Well control // How to understand the input of formation flows in the drilling of petroleum and/or natural gas wells

in steemstem •  4 months ago 


Image created by @carlos84 using Microsoft PowerPoint design tools
Image source. Pixabay

Welcome friends to my blog about Petroleum Engineering

In this opportunity I want to explain the causes that originate the entry of fluids such as oil and/or gas from the reservoir to the well while we are in drilling operations.

It is important that we analyze the existing risks of having an inlet of fluids such as oil, gas and water from the formation to the well.

There are many reasons for this fluid invasion from the reservoir to the well, but the most prevalent is that the pressure exerted by the drilling fluid is less than the pressure exerted by the formation fluids.

What are the consequences of an invasion of formation fluids into the well?

[1] One of the immediate consequences is the loss of time due to monitoring operations to determine if there really is fluid invasion from the formation to the well.

[2 ] Once control of the influx of unwanted fluids into the well has been lost, the least desired event in well drilling occurs, which is the uncontrolled flow of gas and oil to the surface, resulting in what is known as a blowout.


Image source.

What causes unwanted fluids to enter the well?

There is only one condition for fluids such as oil and gas to migrate from the bottom of the well to the surface, this condition occurs when the formation pressure (also known as reservoir pressure) is greater than the pressure exerted by the drilling fluid column, fluids such as petroleum, gas and water invade the well and can migrate to the surface can cause an onslaught of unwanted flows and subsequently cause a blowout, this is summarized in:


Author of the image: @carlos84

What causes the condition that the formation pressure is greater than the drilling fluid pressure?

[1] One of the most common reasons why the pressure of the drilling fluid is lower than the pressure of the forming fluids is due to low density or insufficient density on the part of the drilling fluid.

[2] Poor well filling occurs when drill pipe is put in or taken out of the well. This is because the drilling fluid column falls below acceptable levels, so the hydrostatic pressure exerted by the drilling fluid also drops considerably. However, one of the reasons why hydrostatic pressure can drop is because the well loses fluid level, and this lost fluid from the well is not recovered by adding more drilling fluid. When the well is poorly filled in the maneuvers performed to take the drill string out of the well, there is a possibility that the formation pressure will exceed the pressure exerted by the drilling fluid causing an onslaught of unwanted fluids into the well and possible migration to the surface.

[3] Loss of circulation: “I would like to start this point by saying that "no more is always better". The hydrostatic pressure of the drilling fluid has to be greater than the formation pressure to keep the well under control, however this hydrostatic pressure should not be so great, if it is true that it has to be greater than the formation pressure, but in turn it has to be less than the fracture pressure.

[3] Loss of circulation: “I would like to start this point by saying that "no more is always better". The hydrostatic pressure of the drilling fluid has to be greater than the formation pressure to keep the well under control, however this hydrostatic pressure should not be so great, if it is true that it has to be greater than the formation pressure, but in turn it has to be less than the fracture pressure.

The density of the drilling fluid must be such that a hydrostatic pressure is generated which can withstand the formation fluids, but which in turn does not exceed the pressure with which the formation can be fractured. That density value must be such that for example for drilling activities in PDVSA petroleum industry it must be a pressure gradient in favor of the drilling fluid of more or less 500 psi overbalance.

[4] Another cause that causes the pressure of the formation fluids to exceed the hydrostatic pressure of the drilling fluid is abnormal pressures. In the case of abnormal pressures their existence is due to various geological factors such as faults, anticlines, discordances that cause during drilling to enter into an abnormal pressure formation, what is recommended to do when entering into an abnormal pressure formation is to begin to control the well by gradually increasing the density of the drilling fluid until the abnormal pressure zone is passed.

In Venezuela it is difficult to find oil fields that have formations with abnormal pressures, however, my recommendation and based on my experience in well drilling operations is that if there is not enough geological information, we should always be attentive to the supervision of drilling parameters such as penetration rate, torque, drilling mud density, the best thing is not to be confident, always be attentive, and to have all the densifying chemical material in place, so that if we get to it with abnormal pressure we can gradually increase the density of the drilling fluid until we pass the abnormal pressure zone.

How to detect an inlet of formation fluids into the well and their possible migration to the surface?

Understanding then the main cause that originates an unwanted entry of fluids such as gas and / or petroleum to the well, where they can migrate and reach the surface causing a fire at larger scales and possible material and human loss, we must then interpret the elements that are indicators that an onslaught of unwanted fluids is migrating to the surface.

Understanding then the main cause that originates an unwanted entry of fluids such as gas and / or petroleum to the well, where they can migrate and reach the surface causing a fire at larger scales and possible material and human loss, we must then interpret the elements that are indicators that an onslaught of unwanted fluids is migrating to the surface.

Variations in drilling speed represent a strong indicator of the presence of an onslaught or threat of formation fluids, if the speed with which we are drilling in the well changes, and begins to increase suddenly, it is because perhaps we are in the presence of an increase in formation pressure.

These sudden changes of increase of the penetration rate in the perforation should not occur, since as the perforation deepens the speed of perforation tends to diminish, since as we deepen in the perforation we obtain with strata that are increasingly hard and difficult to cut with the wick.

What happens to the drilling speed when formation fluids invade the well?

As there are more fluids at the bottom of the well resulting from the invasion of formation fluids, the wick cuts the formation strata faster because this excess fluid softens the formation, and a soft formation may be easier to cut through the wick.

The driller in your booth has a control panel and reading parameters where you can observe if the penetration rate suddenly increases, if this happens the drill should immediately communicate to the supervisor of the drill or to the engineer of drilling operations.


Image source. Pixabay

When the torque and / or drag increases is an indication that you are in the presence of a threat or inset.

The increase in torque is related to the increase in the penetration rate, if you increase the speed with which the wick is cutting the formation are many debris cuts that will accumulate around the drill string, preventing it from turning, so the torque and drag will rise.

We can therefore say that the increase in torque is a good indicator of the increase in formation pressure and the invasion of undesirable fluids in the well.

If gas is present in the drilling fluid, this is a clear indication that a geological stratum containing abnormal pressures is being passed through. However, gas cuts are not always the result of an unbalanced condition, so a proper understanding of gas trends is important.

We must also be alert if we find the presence of chlorine in the drilling fluid, as the presence of chlorine is a strong indicator that formation gas is entering the well.

Conclusions and recommendations

When any personnel on location, be it the driller, surveyor, supervisor or operations engineer, can correctly interpret any of the indicators of an assault, this would prevent a possible disaster in which human lives and large amounts of equipment are lost.

All the personnel necessary for the execution and construction of an petroleum well, especially the personnel without experience in relation to the topic of well control should be trained so that they know which the causes that originate an attack are and so that they learn to detect their possible indicators. Once trained, they must practice in the location and simulate the behaviors of threat of an influx of unwanted fluids towards the well to acquire skills and abilities that allow them to face a situation of such magnitude.

Reference consulted and recommended

[1] Well Control Manual: Well control School. Harvey, Louisiana.

[2] Well Control

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