What is Underbalanced Drilling?
Underbalanced drilling is a drilling method which reduces the hydrostatic pressure of the drilling fluid column so that the wellbore pressure is less than the pressure of the formation. The use of underbalanced drilling methods is improving production rates and the ultimate recovery of oil and gas reserves in many of the areas where hydrocarbon reservoirs are being found today, including North America, the North Sea, the Middle East, and Indonesia.
The potential exists for underbalanced drilling to improve field economics by increasing production rates and improving access to oil and gas reserves. Underbalanced drilling is considered one of the more exciting recent developments in drilling technology. Initially used to exploit lower quality reserves, underbalanced drilling has also proven to be an economical and viable method for drilling in low pressure and depleted or clay-rich reservoirs. As opposed to conventional drilling, hydrocarbon flow rates can be recorded during the process making it easier for operators to accurately identify pay intervals and stop drilling operations as soon as target zones are penetrated, thereby minimizing formation damage caused by exposure to drilling fluids.
Underbalanced versus Conventional Drilling
Conventional drilling requires the hydrostatic pressure of the drilling fluid to remain above the formation's pore pressure and below its fracture pressure. The drilling fluid circulates continuously in the wellbore to control the formation fluids and bring the cuttings to the surface while helping to stabilize the wellbore and lubricate the drill bit. Problems with conventional drilling methods include lost circulation, differential sticking, low drilling rates and formation damage. Underbalanced drilling can reduce these problems. Even though it is initially more costly, the rate of return from underbalanced wells can be significantly higher.
Benefits of Underbalanced Drilling
Underbalanced drilling can reduce lost circulation, minimize differential sticking, and minimize formation damage thereby creating higher productivity completions. Early detection and testing of productive intervals while drilling are also possible added benefits. Underbalanced drilling can help in detecting potential hydrocarbon zones, especially zones that could have been bypassed with conventional drilling methods. With decreased pressure at the bit, underbalanced drilling operations have demonstrated superior penetration rates. Underbalanced drilling operations often increase bit life and reduce drilling times. Since conventional drilling fluids are not used, there is a reduced environmental impact and less potentially hazardous drilling fluids for disposal.
Limitations of Underbalanced Drilling
Underbalanced drilling is not appropriate for most shales, salt, shattered coal sections, unconsolidated sections and chalk sections. In these cases, it may be more appropriate to drill conventionally, case the wellbore and then switch to underbalanced drilling to penetrate the target formation. When drilling underbalanced, it is important to keep the well underbalanced at all times to minimize formation damage. Underbalanced drilling is still an expensive technique and depending on the drilling fluid used, the cost can be significant, particularly for extended reach horizontal wells.
Techniques of Underbalanced Drilling
There are four techniques currently available to create the appropriate underbalanced conditions while drilling: 1) lightweight drilling fluids 2) gas injection through the drilling column 3) gas injection into a parasite string 4) foam. Formation knowledge will dictate which technique is used.
Underbalanced Drilling With Gas
Gases used for underbalanced drilling include air, nitrogen, CO2 or natural gas. When natural gas is available, it can be the most cost-effective method to achieve an underbalanced condition because it can be recovered and re-injected into the supply/sales line, but it is not always an option. Air injection drilling is common, but significant corrosion problems and the potential for a downhole fire or explosion are deterring factors. In general, there is desire to minimize CO2 emissions. Therefore, nitrogen has often become the medium of choice.
Nitrogen is an appealing medium because of its relatively low generation cost, scale control, and low potential for downhole fire or explosion. Injecting nitrogen through the drill pipe can be most cost effective when electromagnetic measurement-while-drilling (MWD) is used. Despite the added cost and time, parasite injection of nitrogen is the preferred method when electromagnetic MWD is not possible. Foams are more stable than aerated systems, but they are more costly.
History of Underbalanced Drilling
Drilling with low pressure circulating fluid was first patented in the United States in 1866. Its early applications used compressed air to drill the hole. The technique called flow drilling was first developed in South Texas in the late 1950's and 1960's, and became very popular worldwide with early successful applications in Canada, Australia and China. It was primarily used for re-development of gas depleted or low pressure fields. As equipment and techniques became more refined, the use of underbalanced drilling expanded to the development of tight formations in the late 1980's. During the 1990's, underbalanced drilling was successfully applied in onshore and offshore drilling operations throughout the US, Canada and Europe. Underbalanced drilling is quickly being recognized as one of the top drilling technologies today.