95
Reservoirs and Reservoir Fluids
4.6 RESERVOIR EVALUATION
It has been customary to attempt to define conditions—usually a limited list of reservoir and fluid
properties and operating conditions—under which thermal recovery operations are likely to be
commercially attractive. These conditions have been called by a variety of names, including screening criteria, screening guidelines, preferred criteria, and selection criteria. Any such set of criteria
reflects its authors’ opinions at a particular time and usually are affected by the current and local
economic climate.
Although most available guidelines attempt to include the effect of economic considerations in
the technical variables, there are few technical considerations that in themselves limit the operability of thermal recovery processes. Important technical considerations are those that affect
(1) the ability to generate heat within or inject heat into an oil-containing reservoir at efficient rates,
(2) the ability to displace the oil, and (3) the ability to recover the oil, all in a controlled manner.
It is difficult, if not impossible, to translate these broad considerations into specific values of properties or groups of properties that would limit the range of applicability of a thermal process. Some
examples will help clarify this point.
In steam drives, the gravity of the crude plays no role in the technical considerations just listed
except as it might affect plugging and, thus, the ability to maintain adequate communication
between wells in reservoirs containing relatively heavy crudes. With regard to loss in injectivity
or productivity, gravity override (or bypassing) of steam reduces the tendency of the formation to
plug. Also, hydraulic fracturing, control of the injection temperature, and cyclic steam injection
have been used successfully in thermal operations to avoid or minimize plugging. Thus, it seems
impractical to place a limit on the range of API gravity of crudes to be considered for steam drive
processes.
As another example, consider injectivity. Reduced ability to inject (or produce) fluids is a factor
affecting the economics of all enhanced recovery processes, especially those requiring the injection of heated fluids. When the injection rate is reduced, the project life must be longer and more of
the heat entering the formation is lost to the adjacent zones. This tends to reduce the oil displacement and production rates (i.e., the rate at which income is generated), which in turn reduces the
economic attractiveness. But what may be an acceptable level of injectivity in one project could be
economically disastrous in another.
Although guidelines giving parameter limits are useful in helping government bodies and oil
companies decide how to allocate funds to support their general research and development efforts,
they are of little use to an operator having a specific reservoir requiring either improved performance or special development. Each reservoir should be examined individually (at least briefly) as
though there were no guidelines, especially where the reserves are great enough to support some
engineering studies. A screening economic analysis for each likely process is the preferred method
for forming initial decisions.
The approach is to indicate some of the elements necessary to screen a potential venture, including an understanding of the possible effects of uncertainties on a parameter or group of parameters.
This approach, which requires only a few hours to perform, should allow the engineer to make preliminary decisions or recommendations as to the merits of a prospective thermal project.
Reservoir thickness is particularly important because it has a pronounced effect on the fraction
of the introduced heat that remains in the reservoir—a quantity known as the heat efficiency. For
a given reservoir (which has a fixed thickness), the only way to increase the heat efficiency is to
reduce the time t required to produce the oil from the zone between an injector and a producer. This
generally can be done by increasing the injection rate (one also must be able to recover the oil at an
increased rate) or by reducing the spacing. Each of these alternatives would cost money to implement but may be attractive compared with a slowly expanding steam front that results in low oil
production rates and a long operating life.
Reservoirs and Reservoir Fluids
4.6 RESERVOIR EVALUATION
It has been customary to attempt to define conditions—usually a limited list of reservoir and fluid
properties and operating conditions—under which thermal recovery operations are likely to be
commercially attractive. These conditions have been called by a variety of names, including screening criteria, screening guidelines, preferred criteria, and selection criteria. Any such set of criteria
reflects its authors’ opinions at a particular time and usually are affected by the current and local
economic climate.
Although most available guidelines attempt to include the effect of economic considerations in
the technical variables, there are few technical considerations that in themselves limit the operability of thermal recovery processes. Important technical considerations are those that affect
(1) the ability to generate heat within or inject heat into an oil-containing reservoir at efficient rates,
(2) the ability to displace the oil, and (3) the ability to recover the oil, all in a controlled manner.
It is difficult, if not impossible, to translate these broad considerations into specific values of properties or groups of properties that would limit the range of applicability of a thermal process. Some
examples will help clarify this point.
In steam drives, the gravity of the crude plays no role in the technical considerations just listed
except as it might affect plugging and, thus, the ability to maintain adequate communication
between wells in reservoirs containing relatively heavy crudes. With regard to loss in injectivity
or productivity, gravity override (or bypassing) of steam reduces the tendency of the formation to
plug. Also, hydraulic fracturing, control of the injection temperature, and cyclic steam injection
have been used successfully in thermal operations to avoid or minimize plugging. Thus, it seems
impractical to place a limit on the range of API gravity of crudes to be considered for steam drive
processes.
As another example, consider injectivity. Reduced ability to inject (or produce) fluids is a factor
affecting the economics of all enhanced recovery processes, especially those requiring the injection of heated fluids. When the injection rate is reduced, the project life must be longer and more of
the heat entering the formation is lost to the adjacent zones. This tends to reduce the oil displacement and production rates (i.e., the rate at which income is generated), which in turn reduces the
economic attractiveness. But what may be an acceptable level of injectivity in one project could be
economically disastrous in another.
Although guidelines giving parameter limits are useful in helping government bodies and oil
companies decide how to allocate funds to support their general research and development efforts,
they are of little use to an operator having a specific reservoir requiring either improved performance or special development. Each reservoir should be examined individually (at least briefly) as
though there were no guidelines, especially where the reserves are great enough to support some
engineering studies. A screening economic analysis for each likely process is the preferred method
for forming initial decisions.
The approach is to indicate some of the elements necessary to screen a potential venture, including an understanding of the possible effects of uncertainties on a parameter or group of parameters.
This approach, which requires only a few hours to perform, should allow the engineer to make preliminary decisions or recommendations as to the merits of a prospective thermal project.
Reservoir thickness is particularly important because it has a pronounced effect on the fraction
of the introduced heat that remains in the reservoir—a quantity known as the heat efficiency. For
a given reservoir (which has a fixed thickness), the only way to increase the heat efficiency is to
reduce the time t required to produce the oil from the zone between an injector and a producer. This
generally can be done by increasing the injection rate (one also must be able to recover the oil at an
increased rate) or by reducing the spacing. Each of these alternatives would cost money to implement but may be attractive compared with a slowly expanding steam front that results in low oil
production rates and a long operating life.