Downhole drill cuttings cleanup considerations

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While drilling an oil well the drill bit cuts the geological strata of the subsoil, these in turn are transported by the drilling fluid from the bottom of the well to the surface, thus allowing the hole cleaning process, however for this process to be executed efficiently it is necessary to address a number of very important considerations, which will be the focus of explanation of this post.

The geometry of the well is very influential in the cleaning of the hole while we are drilling, since for example it is not the same cleaning capacity for a well that is drilled horizontally (directional drilling) than for a vertical well. Hydraulics gives us an indication that for a horizontal well the drilling fluid needs more power to remove the cuttings from the bottom of the hole.

All these problems in hole cleaning in wells that have an inclination angle greater than 30º measured from the vertical have provided us through laboratory studies that in this type of wells there are greater difficulties for hole cleaning while drilling than in vertical wells or that its inclination angle measured with respect to the vertical is below 30º.

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The complexity in the design of these wells begins when the design engineer intends to use the same flow rate values as for a vertical well, since he may think that as the geological data are the same, the same flow rate values of the drilling fluid should be used, however this is wrong in the fact that the geometry of the well is influential in the flow rate values of the drilling fluid.

The hole cleaning in well drilling is a variable that is taken into account by the simple fact of taking care of the non-occurrence of the negative consequences that occur when there is not a good hole cleaning, that is why when we think of the additional costs that may arise from the fact that the pipe gets stuck due to inefficient hole cleaning, or that there may be premature wear of the drill bit, is when all possible efforts will be made in the hydraulic design that is relevant to maintain an efficient hole cleaning.

What is hole cleaning like when drilling under balance?

In order to understand hole cleaning under this condition it is important to understand the meaning of underbalanced drilling, so it is necessary to understand the following:

For underbalanced drilling to be executed it has to be fulfilled that the hydrostatic pressure of the drilling fluid is less than the formation pressure, which translates to:

Hydrostatic pressure < Formation pressure (Underbalanced condition).

This condition makes it necessary to use a low density drilling fluid, which translates into fluid parameters whose rheology can become critical in directional wells whose angle measured from the vertical is greater than 45º, so the recommendation is to closely monitor the hole cleaning when drilling directional wells in the underbalanced drilling condition.

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The conclusion for considerations in hole cleaning while drilling is to evaluate the conditions that are influential in an optimized hole cleaning, one of these examples are the wells that are deviated, that is, wells whose trajectory is directional, the other important thing is to analyze the underbalanced condition, since the low hydrostatic pressure conditions the other properties of the drilling fluid to perform a rheology that does not help the hole cleaning.

References

Sewell, M. and Billingsley, J. 2002. An Effective Approach to Keeping the Hole Clean in High-Angle Wells. World Oil 223.

Pilehvari, A.A., Azar, J.J., and Shirazi, S.A. 1999. State-of-the-Art Cuttings Transport in Horizontal Wellbores. SPE Drill & Compl 14 (3): 196–200. SPE-57716-PA.