Effective porosity

About: Effective porosity is a research topic. Over the lifetime, 1199 publications have been published within this topic receiving 26511 citations.

Chapter 4 Diagenesis in Argillaceous Sediments

Abstract: Summary Argillaceous muds with an initial porosity of about 80% undergo a marked physical change during the compaction. The porosity decreases continuously with increasing depth of burial; very rapidly down to about 500 m and more slowly below that depth. An increase in the orientation of the clay-mineral particles vertically to the stress direction accompanies the decrease in porosity. The behavior of porosity, permeability and structure is discussed in relation to grain size (clay content), concentration of electrolytes in superjacent and interstitial waters, mineral composition of clays, and the exchangeable cations present during the pre-burial (initial porosity and structure) and shallow-burial (depth of overburden ranges from 0 to about 500 m) stages of diagenesis. Only grain size, clay-mineral composition and temperature play an important role in the porosity reduction below a depth of about 500 m (deep-burial stage). At this depth the clay mud becomes a mudstone (or shale if fissile) with a porosity of about 30%; the total volume of the sediment having decreased by about 50%. With a further decrease of porosity in the deep-burial stage, the mudstone (or shale) becomes an argillite with a porosity of only about 5-4%. The lowest level of the deep-burial stage is probably at a maximum burial depth of 10, 000 m. Slate is the product of metamorphism. The chemical and mineralogical changes are quite varied: during the transportation, weathering and the pre-burial stage of diagenesis in the fresh-water environment fine-grained mica minerals are mainly altered into clay micas (illite and ledikite) as a result of loss of K. In the marine environment, the cations in the exchange positions of clay minerals are changed (Mg substituted for Ca). In some marine environments chlorite- and illite-like minerals are formed by the fixation of Mg and K in montmorillonite or degraded illite brought by rivers. Volcanic glass can be altered into montmorillonite, zeolites and SiO 2 -minerals. In the geologic past palygorskite and sepiolite as well as huge accumulations of analcime may have formed from ionic solutions or from gels under extreme conditions and, probably, also during the pre-burial stage of diagenesis. Today glauconite and manganese nodules form on the surface of the sediments in many marine environments having a slow sedimentation rate. The formation of carbonate concretions and sulfides, as well as kaolinite, takes place during the shallow-burial stage. Many processes which start in the preburial stage continue slowly in the upper layers of the shallow-burial zone. During the deep-burial stage illite is formed at the expense of montmorillonite (via mixed-layer intermediate stages); and chlorite (+ illite), at the expense of kaolinite. During these processes large quantities of SiO 2 are liberated. The deepest stage of diagenesis is characterized by a uniform clay-mineral association “illite—chlorite”; in the transition to metamorphism it changes to a paragenesis “sericite-chlorite”.

Three-Dimensional Structural and Petrophysical Modeling for Reservoir Characterization of the Mangahewa Formation, Pohokura Gas-Condensate Field, Taranaki Basin, New Zealand

Abstract: This study integrated three-dimensional (3D) structural and petrophysical models to establish the reservoir characteristics of the Mangahewa Formation within the Pohokura Gas-Condensate Field. The 3D structural model, in which 3 horizons and 51 faults were interpreted, was developed using an algorithm for volume-based modeling. The complex structural mechanism was observed as compressional and extensional stresses resulting in steeply dipping normal and reverse faults. The fault throw was estimated to be up to 85 m, and 33% of the faults had throws of less than 10 m. This explains the fault growth system as small, younger faults have merged to develop larger faults. Well log analysis was used to evaluate important petrophysical parameters such as effective porosity, net-to-gross ratio, shale volume, and water and hydrocarbon saturation. After applying a cutoff, the estimated values for effective porosity, net-to-gross ratio, shale volume, and water and hydrocarbon saturation were 12–18%, 13–31%, 13–26%, 6–22%, and 78–94%, respectively. The estimated values were then incorporated into the grid cells to design 3D petrophysical modeling using the algorithm for sequential Gaussian simulation. The structural model indicated effective trapping and the presence of a conduit mechanism for hydrocarbons. The well log analysis identified significant effective porosities containing substantial hydrocarbon saturation, whereas the petrophysical models showed very good dissemination of the petrophysical parameters. From these models, which also incorporate the gas–water contact, proposed drilling sites for future exploration and well development were proposed. The results characterize the Mangahewa Formation as a good reservoir within the Pohokura Gas-Condensate Field.

Modelling of porosity evolution and mechanical compaction of calcareous sediments

Abstract: A continuum mechanics model for the gravitational compaction of sediments is derived by assuming that the sediments are normally pressured and in a one-dimensional state of stress. Sediment strength is characterized in terms of effective stress laws adopted from soil mechanics. The model is a relatively simple mathematical formula that gives the porosity as a function of burial depth. The shape of the porosity profile is controlled by two mechanical parameters, the compression index and the void ratio at an effective stress of 100 kPa. The model was verified by analysing the porosity—depth data of oozes and chalk from the Ontong Java Plateau, gathered during Leg 130 of the Ocean Drilling Program. The mechanical parameters of the sediments were estimated using a least-squares method to fit the theoretical profile to the porosity data. The theoretical profile described accurately the ooze porosity data over depth ranges of 100 m or more. However, over smaller length-scales of 10–50 m there were systematic deviations between the theoretical porosity values and the ooze porosity data. The porosity deviations correlated with variations in the mean grain size of the sediments, due in part to changes in the foraminifera abundance. In the case of the oozes, the estimated mechanical parameters were consistent with published values obtained from one-dimensional compression tests. In contrast, the estimated mechanical properties for the chalks differed from published values. The chalk porosities were lower than could be explained by mechanical compaction. This explanation is supported by the compressional (P-wave) velocity data. In the chalk sections, the P-wave velocity increases more rapidly with burial depth than it does in the ooze sections, suggesting that sediment elastic properties are increasing due to interparticle binding.

Comparative Porosity and Pore Structure Assessment in Shales: Measurement Techniques, Influencing Factors and Implications for Reservoir Characterization

Abstract: Porosity and pore size distribution (PSD) are essential petrophysical parameters controlling permeability and storage capacity in shale gas reservoirs. Various techniques to assess pore structure have been introduced; nevertheless, discrepancies and inconsistencies exist between each of them. This study compares the porosity and PSD in two different shale formations, i.e., the clay-rich Permian Carynginia Formation in the Perth Basin, Western Australia, and the clay-poor Monterey Formation in San Joaquin Basin, USA. Porosity and PSD have been interpreted based on nuclear magnetic resonance (NMR), low-pressure N2 gas adsorption (LP-N2-GA), mercury intrusion capillary pressure (MICP) and helium expansion porosimetry. The results highlight NMR with the advantage of detecting the full-scaled size of pores that are not accessible by MICP, and the ineffective/closed pores occupied by clay bound water (CBW) that are not approachable by other penetration techniques (e.g., helium expansion, low-pressure gas adsorption and MICP). The NMR porosity is largely discrepant with the helium porosity and the MICP porosity in clay-rich Carynginia shales, but a high consistency is displayed in clay-poor Monterey shales, implying the impact of clay contents on the distinction of shale pore structure interpretations between different measurements. Further, the CBW, which is calculated by subtracting the measured effective porosity from total porosity, presents a good linear correlation with the clay content (R2 = 0.76), implying that our correlated equation is adaptable to estimate the CBW in shale formations with the dominant clay type of illite.