When drilling a well that targets naturally fractured formations, or a well that must drill through natural fractures, there are a few drilling methods to consider, among those vertical drilling, horizontal drilling and sloping drilling. Of which, horizontal and sloping drilling are known as directional drilling.

Directional drilling is the intentional deviation of the vertical well path. Downhole motors rotate and orient the entire drill string in the preferred direction. In the simplest cases, directional drilling requires accounting for fields of stresses in situ and the drilling objective.

In naturally fractured deposits, the baseline porosity of the formation and the porosity of natural fractures caused by telluric movements and geomechanical stresses contribute to the flow of fluids towards the direction of the fracture, thus creating an extremely important option for production.

Operators of directional drilling must therefore have substantive geological knowledge of the area, and it must be performed by a highly qualified staff.

Growth and use

According to the U.S. Energy Information Administration (EIA), directional drilling in the U.S. was almost nonexistent until the 2000s, when its usage increased for shale gas wells. By 2016, directional wells accounted for approximately 68% of crude oil and natural gas wells in the U.S., of the total, closing in 2019 with 65% of the wells drilled in the country.

Geological fractures. Source: Crain's Petrophysical Handbook

Directional drilling is applied for different objectives. Reasons include:

• Drilling several wells from the same location
• Drilling below inaccessible locations
• Lateral deviation and straightening
• Drilling of salt domes
• Creating relief wells
• Shale gas wells
• Drilling in geological faults

Drilling in geological faults, it is of the utmost importance for the development of this article.

With the existence of geological fractures, either in the target area or in areas that must be drilled through to reach the target area, a detailed structural framework for the geological faults of the area must be made.

The use of seismic records, in conjunction with sonic records and images, provides information on the direction and dip of sub-soil faults. Likewise, it serves for a geomechanical study of the area.

Images log. Source: Crain's Petrophysical Handbook

Geomechanics infers the magnitude and direction of the main stresses, knowing that fractures do not have a purely vertical or horizontal direction, and instead are often at angles.

With data for the direction of minimum and maximum stress, and the stresses’ vertical and horizontal magnitude, the stress magnitude for directional drilling can be calculated by taking one direction or another, and using the Pythagorean Theorem for the calculation of the different drilling angles.

As a result, the question arises: what should be the drilling direction in naturally fractured wells? And the answer is congruent with the goal.

Drilling direction

When drilling to reach the target area and natural fractures are drilled through on the way, the stresses that influence the area must be taken into account.

Drilling mud withstands the external stresses to the pit, so the density of the hole should be able to withstand the different magnitudes. With directional drilling, operators drill in an angle parallel to the fracture, avoiding contact with the stress that caused the fracture, commonly the maximum one. This eliminates part of the stress that caused the fracture, because the drilling is above it. The drilling mud will not its weight and will not affect the superficial zone during the drilling, leaving as the only drawback to drill through the fracture. For this part, a viscous mud that is capable of creating walls more resistant for the hole is essential.

Directional drilling. Source: Schlumberger Ltd.

In cases where the target zone is a natural fracture, directional drilling is used to reach the deposit and navigate within the fracture. Unlike what it is shown above, this procedure seeks to drill perpendicularly on the area fractured to take advantage of the drainage radius that this leaves, and it seeks to navigate in the direction of the fracture. Generally, these wells are more extensive in drilling performed, and in these circumstances the well is under the stress magnitude that caused the fracture, so the drilling mud must be capable of maintaining the stability of the hole until it is cemented or packed to move to production.

Naturally fractured reservoir. Source: Oil & Gas Middle East

Every day there are more fractured deposits, either natural ones or modified ones, such as fracking deposits. In any case, directional drilling has become the spearhead for the exploitation of hydrocarbon flow paths increasing recovery and facilitating economic feasibility when creating new wells. In this way, production has increased in the United States, the Emirates. and Saudi Arabia, passing the 10 million barrels per day. This is just the beginning, as directional drilling is not poised to decrease any time soon.