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POROSITY Many slides contain more detailed notes that may be shown using the “Notes Page View” Acknowledgments • Dr Walt Ayers, PETE 311, Fall 2001 • NExT PERF Short Course Notes, 1999 – Note that many of the NExT slides appears to have been obtained from other primary sources that are not cited RESERVOIR POROSITY Definition: Porosity is the fraction of a rock that is occupied by voids (pores) Discussion Topics • Origins and descriptions • Factors that effect porosity • Methods of determination ROCK MATRIX AND PORE SPACE Rock matrix Pore space Note different use of “matrix” by geologists and engineers POROSITY DEFINITION Porosity: The fraction of a rock that is occupied by pores • Porosity is an intensive property describing the fluid storage capacity of rock Vp Vb − Vma Porosity = φ = = Vb Vb ROCK MATRIX AND PORE SPACE Rock matrix Water Oil and/or gas OBJECTIVES To provide an understanding of • The concepts of rock matrix and porosity • The difference between original (primary) and induced (secondary) porosity • The difference between total and effective porosity • Laboratory methods of porosity determination • Determination of porosity from well logs CLASSIFICATION OF ROCKS Rock-forming Source of process material IGNEOUS SEDIMENTARY METAMORPHIC Molten materials in deep crust and upper mantle Weathering and erosion of rocks exposed at surface Rocks under high temperatures and pressures in deep crust Crystallization (Solidification of melt) Sedimentation, burial and lithification Recrystallization due to heat, pressure, or chemically active fluids SEDIMENTARY ROCKS • Clastics •Carbonates •Evaporites CLASTIC AND CARBONATE ROCKS Clastic Rocks Consist Primarily of Silicate Minerals Are Classified on the Basis of: - Grain Size - Mineral Composition Carbonate Rocks Consist Primarily of Carbonate Minerals (i.e Minerals With a CO3-2 Anion Group) Limestone - Predominately Calcite (Calcium Carbonate, CaCO3) Dolomite - Predominately Dolostone (Calcium Magnesium Carbonate, CaMg(CO3)2 ) PORE-SPACE CLASSIFICATION Total Pore Volume Bulk Volume • Total porosity, φt = • Effective porosity, φe = Interconnected Pore Space Bulk Volume • Effective porosity – of great importance; contains the mobile fluid COMPARISON OF TOTAL AND EFFECTIVE POROSITIES • Very clean sandstones : φe → φt • Poorly to moderately well -cemented intergranular materials: φt ≈ φe • Highly cemented materials and most carbonates: φe < φt MEASUREMENT OF POROSITY • Core samples (Laboratory) • Openhole wireline logs SANDSTONE POROSITY MEASURED BY VARIOUS TECHNIQUES Total Porosity - Neutron Log Total Porosity - Density Log Absolute or Total Porosity Rock Matrix Oven-Dried Core Analysis Porosity Humidity-Dried VShale Core Analysis Porosity bSand yVariousTtneP echniqustyMa red TotalPr rosity-DenLgNu VRockMatrix ShaleOvn-DridCo Humidty-DreCoAnalsPb ityToalPrs S(FramQu ClayLers&tzucwok) WaCpiPorSmIntelysufc PoresLag,Intcly PoresIlatncd d (O -)WaterBH IreduciblomWatHyn PoreVluHydcab meon (modifer slingeradPv,198E ) Clay Clay Surfaces (Framework) Layers & Interlayers Quartz Structural (OH -) Water Hydration or Bound Water Small Pores Capillary Water Irreducible or Immobile Water (modified from Eslinger and Pevear, 1988) Large, Interconnected Pores Hydrocarbon Pore Volume Isolated Pores INFORMATION FROM CORES* Standard Analysis • Porosity Special Core Analysis • Vertical permeability to air • Relative permeability • Horizontal permeability to air • Capillary pressure • Grain density • Cementation exponent (m) and saturation exponent (n) *Allows calibration of wireline log results CORING ASSEMBLY AND CORE BIT Drill collar connection PDC Cutters Thrust bearing Outer barrel Inner barrel Core retaining ring Core bit Fluid vent COMING OUT OF HOLE WITH CORE BARREL WHOLE CORE Whole Core Photograph, Misoa “C” Sandstone, Venezuela Photo by W Ayers SIDEWALL SAMPLING GUN Core bullets Formation rock Core sample SIDEWALL CORING TOOL Coring bit Samples WHOLE CORE ANALYSIS vs PLUGS OR SIDEWALL CORES WHOLE CORE • Provides larger samples • Better and more consistent representation of formation • Better for heterogeneous rocks or for more complex lithologies WHOLE CORE ANALYSIS vs PLUGS OR SIDEWALL CORES PLUGS OR SIDEWALL CORES • Smaller samples • Less representative of heterogeneous formations • Within to 2% of whole cores for medium-to highporosity formation • In low-porosity formations, φ from core plugs tends to be much greater than φ from whole cores • Scalar effects in fractured reservoirs CORE PLUG Sparks and Ayers, unpublished NEXT: LABORATORY DETERMINATION OF POROSITY Student Questions / Answers • intraparticle porosity in carbonates (JC1): – vugs and fractures • why are clays important (JC1): – one major reason is that clays conduct electricity, this can effect water saturation calculations if not accounted for • fines (ABW): – solid particles so small that they can flow with fluids through pores - but they can also plug pore throats • tortuousity (ABW): – the indirect curvy flow path through the pore system to get from point A to point B • holocene: – referring to the Holocene Epoch (geology) or in general meaning about the last 10,000 years [...]... ORIGINS OF POROSITY IN CLASTICS AND CARBONATES (Genetic Classification) • Primary (original) • Secondary (induced) (Generally more complex than primary porosity) PRIMARY (ORIGINAL) POROSITY • Developed at deposition • Typified by – Intergranular pores of clastics or carbonates – Intercrystalline and fenestral pores of carbonates • Usually more uniform than induced porosity SECONDARY (INDUCED) POROSITY. .. 1982) CUBIC PACKING OF SPHERES Porosity = 48% Porosity Calculations - Uniform Spheres • Bulk volume = (2r)3 = 8r3 • Matrix volume = 4 π r3 3 • Pore volume = bulk volume - matrix volume Pore Volume Porosity = Bulk Volume Bulk Volume − Matrix Volume = Bulk Volume 8 r3 − 4 / 3 π r3 π = = 1 − = 47 6 % 2 ( 3) 8 r3 RHOMBIC PACKING OF SPHERES Porosity = 27 % FACTORS THAT AFFECT POROSITY PRIMARY • Particle sphericity... SECONDARY (diagenetic) • Cementing materials • Overburden stress (compaction) • Vugs, dissolution, and fractures SPHERICITY Porosity ROUNDNESS AND SPHERICITY OF CLASTIC GRAINS High Low Very Angular Angular SubSubWellRounded Rounded Angular Rounded ROUNDNESS Porosity FACTORS THAT AFFECT POROSITY PRIMARY • Particle sphericity and angularity • Packing • Sorting (variable grain sizes) SECONDARY (DIAGENETIC)... sandstones, shales, and carbonates SANDSTONES POROSITY TYPES Intergranular (Primary) Interstitial Void Space Between Framework Grains Micropores Small Pores Mainly Between Detrital Framework Grains or Cement Dissolution Partial or Complete Dissolution of or Authigenic Grains (Can Also Occur Within Grains Fractures Breakage Due to Earth Stresses FACTORS THAT AFFECT POROSITY PRIMARY • Particle sphericity and... and angularity • Packing • Sorting (variable grain sizes) SECONDARY (DIAGENETIC) • Cementing materials • Overburden stress (compaction) • Vugs, dissolution, and fractures Packing of Two Sizes of Spheres Porosity = 14% Grain-Size Sorting in Sandstone Very Well Sorted Well Sorted Moderately Sorted SORTING Poorly Sorted Very Poorly Sorted TYPES OF TEXTURAL CHANGES SENSED BY THE NAKED EYE AS BEDDING Sand
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