01-Porosity - Porosity Instructional Objectives: - Define...

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Porosity 1 Porosity Instructional Objectives: - Define porosity, total porosity, and effective porosity. - Explain the difference between original and induced porosity. - List the factors that affect porosity. - Calculate effective porosity using data from the gravimetric method. - List the common techniques for estimating porosity from well logs. Reservoir Composition: - Rock matrix - Pore space - Water - Oil and gas Rock Matrix and Pore Space: - Rock matrix - the grains of sandstone, limestone, dolomite, and/or shale. - Pore space - filled with water, oil, and/or gas. - Water exists as a thin film coating the rock grain surface. - Oil and gas occupy the larger pore spaces with a film of water between the rock surface and the hydrocarbon. Rock matrix Pore space Rock matrix Water Oil and/or gas
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Porosity 2 Porosity Definition: The fractional void space within a rock that is available for the storage of fluids. φ = Porosity, expressed in fraction or % V b = Bulk volume of reservoir rock, ft 3 V ma = Matrix volume, ft 3 V p = Pore volume, ft 3 V b = V ma + V p Packing: Cubic Packing of Spheres Porosity = 48% Porosity Calculations – Uniform Spheres: Bulk volume = (2 r ) 3 = 8 r 3 Matrix volume = 3 4 3 r π Pore volume = bulk volume - matrix volume Porosity = Volume Bulk Volume Matrix Volume Bulk () % 6 . 47 3 2 1 8 3 / 4 8 3 3 3 = = = r r r Note that for uniform-sized spheres with cubic packing, porosity is independent of grain size. Explanation: Bulk volume is a cube with length of side = 2 r Volume of cube = A 3 or (2 r ) 3 = 8 r 3 Volume of sphere is 3 4 3 r b ma b b p V V V V V Porosity = = φ =
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3 1/8 of sphere = 24 4 3 r π 8 of these = 3 4 24 4 8 3 3 r r = × Mixing of larger and smaller particles clearly has a significant affect on porosity, reducing the original porosity of 47.6% to 14%. Real formations do not consist of these perfectly shaped spheres, but these theoretical packing models help us understand the effects of particle size and distribution on porosity. Classification of Porosity Based on Mode of Origin: - Primary (original). - Secondary (induced). Primary (Original) Porosity: - Developed at deposition. - Typified by: - Intergranular sandstones. - Intercrystalline, oolitic carbonates. - Usually more uniform than induced porosity. Secondary (Induced) Porosity: - Developed by geologic process occurring after deposition (e.g. faulting and uplifting). - Typified by: - Fracture development in some shales and carbonates. - Vugs and solution cavities in limestones and dolomites. Pore-Space Classification: - Total porosity, φ t = volume Bulk space pore Total - Effective porosity, e = volume Bulk space pore cted Porosity = 27 % Packing of Two Sizes of Spheres Porosity = 14%
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01-Porosity - Porosity Instructional Objectives: - Define...

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