Determine Bottom Hole Pressure from Wellhead Pressure in a Dry Gas Well

Gas behaves differently from fluid therefore you cannot use a simple hydrostatic formula to determine reservoir pressure. Gas is compressible but fluid is incompressible.


The formula to determine the bottom hole pressure of dry gas well is shown below;

equation 1


Where; Pbh = bottom hole pressure in psia (absolute pressure)

Pwh = wellhead pressure in psia (absolute pressure)

H = true vertical depth of the well

Sg = specific gravity of gas

R = 53.36 ft-lb/lb-R (gas constant for API standard condition air)

Tav = average temperature in Rankin (Rankin = Fahrenheit + 460) Continue reading

Fracture Gradient Reduction Due to Water Depth

Fracture gradient is one of the critical information which drilling engineers need to know in order to design drilling programs. For the well control stand point, the fracture gradient directly affects on how much influx volume can be successfully contained in the wellbore. If the wellbore pressure is over the fracture pressure, formations would be broken down and this situation will result in loss of drilling fluid into formations. Additionally, it might lead to well control situation because of loss of hydrostatic pressure. Fracture gradient is quite straight forward for land operation because it will not be reduce due to water column. However, the fracture gradient will be reduced in deepwater environment. In this article, we will discuss why water depth can cause the reduction in fracture gradient.

Fracture Gradient Reduction Due to Water Depth

Basically, the fracture gradient is related to fluids occupying in pore spaces of rock and weight of rock which are called overburden pressure. Generally, the overburden of a typical sedimentary is about 1.0 psi/ft (19.2 ppg). Rocks will be fractured when the wellbore pressure exceeds the confining stresses acting on it. If we make a general assumption that the overburden pressure causes the minimum confining stress of the rock. Then the formation fracture gradient will not be 1.0 psi/ft if the location is offshore.

Note: this assumption is made in order to help you get more understanding on how and why water depth can decrease the formation fracture gradient.

Why does the water depth reduce fracture gradient?

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Block and Drilling Line Calculation

This article will focus on block and drilling line calculations as block efficiency, drilling power input/output, etc. Additionally, there are some examples which will help you understand how the formulas work.


Block and drilling line efficiency formula is described below;

Block and Drilling Line Calculation 1

Power Output (Po) = Fh x Vtb

Power Input (Pi) = Ff x Vf

Block and Drilling Line Calculation 2

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Rig Engine Power Consumption and Engine Efficiency

In this article, we will focus on the rig engine power consumption and overall engine efficiency and there are few examples for you to get more understand on how to use the formulas as well.



Input Power of Engine

The formula is listed below;

input power


Pi = input power in horse power (hp)

H = fuel heating value in BTU/lb

Qf = fuel consumption rate in lbm/hr

Fuel Consumption

The formula is listed below;

Fuel Consumption formula

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How Much Force Applied to a Rig Tong To Get The Right Torque at The Connection

One application of the rig tong is to use to make up connection. The question is asked about how to get the right torque value to the connection if you use the rig tong because you will not see the torque value on the gauge. This article will describe how to determine the correct force applied to get the correct torque value when you use the rig tong to make up the connection.


This formula below is used to determine force at required make up torque value.


Oilfield Unit

Force = Torque ÷ Length of the tong


Torque in ft-lb

Force in lb

Length of the tong in ft

Force is perpendicular to the tong length.

The illustration above demonstrates the direction of force.

Metric Unit

Force = Torque ÷ Length of the tong


Torque in N-m

Force in

Length of the tong in m

Force is perpendicular to the tong length.


Connection 4-1/2” IF – required make up torque = 30,700 ft-lb.

Tong length is 4 ft


How much pulling force do you require to achieve the required make up torque?

Force = Torque ÷ Length of the tong

Force = 30,700 ÷ 4.0

Force = 7,675 lb

Answer: 7,675 lb pulling force is applied to 4-ft tong in order to get the torque at connection at 30,700 ft-lb.

You can download the spreadsheet from the  download link.

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.