Successful underbalanced operations demand a systematic approach with integration of engineering, operations, production, reservoir, and geology.
There are significant advantages to underbalanced drilling: formation damage can be minimized if proper circulating fluids and operating parameters are chosen; there is an opportunity for reservoir evaluation and characterization while drilling; lost circulation and differential sticking can be avoided, combined with improvements in drilling rate of penetration; and it also provides an alternative method of well control.
This paper discusses engineering challenges for underbalanced drilling and completion operations, as well as advances made during underbalanced drilling with rotary jointed pipe. Much of the discussion and concepts also apply to coil tubing underbalanced operations.
Underbalanced drilling has firmly evolved into a new facet of the oil and gas industry. Operators are applying this new technology to improve ultimate reservoir recovery and maximize the economics of production. The introduction of any technology is associated with initial difficulties, uncertainties, potentially high costs, and steep learning curves. Successful underbalanced drilling depends upon a complete understanding of the reservoir and it's geology, as well as appropriate design and computer modeling of a multi-phase flow circulating system. The ability to properly monitor and execute operations plays an important role in optimization of the drilling parameters, which is dictated by actual reservoir inflow performance and well conditions. Advanced underbalanced drilling techniques have passed through the development stages, and operational inefficiencies have been minimised.
Jointed pipe underbalanced drilling is influenced by a nonsteady state system and requires specialized drilling procedures, circulation system multi-phase flow considerations, specialized equipment, and on-site personnel coordination. The effects of non-steady state, jointed pipe drilling must be properly managed and controlled to obtain the maximum benefits, while remaining cost effective. Standard operating procedures for conventional drilling are modified to accommodate underbalanced drilling with jointed pipe. For example, precautions must be taken while drilling, not only for safety of personnel, especially while handling fluids, but to avoid serious damage to the reservoir.
Conventional overbalanced drilling invades the reservoir with solids and foreign fluids, resulting in skin formation damage and reduced deliverability. To successfully remove formation damage, expensive stimulations may be required. Often, the original productivity of the formation is never regained. Formation damage is of particular concern with high angle and horizontal wells, as formations are exposed to an overbalance of drilling fluids and solids for a considerable period of time.
Improvements in productivity have been achieved using underbalanced techniques. Essentially, it is production optimisation at the drilling phase. The objective of underbalanced drilling is to achieve near-zero skin damage and offer every interval of the reservoir an opportunity for production - fluid and solid invasion is minimized or completed avoided. It should not be considered a stimulation technique. Continuous production evaluation and reservoir characterisation is possible while drilling. Through reservoir characterisation permeability and skin, production fluid types and rates, reservoir pressure, inflow performance, interval production, and optimal horizontal hole lengths can be determined. Underbalanced drilling typically has increased operating costs, from 1.3 to 2.0 times the conventional, but is countered by the value of reduced formation damage.
Underbalanced drilling not only enhances production, but it provides an alternative method for well control and evading conventional drilling problems.