Horizontal Underbalanced Drilling
Authors: Van Regen Nick 
Published: March 2010

The principles of horizontal low head (i.e.,-reduced overbalance) and underbalanced (pressure differential into the wellbore) drilling were developed in the United States and Canada. The mid to late 1980s surge in horizontal drilling encountered operational problems which were not solvable with conventional overbalanced techniques. In older, depleted zones, where considerable volumes of recoverable hydrocarbons are yet to be accessed, lost circulation and differential sticking were possible. In other zones, the hardness of the reservoir or overburden lead to low, occasionally uneconomically low, penetration rates. These problems, singly or in combination, rendered economic installation of conventionally drilled wells unlikely.
All of these drilling problems respond well to underbalanced and low head drilling techniques. The amount of drilling fluid lost to a depleted or underpressured reservoir is dependent on the difference in pressure between the wellbore and the formation: lower pressure equals lower losses. The risk of differential sticking is lessened as the differential pressure is reduced or eliminated. Higher penetration rates are achievable with minimum or negative wellbore-to-reservoir pressure differentials. Low head and underbalanced techniques have installed wellbores in formations where conventional overbalanced techniques have been unable to produce economic results.
Other collateral benefits include inflicting less damage in reservoir permeability, which may reduce or eliminate the need to stimulate, and availability of well flow information as the well is drilled.
Lower bottomhole circulating pressures are delivered via tailored-density single-phase drilling fluids, or by property-tailored two-phase fluids (i.e., membrane-generated or cryogenic nitrogen, or natural gas may be used as the density-reducing phase).
The circulating conduit to the bottom of the hole may be conventional or special-manufacture jointed pipe, or it may be coiled tubing. Tools and techniques for jointed pipe are generally off-the-shelf now, and concerted efforts are being made to develop the full potential of coiled tubing. With no connections, coiled tubing has an inherent safety advantage over jointed pipe operations.
The rollout of any new operational technique relies on the pre-existence of available technical resources suitable or adaptable for use, as well as the possibility of their successful field use. Horizontal drilling's requirements for longer-lasting and more reliable downhole power sources have produced impressive improvements in mud motor and turbine technology. The need for reliable monitoring equipment has provided a variety of Measurement While Drilling (MWD) tools and has increased reliability. Conventional MWD tools rely on the presence of an incompressible drilling fluid between MWD sensor and surface to allow transmission of the tool signal. The advent of multiphase returns from drilling wells has lead to the development of the electromagnetic MWD, which is indifferent to fluid compressibility.
Operational demands from the new technique resulted in development of active and passive rotating diverters which allow secure containment of pressure at surface, while still permitting rotation of the drillpipe.
Conventional overbalance drilling maintains hydrostatic pressure on the productive zone, and monitors returns from the system and fluid added to it. The pressure and flow data resulting from a conventional peration is used to exercise control to prevent reservoir fluids from being produced into the wellbore.