Showing papers on "Effective porosity published in 1995"

Critical porosity; the key to relating physical properties to porosity in rocks

Abstract: Many classes of rock such as sandstones, dolomites, chalks, and cracked igneous rocks have each a distinct characteristic porosity above which the material behaves as s suspension. The porosity at which this system changes, or transforms from isostress to solid load-bearing is defined here as the critical porosity {phi}{sub c}. It is easy to envision that at {phi}{sub c} not only the mechanical moduli, but also other properties such as strength and electrical conductivity, may also undergo transformations. Consequently, the critical porosity must be a fundamental property of a given porous system, not just of one of its physical properties. The observed values of {phi}{sub c} range from .005 for cracked granites to .30 or .40 for limestones, dolomites and sandstones, .60 for chalks and .90 for volcanic glasses. The data suggest that (1) A critical porosity value {phi}{sub c} exists which is typical of a given class of porous materials. Each class is defined on the basis of its common mineralogy or diagenetic porosity reduction processes. (2) Given {phi}{sub c} it may be possible to closely approximate the relation between porosity and velocity, over the entire range of porosity, with a modified mixture relation, in which the mixed componentsmore » are the pure solid on one end, and a critical suspension on the other. (3) Without {phi}{sub c}, theory cannot yield reliable or useful velocity-porosity relations.« less

Measuring sandstone compaction from modal analyses of thin sections; how to do it and what the results mean

Abstract: Calculation of compactional porosity loss from modal analyses of thin sections is a useful technique for characterizing the diagenesis and porosity evolution of sandstone reservoirs. To obtain meaningful results, however, it is essential to assume an appropriate value for the "original porosity" and to use point-counting categories that clearly distinguish between grains and intergranular materials. Because of the latter requirement, it is commonly not appropriate to make calculations of compactional porosity loss from modal analyses that were not performed for the specific objective of measuring intergranular volume. Suites of modal analyses from deeply buried sandstone units typically show wide variations in the relative importance of compaction versus quartz cementation. This heter geneity must reflect subtle variations in depositional sand characteristics, in particular the distribution of detrital intergranular clays that tend to promote both mechanical grain rearrangement and stylolitic grain dissolution, as well as locally inhibiting quartz cement growth. Despite wide variation in the mechanism of porosity loss, total (helium) porosity tends to be relatively constant at individual well locations, suggesting a close interdependence between compaction and quartz cementation, with total porosity loss dependent mainly upon thermal exposure (either maximum temperature or time-temperature integral).

Geochemical Models for the Origin of Macroscopic Solution Porosity in Carbonate Rocks

Abstract: Any single geologic setting may include a variety of geochemical environments, each capable of producing a different type of carbonate solution porosity. Also, many types of porosity can form in more than one geologic setting. Thus, the interpretation of solution porosity is best approached by first delineating the geochemical processes necessary to form the observed pattern of porosity, and then using these insights to assess the broader geologic context. Throughout most of any carbonate formation the solution process is highly selective, and only those openings of maximum groundwater flow are enlarged, while surrounding openings undergo little or no enlargement. Pervasive macroscopic porosity, in which nearly all initial openings are enlarged by solution, is formed by: (1) meteoric water with high discharge and/or low flow distance, (2) mixing of waters of disparate chemistry, (3) oxidation of hydrogen sulfide, or (4) production of acids by redox reactions involving carbon compounds in reducing environments. Areally extensive solution porosity within a narrow stratigraphic range usually indicates solution or reduction of sulfates. Cavernous solution porosity is negligible where aggressive infiltration is lacking, in deep zones where groundwater chemistry is uniform, and in low-flow areas of diagenetically mature carbonate rocks far from sources of groundwater recharge.

The Effect of Zones of High Porosity and Permeability on the Configuration of the Saline-Freshwater Mixing Zone

Abstract: Coastal karst aquifers have highly variable distributions of porosity and permeability. The ability to assess the volume of aquifer occupied by freshwater in coastal karst aquifers is limited by both the lack of understanding of the effect that regions of cavernous porosity and permeability have on the configuration of the saline-freshwater mixing zone and by the limited knowledge of the location of the cavernous regions. A dual-density ground-water flow and solute transport model was used to explore the effect that the depth, lateral extent, and proximity to the coast of zones of high porosity and permeability has on the configuration of the saline-freshwater mixing zone. These aquifer heterogeneities tend to shift the mixing zone upward relative to the position it would have in aquifers with homogeneous porosity and permeability. Zones of high porosity and permeability located at positions shallow in the aquifer or nearer to the coast had the greatest effect. In fact, for the conditions modeled, position was more important in modifying the configuration of the mixing zone than was changing the ratio of the intrinsic permeability of the cavernous zone to the aquifer matrix from 100 to 1000. Modeling results show that ground-water flow is concentrated into the zones of high porosity and permeability and that flow configuration results in steep salinity gradients with depth. Field observations of the location of the halocline and of step changes in ground-water composition coincident with regions of cavernous porosity in coastal karst aquifers corroborate the model results. In a coastal setting with saline water intruding into an aquifer, the effect of cavernous porosity and associated high permeability is to decrease the volume of aquifer in which freshwater occurs by a greater degree than would occur in an aquifer with homogeneous porosity and permeability.

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.

Processing of data from an NMR logging tool

Abstract: A new pulsed nuclear magnetic resonance (NMR) logging tool, known as the CMR* Combinable Magnetic Resonance tool, is being used worldwide. During the CMR tool development phase, one challenge was to design a robust and economical data acquisition and signal processing scheme for the hundreds or thousands of spin-echo amplitudes that can be acquired during the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence. This challenge was met by developing a new signal processing and associated downhole data compression algorithm. Data compression is essential to reducing the processing times so that formation T2-distributions can be estimated in real time. Compression of the digital data is possible, without loss of information, because the linear dependency of the NMR measurement kernels results in gross redundancy of the measured spin-echo amplitudes. An attractive feature of the algorithm described in this paper is that the compression can be performed in the downhole tool, thus substantially reducing the telemetry requirements. The raw spin echoes can also be sent uphole and made available for additional processing. Logs of CMR porosity, free-fluid porosity, mean relaxation time and rock permeability are computed from the estimated T2-distributions. The accuracy and precision of the CMR log outputs are demonstrated by repetitive Monte Carlo computer simulations in which noisy, synthetic spin-echo amplitudes are generated from known T2-distributions and then processed to obtain log outputs. Monte Carlo simulations are used to elucidate CMR log responses in typical clean sand, shaly sand, and carbonate rocks. The relative insensitivity of the measurements to short relaxation times (e.g., those less than a few milliseconds) is discussed and used to explain the differences between CMR log porosity and total formation porosity in shaly formations. We show that CMR porosity is an effective porosity that does not include clay bound water and microporosity having relaxation times less than a few milliseconds. We give examples of statistical fluctuations that can occur on estimated T2-distributions to assist log analysts in recognizing artifacts that are not indicative of actual reservoir rock properties. Field logs that display many of the features of CMR log responses revealed by the simulations are also presented.

H2S-Related Porosity and Sulfuric Acid Oil-Field Karst

Abstract: "H2S-related porosity" refers to porosity created in a H2S system where dissolution can be produced by the mixing of waters of different H2S content or by the oxidation of H2S. "Sulfuric acid oil-field karst" refers to a specific kind of H2S-related porosity where carbonate reservoirs of cavernous size have been dissolved by a sulfuric acid mechanism. In a H2S system, porosity can be produced entirely in the deep subsurface and does not have to represent a paleokarst surface or dissolution in the shallow-phreatic or vadose zones. H2S-related porosity is characterized by the large volume of hydrocarbons it can host, by extensive fracture permeability interconnected with "spongework" cavities or caves of tens to hundreds of meters in extent, by porosity related to structural and/or stratigraphic traps, and by the presence of high uranium and/or iron. Possible examples of H2S-generated porosity systems are the Lisburne field, Prudhoe Bay, Alaska, and some of the extremely productive fields of the Middle East.

Porosity Evolution in the Batu Raja Carbonates of the Sunda Basin — Windows of Opportunity

Abstract: Much of the effective porosity within the productive carbonates of the Batu Raja Formation has a meteoric fresh water origin. Pervasive neomorphic alteration of the original textures, the prevalence of radial fibrous cements, and de-dolomite are among the petrographic features that support this argument. Sunda Basin palaeogeography and basin evolution suggest that the timing for this porosity generation was early. Hot cathode luminescence, electron microprobe and isotopic analyses supplement basic petrographic studies and prove invaluable in resolving the paragenesis and timing of porosity development. Results indicate that there have been six phases of cementation interrupted by two phases of dissolution and porosity enhancement, of which the earlier is the more important. Burial history of the Sunda Basin, throughout the Miocene, is one of progressive subsidence, yet the cyclic patterns evident on electric logs coupled with the petrographic and isotope data imply that during its early history this subsidence was episodic. Rates of carbonate dissolution under tropical conditions are not only exceptionally rapid but can also vary locally. Nevertheless, the enhanced pore throat sizes so created, even when small, can be extremely effective conduits capable of delivering producible hydrocarbons at very high flow rates. Zones of only a few feet thickness are potentially viable pay intervals and, thus, become attractive. It is suggested that hiatuses resulting from 4th and 5th order cycles of sea level change, within the early Miocene, provided ready opportunities to both create and preserve the cavernous porosity and associated multi-darcy permeabilities that allow for locally prolific hydrocarbon production. While the brief time spans available preclude any major modification of the paleo-surface relief, rendering it all but invisible to seismic, certain amplitude and reflector anomalies are attributable to karst activity. These are now being actively pursued as infill and exploration drill targets within the Lower Miocene carbonates of the Sunda Basin. The rocks hold the key to the recognition of karstic features and identification of attendant porosity development. When combined with basin evolution and migration studies, the rock data become a powerful exploration and development tool.

The prediction of high porosity chalks in the East Hod Field

Abstract: The East Hod Field in the Norwegian Block 2/11 produces from the Lower Tertiary and Upper Cretaceous Chalk Group. Although at first sight the trapping mechanism of the East Hod Field appears to be structural, increased understanding of porosity preservation, combined with new seismic interpretation, demonstrates that there is also a strong stratigraphic component. Both autochthonous and allochthonous facies occur, the latter forming the principal reservoirs. The extremely homogeneous nature of the matrix means that variations in seismic response directly reflect changes in pore volume, and a good correlation between seismic impedance and porosity is observed. High quality 3D seismic data made it possible to undertake seismic inversion which, together with the correlation between seismic impedance and porosity, allows prediction of porosity variations within the inter-well volume. The results indicate that within the allochthonous reservoir units, porosity is maintained in down-flank areas at depths previously believed to be predominantly tight. Seismic inversion is particularly accurate for predicting porosity because of the exceptionally uniform nature of the chalk matrix.

Dynamic programming analysis of pore space

Abstract: Summary A method based on the principles of dynamic programming is developed and applied to digital soil images to determine tortuosity, effective porosity, and the shortest paths available for flow. Pathways for flow are determined by minimizing a resistance factor. Analysis of two numerically simulated soils finds that a platy soil has greater effective porosity and smaller tortuosity than a layered soil. The ability of the dynamic programming model to analyse large digital images with complex pore structure is subsequently demonstrated by the analysis of a digital image of an agricultural soil. The results from the agricultural soil point to the need for digital soil image analysis to be three–dimensional if convective and diffusive soil properties are to be examined. It is demonstrated that, because determination of flow paths can be treated as a routing problem solved by applying a few simple rules, dynamic programming analysis is not limited by complexity of pore structures.

The origin and distribution of porosity in the Zechsteinkalk (Upper Permian) of Hewett Field, southern North Sea

Abstract: The Zechsteinkalk of the Hewett Field is characterized by extremely heterogeneous porosity distribution. The two most closely spaced wells (only 200 m apart) show one of the largest differences in average porosity (1.8 to 7.6%). In order to locate future wells and achieve 9reasonable9 OGIP estimates, it is therefore necessary to model porosity distribution. Facies type has some influence on porosity development. Tidal flat muds developed at the shoreward margins of shoals have only very low porosity, whereas inter-shoal and main shoal facies have moderate porosity, on average. Extreme porosity variation within facies is caused by differential diagenesis. Porosity has been created by leaching and dolomitization and destroyed by early carbonate cementation and by anhydrite cementation. Anhydrite is volumetrically the most significant, occluding 20% (bulk volume) porosity in many intervals and completely cementing many fractures. The scale of the variability that can be derived from modelling of porosity-affecting diagenetic processes is mostly much greater than that found between closely spaced wells, particularly in the case of anhydrite cementation. In spite of this uncertainty the geological model represents an improvement on simple independent porosity contouring. The gross trends of porosity distribution, defined by the sedimentological model, have helped enhance confidence in a seismic porosity model and have helped with estimates of gas resources.

Application of Kozeny-Carman Equation for the Estimation of Saturated Hydraulic Conductivity of Soils

Abstract: The generalized Kozeny-Carman equation was applied in the estimation of saturated hydraulic conductivity (Ks)from the estimated values of effective porosity of soils from six major subgroups in Kamarajar district of Tamil Nadu, India The a values of the equation for different soil subgroups varied very widely from 042 to 21 and the B values from 0004 to 1047, indicating that it cannot be universally adopted However, when profiles or each subgroup is considered separately, it is possible to have a better fit with KozenyCarman equation

Determination of effective porosity of mudrocks: a feasibility study

Abstract: Matrix diffusion is believed to be an important transport process within the double-porosity (primary sedimentary porosity and secondary fracture porosity) mudrock-dominated stratigraphic units on the Oak Ridge Reservation (ORR). Effective porosity is identified as an important parameter for evaluating and modeling matrix diffusion as a transport process. This report identifies, summarizes and evaluates petrophysical techniques, which can be used to determine the effective porosity of mudrock. Most of the techniques found their original application in the petroleum industry for the evaluation of reservoir rocks.