Underbalanced drilling (UBD) is defined as ‘‘deliberately drilling into a formation in which the formation pressure, or pore pressure, is greater than the pressure exerted by the annular fluid or gas column’’ (IBD HSE Forum, IADC 2002). In this respect, ‘‘balanced’’ pressure drilling is a subcategory of underbalanced drilling because the annular pressure may fall below the formation pressure during pipe movement. Underbalanced drilling is used to avoid or limit lost circulation and as a method to protect reservoirs, prevent differential sticking, and increase the drilling rate.
Drilling with a hydrostatic pressure that is less than formation pressure may create a condition similar to a ‘‘kick,’’ or well-control condition. Controlling these pressures and maintaining a safe environment require special surface pressure control equipment and a properly trained crew (Figure 1). The type of equipment required depends on primarily the lithology, permeability, and pressure of the formations that are to be drilled.
UBD systems require surface handling equipment similar to that used on wells that are drilled overbalanced, in addition to specialized equipment. Regardless of the relationship between fluid and formation pressures, the drilling fluid must remove the cuttings from the bottom of the hole and carry them to the surface. At the surface, there must be adequate equipment to remove the solids from the fluid before it is circulated back down the drill string. In cases where the fluid is not recirculated, such as air drilling and some foam drilling, provisions must be made for handling the solids and liquids.
Solids control in UBD does not get as much attention as does solids control in overbalanced drilling. There are a number of reasons for the lack of attention. Probably the major reason is the attitude that solids control is not necessary when well-bore pressure exceeds hydrostatic pressure. This is not even close to the truth. Reservoir damage from high solids content can occur because the annulus is not underbalanced at all times. Also, excess solids decrease efficiency just as they do in overbalanced drilling. All of the existing solids-control equipment can be used in underbalanced drilling, but the system should be redesigned for each particular set of conditions.
The purpose of this chapter is to provide information regarding how well-bore solids are collected and sent to the solids-control equipment in all types of UBD. Some techniques have been developed specifically for UBD, but most are the same techniques that are used in overbalanced drilling. A description of each type of UBD will be given, followed by techniques used for proper solids control.
Underbalanced Drilling Methods
Underbalanced drilling may be conducted using any type of fluid, provided that the hydrostatic column of that fluid is less than formation or pore pressures. The wide variety of ‘‘fluids’’ include:
- Dry air, or air supplied by one or more air compressors.
- Natural gas, or naturally occurring hydrocarbon (primarily methane) gas or gases.
- Mist, created by injecting water into a gas or air stream.
- Foam, created by adding a foaming agent (surface active agent) to injected water.
- Gaseated water or gaseated oil.
- Nitrogen gas, or N2 gas supplied either as bulk cryogenic liquid or from a filter (membrane) unit.
- Stiff foam, created by adding a foaming agent to a specially prepared drilling fluid injected into the air stream.
- Drilling fluid, of any type, that creates a hydrostatic pressure less than the pore pressure.