Sunday, 30 July 2017

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Reservoir Fluid & rocks: Natural gas, crude oil composition and Properties of reservoir rocks

oil distribution
Reservoir fluid distribution.


Reservoir science

 A reservoir science is the science of the study of porous rock and accumulated fluid (Petroleum + water). Reservoirs are formed from the consolidation of permeable sediments moved and deposited by fluid medium on the earth surface from a ‘source rock’.

All reservoirs are of the sedimentary structure and hydrocarbons are trapped within the structure by deformation force upon layers at high temperature (structural trap) or by the depositional environmental changes in lithology.


Reservoir fluids

Composition and properties

   Petroleum: This consists of natural gas, crude oil ,condensate  and constituent(Nitrogen oxide, hydrogen sulfide, carbon (iv) oxide etc). It is referred to a mixture which is complex, of hydrocarbons (compounds containing hydrogen and oxygen only) and other constituents depending on composition, temperature, pressure etc.

Natural gas:

This is mixture of individual gas component. Such as;
  • Dry Gas: No liquid just methane.
  • Wet gas: contains liquid droplet with an API gravity of 50-70°API.
  • Acid gas: contains carbon (iv) oxide.
  • Sweet gas: Does not contain hydrogen sulfide.
  • Sour gas: contains hydrogen sulfide.


Properties of Natural gas: 

    Apparent molecular weight, Density, Specific gravity, Specific volume, Gas formation volume factor, Gas isothermal compressibility etc.

Crude oil

  This is a hydrocarbon mixture of individual oil components. The quality is measured in terms of API gravity.
  •     Bitumen: 10°API
  •     Heavy oil: 20 -25°API
  •     Black oil: 30-40°API
  •     Volatile oil: 40-50°API


Properties of crude oil: 

Fluid gravity, Oil density, Oil formation volume factor, Specify gravity, Surface tension, Isothermal compressibility, Total formation volume factor etc.

Condensate

  Gas condensate are hydrocarbon in which at certain conditions of temperature and pressure has led to the condensation of heavy gas in the reservoir. It occurs at critical temperature and pressure conditions.


Reservoir rocks: 

All reservoir rocks are sedimentary rocks for example sandstone, carbonate, evaporates etc. Other types of rocks like; Igneous and sedimentary rocks cannot form reservoir rock because of their source of formation, structure and properties.

Classification of reservoir rocks includes

             According to Lithology
        Sand stones
        Carbonates
        Evaporites
          According to Strength
        Unconsolidated
        Partially Consolidated
        Friable
        Consolidated
          According to Pressure
                  -- Normal Pressure
                  -- Abnormal Pressure
                 -- Subnormal
                 -- Geo pressured
          According to Fluid Content
                 -- Gas Reservoir
                 -- Oil Reservoir
                 -- Condensate Reservoir
                 -- Multiphase Reservoir

Properties of reservoir rocks includes

Flow properties
  • Porosity: This refers to the fractional void space within the reservoir rock that is available for storing fluid.

        Porosity = pore volume/bulk volume

  • Permeability (K): this is a property of porous medium which measures capability of rock formation to transmit fluid. It is the interconnection of void spaces to enable transmission or flow of fluids. Its S.I unit is in Darcy.

Elastic properties This is related to the strength of rock. It is the reversible deformation by the action of stress under a given external of or internal pressure . It includes; stress, strain Poisson Ratio ,Bulk Modulus (Function of Change in Volume) , Shear Modulus (Function of Change in Shape) , Compression Strength etc

Electrical properties
                   These properties help to measure of Voltage Resistance and measure of ease with which current will pass through. They include; Resistivity, Conductivity.


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