quiz Ch6 - Porosity r atio of void space volume compared to...

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Sedimentation Cementation Solution Porosity – ratio of void space volume compared to whole rock volume Caternary Pores – pores that communicate with others by more than one throat passage ( effective porosity) Hydrocarbons can be flushed out by natural and artificial water drive Cul-de-sac / dead-end Pores – pores that have only one throat passage connecting with another pore (effective porosity) Closed Pores – pores that have no communication with other pores (ineffective porosity) Primary Porosity (depositional) Intergranular, interparticle o Intragranular, intraparticle Intercrystalline o Fenestral Secondary Porosity (postdepositional) Vuggy o Moldic o Fracture Moldic porosity – porosity created through the dissolution of pre- existing constituent of a rock such as a shell or rock fragment. Biomoldic relates to porosity from dissolution of a shell or something organic Vug – a cavity, void, or large pore in a rock Brittle rock geologic settings
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o Stress induced folding o Fractures associated with faults o Fractures beneath unconformities Porosity can be measured….. o Directly from cores o Indirectly from geophysical well logs o From seismic data Washburn-Bunting Method: o Gas expansion technique; are within pores of sample is extracted when a vacuum is created by lowering and raising the mercury bulb. o Vol. gas extracted / bulk vol. of rock Boyle’s Law Method o Measure pore volume by sealing sample in a pressure vessel, decreasing the pressure by a known amount and measuring the increase in volume of the contained gas o Ruska Porosimeter used in technique Darcy’s Law – unit of permeability; *only valid if there are no chemical reactions between fluid and the rock and when only one fluid phase completely fills the pores o Q = [K(P 1 –P 2 )A] / L μ Q = rate of flow K = permeability (P 1 – P 2 ) = pressure drop across the sample A = cross-sectional area of the sample
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L = length of the sample = viscosity of fluid μ petrophysical properties of the facies at core-plug scale are generally controlled by matrix grain size fracturing of lithologies enhances permeability but does not add to porosity vuggy pores can be connected where vuggy porosity is extensive near an unconformity surface Ways to measure Permeability: o Drill stem or production test: measuring flow rate and pressure drop as flow is allowed through each testing depth interval o Wireline log; from SP and caliper logs o Permeameter – measures the rate of flow of a gas through a cut of the core Capillary pressure – difference between the ambient pressure and the pressure exerted by the column of liquid
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