Directional Drilling in Oil and Gas

Directional Drilling

In the Directional Drilling lesson, we’ll discuss some common types of deviated oil and gas wells.

Deviated Wells – Directional Drilling and Horizontal Drilling

Deviation is defined as the angle at which a wellbore diverges from the 90° vertical position.

In the drilling operation, most deviations occur naturally because of the flexibility of the drill string and its reaction to downhole geological structures, such as changes in formation density, or faults and dips in the rock strata that is being drilled through.

A wellbore is often deliberately deviated to miss high-risk obstacles, such as adjacent producing or injection wells in a more developed field location.

Additionally, deviation can be used to reach additional reservoirs when drilling “multi-wells” from a single drilling location – especially offshore.

Finally, wells are also deviated to avoid risky drilling subsurface formations like salt layers.

“Today, most wells drilled are deviated or horizontal. Note that a horizontal well is deviated, but a deviated well may not be horizontal.”

Directional Drilling

Directional drilling is the intentional deviation of a wellbore from the path it would naturally take.

This is accomplished through the use of specialized steering mechanism or tools attached to the drillstring.

Some examples of these type of drilling tools are: mud motors, whipstocks, and sophisticated bottom-hole assembly (BHA) configurations.

The directional driller also can alter other drilling parameters such as weight on the drill bit and rotary table speed to help direct the bit to the desired directional path.

Downhole instruments continuously measure the path of the wellbore in three-dimensions. The term for this technology is Measurement While Drilling, or MWD.

Data links from the drill bit to the surface communicate these measurements to allow constant positional adjustment to the mud motors and BHA components.

Horizontal Drilling

Prior to the 1990’s vertical well drilling was the preferred method. As shale plays which are wider than they are thick became economic, horizontal wells became dominant.

Though horizontal drilling is more costly, it can reach subsurface objectives that could not easily be reached with a vertical borehole.

Today, most wells drilled are deviated or horizontal. Note that a horizontal well is deviated, but a deviated well may not be horizontal.

Wells commonly are turned to a horizontal direction to:

increase exposure to producing zones structures
intersect a larger number of fractures in certain geologies
follow a complex structure, like a shale layer
drill numerous (multi) wells from one location or pad

Because horizontal drilling can drain a larger area, fewer are needed, which means less surface infrastructure.

This reduced footprint makes horizontal drilling ideal for shale reservoirs that are shallow and spread out, fractured reservoirs like the Austin Chalk in Texas or for in drilling in sensitive environments.

These factors, in addition to drilling equipment advances, have made horizontal wells commercially viable.

There has been a significant increase in the number of horizontal wells in the US over the past two decades.

Technology advances have allowed horizontal laterals to be drilled in excess of a mile under very controlled conditions.

With a vertical well a geologist’s role is primarily to evaluate “what has been drilled.”

With horizontal wells, geologists now become part of the drilling operation, actively involved in steering the well along a desired geological profile and responding to feedback data in real-time as drilling occurs.

Multilateral Wells

Multiple horizontal wells being drilled from the same pad decreases the site footprint. Historically a driller moved the rig a few yards and starts another vertical shaft.

This is not needed today. It is now possible and common to drill multiple deviated wells from the same vertical wellbore.

In this technique, called multilateral drilling, two or three horizontal boreholes, each a few hundred to a few thousand feet long, can be drilled from the main vertical wellbore, like the branches of a tree or a candelabra.

This technique is especially common offshore or in environmentally sensitive onshore locations.

Multilateral drilling

creates a highly productive drainage network in reservoirs with many isolated pockets of oil or gas.

Advances in multilateral drilling and completion technology promise reduced well costs, greater flexibility and increased profit potential for the operator.

The TAML (Technical Advancement of Multilaterals) joint-industry project was formed in 1997 to educate the industry on the advantages of multilateral wells.

Multilaterals have proven successful in various drilling- production applications, but members of TAML feel that the industry is just starting to recognize the full potential of the technology.