Well Control Considerations for Underbalanced Drilling
Authors: Bourgoyne Adam T. 
Published: December 2007

Abstract
The number of wells being drilled underbalanced is rapidly increasing as new applications of this technology are found in many different sedimentary basins and as advancements are made in rotating control-head technology. Conventional well-control training programs do not apply and are of little benefit to this emerging technology. This paper describes the special equipment that makes underbalanced drilling possible and explains why that well control procedures can be different when using this technology. The importance of the rotating seal is discussed and wear rate data are provided for a typical sealing element. A suggested outline of special well control training is presented.

Introduction
Conventional well control practices are based on maintaining the bore-hole pressure more than the pore pressure of all formations exposed by the drill bit. An undesirable well control event, which is called a kick, occurs when the borehole pressure inadvertently is allowed to fall below the pore pressure of a permeable zone and formation fluids begin to flow into the well. A number of schools that provide training for drilling personnel in conventional well control practices are available to the industry. The conventional practices covered in these schools focus primarily on methods for avoiding kicks, methods for detecting kicks, and procedures for suspending drilling operations and removing formation fluids from the well.
Underbalanced drilling refers to the practice of intentionally drilling a well with bore-hole pressure less than the formation pore pressure. Specialized well control equipment has been developed to allow these wells to be drilled while the formations are flowing. A key element of this equipment is a rotating control-head. Rotating control-heads are now used on about 25% of the well drilled in North America. Underbalanced drilling of horizontal wells has allowed hydrocarbon-bearing formations to be developed in areas where previous conventional drilling practices were not economical. in the past, this practice has been limited to a few areas where the formations were known to have a high strength and a low permeability. Rotating control-heads are now available with working pressures more than 14,000 kPa (2000 psi). This is allowing underbalanced drilling methods to be applied to a much wider range of formation depths and pore pressures.
Field personnel have often commented to the author that well-control training programs do not prepare them for underbalanced drilling operations. A comment that has been often heard is "We take what you teach in your well-control school and throw it out the window." To better understand this comment, compare the following well control practices generally taught in well-control schools to the practices followed in underbalanced drilling operations. During conventional drilling operations, the field personnel are taught the following rules.
Well-Control Practices during Conventional Drilling:–Attempt to avoid kicks by keeping the drilling fluid density high enough to prevent flow of formation fluids into the well.–Stop drilling when formation fluids flow into the well.–Stop drilling when there is a loss of drilling fluid to the well.–Do not leave bottom with the well flowing.–Keep the bottom-hole pressure constant and slightly above the formation pore pressure.–Do not maintain the pit level constant when circulating the well under pressure.
P. 207^