Reservoir Rocks:
“The rock, which is porous and permeable, contains oil and gas becomes a reservoir rock.” OR
“Permeable subsurface rock that contains petroleum”
It can be different from source rock with respect to grain size and pore spaces.
Nearly all rocks are reservoir and can be classified accordingly as
· Clastic Rocks
· Carbonate Rocks
· Igneous/Metamorphic Rocks
Clastic Rocks
Aggregates of a particle, fragments of older rocks
Range in size from fine clay to boulder size forming various clastic rocks, including claystone, shale, siltstone, sandstone, conglomerates.
Most important reservoir rocks in this category are sandstone and sometimes conglomerate due to porosity and permeability. The fractured shale reservoir is also possible but uncommon in nature.
Carbonate Rocks
Formed by chemical precipitation or biological precipitation.
The most important reservoir in this category are limestone and dolomite.
Igneous/Metamorphic Rocks
These are rare comparatively to clastic and carbonate reservoir rocks, from which commercial oil and gas are produced.
e.g., there are number of volcanic oil fields, which contains oil and gas includes: Columbia Plateaue of Washington and Oregon, the Mexico-Arizona Volcanic field, the Deccan Traps of India and volcanic of pacific.
Reservoir Rocks Properties:
There are two main properties on the basis of which reservoir quality of rocks is controlled.
· Porosity
· Permeability
Porosity:
“It is the ratio of pore spaces volume to the total volume (bulk volume) of a reservoir rock.”
It is expressed in %age.
Mathematically: Porosity = Pore volume/Bulk volume x 100
Porosity on the basis of origin
· Primary Porosity
· Secondary Porosity
Primary Porosity:
Porosity which developed at time of deposition.
Depends upon; size, shape and pattern of arrangement of grains
e.g., uniform grain size higher the porosity, perfectly rounded shape higher the porosity. In last when packing is cubic of same size grain higher the porosity (47.6%) rather than rhombohedral grains (25.9%).
Secondary Porosity:
Porosity which is developed after the sedimentation process due to physical and chemical effects.
The physical and chemical effects includes
· Compaction
· Fracturing
· Dissolution
· Cementation
· Recrystallization
· Dolomitization
Porosity based on the pore space connectivity Both primary and secondary porosity could be:
· Absolute Porosity
· Effective Porosity
Absolute Porosity:
The ratio of total volume of pore spaces to total volume of rock
Effective Porosity
The ratio of the volume of interconnected pore spaces to the total volume of the rock.
There are four basic porosity types within sandstone (after, Pittman, 1979)
· Intragranular Porosity
· Intergranular Porosity
· Dissolution porosity
· Fractured Porosity
Porosity in Carbonates (Limestone):
There are many types of porosities within limestone but can be groups together in three groups (after Choquette and Pray, 1970) such as:
2.non fabric selective
3.fabric selective or Not
Porosity in Igneous/Metamorphic Rocks:
Permeability:
“Ability of a rock to permit fluidflow” through the inter-conneted pores.
It is expressed by m2. Mathematically it can be written as by Darcy’s law:
q/A= KAP/p AL
q= volumetric flow rates, m3/s A= cross sectional area, m2 K= permeability, m2
AP/AL= pressure gradient in direction of flow, Pa/m p= viscosity, Pa.s
Types of Permeability
Ø Absolute Permeability
When only one fluid saturate the effective pore spaces and can pass through effective pores.
Effective Permeability
When only one fluid can pass through effective pores in presence of other fluid.
Ø Relative Permeability
The ratio of effective permeability to the absolute permeability
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