Comparative Study Between Overbalanced and Underbalanced Oil Drilling Methods
Chapter One
Aims and Objectives
In view of the comparative analysis of overbalance and underbalance drilling operations in the Oil and Gas industry, this project work seeks to investigate the following:
- To have an understanding of both the underbalance and overbalance drilling operations.
- To know reservoirs suitable for either underbalance or overbalance drilling operations.
- To also know the various effects and challenges associated with these operations.
CHAPTER TWO:
LITERATURE REVIEW
HISTORICAL OVERVIEW OF DRILLING IN THE OIL INDUSTRY
Drilling was developed by the Chinese more than 4000 years ago. They used a cutting head secured to bamboo rods, that were linked together to drill to depths of 3000 feet (915m). The raising and dropping of the bamboo drill string allowed it to impact and fracture the less dense rock formations. It was reported to often take two to three generations of workers to complete large wells (Treadway, C., 1997). In 1859 at Titusville, Pennsylvania, Colonel F. L. Drake completed the first oil well using a cable tool percussion-type machine. One of the earliest reports of percussion drilling technique occurred in 1949 (Harpst and Davis, 1949).
Major developments and research in drilling have been reported between the 1950s and 1960s (Wanamaker, 1951; Faihust and Lacabanne, 1956; Topanelian, 1958; Fish, 1961; Simon, 1964; Hartman, 1966; McGregor, 1967). Significant gains in understanding the percussive mechanism have been achieved in lab. Some single-well applications have been reported in oilfields for the purpose of demonstrating the effectiveness of drilling technology (Smith and Kopczynksi, 1961; Bates, 1964).
Since the 1990s, oil wells have been drilled deeper and deeper, and consequently, with increasing depth, the rocks become harder and harder. A hydraulic hammer or water hammer has been developed to accommodate these new challenges and efficient mechanical designs have been achieved (Kong et al, 1996; Giles et al, 2001; Tibbitts et al, 2002).
These designs, however, are still in pre-field stage. Throughout its history, theoretical development of drilling technology has relatively lagged behind, compared to its improvement in mechanical designs. This phenomenon is not uncommon in the drilling industry as the integrated process of rock drilling involves so many disciplines and complicated physics that modeling it rigorously faces prohibitive theoretical challenges.
The first oil wells drilled in the 1800s were drilled underbalanced. These wells drilled with insufficient fluid pressure in the annulus. Consequently, when a permeable formation was encountered, the wells flowed. The well flow was uncontrolled, thus it resulted in lost reserves. The earliest UBD patent can be traced back to the mid-1800s when a patent was issued for using compressed air to clean out cuttings from the bottom of a hole.
Advances in the industry continued through the mid-1900s. People began to understand the use of mist and multiphase fluids to control downhole fires and provide a higher tolerance to water influxes. Advances were made in understanding and modeling of air and multiphase systems. Algorithms and equations were developed to predict the amount of gas required to clean holes and bottom hole pressure resulting from circulating mixtures of fluid and gas. The growth of this technology continued into the early 1900s with the first application of multiphase fluids occurring in the 1930s.The use of multiphase fluids (either air or natural gas with water or oil) became popular in oil well drilling throughout the southern United States at this time.
During the 1970s, UBD technology was used in limited applications. However, problems with UBD techniques limited the growth of the industry. Environmental problems were the largest obstacle, particularly in gas drilling systems, where large amounts of dust were released into the atmosphere. Most wells drilled underbalanced prior to 1985 were low pressure applications with the aim of increasing rate of penetration (ROP) in non-productive zones.
THE EMERGENCE OF DRILLING TECHNIQUES
Throughout time, the petroleum industry witnessed vast advancements and evolutions and still does. The vitality of the presence of hydrocarbons (oil and natural gas) in everyday lives dictates the commitment that has to be made towards the petroleum industry. Consequently, the latter became one of the most important industries in modern day life. Drilling problems are inevitable and there is no drilling operation throughout the world that occurs without encountering problems. Such problems may be related to the equipment being used, difficulties met within the well, human errors or accidents. Naturally, any problems or accidents might have fatal as well as economical consequences. An ideal drilling operation is one where there are no problems faced, hence no additional time and cost requirements and no life threats.
Therefore, it can be deduced that drilling engineers aim to achieve optimum drilling operations with the least time and costs possible. To achieve this, researches are being conducted globally to tackle tedious problems associated with drilling. The solutions may be in the form of drilling equipment modifications and chemicals or introduction of new drilling techniques and new equipment designs.
CHAPTER THREE:
METHODOLOGY
REVIEW OF THE DIFFRENCES BETWEEN OVERBALANCE AND UNDERBALANCE DRILLING
The major difference between conventional overbalanced drilling and underbalanced drilling is the hydrostatic pressure exerted by drilling fluids in each technique. To elaborate, hydrostatic pressure of the drilling fluid is proportional to its density. When it comes to overbalanced drilling, the equivalent circulating density (ECD) of the drilling fluid is adjusted so that the pressure due to the drilling fluid column is higher than the formation pressure, resulting in what can be referred to as a “killed” state where there is “no inflow of formation fluids. It should be noted that the density of the drilling fluids is adjusted by the use of suitable additives.
On the other hand, UBD is the technique in which the circulating pressure of the drilling fluid is less than the formation pressure, such condition if applied, should be present along the entire section of the pay zone. The result is the flow of formation fluids (water, oil or gas) into the wellbore. The low pressure of the drilling fluid is attributed to its low density, which is attained by the injection of non-condensable gases into the circulating fluid, reducing its “effective hydrostatic density”.
The gases used could be Nitrogen, Air, Natural gas, processed flue gas, reduced oxygen content air. Injection of gases into drilling fluids to reduce their density is an artificial induction of the underbalanced state. Naturally occurring underbalanced state can be found in over pressured reservoirs where low density mud can be utilized; this is known as flow drilling. The technique used worldwide while drilling is known as overbalanced drilling. Which is defined as the drilling process where the hydrostatic pressure used exceeds the formation pressure.
CHAPTER FOUR:
ANALYSIS
LABORATORY SCREENING TECHNIQUES
A variety of laboratory techniques are available to quantify the effect of UBD and OBD on a given formation.
CHAPTER FIVE:
CONCLUSION AND RECOMMENDATION
CONCLUSION
The field cases selected have shown scenarios where the project was initially planned to be overbalanced but were switched to Underbalanced based on the project drivers and the well conditions. The cases have also shown situations where OBD was implemented, but reservoir performance encouraged the consideration of UBD. It is important to implement lessons learned and be flexible in the project plan to best address the situation faced. A tendency that should be avoided is to preclude one method over the other solely based on subjective considerations. UBD is often viewed as complex and more costly by the industry. It is safe to conclude that OBD cannot match UBD in terms of minimizing formation damage/improved productivity and allowing characterization of the reservoir; and this aspect needs to be considered in the technical and economic comparison of the methods before a final decision is made.
What is the best operational system to use? This question must be answered from a technical, safety and economic point of view. Technically, the system which will yield the best bottomhole pressure control with a continuously underbalanced condition is optimum with the required necessity of continuous real time bottomhole location and pressure measurements.
Which to use and how to choose? While both underbalanced and overbalanced drilling provide means of controlling downhole pressures during drilling, the methods differ significantly in how they do it. During candidate well selection, the benefits and limitations of each technique must be considered qualitatively and quantitatively to determine which should be applied.
RECOMMENDATION
When determining whether underbalanced drilling or overbalanced drilling should be applied as a solution, the benefits and limitations of each should be both qualitatively and quantitatively considered, and a decision should be reached depending on the merits of each technique. UBD addresses drilling problems, reducing NPT by minimizing losses, and differential sticking and the time associated with well control events typically associated with conventional overbalanced drilling.
Where the primary drivers are reservoir related, UBD has been found to be the best option. Reservoirs benefiting most from UBD are those formations prone to damage. Additionally, if reservoir characterization while drilling is of importance then underbalanced drilling is the option that should be selected, although UBD can be more costly than OBD due to additional equipment that may be required to achieve and maintain underbalanced conditions.
When properly designed and executed UBD provides a whole new approach to complex reservoir management problems and may facilitate the economic completion and exploitation of reserves unobtainable by any other type of currently available technology.
REFERENCES
- Ansah, J., Shayegi, S., and Ibrahim, E.: “Maximizing Reservoir Potential using Enhanced Analytical Techniques with Underbalanced Drilling,” paper SPE 90196 presented at the 2004 SPE Annual Technical Conference and Exhibition, Houston, TX, Sept. 26-29.
- Benesch, J.M. and Nor, N.“Optimization of Big-Bore Wells to Exploit a Low Pressure Reservoir in Indonesia,” paper SPE 87171 presented at the 2004 SPE/IADC Drilling Conference, Dallas, TX, Mar. 2-4.
- Bennion, D.B. and Thomas, F.B.: “Recent Investigations Into Formation Damage in Horizontal Wells During Overbalanced and Underbalanced Drilling and Completion Procedures.~ paper presented at the 1994 Annual Conference on Emerging Technology-Coiled Tubing-Horizontal Wells-Extended Reach and Multilaterals, Aberdeen, 1-3 June.
- Bennion, D.B., et al, (2000), “Using Underbalanced Drilling to Reduce Invasive Formation Damage and Improve Well Productivity—An Update”. Petroleum Society of Canada, Journal of Canadian Petroleum Technology, Vol. 39. 0021-9487.
- Bennion, D.B., et al, (2012), “Underbalanced Drilling: Praises and Perils”. [book auth.] Steve Nas and Deepak M. Gala. Getting Up to Speed : Underbalanced Drilling. Richardson Press, Michigan USA pg 22 – 27.
- Doane, R. et al.: “Successful Drilling of an Underbalanced Horizontal Well in the Rigel Halfway Pool-Laboratory Screening and Field Results,” paper SPE 31065 presented at the 1996 SPE International Conference on Horizontal Well Technology, Calgary, 18-20 November.
- Finley, D., Shayegi, S., Ansah, J., and Gil, I.: “Reservoir Knowledge and Drilling – Benefits Comparison for Underbalanced and Overbalanced Drilling Operations,”. Paper SPE/IADC 104465 presented at the 2006 SPE/IADC Indian Drilling Technology Conference and Exhibition, Mumbai, India, 16-18 October.
