Abstract

The economics of exploration and production (E&P) continue to encourage use of new methods to reduce problems and costs. Underbalanced technology is one such method, which offers effective solution to conventional drilling problems. Underbalanced drilling or UBD is a procedure used intentionally to drill oil and gas wells where the equivalent circulating density (ECD) of the drilling fluid is less than the pore pressure in the formation being drilled. Underbalanced drilling (UBD) has been used with increasing frequency to minimize problems associated with invasive formation damage, which often greatly reduce the productivity of oil and gas reservoirs, particularly in openhole horizontal well applications. UBD, when properly designed and executed, minimizes or eliminates problems associated with the invasion of particulate matter into the formation as well as a multitude of other problems such as adverse clay reactions, phase trapping, precipitation, and emulsification, which can be caused by the invasion of incompatible mud filtrates in an overbalanced condition. In many UBD operations, additional benefits are seen because of a reduction in drilling time, greater rates of penetration, increased bit life, a rapid indication of productive reservoir zones, and the potential fordynamic flow testing while drilling.

Introduction

The drilling technology has evolved over the years. The prime reasons for the advancements in this technology are the ever increasing demand for energy i.e. Oil and Gas, and the increase in 'energy price'. These reasons are actually driving the oil industry to drill the zones which were considered a 'No No' in the past due to the economic and technological constraints.

Underbalanced Drilling (UBD) is one such technology which is helping the industry in its hunger for new horizons for the exploration and production of oil and gas. Underbalanced drilling as a concept is not new to the industry; it is as old as Drilling. Actually, this technology was considered as hazardous to the health and safety of the individuals working at the site. It has evolved in terms of safety and technology in the last 100 years or so, and the industry no longer considers it to be unsafe for operations both on land and off-shore. But in order to keep this technology rosy there is a constant need to innovate and develop the existing technology that would raise the bar in terms of value added to the industry.

With this view in mind the concept of "Underbalanced Drilling" or UBD is developed and is successfully applied in onshore and offshore drilling operations. It is predicted that this technology would save the industry's millions of dollars by increasing the amount of recoverable oil within a shorter time frame. Almost every drilling operation is a potential source of damage to well productivity, lost circulation, differential sticking and other related conventional drilling problems. In UBD, the basic idea is to maintain the wellbore mud column pressure below the formation pressure, while drilling, so as to avoid formation damage, which ultimately greatly reduces productivity of oil or gas reservoirs1, 2. This would in turn lead to minimization of the stimulation requirements and hence considerable savings. UBD, when properly designed and executed, minimizes problems associated with the invasion of particulate matter into the formation as well as a multitude of other problems such as adverse clay reactions, phase trapping, etc. In many UBD operations, additional benefits are seen because of a reduced differential sticking, reduced lost circulation problems, reduction in drilling time, greater rates of penetration, increased bit life, a rapid indication of productive reservoir zones, and the potential for dynamic flow testing while drilling. However, today, it is always said that 'UBD is done for the love of reservoir'. Thus the practice of UBD is done with a due consideration towards the reservoir oriented benefits and little towards the drilling oriented benefits.

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