How is the rotary drilling technique taken care of if the equivalent circulating density is monitored?

Image source. Wikimedia Commons

Rotary well drilling is a technique that involves multiple technologies to achieve minimum requirements in the control of the parameters in the drilling of an oil well.

There are elements of technology such as downhole well logging tools that must be taken care of, for that reason it must be avoided that there is a sticking of pipe in which the well has to be abandoned and leave all those tools at the bottom of the hole.

The purpose of this post is to teach everyone one of the ways to calculate and monitor a drilling parameter that is of multiple importance to preserve the rotary drilling technique and to take care of all the elements involved, for it made the following technical description:

As I have explained in my multiple previous posts, in order to drill an oil well, a drilling fluid, also known in the oil industry as drilling mud, must circulate. This fluid is pumped from the surface to the bottom of the well to fulfill a wide variety of functions, among which is to generate a backpressure in the well and to hold the fluids that are in the formation such as gas and/or oil.

There are two conditions in which the drilling fluid can be in the well, which are:

• In static condition: this is a condition in which the drilling fluid is in the well without circulation, since the mud pumps are off, under this condition the drilling fluid exerts a pressure on the walls of the well called hydrostatic pressure, this pressure has also been explained based on its operation and calculation in previous post.

• In circulating condition: this is a condition in which the mud pumps are turned on circulating the mud from the surface to the bottom of the well, the drilling mud is returned through the space between the drill pipe string and the walls of the well (annular space) and returns again to the surface, where it goes through a series of controls to restore its properties, to be constantly circulated and thus comply with the drilling mud circulation circuit. When the fluid is circulated an additional pressure is added as a result of the effort that the mud pumps have to make to overcome the friction resistances, there is also a resistance by elbows, valves and any accessory, the conclusion of the case is that this additional pressure added makes that there is equivalent density to that pressure and is called equivalent circulating density, known by its acronym in English as ECD.

The advantage of knowing the values of the equivalent circulating density is that we can estimate the increase in drilling mud density in relation to when the mud pumps are off when the drilling mud is at rest, so if we know the formation pressure we will know if we can control the well by circulating the drilling fluid, in case there is no control of the formation pressure when the well is without circulation.

How is the Equivalent Circulation Density (ECD) calculated?

The equation that I am going to show you below has the ability to calculate the equivalent circulating density in units of pounds / gallon which are the units of the English system used in the oil industry in the drilling of wells, the variables that we must take into account to calculate the equivalent circulating density (ECD) is:

• Density of the mud in use measured in pounds/gallon.
• Pressure losses in the annulus measured in Psi.
• True vertical depth of the well being drilled measured in feet.

Example of a practical exercise in the field:

An oil well is being drilled to a true vertical depth (TVD) of 9000 ft, using a drilling fluid (drilling mud) with a density of 9.5 lb/gallon and having an annular pressure drop of 200 Psi. What is the equivalent circulating density (ECD) at these conditions?

In order to solve and find the value of the equivalent circulating density (ECD) it is convenient to consider the following field equation:

For this case we see that to calculate the equivalent circulating density (ECD) we simply divide the pressure losses in the annulus by the conversion factor of 0.052 by the true vertical depth, and whatever we get from these divisions we add the density of the drilling fluid in use, as I explain below:

With the result of the equivalent circulating density (ECD) equal to 9.93 lbs/gallon we realize that at that depth of 9000 ft when the pumps are turned off the drilling fluid density is 9.5 lbs/gallon, however when the mud pumps are turned on and the mud is circulated with that 200 psi annular pressure loss the density increases to 9.93 lbs/gallon, thus having an increase of 0.43 lbs/gallon over the density in use.

The only way for this increase to be greater and greater at that same depth is for the pressure losses in the annulus to increase, and this can only happen if the circulation flow rate of the pump is increased or if some accessory is changed in the drilling mud circuit that causes a higher pressure to be required from the mud pump.

Conclusion

In conclusion we can define the monitoring of drilling parameters as that essential piece in the care of technological tools that are indispensable in well drilling operations, while at the same time stimulating science and engineering to continue studying and innovating on the technological future in the area of well drilling.

Recommended and consulted bibliography

Well Control Manual. Well Control School (WCS). Harvey Louisiana. Year 2003.

Why does equivalent traffic density exist and how is it calculated? Spanish version by author @carlos84

Note: Equation images and exercise examples were made using Microsoft Word equation insertion tools.