Drilling Directional Wells Underbalanced
The main justifications for drilling a directional well underbalanced are to (1) reduce drilling costs, and/or (2) improve productivity. The combination of horizontal/multilateral and UBD technology is a relatively new one, and which has been explored more in international fields (especially the Middle East), than in the United States.
Favorable conditions for UBD include:
- depleted reservoir pressure
- reservoirs sensitive to invasion of foreign materials
- hard rock formations
- fractured reservoirs
- hard-to-stimulate reservoirs
The main issues involved in underbalanced directional drilling are similar to those for conventional directional drilling, namely directional control, surveying, hole cleaning and drill string friction. Specific problems associated with underbalanced directional drilling include:
- Shorter life and lower efficiency for conventional downhole drilling motors.
- Inability of conventional MWD systems to work with compressible fluids. Specifically, there is need for a directional drilling system and service including steerable motors, and providing EM telemetry of survey and steering information, for use on all underbalanced drilling fluids including aerated fluids, foam, and air/mist.
- Hole cleaning problems when inclination angles exceed 50 degrees.
- Increased drag, which may require reducing the length of the horizontal section.
- Unsuitability of UBD methods in certain formations and lithologies.
Pressure drops in UBD operations are controlled by maintaining a constant flow rate. For gas reservoirs, horizontal wells drilled with UBD have been very successful, especially when the wells are drilled with coiled tubing, which allows for constant pressure maintenance (and thus a constant flow rate), and eliminates the pressure fluctuations that arise at the joints of a multi-cased well.
The use of compressible drilling fluids is a complicating factor directional and horizontal drilling in part because conventional MWD signals cannot be transmitted through compressible fluids. Steering the well trajectory thus becomes an issue. There are steering tools available, but these have a number of shortcomings when compared to MWD; as a result, Electromagnetic MWD (EMWD) tools have been developed for UBD applications.
There are limitations with respect to the types of lithologies that can be drilled underbalanced. Young, poorly consolidated rocks, for example, are usually not good candidates for air or foam drilling. And depending on the lithology and downhole stress environment, directional wells sometimes must be drilled overbalanced to prevent wellbore collapse.
