Using outdated decision-making processes during the implementation of new technology can lead to improper summarization and possible failure of the new technology. Often, acceptance is slowed - sometimes halted - due to a lack of understanding of the technology and, especially, when it should be implemented.

Oil and gas drilling has evolved in the last two decades to embrace many unconventional techniques, including underbalanced and managed pressure drilling, which have increased efficiency and significantly reduced cost. These new technologies have also altered the process that petroleum engineers use to analyze a formation and apply the appropriate drilling application.

A simple analogy exists that can shed light on successful decision-making and help those considering new drilling technology obtain a clearer perspective of when (and if) the technology should be implemented.

Coming down the mountain

Imagine trying to safely navigate a car down a steep mountain road. As the speed of the vehicle increases, several options are available to avoid a collision at the bottom of the decline:

1. Coasting faster and faster until a critical speed is reached, then slamming on the brakes. This process can be repeated all the way down the mountain;
2. "Speed braking" by using a lower gear to allow the compression of the engine to help control speed. The vehicle's brakes can be slightly applied to assist in speed braking; and
3. "Feathering the brake" by lightly tapping on the brake pedal. This controls the vehicle's acceleration and allows the driver to effectively maneuver the mountain.

Conventional vs. unconventional

Conventional drilling is typically a "stop or go" situation, similar to the first option in the mountain road analogy. Methods involve a static mud column to provide downhole stability and avoid formation influx. The mud column is the primary barrier to well influx, and the blowout preventer (BOP) stack is a secondary barrier to "correct" influx. It is the function of the secondary barrier to return the primary barrier to its intended purpose.

Once well control is achieved, the BOP stack returns to its role as secondary barrier. This situation obviously precludes drilling through a closed BOP. Drilling ceases until the mud column once again becomes the primary barrier to formation influx.

In other words, conventional drilling continues down the mountain road until the well begins flowing, when the brakes are suddenly applied and drilling abruptly stops. Only after the problem is addressed does progress continue
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Managed pressure drilling

Managed pressure drilling (MPD) intends to avoid influx and drill with little hydrostatic pressure above downhole pore pressure. Akin to "speed braking" in the mountain road analogy, MPD allows the driller to use a lower gear to help control speed while covering the brake in the event of wellbore influx.
MPD can lower downhole pressure to avoid loss of circulation through a particular section of rock. The technique can also be used to provide a custom downhole drilling pressure profile to simultaneously deal with lost circulation, influx and rock collapse. In this method, the mud column density is designed to address the highest pore pressure in permeable rock that is exposed in the well bore.

Equipment includes a rotating control device (RCD), an enclosed flow line, a flow choke manifold and a mud/gas separator system. The separator removes gas from the drilling fluid and dumps cuttings and drill fluid into the rig pit system for solids removal. This allows the well to flow at a controlled rate while continuing to drill.

The primary barriers to wellbore influx include mud column density, the mud pump rate and associated friction parameters, and annular backpressure. When influx is noted at surface, pump rate can be increased. The pump rate can give an indication of the mud weight required to balance the well. The secondary barrier is the BOP stack.

Underbalanced drilling

UBD uses mud column density, downhole friction and surface equipment as primary barriers to control formation influx. In this case it is assumed that well influx will occur and the method involves controlling influx at an acceptable and safe rate. The secondary barrier (as in the previous two techniques) is the BOP stack.

The surface equipment choices are similar to those used for MPD. Similar to the mountain road analogy, UBD "feathers the brake" by adjusting downhole and surface equipment friction to control wellbore influx. The mud density may also be adjusted slightly to maintain influx within a desired range. Drilling continues as pressure at the surface is managed by adjusting operational parameters. Total depth can be achieved with a minimum of lost time.

Understanding the risks

In conventional drilling, the operator's objective is to totally avoid formation influx, which minimizes drilling progress. This "stop and go" philosophy exists because the return flow exits the well bore into an open flow line that dumps into the flow plenum of the shakers. Gas is closely monitored and the mud weight is increased upon notice of increased gas to the surface.
However, it is sometimes difficult to determine the nature of the well bore simply by monitoring the gas at the surface. Drilling can fail the rock and generate "core gas" that begins to expand within the top 500 ft (152.5 m) of the hydrostatic column. This does not automatically mean that the well is flowing, but is rather a function of rate of penetration. The gas monitoring system logs core gas as background gas (BGG).
Typically a baseline gas volume is established in dimensionless "units" of gas. With vertical drilling, an increase in pore pressure and the presence of permeable rock, BGG should increase. NOTE: Increasing pressure and the corresponding requirement for increased mud density will also result in an increase in BGG. Interpreting the increase in BGG and determining when the mud weight should be increased is often speculative and may be influenced by drilling experience, other surface parameters or a simple desire to pre-empt a well-control problem.

It is possible for BGG to increase significantly and the well not flow. Better indicators of insufficient mud density are short trip gas (STG) and connection gas (CG). If the driller notices the well flowing and an increase in connection gas upon "bottoms up" after a connection, he may elect to make a short trip and simulate tripping conditions to determine whether the mud weight should be increased. Upon returning to bottom after a short trip and before drilling further, the well should be circulated while monitoring the gas that comes off bottom. If there are doubts about the volume of short trip gas that may surface, the return flow may be routed through the rig choke manifold with the annular preventer closed.
This requires time and money, and many operators will instead choose to increase mud weight on increased BGG and connection gas. An interesting fact is that mud density increases do not impact core gas except through the effect of decreased rate of penetration. The core gas is still generated per foot of hole. (It is really a function of kH, not just H).

Ripe for a lane change

New technologies tend to show that conventional drilling should be phased out - with the exception of the most inert drilling environments - and MPD be used in any environment that suffers from lost circulation; stuck pipe; kicks; weak formations; hole collapse; fractured gas-bearing rock at any pore pressure; and deep, high-pressure gas wells.

Most operators recognize the importance of an RCD to change the decision-making process and improve safety. However, many rig operations continue to use the "stop and go" process in spite of the opportunity to lower drilling time and costs by applying MPD/UBD. On this path, drillers will still be slamming on their brakes every time a problem is encountered. The hazards of driving down steep mountain roads continue to demand a more efficient method of navigation.

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