- Title : Underbalanced Drilling Manual [ pdf ]
- Publish : Gas Research Institute Chicago, Illinois
- Type Document : pdf
- Release : December 1997
- Total Page : 566 Page
- Size : 4.45 Mb
Download Free by Netload : [ http://adf.ly/XhTAi ]
Decrypted Contents
Liquid Drilling Fluids
The formation pore fluid pressure often exceeds the hydrostatic pressure of fresh or saline water at the same depth. In this environment, it is possible to drill under-balanced using a liquid. It is not uncommon for conventional drilling oper-ations to become underbalanced (un-intentionally) if the wellbore penetrates a region of higher than anticipated pore pressure. In certain circumstances it is possible to achieve underbalanced conditions even though the drilling fluid has a density exceeding the pore pressure gradient. For example, loss of drilling fluid into a low pressure zone can reduce the wellbore pressure, allowing formation fluids to flow into the well from higher up the hole. The inflowing fluids then reduce the drilling fluid density until circulation is regained and a mixture of drilling and formation fluids flows to the surface. This is the case in the Pearsall Field in Texas, which has seen one of the most extensive and successful recent applications of underbalanced drilling in the United States.14 Surface Systems
Probably the key distinction between underbalanced and conventional drilling operations is that additional surface equipment is required if a well is to be drilled underbalanced. This equipment essentially diverts all return flow away from the rig floor and separates produced hydrocarbons from the drilling fluid in a way that allows them to be contained. In this way, underbalanced drilling can continue safely once a permeable formation is penetrated. The complexity of the surface system is influenced by the choice of drilling fluid and the nature and quantity of formation fluids produced while drilling. In the case of dry air drilling, with natural gas as the only potential inflow and no potential for hydrogen sulfide, it is often sufficient to have the blooie line discharge flared over an open, earthen pit in which the cuttings collect. At the other extreme, a closed, multi-phase separator, used with a nitrified water drilling fluid, has to handle cuttings, produced oil, produced gas, circulating water, and nitrogen. Such systems allow oil to be collected for storage, gas to be flared, and water to be re-cycled to the rig pumps.
Broadly, it is possible to characterize the separation systems as open or closed, depending on whether or not the separation vessels themselves are open to the atmosphere or sealed. Closed separators are not normally used with drilling fluids containing air, in order to minimize any explosion hazard when hydrocarbons are encountered. Conversely, a closed system should be used if hydrogen sulfide may be present in the produced fluids. Specific requirements for various drilling fluids will be discussed in more detail in the relevant sections of Chapter 2. In many instances, surface equipment incorporates an adjustable choke in the drilling fluid return line, between the diverter and the separation system. Back pressure on the well provides some degree of control over the wellbore pressure, independently from the drilling fluid density and rheology. If this is to be done, a rotating seal element in the stack is normally required, to provide sufficient pressure bearing capacity to seal the back pressure generated by the choke.
This technique provides the flexibility in controlling wellbore pressure that can be particularly important when drilling through poorly consolidated or very productive formations, where it may be necessary to restrict the underbalance pressure (differential) to a few hundred psi. In air or mist drilling, if back pressure is increased, annular velocities are reduced and hole cleaning may be jeopardized. Applying a back pressure can also help to control changes in the liquid volume fraction with depth. This may be required if a foam is to be maintained throughout the annulus.15
The formation pore fluid pressure often exceeds the hydrostatic pressure of fresh or saline water at the same depth. In this environment, it is possible to drill under-balanced using a liquid. It is not uncommon for conventional drilling oper-ations to become underbalanced (un-intentionally) if the wellbore penetrates a region of higher than anticipated pore pressure. In certain circumstances it is possible to achieve underbalanced conditions even though the drilling fluid has a density exceeding the pore pressure gradient. For example, loss of drilling fluid into a low pressure zone can reduce the wellbore pressure, allowing formation fluids to flow into the well from higher up the hole. The inflowing fluids then reduce the drilling fluid density until circulation is regained and a mixture of drilling and formation fluids flows to the surface. This is the case in the Pearsall Field in Texas, which has seen one of the most extensive and successful recent applications of underbalanced drilling in the United States.14 Surface Systems
Probably the key distinction between underbalanced and conventional drilling operations is that additional surface equipment is required if a well is to be drilled underbalanced. This equipment essentially diverts all return flow away from the rig floor and separates produced hydrocarbons from the drilling fluid in a way that allows them to be contained. In this way, underbalanced drilling can continue safely once a permeable formation is penetrated. The complexity of the surface system is influenced by the choice of drilling fluid and the nature and quantity of formation fluids produced while drilling. In the case of dry air drilling, with natural gas as the only potential inflow and no potential for hydrogen sulfide, it is often sufficient to have the blooie line discharge flared over an open, earthen pit in which the cuttings collect. At the other extreme, a closed, multi-phase separator, used with a nitrified water drilling fluid, has to handle cuttings, produced oil, produced gas, circulating water, and nitrogen. Such systems allow oil to be collected for storage, gas to be flared, and water to be re-cycled to the rig pumps.
Broadly, it is possible to characterize the separation systems as open or closed, depending on whether or not the separation vessels themselves are open to the atmosphere or sealed. Closed separators are not normally used with drilling fluids containing air, in order to minimize any explosion hazard when hydrocarbons are encountered. Conversely, a closed system should be used if hydrogen sulfide may be present in the produced fluids. Specific requirements for various drilling fluids will be discussed in more detail in the relevant sections of Chapter 2. In many instances, surface equipment incorporates an adjustable choke in the drilling fluid return line, between the diverter and the separation system. Back pressure on the well provides some degree of control over the wellbore pressure, independently from the drilling fluid density and rheology. If this is to be done, a rotating seal element in the stack is normally required, to provide sufficient pressure bearing capacity to seal the back pressure generated by the choke.
This technique provides the flexibility in controlling wellbore pressure that can be particularly important when drilling through poorly consolidated or very productive formations, where it may be necessary to restrict the underbalance pressure (differential) to a few hundred psi. In air or mist drilling, if back pressure is increased, annular velocities are reduced and hole cleaning may be jeopardized. Applying a back pressure can also help to control changes in the liquid volume fraction with depth. This may be required if a foam is to be maintained throughout the annulus.15
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