Chapter 2 Petroleum Geology and Reservoirs

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Chapter 2 Petroleum Geology and Reservoirs
2
Petroleum Geology (????)

• Geology (??)
• ---??(1)????????
• (2)????????(?)??
• Petroleum Geology(????)
• ---??????????????

3
????????

• ????? 4050???????(Cosmic dust)?????
• ???????
• Core--- heavy (4,400 miles)
• Mantle--- Lighter (1,800 miles)
• Crust--- 1030 miles

4
???????
5
????,???????,??????(Rock)?????????,?????????20??
20?????
6MILES 9.6 KILOMETERS 20MILES 32 KILOMETERS
????????6??
??20????????3??,?????????
6
??????? -- Rock cycle
Primeval(???) Atmosphere(??)
??
??
????
Water vapor and gases
Magma (??)
????
heat
?????????
Igneous rocks (???)
Metamorphic rocks
erosion
??
heat pressure
erosion
erosion
Sediments (???)
Sedimentary rocks
pressure cementation
7
Reservoir Rock
Clastic Chemical Chemical Organic Other
Conglomerate Sandsonte Siltstone Shale Carbonate Evaporite Peat Coal Diatomite Limestone Chert
Conglomerate Sandsonte Siltstone Shale Limestone Dolomite Gypsum Anhydrite Salt Potash Peat Coal Diatomite Limestone Chert
8
??????
??? ??? ??? ??? ??
?? ?? ??? ?? ??? ??? ?? ? ??? ??? ??
?? ?? ??? ?? ??? ??? ?? ??? ?? ???(????) ?? ? ??? ??? ??
Clastic Chemical Chemical Organic Other
Conglomerate Sandsonte Siltstone Shale Carbonate Evaporite Peat Coal Diatomite Limestone Chert
Conglomerate Sandsonte Siltstone Shale Limestone Dolomite Gypsum Anhydrite Salt Potash Peat Coal Diatomite Limestone Chert
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?????

• ???(Cambrian)??5.5????????????????(??)
• ???????????(Precambrian)
• ??????????
• ?gt ?????(Geologic Time Scale)
• ???(Devonian)????3.3??????????????

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Geological Time Scale
11
?????
12

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Petroleum accumulation(????)

• Petroleum accumulation(????)????
• (1)Oil gas ???
• (2)????(porosity)????(permeability)?Reservoir
  Rock
• (3) ??trap(??)????????

14
?????
15
????????????????????????,????????
,
?????????,??????????????
??,??????????????????
16
????,?????????,????????????????????,??????????????
?????
??,????,?????????????????,?????,??,?????????????
???????????,????????????????
????????????????????????????????
?????????,???????????????,???????????,????????????
??,??????????? ???????????,????????,??????????????
??????????? ?????????????????????,????????????????
17
?????

• -????????????
• -???????????,?????????
• ?????????,???????????,
• ????(?????)?????????????
• ??
• -?????Sand(?),Clay(??)?debris??????
• ???????
• -???????????
• ???????????,????????,
• ???????
• -????????????????,
• Clays??Shales ? ????

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• Petroleum formation requires that organic source
  clays become mature by subjection to pressure and
  temperature.

19
?????????

• 225? lt temperature lt 350? ????
• temperature lt 150? ???????
• temperature gt 500? ??????,
• 
  ??????

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Generation of gas and oil
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• In geology and oceanography, diagenesis is any
  chemical, physical, or biological change
  undergone by a sediment after its initial
  deposition and during and after its
  lithification, exclusive of surface alteration
  (weathering) and metamorphism.

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• Catagenesis can refer to
• Catagenesis (geology) The cracking process in
  which organic kerogens are broken down into
  hydrocarbons
• Catagenesis (biology) Retrogressive evolution,
  as contrasted with anagenesis.

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• Metamorphism can be defined as the solid state
  recrystallisation of pre-existing rocks due to
  changes in heat and/or pressure and/or
  introduction of fluids i.e without melting. There
  will be mineralogical, chemical and
  crystallographic changes

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• Prolonged exposure to high temperatures, or
  shorter exposure to very high temperatures, may
  lead progressively to the generation of
  hydrocarbon mixtures characterized as
  condensates, wet gases and gas.
• The average organic content of potential source
  rocks is about 1 by weight.
• The Kimmeridge clay, the principal source rock
  for North Sea oil average about 5 carbon (7
  organic mater) with local rich streaks greater
  than 40.
• The hydrogen content of the organic matter
  should be greater than 7 by weight for potential
  as an oil source.

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• It is a rule of thumb that for each percentage
  point of organic carbon in mature source rocks,
  some 13001500 cubic meters of oil per km2-m (or
  1040 barrels of oil per acre-ft or 56-225 ft3/
  43560 ft3) of sediment could be generated.
• It is not, however, necessarily true that all
  the oil generated will be expelled or trapped in
  porous rock.

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????

• The migration process involves two main stages,
  namely from the source rock and then through a
  permeable system.

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Migration of petroleum -- from the source rock
29
??????,??????????,???????,??????
???????
30
??????,????????????(????,Faults)
????????????????????(Seep),??????????,???????????
Jack,????????????
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?????????,??????????????,?????,???????
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???????????????????????????(????,Cap
Rock)???????,???????
???????????(Cap Rock)???????????,????????????(Oil
and Gas Reservoirs)
???????????(Oil and Gas Reservoirs)?????,????(Sand
stone),??????????????????????,???????????????
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Migration of petroleum -- from the source rock

• Capillary effect
• Microfractures
• Since the generation of petroleum is
  accompanied by volume changes which can lead to
  high local pressures, there may well be an
  initiation of microfractures which provide an
  escape route into permeable systems such as
  sedimentary rocks or fault planes.
• The source rock microfractures are
  believed to heal as pressures are dissipated.

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Migration of petroleum --through a permeable
system

• Fluid potential gradient or gravity effect
• In the permeable system the transport occurs
  under conditions of a fluid potential gradient
  which may take the hydrocarbon to surface or to
  some place where it becomes trapped.
• It might be assumed that less than 10 of
  petroleum generated in source rocks is both
  expelled and trapped, as shown in the example of
  Fig. 2.5.

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Petroleum traps(????)

• The characteristic forms of petroleum trap are
  known as
• structural traps(????) and
• stratigraphic traps(????),
• with the great majority of known accumulation
  being in the former style.

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????(Geological Structures)
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Figure 1.13. Basic hydrocarbon reservoirs are
structural and / or stratigraphic traps.
Figure 1.12. Two general kinds of unconformities
are disconformity (A) and angular unconformities
(B) and (C).
39
??(traps)
Combination traps
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Cap rock

• Impermeable rocks provide seal above and below
  the permeable reservoir rocks.
• At equilibrium conditions, the density
  differences between the oil, gas and water phases
  can result in boundary regions between them known
  as fluid contacts, i.e. gas-oil and oil-water
  contacts.

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Structural trapa(????) -- Anticline
Longitudinal view of a typical anticline. The oil
cannot escape upward because of the impervious
shale bed above the oil sand neither can it
travel downward because of the water that is
associated with an accumulation of this type.
Anticlines- Of the many types of structural
features present in the upper layers of the
earths crust that can trap oil, the most
important is the anticlines-the type of structure
from which the greater part of the words oil has
been produced. Anticlines are upfolds of beds in
the earths crust, and, when the proper
conditions are present, oil accumulates within
the closure of there folds.
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Structural trap-- Anticline
Plan view of a typical anticline, showing
locations of longitudinal view A-B and lateral
view C-D.
Lateral, or end view, of a typical anticline.
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Structural traps
Figure 1.7. Schematic cross section shows
deformation of earths crust by bucking of layers
into folds
Figure 1.8. Simple kinds of folds are symmetrical
anticline (A), plunging asymmetrical anticline
(B), plunging syncline (C), and dome with deep
salt core (D).
Figure 1.9. Simplified diagram of the Milano,
Texas, fault.
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Structural traps dome anticline
Figure 1.15. Oil accumulates in a dome-shaped
structure (A) and an anticlinal type of fold
structure (B). An anticline is generally long and
narrow while the dome is circular in outline.
(Courtesy of American Petroleum Institute)
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Structural traps -- faults
Figure 1.10. Simple kinds of faults are normal
(A), reverse (B), thrust (C), and lateral (D).
Figure 1.11. Variations of normal and reverse
faulting are rotational faults (A) and upthrust
faults (B).
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Structural traps
Figure 1.14. Common types of structural traps
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Structural trap fault anticline
Figure 1.16. Gas and oil are trapped in a fault
trap-a reservoir resulting from normal faulting
or offsetting of strata. The block on the right
has moved up from the block on the left, moving
impervious shawl opposite the hydrocarbon-bearing
formation. (Courtesy of American Petroleum
Institute)
Figure 1.17. Shown in map view, fault traps may
be simple (A) or compound (B).
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Stratigraphic traps(????)
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Figure 1.13. Basic hydrocarbon reservoirs are
structural and / or stratigraphic traps.
Figure 1.12. Two general kinds of unconformities
are disconformity (A) and angular unconformities
(B) and (C).
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Stratigraphic traps
Unconformity -Disconformity -Angnlar
unconformity Pinctout Sand lenses Changes in
sedimentation
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Figure 1.22. Oil is trapped under an
unconformity. (Courtesy of API)
Figure 1.23. Lenticular traps confine oil in
porous parts of the rock. (Courtesy of API)
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Stratigraphic trap
A stratigraphic trap where sand lenses are
interspersed in a shale bed. The shale acts as a
permeability barrier
An example of a stratigraphic trap where the oil
zone pinches out.
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Stratigraphic Traps
A stratigraphic trap where changes in
sedimentation act as a permeability barrier.
An angular unconformity as an oil trap. The
flat-lying shale bed above the oil zones acts as
a permeability barrier.
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Stratigraphic traps

• Stratigraphic traps result when a depositional
  bed changes from permeable rock into fine-grain
  impermeable rock (Fig. 2.8).

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Combination traps

• Many reservoirs exist as the result of a
  combination of structural and stratigraphic
  features.
• In the Viking Graben area of the northern
  North Sea, the Brent Sand reservoirs are
  characteristically faulted deltaic sands
  truncated by the Cretaceous unconformity.

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Reservoirs

• Reservoir(???)
• We may define a reservoir as an accumulation of
  hydrocarbon in porous permeable sedimentary rock.
• The accumulation, which will have reached a fluid
  pressure equilibrium throughout its pore volume
  at the time of discovery, is also sometimes known
  as a pool.
• A hydrocarbon field may comprise several
  reservoirs at different stratigraphic horizons or
  in different pressure regimes.

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Field

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Lease

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Reservoir(???)

• ?????????(????)??--reservoir,???????
• (1)??????? (Shape/Configuration- traps)
• (2)??? (cap rock, rock seal)
• (3)??????(area)?
• (4)??????(thickness)?
• (5)???????(porosity)?
• (6)?????????(water saturation)?
• (7)???????(permeability)?

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??????Original oil in place (OOIP)

• OOIP A h ? (1-Sw) 1/Bo
• where
• A??????(area)
• h??????(thickness)
• ????????(porosity)
• Sw ?????????(water saturation)
• Bo ????????(oil formation
• volume factor)

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??????Original oil in place (OOIP)

• OOIP 7758 A h ? (1-Sw) 1/Bo
• where
• OOIP ??????, STB
• A??????(area), acres
• h??????(thickness), ft
• ????????(porosity), fraction
• Sw ?????????(water saturation), fraction
• Bo ????????(oil formation volume factor)
• , bbl/STB

1 acres 43560 ft2 1 bbl 5.61458 ft3
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?????????

• ???
• (Petroleum Resources, ? Resources, ? Total
  Petroleum in place , ? Original oil in place )
• ?????????????(????)???,??????
• ???(Petroleum Reserves,? Reserves )
• ??????????,????????,?????????(E)?????(F)??????(G
  )?,??????????????(????)???????,???????

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Reserves (???)

• Reserves OOIP recovery factor
• where OOIP A h ? (1-Sw) 1/Bo
• recovery factor (????)
• f( k, E, P, T )
• k permeability (???)

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• The setting for hydrocarbon accumulation is a
  sedimentary basin that has provided the essential
  components for petroleum reservoir occurrence,
  namely
• (a) a source for hydrocarbons,
• (b) the formation and migration of petroleum,
• (c) a trapping mechanism, i.e., the existence of
  traps in porous sedimentary rock at the time of
  migration and in the migration path.
• The discovery of oil by exploration well drilling
  in some of the worlds sedimentary basin is shown
  in Figs. 2.1 and 2.2

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Lower right line (0.1 103 m3 oil / km2 ) /
(100 willcat wells/104 km2 ) 104 m3 oil /
willcat well 6.289104 bbl3 oil / willcat well
Upper left line (10 103 m3 oil / km2 ) / (1
willcat well/104 km2 ) 108 m3 oil /
willcat well 6.289108 bbl oil / willcat well
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Lower right line (0.01 106 m3 oil discovered
/ willcat ) / (1 106
m3 oil discovered/ successful wildcat )
1 successful wildcat / willcat
Upper left line (0.1 106 m3 oil discovered /
willcat ) / (0.1 106
m3 oil discovered/ successful wildcat )
100 successful wildcat / willcat
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??????????????????????????????????
?????????????????????
??????????,???????????????
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Reservoir fluids and pressure

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• From a petroleum engineering perspective it is
  convenient to think of sedimentary basins as
  accumulations water in areas show subsidence into
  which sediments have been transported.

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Reservoir fluids and pressure
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Pressure Kick Oil and Water
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pressure kick-gas and water
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pressure kick-gas, oil and water
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Pressure Kick Oil and Water
P(psia)
Pw2265
Po2315
5000
oil
5200
Pw2355
Po2385
OWC
5500
D5500ft
PwPo2490
5600
Pw2535
water
Depth(ft)
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pressure kick-gas and water
P(psia)
Pw2265
Pg2450
5000
Gas
Pg2466
Pw2355
5200
GWC
5500
PwPg2490
D5500ft
5600
Pw2535
water
Depth(ft)
84
pressure kick-gas, oil and water
P(psia)
Pg2396
Pw2265
5000
Gas
Pw2355
5200
Pg2412
Pw2400
5300
Po Pg2420
GOC
D5300ft
Pw2445
5400
Po2455
oil
5500
Pw Po2490
OWC
D5500ft
5600
Pw2535
water
Depth(ft)
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Fluid pressures in a hydrocarbon zone
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Pressure gradient equation

• In a water column representing vertical pore
  fluid continuity, the pressure at any point (Px)
  is approximated by the relationship
• Px X.Gw
• where X the depth below a reference
  datum (such
• as sea level)
• Gw the pressure exerted by unit
  height of
• water, or pressure
  gradient
• Gw f (T, salinity)
• Gw 0.433 psi/ft (or 9.79 kpa/m) for fresh
  water
• Gw 0.44 psi/ft (10 kpa/m) 0.53 psi/ft
  (12 kpa/m)
• for reservoir water system

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Hydrocarbon pressure regimes

• In hydrocarbon pressure regimes
• psi/ft
• psi/ft
• psi/ft

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Pressure gradient ranges

• In reservoir found at depth between 2000 m SS and
  4000 m SS, we might use a gradient of 11 kpa/m to
  predict pore fluid pressures around 220 bars to
  440 bars.

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Reservoir pressures

• Hydrocarbon reservoirs are found over a wide
  range of present day depths of burial, the
  majority being in the range 500 4000 m ss.
• In our concept of the petroliferous sedimentary
  basin as a region of water into which sediment
  has accumulated and hydrocarbons have been
  generated and trapped, we may have an expectation
  of regional hydrostatic gradient.

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Reservoir pressure
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• The primary depositional processes and the nature
  of the sediments have a major influence on the
  porosity and permeability of reservoir rocks.

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• Secondary processes, including compaction,
  solution, chemical replacement and diagenetic
  changes, can act to modify further the pore
  structure and geometry.
• With compaction, grains of sediment are subject
  to increasing contact and pore fluids may be
  expelled from the decreasing pore volume. If the
  pore fluids cannot be expelled, the pore fluid
  pressure may increase.

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Abnormal pressure

• Under certain depositional conditions, or because
  of movement of closed reservoir structures, fluid
  pressures may depart substantially from the
  normal range.
• One particular mechanism responsible for
  overpressure in some North Sea reservoirs is the
  inability to expel water from a system containing
  rapidly compacted shales.
• Abnormal pressure regimes are evident in Fig.
  2.11.

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Abnormal high pressure

• All show similar salinity gradients but different
  degrees of overpressure, possibly related to
  development in localized basins.
• Any hydrocarbon bearing structure of substantial
  relief will exhibit abnormally high pressure at
  the crest when the pressure at the
  hydrocarbon-water contact is normal, simply
  because of the lower density of the hydrocarbon
  compared with water.

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2.3 Fluid pressures in a hydrocarbon zone
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Are the water bearing sands abnormally pressured ?

• If so, what effect does this have on the extent
  of any hydrocarbon accumulations?

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Pressure Kick

• Assumes a normal hydrostatic pressure regime P?
  0.45 D 15
• In water zone
• at 5000 ft P?(at5000) 5000 0.45 15
  2265 psia
• at OWC (5500 ft) P?(at OWC) 5500 0.45 15
  2490 psia

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Pressure Kick

• In oil zone Po 0.35 x D C
• at D 5500 ft , Po 2490 psi
• ? C 2490 0.35 5500 565 psia
• ? Po 0.35 D 565
• at GOC (5200 ft) Po (at GOC) 0.35 5200
  565 2385 psia

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Pressure Kick

• In gas zone Pg 0.08 D 1969 (psia)
• at 5000 ft Pg 0.08 5000 1969 2369
  psia

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Pressure Kick

• In gas zone Pg 0.08 D C
• At D 5500 ft, Pg P? 2490 psia
• 2490 0.08 5500 C
• C 2050 psia
• ? Pg 0.08 D 2050
• At D 5000 ft
• Pg 2450 psia

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Overburden pressure

• There is a balance in a reservoir system between
  the pressure gradients representing rock
  overburden (Gr), pore fluids (Gf) and sediment
  grain pressure (Gg).
• The pore fluids can be considered to take part
  of the overburden pressure and relieve that part
  of the overburden load on the rock grains.
• Gr Gf Gg

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Overburden gradient

• The magnitude of the overburden gradient is
  approximately 1 psi/ft (22.6 kpa/m).
• For 100 rock (sand) Gg 0.433 x 2.7 1.169
  psi/ft
• For 100 water Gf 0.433 psi/ft
• For ? 20 rock Gr 0.2 x 0.433 0.8 x
  1.169
• 1.022
  psi/ft

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Causes of abnormal pressure

• Abnormal fluid pressures are those not in initial
  fluid equilibrium at the discovery depth.
• Magara (1978) has described conditions leading
  to abnormally high and abnormally low pressures.
  Some explanations lie in reservoirs being found
  at pressure depths higher or lower than the
  depths at which they became filled with
  hydrocarbon. This may be the result of upthrust
  or downthrown faulting.

107
Causes of abnormal pressure

• Overpressure from the burial weight of glacial
  ice has also been cited.
• In Gulf coast and North Sea reservoirs,
  overpressure is most frequently attributed to
  rapid deposition of shales from which bound water
  cannot escape to hydrostatic equilibrium. This
  leads to overpressured aquifer-hydrocarbon system.

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Fluid Pressure Regimes

• The total pressure at any depth
• weight of the formation rock
• weight of fluids (oil, gas or water)
• 1 psi/ft depth(ft)

109
Fluid Pressure Regimes

• Density of sandstone

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Pressure gradient for sandstone

• Pressure gradient for sandstone

111
Overburden pressure

• Overburden pressure (OP)
• Fluid pressure (FP) Grain or matrix
  pressure(GP)
• OPFP GP
• In non-isolated reservoir
• PW (wellbore pressure) FP
• In isolated reservoir
• PW (wellbore pressure) FP GP
• where GPltGP
• In a perfectly normal case , the water pressure
  at any depth

112
Normal hydrostatic pressure

• In a perfectly normal case , the water pressure
  at any depth
• Assume (1) Continuity of water pressure to the
  surface
• (2) Salinity of water does not
  vary with depth.
• 
  psia
• 
  psi/ft for pure water
• 
  psi/ft for saline water

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Abnormal hydrostatic pressure ( No continuity of
water to the surface)

• psia
• Normal hydrostatic pressure
• c 0
• Abnormal (hydrostatic) pressure
• c gt 0 ? Overpressure (Abnormal high
  pressure)
• c lt 0 ? Underpressure (Abnormal low
  pressure)

114
Conditions causing abnormal fluid pressures

• Conditions causing abnormal fluid pressures in
  enclosed water bearing sands include
• Temperature change ?T 1? ? ?P 125 psi in a
  sealed fresh water system
• Geological changes uplifting surface erosion
• Osmosis between waters having different salinity,
  the sealing shale acting as the semi permeable
  membrane in this ionic exchange if the water
  within the seal is more saline than the
  surrounding water the osmosis will cause the
  abnormal high pressure and vice versa.

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GWC error from pressure measurement

• Pressure 2500 psia Pressure
  2450 psia
• at D 5000 ft at
  D 5000 ft
• in gas-water reservoir in
  gas-water reservoir
• GWC ?
  GWC ?
• Sol. Sol.
• Pg 0.08 D C Pg 0.08 D
  C
• C 2500 0.08 5000 C 2450
  0.08 5000
• 2100 psia 2050
  psia
• ? Pg 0.08 D 2100 ? Pg 0.08
  D 2050
• Water pressure P? 0.45 D 15 Water
  pressure P? 0.45 D 15
• At GWC Pg P? At GWC Pg
  P?
• 0.08 D 2100 0.45 D 15 0.08 D
  2050 0.45 D 15
• D 5635 ft (GWC) D 5500 ft
  (GWC)

116
Results from Errors in GWC or GOC or OWC

• GWC or GOC or OWC location
• affecting
• volume of hydrocarbon OOIP
• affecting
• OOIP or OGIP
• affecting
• development plans

117
2.4 Reservoir Temperature

• Reservoir temperature may be expected to conform
  to the regional or local geothermal gradient.
• In many petroliferous basins this is around
  0.029 k/m (1.6oF/100 ft).
• The overburden and reservoir rock, which have
  large thermal capacities, together with large
  surface area for heat transfer within the
  reservoir, lead to a reasonable assumption that
  reservoir condition processes tend to be
  isothermal

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