Though not as common as overbalanced drilling, underbalanced drilling is achieved when the pressure exerted on the well is less than or equal to that of the reservoir. Performed with a light-weight drilling mud that applies less pressure than formation pressure, underbalanced drilling prevents formation damage that can occur during conventional, or overbalanced drilling processes.
The negative differential pressure obtained during underbalanced drilling between the reservoir and the wellbore encourages production of formation fluids and gases. In contrast to conventional drilling, flow from the reservoir is driven into the wellbore during underbalanced drilling, rather than away from it.
Although initially more costly, underbalanced drilling, also known as managed-pressure drilling, reduces common conventional drilling problems, such as lost circulation, differential sticking, minimal drilling rates and formation damage. Additionally, underbalanced drilling extends the life of the drill bit because the drilling gases cool the bit while quickly removing cuttings.
To establish pressure control, a rotating control head with a rotating inner seal assembly is used in conjunction with the rotating table. An important factor to successful underbalanced drilling, drilling and completion operations must remain underbalanced at all times during operations. To accomplish this, pre-planning and onsite engineering are critical to the success of underbalanced drilling procedures.
Typically used for only a section of the entire drilling process, underbalanced drilling cannot be used in most shale environments.
Gases used for underbalance include air, nitrogen and natural gas. Although it is not typical, if natural gas is recovered from the well, it can be reinjected into the well to establish underbalance, resulting in the most cost-effective solution for underbalanced drilling.
Commonly used in underbalance operations, nitrogen is preferred for its somewhat low cost of generation, scale of control and minimal potential for downhole fires. While pure nitrogen can be purchased, it is cost-prohibitive. Therefore, nitrogen is more commonly produced onsite with a membrane unit, resulting in a 95% level of purity.
There are four main techniques to achieve underbalance, including using lightweight drilling fluids, gas injection down the drill pipe, gas injection through a parasite string and foam injection.
Using lightweight drilling fluids, such as fresh water, diesel and lease crude, is the simplest way to reduce wellbore pressure. A negative for this approach is that in most reservoirs the pressure in the wellbore cannot be reduced enough to achieve underbalance.
The method of injecting gas down the drillpipe involves adding air or nitrogen to the drilling fluid that is pumped directly down the drillpipe. Advantages to this technique include improved penetration, decreased amount of gas required, and that the wellbore does not have to be designed specifically for underbalanced drilling. On the other hand, disadvantages include the risk of overbalance conditions during shut-in and the requirement of rare MWD tools.
In performing the gas injection via parasite string, a second pipe is run outside of the intermediate casing. While the cost of drilling increases, as does the time it takes, this technique applies constant bottom hole pressure and requires no operational differences or unique MWD systems.
A less common underbalanced application, nitrogen foam is less damaging to reserves that exhibit water sensitivities. While the margin of safety is increased using foams, the additional nitrogen needed to generate stable foam makes this technique cost prohibitive. Additionally, there are temperature limits to using foam in underbalanced drilling, limiting using the technique to wells measuring less than 12,000 feet deep.