bioclastic and reef limestones are largely a result of the
palaeo environment, which determined the distribution
of fauna.
Fractures are generally much more important in
carbonate than in sandstone reservoirs. Stylolites are
sometimes found in sandstone reservoirs that have
been buried to at least 3.4–4 km. In limestones
stylolites develop at much shallower depths, often
forming a thin clay surface that is quite impermeable,
and which may divide a reservoir up into compartments or even serve as a cap rock.
In fractured reservoirs there is a high percentage of
porosity due to fractures rather than more uniformly
distributed porosity. Fractures are most extensively
developed in brittle rocks like well-cemented
limestones and chert. They may also form in metamorphic and igneous rocks. Fractures are often invaded by
drilling mud, causing lost circulation, and this may
damage the reservoir near the well. The flow of petroleum through a fractured reservoir during production
is very complex, and it is difficult to map out the
fracture system of the subsurface in sufficient detail.
As a result of reduced pressure during production,
fractures may start to close due to increased net stress.
By injecting water at very high pressures (higher than
the geostatic pressure) fractures may be widened
(hydrofracturing), a method also sometimes used in
other reservoirs to create or widen fractures close to
wells and improve the permeability of the critical area
around the well. The introduction of some coarsegrained material like sand and also other types of
granular materials may help to wedge the fractures
and prevent them from closing.
Onshore oil fields have much closer well spacing
than offshore fields because of the lower drilling costs.
This means that many of the enhanced recovery
methods can be used much more effectively. At high
oil prices mature oil fields can have a very long tail
production and be economical even with very high
water production (>90% water).
21.13 Monitoring Production
During production, comparison of pressure changes
in the production and injection wells will provide
information about fluid communication within the reservoir. These data are fed into vary large and complex
reservoir simulation models.
4D seismic may be successful in some fields for
detecting changes in the gas/oil contact (GOC) and in
some instances also the oil/water contact (OWC).
Repeated seismic surveys at 1–3 years intervals may
detect an upward movement of the OWC as the reservoir is depleted. The difference in impedance may,
however, not be as pronounced at the primary OWC
because there may be 30–40% oil saturation in the
drained intervals, It may then be possible to detect
parts of the reservoir which are not drained and
which are large enough to justify an additional well.
21.14 Well-to-Well Tracers
Tracers can be added to the water in the injection well
and their arrival in the production well can provide
important information about the reservoir. If the tracer
is recorded in the production well relatively soon after
the injection there must be a high permeability connection in the form of an open fault or fracture or a
high permeability sand layer. The time of the arrival of
the tracers makes it possible to calculate the flow
velocity of the water phase. The tracers must remain
dissolved in the water and should not be adsorbed on
the minerals. Since they become part of the produced
water they should be environmentally acceptable.
Many of the tracers are radioactive but only with
β radiation, such as tritiated water (HTO),
36 Cl
À and
22 Na
þ . Most of them have a relatively short half life,
except
36 Cl
À .
It is also possible to use non-radiocative tracers, but
extremely low levels must be detectable (in the ppb
range) because of the strong dilution. Gas tracers may
also be used.
The concentration of the tracer as a function of time
during production may provide very important information about the rock properties and the flow in the
reservoirs. Different types of tracers may to different
degrees interact and be adsorbed onto the reservoir
rocks.
21.15 Reservoir Models and Field
Analogues
When planning the production of a reservoir, a relatively detailed reservoir model is required.
21 Production Geology
557
palaeo environment, which determined the distribution
of fauna.
Fractures are generally much more important in
carbonate than in sandstone reservoirs. Stylolites are
sometimes found in sandstone reservoirs that have
been buried to at least 3.4–4 km. In limestones
stylolites develop at much shallower depths, often
forming a thin clay surface that is quite impermeable,
and which may divide a reservoir up into compartments or even serve as a cap rock.
In fractured reservoirs there is a high percentage of
porosity due to fractures rather than more uniformly
distributed porosity. Fractures are most extensively
developed in brittle rocks like well-cemented
limestones and chert. They may also form in metamorphic and igneous rocks. Fractures are often invaded by
drilling mud, causing lost circulation, and this may
damage the reservoir near the well. The flow of petroleum through a fractured reservoir during production
is very complex, and it is difficult to map out the
fracture system of the subsurface in sufficient detail.
As a result of reduced pressure during production,
fractures may start to close due to increased net stress.
By injecting water at very high pressures (higher than
the geostatic pressure) fractures may be widened
(hydrofracturing), a method also sometimes used in
other reservoirs to create or widen fractures close to
wells and improve the permeability of the critical area
around the well. The introduction of some coarsegrained material like sand and also other types of
granular materials may help to wedge the fractures
and prevent them from closing.
Onshore oil fields have much closer well spacing
than offshore fields because of the lower drilling costs.
This means that many of the enhanced recovery
methods can be used much more effectively. At high
oil prices mature oil fields can have a very long tail
production and be economical even with very high
water production (>90% water).
21.13 Monitoring Production
During production, comparison of pressure changes
in the production and injection wells will provide
information about fluid communication within the reservoir. These data are fed into vary large and complex
reservoir simulation models.
4D seismic may be successful in some fields for
detecting changes in the gas/oil contact (GOC) and in
some instances also the oil/water contact (OWC).
Repeated seismic surveys at 1–3 years intervals may
detect an upward movement of the OWC as the reservoir is depleted. The difference in impedance may,
however, not be as pronounced at the primary OWC
because there may be 30–40% oil saturation in the
drained intervals, It may then be possible to detect
parts of the reservoir which are not drained and
which are large enough to justify an additional well.
21.14 Well-to-Well Tracers
Tracers can be added to the water in the injection well
and their arrival in the production well can provide
important information about the reservoir. If the tracer
is recorded in the production well relatively soon after
the injection there must be a high permeability connection in the form of an open fault or fracture or a
high permeability sand layer. The time of the arrival of
the tracers makes it possible to calculate the flow
velocity of the water phase. The tracers must remain
dissolved in the water and should not be adsorbed on
the minerals. Since they become part of the produced
water they should be environmentally acceptable.
Many of the tracers are radioactive but only with
β radiation, such as tritiated water (HTO),
36 Cl
À and
22 Na
þ . Most of them have a relatively short half life,
except
36 Cl
À .
It is also possible to use non-radiocative tracers, but
extremely low levels must be detectable (in the ppb
range) because of the strong dilution. Gas tracers may
also be used.
The concentration of the tracer as a function of time
during production may provide very important information about the rock properties and the flow in the
reservoirs. Different types of tracers may to different
degrees interact and be adsorbed onto the reservoir
rocks.
21.15 Reservoir Models and Field
Analogues
When planning the production of a reservoir, a relatively detailed reservoir model is required.
21 Production Geology
557