When does formation damage occur and how does it affect future well productivity?

11 months ago

Hello friends and lovers of STEM content, especially content related to engineering.

In this opportunity I bring you a very important topic in petroleum engineering, as is the case of formation damage, when drilling an oil well a drilling fluid is used with a density such that it can generate a hydrostatic pressure that is greater than the formation pressure, which is what is known as the overbalance drilling technique.

However, when the overbalance drilling technique is used, it has the precious benefit of safety while drilling, since the pressure exerted by the drilling fluid on the walls of the well makes the fluids such as oil and/or gas remain in the reservoir and do not enter the well, the disadvantage of applying this technique is that a large amount of the drilling fluid penetrates the formation, Especially in lithological formations that are highly permeable, this drilling fluid that enters the well into the formation is what is called formation damage, as it plugs the pores of the producing rock or reservoir, reducing the future productivity of the reservoir in relation to its contribution of oil and / or gas.

A formation damage can be quantified in relation to the flow restriction that oil has to flow from the formation to the vicinity of the well, these productivity tests can be done even in the period that the well is being drilled, also productivity tests can be performed once the well has already been drilled and is in production.

A very simple production test is to track the drop in production of barrels of oil being produced per day, in the example of the graph above we can see how oil production falls from the first month of evaluation from 600 barrels per day to reach 200 barrels per day in the fourth month of evaluation, this may already mean some symptoms that in the drilling of that well there was damage to the formation due to the invasion of drilling fluid from the well into the formation.

Another data that can help us to reach the conclusion that a well in production could have been exposed to drilling fluid invasion and therefore to have had a damage to the formation is to compare the production history of the neighboring wells to the well we are submitting to evaluation.

It is important to mention that formation damage not only occurs in the process of drilling an oil well, but also in the production process of a well, in the process of completion and cementing can also occur, however the most common cause is the invasion of drilling fluid, which is why it is what I am trying to study in this post.

Can we mathematically calculate the damage to the formation which we will denote with the letter (S)?

An estimation of the damage can be made, however, obtaining the variables contained in the equation that I am going to show you depends on some petrophysical values of the formation that is involved in the possible damage, these petrophysical variables are:

Permeability of the virgin zone (K): is the permeability of the formation that has not been exposed to damage.
Damaged zone permeability (Kd): is the permeability value of the zone of the formation that is damaged or contaminated with the drilling fluid.
Damaged zone radius (Rd): is the distance from the center of the borehole to the edge of the damaged zone.
Well Radius (Rw): is the distance from the center of the well to the walls of the well.

Once these petrophysical values are obtained, the formation damage can be calculated with the following equation:

As can be seen in the previous equation, if we analyze the variables we can realize that if the permeability values are equal, it is because the permeability of the damaged zone is equal to the permeability of the virgin zone, therefore there is no damage, since their quotient is equal to one, this value is subtracted one, which makes the equation null and void and the damage (S) is zero, i.e. non-existent.

If the permeability of the damaged zone is greater and is far from the value of the permeability of the virgin zone, it implies that the quotient between the two permeabilities will be a very large number, and when we subtract one it will still give a larger number, which is an indication that the damage is greater.

The quotient of the radii will be greater as the radius of the damage is greater, and this value will be increased by the fact that this number is the argument of the neperian logarithm, so if the radius of the damage is greater the damage (s) will be greater.

Conclusion

My recommendation based on my experience as a drilling operations engineer is that during drilling a very strict control must be kept in the supervision of the drilling fluid density values, since such density values must be those indicated in the drilling program as performed for the engineering design of the well being drilled.

This is because any increase in density will result in a higher hydrostatic pressure of the drilling fluid and therefore a greater invasion of the drilling fluid into the formation, which will ultimately result in lower oil production from the well.

I also recommend drilling with drilling fluids whose chemical formulation contains additives that can provide a resistant plaster on the walls of the well so that the drilling fluid itself does not invade the formation.

In conclusion, to perform all the supervision controls of the physicochemical parameters of the drilling fluid to maintain them according to the drilling program will make that the levels of damage to the formation will be very low.