Underbalanced Drilling (UBD) is an alternative method of drilling where the wellbore pressure is maintained lower than the formation pressure while drilling. The primary advantage of Underbalanced Drilling is to reduce formation damage in reservoirs where overbalanced drilling would reduce production due to skin damage. This damage is caused by a number of factors including solids invasion, phase trapping, clay swelling, and emulsification. As a result, a correctly applied Underbalanced Drilling project can provide an increased net present value and increase the amount of economically recoverable reserves.
The use of lightweight drilling fluids and/or nitrogen gas to maintain the bottom hole circulating pressure below formation pressure permits hydrocarbons to flow while drilling. Underbalanced Drilling equipment is designed to manage gas/fluid production from the reservoir and guide excess gas through pressure control equipment at the surface, encouraging hydrocarbon flow to the surface, in a controlled and safe environment.
When correctly applied, Underbalanced Drilling can significantly reduce reservoir damage associated with invasion of solids into the formation. Since mature reservoirs typically have lower pressures, this becomes even more valuable as a reservoir matures. This low pressure makes the reservoir more susceptible to damage from overbalanced drilling and completion technologies due to fluid and solids invasion. Fluid invasion can cause damage to the permeability and inhibit recovery of hydrocarbons.
In addition to mature fields there is an advantage to applying Underbalanced Drilling in a field's early development as well. During underbalanced operations, production from the reservoir can be monitored real-time to accurately identify inflow mechanisms and pay intervals. With this information the reservoir can be characterized while drilling, allowing better placement of the horizontal well path.
While the main advantage of Underbalanced Drilling is from the reduction of reservoir damage, there are also additional benefits to the drilling operation. These include increased rates of penetration, reduction of lost circulation, and differential sticking. Differential sticking is probably the greatest drilling problem worldwide because of the time and money it takes to correct the problem. The nature of Underbalanced Drilling fluids will not create a filter cake on the wellbore, therefore greatly reducing the possibility of differential sticking, and ultimately reducing NPT.
Drilling muds will slow penetration rates due to a chip hold down effect. The degree to which drilling is slowed is a function of the differential pressure, P wellbore - P formation. If this differential is positive, drilling is slowed by inhibiting dislodgment of the chip and by acting as a confining pressure and strengthening the rock. If this differential is negative, as in Underbalanced Drilling, the dislodgement of the chip is encouraged thus increasing the rate of penetration, lengthening bit life and reducing time on the drilling project.
Getting the Best Results
To achieve success, Underbalanced technology must be applied to a suitable reservoir and then executed according to best practices. In the past poor candidate selection, inaccurate estimations of benefits, and sub optimal operations execution resulted in mixed success for Underbalanced Drilling projects.
One of the most important phases in the Underbalanced Drilling operation is the planning phase. The candidate reservoir should be evaluated for its suitability for underbalanced drilling prior to undertaking an Underbalanced Drilling project. It is also imperative to ensure the proper operational engineering steps are taken in the planning phase to ensure the correct techniques and equipment are selected in order to maintain a constant underbalanced state and ensure operations are performed in a controlled and safe environment.
Conventional drilling, a.k.a. drilling overbalanced, can be very invasive on the formation. The drilling "mud" must be heavier than the formation pressure in order to deter kicks and blowouts caused from hydrocarbon pockets that may be encountered while drilling. Having heavy mud circulate through the well bore controls the entry of hydrocarbons, removes cuttings, and lubricates and cools the bit while drilling. The mud allows for higher pressure to be exerted on the formation, however, if a weak spots exist, the drilling mud may then enter the formation causing loss of circulation and in turn, mud loss increasing the expense of the well.
Underbalanced Drilling not only minimizes the risk of formation damage, but also enables you to overcome drilling problems and provides the unique benefit of immediate reservoir characterization. Using Underbalanced Drilling technology allows you to enhance your ultimate recovery by locating 'sweet spots' in mature fields, performing well testing while drilling, and optimizing the intersection hydrocarbon bearing fractures.