Introduction to Underbalanced drilling

Definition of Underbalanced Drilling

The drilling process in which the wellbore pressure is intentionally engineered (i.e., a low “weight” drilling fluid or a gasified drilling fluid) to be less than that of the target reservoir, therefore allowing for the continuous flow of formation fluids into the well. It is important to note that underbalanced drilling is essentially a drilling process whereby the flow of fluids to surface is carefully monitored and controlled, and that it is not a situation in which the well is out of control (i.e., gas kick or blowout).

Definition of “Low Head” Drilling

The drilling process in which the wellbore pressure is designed to be slightly in excess of the reservoir pressure to allow for only a minimum of “overpressure”, while partially capturing the benefits of “true” underbalanced drilling.

Depending upon specific well conditions and/or client’s objectives, the application of low head drilling rather than absolute underbalanced drilling may be selected. For example, one of Northland’s clients was unsure as to the potential outcome of underbalanced drilling, and accordingly chose to apply the process of low head drilling first prior to employing underbalanced drilling. In this manner, they were able to establish a basic comfort level, and thus become familiar with the fundamental concept before fully utilizing the technology of underbalanced drilling.

Potential Advantages of Underbalanced/Low Head Drilling

• Reduction of formation damage caused by fluid infiltration throughout normal drilling processes;
• Reduced cost and/or elimination of primary stimulation processes (i.e., acidizing, fracturing);
• Reduction/elimination of expensive drilling fluid programs;
• Reduction/elimination of experiencing lost circulation;
• Elimination of drill string differential sticking;
• Increased productivity;
• Increased drilling penetration rates; and/or
• Increased bit life.

Potential Disadvantages of Underbalanced/Low Head Drilling

•  Additional engineering and operational complexity;
• Possibility of sloughing shales;
• Increased operational risks (i.e., higher surface pressures and continually flowing well while
• drilling);
• New methods required for drilling cuttings disposal;
• Specialized equipment required (e.g., surface control system, rotating blow out preventer); and/or
• Possibility of higher operational costs.

Types of Drilling Fluids Used in Underbalanced/Low Head Drilling

• Water-based muds (must be compatible with target formation);
• Oil-based muds;
• Fresh water;
• Produced water;
• Brine solutions;
• Crude oil; or
• Diesel fuel.

Various Media Used in “Gasifying” Drilling Fluid While Underbalanced/Low Head Drilling

• Natural gas (typically methane);
• Nitrogen (both bulk and air-generated); or
• Air.

If the drilling fluid needs to be gasified in order to induce an underbalanced/low head state, nitrogen is typically used. Nitrogen may be generated on-site by employing semi-permeable membranes or by the expansion of bulk liquid nitrogen into a gaseous phase. On-site generated nitrogen, while less expensive than bulk nitrogen, does introduce some oxygen (allowable amount as determined by various studies is about 5%) into the wellbore thereby potentially creating corrosion and downhole fire/explosion problems, as well as introducing oxygen into the surface control separator creating some risk of explosion.

By contrast, bulk nitrogen obviously does not share the same problem of potentially high oxygen content as does on-site generated nitrogen. However, the availability of bulk nitrogen in a given area may be limited. In addition, the cost of bulk nitrogen can be significantly higher than on-site generated nitrogen.

Natural gas may also be employed in creating an underbalanced or low head state, but this process would be entirely dependent on availability of such a gas and accompanying compression equipment at or near the drill site.

While air drilling was popular in the 1960’s, and is still employed in some areas around the world, it is not recommended while using Northland’s surface control system. The primary reason for not using air is the potential explosion hazard created as a result of its high oxygen content.

Use of Jointed Pipe vs. Coiled Tubing While Underbalanced/Low Head Drilling

While drilling in an underbalanced or low head mode, either jointed pipe (conventional drill strings) or coiled tubing may be used. Following are some relative advantages and disadvantages of each: