pressure therefore you will get clearer picture about the frictional pressure in drilling system.
Let take a look at the simple diagram below.
A mud pump creates power to move drilling fluid from point A to C and the frictional pressure or pressure loss is the amount of pressure required to transfer fluid.
Pump pressure is 2,000 psi at the starting point (“A”) and at the end point (“C”), pressure is 0 psi. This tells you that you need 2,000 psi to overcome the frictional pressure in order to move the fluid from point “A” to point “C”.
“Differential pressure between two points in the system is pressure loss while fluid is moved from one to another point.”
Margin of overpull is additional tension to be applied when pulling the stuck drill string without breaking the tensile limit of the drill string. This is the difference between maximum allowable tensile load of drill string and hook load.
There are several factors when consider about the margin of over pull as listed below;
• Overall drilling conditions
• Hole drag
• Likelihood of getting stuck
• Dynamic loading
