Underbalanced Drilling In Canada
Authors: Tudor Robin 
Published: March 2010

History
Underbalanced drilling in Canada is not new; it is an advancing technology. Originally, cable tool rigs would drill underbalanced and bail cuttings out of the hole until a producing formation was encountered. In-field drilling benefited greatly from underbalanced drilling starting in the 1950s. Air drilling was used to speed drilling rates to above the planned production zone and then the zone was drilled overbalanced to control the formation. Air caused problems with downhole fires and corrosion and in the 1960s liquid nitrogen became a choice for lightening hydrostatic columns.
During the 1950s, 60s and 70s the use of a gas in drilling fluid systems was usually done by rule-of-thumb that came from previous experience. The problem was that the compressible nature of gases made it difficult to predict the actual pressure conditions downhole; drawing down the formation too much would create blowout conditions. The development of the personal computer and algorithms for compressibility and friction was a great step to understanding underbalanced drilling.
The personal computer was not the only piece of equipment that has contributed to the current state of underbalanced drilling. rotating blowout preventers (REOPs) and purpose-built separating vessels also helped the current trends in this technology. Coiled tubing has been the most recent major development that allows a continuous closed loop system for drilling vertical and horizontal holes. Coiled tubing also allows the ability to use wireline communication to the bottom hole assembly (BHA) for real-time pressures, gamma, steering and other variables while drilling.

Underbalanced Benefits
The claims of underbalanced drilling include increased rate of penetration, reduced damage to the formation and production while drilling. Although these benefits are qualitative, much work is still required to understand the interactions of the different variables.
Drilling rates are dependent on weight on bit, degree of underbalance and formation characteristics. Coiled tubing drilling has shown the greatest success when completed in the underbalanced state; this is ostly due to the fact that the flexibility of the coil limits the weight on bit (WOB) available and the continuous nature of the rillstring. Drilling operations with underbalanced coiled tubing have seen drilling rates as much as twice that of conventional jointed overbalanced drilling.
Drilling underbalanced has traditionally been done with the aid of gases pumped into the drilling fluid. In Canada this has been predominately a non-stabilized foamed system or nitrified fluids, using the minimum of chemicals. The drilling fluid system is designed to maximize cuttings carrying and minimize formation damage and as long as the formation is kept in the underbalanced state, inflow will help with both of these conditions. Although the theory of underbalanced drilling holds that fluid only comes out of the formation, recent work has shown that "imbibation" may be a damage mechanism and that fluid-formation compatibility should be checked before operations begin.
Production while drilling (PWD) is currently a qualitative benefit from underbalanced drilling. Not only does production while drilling give an indication of the reservoir potential; it can also yield a financial income while drilling.