Porosity Of Reservoir Rocks Research Articles

Zeolites in volcanic-igneous hydrothermal systems: a case study of Pauzhetka geothermal field (Kamchatka) and Oligocene Smrekovec volcanic complex (Slovenia)

Production from geothermal reservoirs in volcanic-igneous hydrothermal systems may be disturbed owing to the formation of authigenic minerals that reduce primary porosity and infill fissure systems. Crystallization may be induced by natural processes or human activity related to the reservoir exploitation. In volcanic and volcaniclastic rocks, zeolites and related authigenic silicate minerals commonly develop. Two selected study sites—Pauzhetka geothermal field in Russia and Smrekovec volcanic complex in Slovenia are characterized by extensive development of progressive stage medium- and low-temperature propylitization and zeolitization with similar mineral assemblages that reduced original reservoir rock porosity and permeability. Retrograde and overprinting reactions of argillitization commonly enhance porosity and permeability of rocks, but some other reactions, like from prehnite to laumontite, from laumontite to heulandite, and from laumontite to analcime, encountered in the Smrekovec volcanic complex reduced it appreciably. Retrograde reactions recognized in the study sites were not induced by the temperature drop only, but were accompanied by the change in chemical composition of reacting fluids. Chemical composition of interstitial waters produced from exploitation wells in the Pauzhetka geothermal field indicate that propylitic and zeolite facies alteration is related to Na–Cl, slightly alkaline waters, while argillitization involved acidic thermal waters with more complex ion composition.

Control of coupling among three major factors for formation of high-efficiency gas reservoir—A case study on the oolitic beach gas reservoir in Feixianguan Formation in the northeast Sichuan Basin

Through a case study of the high-efficiency gas reservoir in Feixianguan Formation in the northeast Sichuan Basin, quantitative and semi-quantitative analyses of key elements such as hydrocarbon generation, migration and accumulation, and reservoir evolution as well as their interplay in the critical moment of reservoir formation controlled by the energy field were carried out, by means of numerical modeling of the energy field. It was found that the climax time for Permian hydrocarbon generation was Late Triassic-Early Jurassic and accumulation of oil and gas has resulted in large-scale paleoreservoirs in paleostructural traps in Feixianguan Formation, a process facilitated by fractures connecting the sources. The paleoreservoirs have been turned into high-efficiency gas kitchens due to pyrolysis, which resulted from deep burial at a temperature of 170–210°C as induced by tremendously thick sedimentation in the foreland basin of Daba Mountain in Late Jurassic-Cretaceous period. Meanwhile, abundant acid gas like H2S produced from thermo-chemical sulfate reduction (TSR) at high temperatures leads to extensive dissolution of dolostone in the paleoreservoirs, which may in turn result in modification of the reservoirs and preservation of the reservoir rock porosity. The present distribution of gas reservoirs was ultimately determined in the processes of adjustment, cooling and decompression of the paleoreservoirs resulting from intense deformation in the front of Daba Mountain during the Himalayan orogeny.

Feldspar dissolution rates measured using phase-shift interferometry: Implications to CO2 underground sequestration

To assess CO 2 underground sequestration from a geochemical viewpoint, the anorthite dissolution rate, which is an important parameter of risk analysis, was measured in a CO 2 –water system. The authors sought to obtain precise dissolution rate data in a short time observing a crystal surface on a nanoscale. For this purpose, phase-shift interferometry was applied. Using this method, uncertainty of the reactive surface area that is imparted on calculation of the dissolution rate constant can also be avoided. The time-course profile of vertical retreat of the surface revealed that the anorthite dissolution process changes from the initial transient state to a later steady state, which is consistent with results of numerous precedent studies. The transient dissolution rate depends strongly on local features (e.g., density of defects, variation of chemical compositions) of the crystal surface, rather than on temperature. Therefore, it is very important to determine the original properties of the anorthite surface for the examination of subsequent dissolution process. Contrary to general expectations, the anorthite dissolution can alter the physical properties of reservoir rock immediately after CO 2 injection. The simple estimation using the anorthite dissolution rate obtained in this study, which was done as a test case for the CO 2 underground sequestration project conducted by RITE, revealed that porosity of reservoir rock increased about 2% (23–23.4%) of initial values during 60 a. That change in physical property in such a short time might enhance the diffusion of injected CO 2 and formation water, and therefore accelerate further geochemical reactions. Results of this study demonstrate that the geochemical water–rock interaction, which is generally regarded as a longer-term phenomenon than various physical processes, can also affect the reservoir system from the initial stage.

FRACTURE‐FILLING CEMENTS IN THE PALAEOZOIC WARBURTON BASIN, SOUTH AUSTRALIA

In the Cambro‐Ordovician Warburton Basin, South Australia, reservoir rock porosity and permeability are enhanced by open fractures and by partially‐open or incompletely mineral‐filled fractures. However in many cases, the fractures are completely filled by cements of several types. A knowledge of the nature and origin of the fracture‐filling cements can help to identify areas where cement precipitation is relatively less intense, and where oil and gas production may be improved.In the present study, fluid inclusion microthermometry, and stable isotope and electron microprobe analyses were used to indicate the nature and timing of fracture‐filling cements. The study shows that fracture‐filling cements in the studied Warburton Basin wells include quartz, siderite, ankerite, kaolin and pyrite. Based upon precipitation temperatures, stable isotope compositions, elemental compositions and quartz cement textures, a hydrothermal source for the silica and carbonate cements is envisaged. The hydrothermal activity may be related to granite intrusions during the Carboniferous.Fluid inclusion microthermometry suggest that Warburton Basin rocks in some areas have experienced temperatures of more than 260°C and cannot therefore be a source of hydrocarbons. The observed low permeabilities occur because most fractures were completely or partially filled by cements precipitated during hydrothermal activity soon after granite intrusion. There is a greater likelihood of finding reservoir rocks with open fractures in areas far from the granite intrusions.

Influence of Effective Stress on the Acoustic Velocity and Log-Derived Porosity

Summary Experimental studies indicate that when effective stress increases, compressional wave velocity in porous rocks increases. Reservoir pressure reduction, resulting from hydrocarbon production, increases effective stress. For a rock with a given porosity the sonic log may show decreasing values as the pressure in the reservoir decreases. This in turn may lead to underestimation of the actual porosity of the reservoir rocks in low pressure reservoirs. The range of such underestimation for liquid saturated reservoirs may not be significant, but since the influence of effective stress on velocity increases as fluid saturation changes to gas, porosity underestimation by conventional velocity-porosity transforms for gas bearing rocks may increase. Examples are taken from partially depleted gas reservoirs in the Cooper basin, South Australia. The stress dependent nature of velocity requires that the in situ pressure condition should be considered when the sonic log is used to determine the porosity of gas producing reservoir rocks.

Solid bitumens: an assessment of their characteristics, genesis, and role in geological processes

ABSTRACTFamiliar since antiquity, and subject in contemporary times to various characterization schemes, the exact nature of solid bitumen is not yet fully known. Bitumens have ‘random polymer‐like’ molecular structures, are mobile as highly viscous fluids or were once fluids but have since turned into solids. Solid bitumens consist mainly of large moieties, of polyclyclic aromatic hydrocarbons, occasionally with finely admixed, fine‐grained cryptocrystalline graphite. Solid bitumens are distinguished from kerogen, which is the syngenetic and generally finely dispersed particulate organic matter in sedimentary rock that virtually does not migrate following its deposition.Occurrences of solid bitumens are relevant to petroleum exploration as well as the search for, and evaluation of, a variety of metallic mineral deposits. Genesis of bitumen is in many cases linked to the thermal and hydrothermal history of organic matter in sedimentary rock. Apparently bitumen, or more specifically organic acids generated along with bitumen during diagenesis, may alter porosity of reservoir rocks or otherwise prepare the ground for ore deposition. Bitumen is also relatively sensitive to alteration processes, some of which, such as oxidative weathering, water leaching, biodegradation (contact) metamorphism and ionizing radiation may likewise affect its nature.Elemental composition of bitumen commonly reflects the nature of mineral deposits. Is is possible that in petroleum exploration, trace metal abundances of bitumen may eventually allow prediction of crude oil types and volumes anticipated from a given source rock? Beside transition elements, notably Ni and V, highly anomalous concentrations of U, Pt and Au occur in some solid bitumens.During the generation of petroleum from kerogen, the trend in δ13C is toward lighter values. The opposite seems to occur when liquid petroleum is subjected to thermal cracking (and /or related processes) yielding solid bitumen enriched in 13C, and isotopically light methane. In fact, except for deasphalting and possibly some irradiation processes, the result of thermal cracking, oxidation, water leaching, inspissation (drying) and bacterial degradation of crude oil is that lower molecular weight hydrocarbons are removed leaving bitumen residues enriched in aromatic hydrocarbons, heteroatomic compounds (NSO) and 13C. Such phenomena are relevant to bitumen paragenesis in petroleum reservoir rocks, to certain Phanerozoic occurrences of multiple generations of bitumens, and to bitumens in mineral deposits.

POROSITY AND PORE STRUCTURE FROM ACOUSTIC WELL LOGGING DATA1

AbstractWyllie's time‐average equation and subsequent refinements have been used for over 20 years to estimate the porosity of reservoir rocks from compressional (P)‐wave velocity (or its reciprocal, transit time) recorded on a sonic log. This model, while simple, needs to be more convincingly explained in theory and improved in practice, particularly by making use of shear (S)‐wave velocity. One of the most important, although often ignored, factors affecting elastic velocities in a rock is pore structure, which is also a controlling factor for transport properties of a rock. Now that S‐wave information can be obtained from the sonic log, it may be used with P‐waves to provide a better understanding of pore structure. A new acoustic velocities‐to‐porosity transform based on an elastic velocity model developed by Kuster and Toksöz is proposed. Employing an approximation to an equivalent pore aspect ratio spectrum, pore structure for reservoir rocks is taken into account, in addition to total pore volume. Equidimensional pores are approximated by spheres and rounded spheroids, while grain boundary pores and flat pores are approximated by low aspect ratio cracks. An equivalent pore aspect ratio spectrum is characterized by a power function which is determined by compressional‐and shear‐wave velocities, as well as by matrix and inclusion properties. As a result of this more sophisticated elastic model of porous rocks and a stricter theory of elastic wave propagation, the new method leads to a more satisfactory interpretation and fuller use of seismic and sonic log data. Calculations using the new transform on data for sedimentary rocks, obtained from published literature and laboratory measurements, are presented and compared at atmospheric pressure with those estimated from the time‐average equation. Results demonstrate that, to compensate for additional complexity, the new method provides more detailed information on pore volume and pore structure of reservoir rocks. Examples are presented using a realistic self‐consistent averaging scheme to consider interactions between pores, and the possibility of extending the method to complex lithologies and shaly rocks is discussed.

Petrography and Reservoir Physics II: Relating Thin Section Porosity to Capillary Pressure, the Association Between Pore Types and Throat Size (1)

Porosity in reservoir rocks is configured into a few types of pores whose size and shape are controlled by depositional fabric and postdepositional processes. The size, shape, and abundance of each pore type can be objectively determined from thin section using image analysis and pattern recognition procedures. Each pore type tends to be associated with a limited range of throat sizes. The association between pore type and throat size can be determined using regression procedures linking pore type data obtained from thin section with capillary pressure data. To do so, a set of samples is required wherein the association between pore type and throat size is fixed, but where pore type proportions vary between samples. This condition is met by a sample suite representing res rvoir facies from a single core or, in many cases, from a single field. The relationship between pore type and throat size is an effective means to relate reservoirs in terms of the efficiency of the porous microstructure to multiphase flow. Parameters derived from the relationship can be used to construct accurate physical models that subdivide physical response in terms of the contributions of each pore type.

Experimental study of aluminum-oxalate complexing at 80 °C: Implications for the formation of secondary porosity within sedimentary reservoirs

An experimental study of aluminum-oxalate complexing at 80 °C indicates that the presence of oxalate in sedimentary basin fluids may play an important role in creating secondary porosity in reservoir rocks. The solubility of gibbsite was measured as a function of oxalate concentration. Geologically realistic oxalate concentrations were used in order to simulate the aluminum mobilities that occur in reservoir fluids. Although the stoichiometry of the important aluminum-oxalate species cannot be determined from these data, the results are used to quantify the minimum extent of aluminum-oxalate complexing that occurs in solution. Assuming that Al(Ox) 3 3- is the only important aluminum-oxalate complex, the data limit the log of the dissociation constant of Al(Ox) 3 3- to be ≤-16.5. Thermodynamic modeling of aluminum systems with fluids that contain NaCl, acetate, and oxalate illustrates that aluminum-oxalate complexing is much more important than aluminum-acetate complexing, even at relatively low oxalate concentrations and at very high acetate concentrations. In addition, these calculations show that aluminum-oxalate complexing can greatly increase aluminum mobility in formation waters and, therefore, may increase aluminosilicate mineral dissolution within sedimentary basin fluid-rock systems.

Origin and significance of redistributional secondary porosity

Redistributional secondary ( RDS) porosity is created by the dissolution of framework grains and cements and the simultaneous precipitation of reaction products in the nearby pore space. Aluminosilicate minerals and in particular feldspars are the usual precursors of RDS porosity in sandstones. Common product minerals are authigenic quartz and clay minerals, usually kaolinite. This type of secondary porosity is almost ubiquitous in sandstones. RDS porosity has little effect on the total porosity of reservoir rocks and, contrary to popular opinion, cannot give rise to higher than expected porosities at depth. Petrographic criteria alone are an insufficient measure that the rock has undergone leaching resulting in porosity enhancement. To demonstrate porosity enhancement due to leaching it is necessary to show that the porosity of the rock is statistically higher than would otherwise be the case for a rock of similar characteristics that has not been leached. The dissolution-precipitation reactions that give rise to RDS arise from the inherent thermodynamic instability of minerals such as the feldspars and other aluminosilicates which formed under magmatic or metamorphic conditions. When these minerals dissolve they add aluminium and silica to the pore fluid. However, the concentration of aluminium and silica in the pore fluid is buffered by clay minerals and quartz, respectively, during burial diagenesis. Thus any increase in the concentration of these components leads to the precipitation of an authigenic clay mineral and authigenic quartz. Carboxylic acids are claimed to have the capacity to complex aluminium and so increase the solubility of aluminium and eliminate the precipitation of a clay phase. However, they are unlikely to have any major effect on aluminium solubility in natural (i.e. complex) pore fluids. In fact, dissolution will occur in acid, neutral or alkaline conditions at a rate that is kinetically controlled. Reaction rates speed up exponentially with increasing temperature and therefore depth, resulting in the destruction of more and more unstable components. This increase in the rate of destruction of unstable components at depth can give rise to what appears to be a late leaching event. What is seen in fact is the natural culmination of the destruction of thermodynamically unstable phases at an ever increasing rate during burial. Thus for a rock of a given composition the fraction of the total porosity that is secondary will increase with burial depth until all labile components are consumed. The behaviour of mineral mixtures in the presence of a moving pore fluid is an additional contributing factor to the ubiquity of RDS porosity. Where two or more minerals add the same ion to the pore fluid, then the steady-state concentration of that ion which is reached at some distance down the flow path (theoretically an infinite distance) is a weighted average of the equilibrium solubilities of that ion with respect to the minerals that contribute the ion. Thus as the pore fluid approaches a steady-state concentration of the ion it will be undersaturated with respect to the least stable minerals and oversaturated with respect to the most stable mineral. Hence, RDS porosity must form as a result of the dissolution of the least stable mineral and the precipitation of the most stable mineral. Although kaolinite is the most common dissolution product of feldspars, with increasing temperature its stability field shrinks with respect to other clay minerals such as illite. Thus it is possible for feldspars and other aluminosilicates to decompose at depth to give illite and authigenic quartz. The growth of fibrous illite together with the change from a dominantly primary to a dominantly secondary pore structure can give rise to a rapid loss of permeability with depth. Where changes in the clay mineral type do not occur, the impact of RDS porosity on the porosity-permeability behaviour of sandstones is minimal up to secondary porosities of about 10%. Beyond this figure there is at present too little data to make a clear statement.

ALUM SHALE BITUMEN MATURATION AND MIGRATION: IMPLICATIONS FOR GOTLAND'S OIL

Thermal maturation of Swedish Alum Shale kerogen and bitumen has been determined from core samples from Eastern and Central Sweden. In samples from Eastern Sweden (Öland and Gotland), the kerogen and bitumen are thermally immature with respect to petroleum generation. In some areas of Central Sweden (Närke, ÖstergÖtland, and Kinnekulle in VästergÖtland), the kerogen is immature, whereas the bitumen is marginally mature to mature. In other areas of Central Sweden (Halleberg‐Hunneberg in VästergÖtland), the kerogen is supermature and the bitumen mature. This suggests that bitumen from a mature source‐rock has migrated into the Alum Shales of Central Sweden. Migration in Central Sweden is further evidenced by the occurrence of obviously‐migrated bituments in vugs and voids in the organicpoor Ordovician limestone overlying the Alum Shale in Central Sweden, and in concretions within the Alum Shale itself.Based on biomarker distributions of extracted bitumen, Alum Shale kerogen pyrolysate and obviously‐migrated oils, and the fact that the Alum Shale in most of the areas studied is the only petroleum source‐rock extant, it is suggested that the migrated bitumen in Central Sweden is from the Alum Shale itself. Bitumen has migrated from areas where the Alum Shale is in close proxmity to Permo‐Carboniferous intrusions, such as Halleberg‐Hunneberg, into nearby areas such as Närke and ÖstergÖtland, where there is no evidence of intrusion and the indigenous organic matter is thermally immature. Other areas, where Alum Shales were associated with intrusions and consequently sourced oil, may have been eroded away.There are producing wells on the island of Gotland, where the Alum Shale is also thermally immature. It is therefore assumed that heating which was responsible for generating Gotland's oil was very localized (such as by an intrusion) or that the oil has migrated from a thermally moremature, distant area. On the basis of reservoir rock porosity, and the fact that the Alum Shale of Gotland contains no migrated component, localized heating is favored.

Three-dimensional trend mapping from wire-line logs

Mapping of lithofacies and porosities of stratigraphic units is complicated because these properties vary in three dimensions. The method of moments was proposed by Krumbein and Libby (1957) as a technique to aid in resolving this problem. Moments are easily computed from wireline logs and are simple statistics which summarize vertical variation in a log trace. Combinations of moment maps have proved useful in understanding vertical and lateral changes in lithology of sedimentary rock units. Although moments have meaning both as statistical descriptors and as mechanical properties, they also define polynomial curves which approximate lithologic changes as a function of depth. These polynomials can be fitted by least-squares methods, partitioning major trends in rock properties from finescale fluctuations. Analysis of variance yields the degree of fit of any polynomial and measures the proportion of vertical variability expressed by any moment or combination of moments. In addition, polynomial curves can be differentiated to determine depths at which pronounced expressions of facies occur and to determine the locations of boundaries between major lithologic subdivisions. Moments can be estimated at any location in an area by interpolating from log moments at control wells. A matrix algebra operation then converts moment estimates to coefficients of a polynomial function which describes a continuous curve of lithologic variation with depth. If this procedure is applied to a grid of geographic locations, the result is a model of variability in three dimensions. Resolution of the model is determined largely by number of moments used in its generation. The method is illustrated with an analysis of lithofacies in the Simpson Group of south-central Kansas; the three-dimensional model is shown as cross sections and slice maps. In this study, the gamma-ray log is used as a measure of shaliness of the unit. However, the method is general and can be applied, for example, to suites of neutron, density, or sonic logs to produce three-dimensional models of porosity in reservoir rocks.

Influence of Depositional Facies on Hydrocarbon Production in Pennsylvanian Tensleep Sandstone, Big Horn and Wind River Basins, Wyoming: ABSTRACT

Primary patterns of reservoir-rock porosity and permeability characteristics in the Pennsylvanian Tensleep Sandstone are the effects of grain-packing fabrics and early-diagenetic cements that are controlled by the environments in which the formation was deposited. These environments are a regressive sequence of shoreface to eolian and sabkha interbedded sandstones and dolomites. The formation has laterally correlatable tabular layers with strongly directional permeability in the eolian section. Tensleep reservoir characteristics can be interpreted from resistivity density and sonic log responses once calibrated to porosity, permeability, saturation, and grain-density analyses from a local full-section core. Directional permeability can be interpreted from dip logs. Because the controls on deposition are systematic, primary reservoir characteristics are predictable. These characteristics may be summarized as follows: lower Tensleep is moderately permeable shoreface sands interbedded with nonreservoir, relatively brittle dolostones; upper Tensleep is significantly more permeable cross-stratified eolian sands with approximately 10:1 ratio of along-bedding-plane vs. through-bedding-plane permeability, appearing cyclically in composite sets of strata, which have interdune sabkha, nonreservoir, horizontal to wavy-bedded, dolomitic sands at their bases. End_of_Article - Last_Page 234------------

Assessment of Reservoir Potential of Bali Geothermal Field, Indonesia: ABSTRACT

A large volume, two-phase reservoir, with a capacity of about 19 mi3 (80 km3), underlies the Donau (lake) Bratan Caldera on the island of Bali. Recent resistivity modeling indicates that the reservoir is at a depth of about 3,300 ft (1,000 m). If the reservoir rock porosity is assumed to be 8%, the saturation to be 32.5% and the temperature to be 245°C, the total energy stored in the reservoir is about 2 × 1012 MJ. This means that a promising geothermal field awaits development on Bali. End_of_Article - Last_Page 984------------

Effects of Burial Stages on Porosity in Reservoir Rocks in Mississippian Strata of Southern Saskatchewan: ABSTRACT

Effects of Burial Stages on Porosity in Reservoir Rocks in Mississippian Strata of Southern Saskatchewan: ABSTRACT

A New Device for Determining Porosity by the Gas-expansion Method

In the calculation of oil and gas reserves by volumetric methods, it isnecessary to know the volume of the reservoir available for the storage of theoil or gas. To obtain this information, a number of methods for determiningboth total and effective porosity of reservoir rocks are used. For thedetermination of the total porosity, or the porosity that considers all of thepore spaces, connected and isolated, the Russell and the Melcher-Nuttingmethods are the best known and most widely used in the petroleum industry.These methods have disadvantages in that they give the total porosity of thesamples and in that the samples are reduced to grain size in the determination.In many sands there may be little or no difference between total and effectiveporosity but in other sands there may be considerable difference between thetwo. For the estimation of oil and gas reserves the effective porosity of thereservoir rock always should be used. Of the methods that have been developed for the determination of theeffective porosity of reservoir rocks, or the porosity that considers onlythose pore spaces that are connected, the best known are the so-called Boyle'slaw and the Washburn-Bunting methods. The Washburn- Bunting method is probablythe most widely used because it gives effective porosity, it is rapid, and theapparatus is inexpensive. This method employs a special porosimeter in whichthe core sample is placed in direct contact with mercury. This method hasdisadvantages in that, as a result of the high vacuum created in the porespaces, mercury is drawn into the openings, making it impossible to use thesample for a check determination and also ruining the sample for any furthertest, such as permeability. The purpose of this paper is to describe anapparatus employing the same principle as the Washburn-Bunting method but sodesigned that the core sample does not come in contact with the mercury in theporosimeter. T.P. 1061