Underbalance Drilling-Surface Control Systems
Authors: Lunan B. 
Published: March 2010

Abstract
A closed system of surface control equipment for underbalance drilling is the safest method of controlling live wells. This new technology is particularly useful in both vertical and horizontal wells where invasion of drilling fluids into virgin formations can result in severe damage and low flow rate wells. The concept is to drill into these formations underbalanced so that there is no restriction to formation flow minimal damage and ultimately higher producing wells. This is achieved by utilizing a rotating BOP or double annular blowout preventers in combination with a choke manifold sample catcher, low stage/high stage separator, flarestack and a "live" flow through control system. The "closed system" is particularly useful in drilling in Jaw pressure and fractured reservoirs. It has been utilized to drill for oil and gas in carbonates and sandstone formations.

Introduction
During the past 18 months a significant number of oil and gas companies in Western Canada have switched from conventional drilling methods to underbalanced drilling technology when the target zone is reached. The normal operation is to drill to casing point with a conventional drilling method, set casing and cement. The heavier mud system is then changed out to a light fluid system and the well drilled ahead in an underbalanced controlled operation. The closed system on surface ensures that the well is being "controlled" during the entire drilling operation. To date over 100 wells in Western Canada have been safely drilled using this technique and it is the purpose of this paper to outline the technical results of drilling these formations underbalanced, The operational aspects are discussed in detail emphasizing the importance of the well control technique. The case studies will include a gas and an oil reservoir with example wells drilled overbalanced and underbalanced.

The Closed System
The closed system for underbalance drilling is shown in Figure 1. The principle components of the system are the wellhead connection. the rigid flowline piping, choke manifold, sample catcher, separator, flarestack and return flow line to the drilling fluid tank, rig pump and the mixing T where the nitrogen is mixed in with the liquid system and the continuation to the drilling head. Experience has shown that the surface equipment can be tied into the drilling contractors blowout preventer system using the following methods:Tie into the flowspoot on the blowout preventer (BOP) stack (Figure 1) orThe rotating blowout preventer (REOP) Figure 2.The Rotating Blow Out Preventor
The rotating blowout preventer maintains surface back pressure up to 1,500 psi to allow drilling with air. gas. water or light muds. Drilling is faster, safer and less expensive in horizontal wells. The RBOP uses a hydraulically actuated kicking element supported on large roller bearings and isolated by mechanical seals Inside a large housing. The housing has a flange for mounting to the BOP stack and a return discharge flange. Hydraulic oil is used to actuate the package elements which ride against the belly or drill pipe.