Underbalanced drilling (UBD) has gained strong momentum in recent years because of a number of advantages of the technology including minimized lost circulation and reduced formation damage. Bottom hole pressure, which is a key factor controlling borehole integrity in UBD, is very sensitive to the combination of liquid and gas injection rates. Due to the complex nature of water, oil, gas and solid multiphase phase flow in the UBD systems, numerous runs of sophisticated computer programs are required in UBD design.

An innovative procedure to delineate the boundary of the safe liquid-gas rate envelope for UBD flow rates is presented in this paper. Formation fluid pressure limits the upper bound of the flowing bottom hole pressure, and borehole collapse pressure serves the lower bound of the circulation-break bottom hole pressure. Fluid's cutting carrying capacity and borehole washout criteria close the envelope. A detailed procedure for developing the safe liquid-gas rate envelope using a spreadsheet program is described in the paper. A successful field application case is presented. This work provides drilling engineers an easy-to-use approach to designing and modifying liquid and gas injection rates in UBD.


Drilling operations where the drilling fluid pressures in the borehole are intentionally maintained to be less than the pore pressure in the formation rock in the open-hole section is called Underbalanced drilling (UBD). The low borehole pressures are achieved by using lightened drilling fluids. The light fluids used in UBD are usually air, gas, foam, and aerated water. However, un-aerated oil, water, even weighted mud can be used for UBD inareas where formation pore pressure gradients are higher than hydrostatic pressure gradient of water.

UBD provides benefits of increasing penetration rate, minimizing lost circulation, increasing bit life, minimizing differential sticking, improving formation evaluation, and reducing formation damage (reducing stimulation requirements). The disadvantages of UBD include personnel and equipment safety issues, handling of produced formation fluids, and wellbore damages (washout, collapse, and cuttings accumulation in the borehole).

Optimized UBD designs are the key to the successfulness of UBD operations. Sever wellbore damages and failures can result from poor UBD designs and/or deviation of the actual drilling programs from the original designs. The combination of mud flow rate and gas injection rate plays a very import role in preventing failure of the UBD. If the combination is chosen such that it gives too high bottom hole pressures, the degree of underbalance is reduced and the benefit of the UBD will be marginal. In the other hand, if the combination is chosen such that it gives too low bottom hole pressures, the UBD operation may fail due to wellbore damage problems.

Computer simulators have been used in drilling industry to design liquid and gas injection rate combinations for UBD. Both steady sate and transient simulators are available.1–5 But the procedure is tedious and not transparent. It is difficult to make an optimum UBD design that balances all the aspects. A graphical method with methodology transparency is highly desirable for drilling engineers who are in charge of UBD designs and UBD field supervisions.

This content is only available via PDF.
You can access this article if you purchase or spend a download.