The Oil and Gas Industry drills thousands of wells globally each year, but what makes a well "successful" depends to a large extent on the perspective of the assessor. To the drilling engineer, a successful well is one that is safely drilled to target depth on time and under budget. The production engineer would add the need for there to be no, or minimum, formation damage and that the well achieve its maximum production potential. Unfortunately, these two aspects are rarely obtained within the same wellbore, due to the problems associated with the managing Equivalent Circulating Density (ECD) and the resulting fluid invasion, while drilling in the reservoir.
The management of ECD is "traditionally" achieved by drilling with mud, suitably weighted, to drill within a pressure margin until a casing point is mandated. This method has both benefits and detriments. The mud column provides primary well control in a conventionally drilled well, while the BOP equipment is secondary well control. This is the standard approach in the drilling industry, as mandated by regulatory agencies. However, it prevents approximately 20–30% of the known world's offshore oil and gas resources from being developed, due to excessive ECD when drilling. The tools and technology associated with underbalanced drilling (UBD) have added a beneficial dimension to ECD management. UBD tools and techniques are key to several emerging offshore technologies such as: drilling with subsea and surface BOP stacks with pressurized mud return systems, the concept of Dual Gradient Drilling, and using subsea rotating diverter control heads to control shallow water flows when drilling riserless. Underbalance drilling moves the primary well control from the mud column's hydrostatic pressure to the surface equipment where it is more easily managed and predictable.
This paper reviews key situations that are typically encountered when drilling offshore and make comparisons between "conventional" methods and UBD technology. The latest technology is reviewed to illustrate how drilling UB offshore can be safe and efficient to achieve a "successful" well from multiple perspectives. Such capability is now viewed by most majors as a requirement in many offshore situations to deal with depleted zones/reservoirs relative to water depth, narrow margins between fracture gradient and pore pressure, and for dealing with abnormally pressured aquifers (shallow water flow hazards). To date, the UBD applications used offshore have achieved an ECD nearer to reservoir pressure, thus minimizing skin damage from drilling fluid invasion.
Offshore Background
In North America, offshore drilling first began in California in 1887, when H. L. Williams built a wharf 300' into the Pacific Ocean and drilled, using cable tools, to find a better producing well than its onshore counterpart. In the Gulf of Mexico, Kerr-McGee Corporation drilled the first well from a fixed platform offshore (out-of-sight of land) in 1947. Its barge and platform combination was a major breakthrough in drilling-unit design for offshore use. This event marked the beginning of the modern offshore industry. The initial drilling concepts were extensions of concepts used on land. Today, the Gulf of Mexico is mature with hundreds of fields and platforms in place to produce and process the oil and gas. In other areas around the world, offshore development is maturing such as the North Sea and in other areas just beginning, such as in deepwater (West Africa, Brazil, Gulf of Mexico, Trinidad, the Faroe Islands). Oil and gas is being discovered offshore in areas once considered too costly or improbable.
