There are many locations worldwide where drilling cost reduction or production improvement is likely to result through application of directional underbalanced drilling (UBD). The main benefit of underbalanced drilling is the minimization of formation invasion related problems. Because the majority of hydrocarbons today are found in existing fields with depleting pressures, or in complex and low quality reservoirs, the economical use of underbalanced drilling has become more popular and the routine of choice for operators drilling in specific geographic areas.

When drilling underbalanced, the hydrostatic weight of the drilling fluid column is intentionally reduced below the known reservoir pressure. This differs from conventional drilling in that the bottomhole circulating pressure is at or below the formation pressure, permitting the well to flow while drilling proceeds. Regardless of whether the underbalanced state is attained naturally or by induction, the resulting situation at the surface is the same and pressure must be controlled to ensure that the well does not blow out.

Choosing fluids for UBD

When the operator makes the call to drill underbalanced, a decision must be made on which fluids will be used for that particular location in order to maintain control. Choices include gas, mist, foam or liquid. The decision of which fluid options to choose is dictated by the conditions of the drilling system.

Figure 1. Nabors Rig 491 drilling underbalanced in Anderson, Texas, near Houston. (All photos courtesy of Nabors)
Selecting a fluid may be extremely complex. Operators must take into consideration issues such as reservoir characteristics, geophysical compatibility, well fluid disposition, temperature and environmental compatibility to name a few. Reservoirs with a high likelihood of benefitting from underbalanced techniques usually have one or more of the following characteristics:

- Reservoir pressure zone is exhausted;

- Reservoir is sensitive to invasion of foreign materials;

- Reservoir is difficult to stimulate;

- Reservoir is fractured; and

- Reservoir is predominantly composed of hard rock.

The fluid depends on where you are in the world. For example, in the Austin Chalk of Texas, UBD can be achieved with unweighted mud. Parasite strings and gas injection techniques are commonly used to reduce pressure and minimize formation damage in Canada. The hard rock in Oklahoma has lower rate of penetration, so air drilling is an option. Each case is extremely different.

High-pressure zones require lightened fluids generally consisting of aerated drilling mud or foam. The fluids are mixed with gas to achieve the desired density and a pump is used to inject the mixture in the gas stream before it enters the well. Because gas and liquid compressibility values differ significantly as pressure and temperature change, the liquid fraction changes as well. Pressure drops are controlled by maintaining a constant flow rate.

Monitoring the operation

Once UBD begins, crews are in a continued state of alertness, poised for well control situations. The compressed gas injected at the surface or produced from the reservoir must be supervised at all times. Consistent manipulation of the chokes as well as gas injection or liquid injection rates help to maintain the target underbalanced conditions of the well.

Figure 2. Nabors Drilling USA toolpusher at choke stand watching pressure gauges, fluid and flare stack.

In order to drill an underbalanced well correctly and safely, the fluctuating pressures, flow rates and temperatures must be constantly and accurately monitored. As a result, it is imperative that the drillers and crew members are expertly trained in UBD techniques to ensure absolute control over all drilling situations. Technology allows us to gather and study valuable rig data so that we are always on guard against potential problems that may arise. We have to read the signs the well gives us.

Predicting the behavior of compressed fluids is not easy. Continuous monitoring of all systems is the key. With the well in a constant state of influx, control of the annular flow is very important. This is achieved by using a rotating head to circulate the gas or mud while taking fluid from the well bore in order to control the flow at surface. The blowout preventer (BOP) system remains closed at surface and flow and cuttings are directed through pressure control equipment so flowing pressure can be regulated. Excess gas is generally flared.

When drilling underbalanced, an extra crew member experienced in this technique is brought on board who is strictly in charge of monitoring bottomhole pressure. The choke hand works alongside the driller constantly watching pressure gauges, fluid outtakes and the flare for signs of building pressure. The choke hand also listens for audible warnings, such as whistling near the BOP or rotating head.

Nabors conducts a BOP drill with each 12-hour shift change. This ensures all equipment is in proper working order and all employees understand proper procedure should a situation arise.

Reaping the benefits

Several factors make drilling underbalanced beneficial to an operation. Higher penetration rates are achieved, lost circulation is minimized, the amount of recoverable oil is increased within a shorter time frame and the chances of sticking are reduced. As a result, productivity is increased. The technique also has environmental advantages. Because of its constant

Figure 3. Excess gas from underbalanced drilling is flared out.
state of influx, formations are not blocked by drilling debris. The well flows naturally with little fluid entering the zone, eliminating chance of formation damage.

Economically, drilling underbalanced does require additional equipment. However, fluids and disposal expenses are significantly less, bit life is increased and overall costs may be lower because the drilling is faster. Drilling underpressured formations is also beneficial to geologist and engineers manipulating rig data. Gas is more easily seen on mud logs and there is less mud invasion in the formation which makes interpreting electric logs easier. It can also indicate production in formations that have not been tested due to a lack of a show while drilling.

Another benefit is the way we can manipulate with this sensitive type of drilling. We are able to reinject or produce gas while drilling, test production values, modify programs to suit situations that may arise and gather real-time data to help squeeze more production from these delicate formations.

It also has disadvantages that can prove detrimental to the outcome of the drilling process. Just as producing gas while drilling is an advantage, it can also bring higher risk. It is not always possible to maintain a continuously underbalanced condition. Because there is not a filter cake around the well bore, any instantaneous pulse of overbalance might cause severe damage to the unprotected formation.

There are easier ways to drill but, depending on the specifics of the candidate well, there may be no better option than to choose underbalanced drilling. With the right team of geologists, engineers and drillers, an operator may have a significant increase in production of the well.

Comments