Showing papers on "Effective porosity published in 1994"

Models of Porosity Formation and Their Impact on Reservoir Description, Lisburne Field, Prudhoe Bay, Alaska

Abstract: The Lisburne field at Prudhoe Bay, Alaska, produces from shelfal carbonates in the Pennsylvanian Wahoo formation. Four major factors control reservoir behavior: (1) depositional stratification, (2) a fractured, permeable subunconformity alteration zone (SAZ), (3) multiple episodes of porosity formation, and (4) faulting. This paper is the first written description of facies, cyclicity, and diagenetic processes as they apply to porosity formation and reservoir modeling in the Lisburne field. Successive depositional cycles in the Wahoo formation pass from ooid/skeletal grainstones deposited in shoal complexes to oncolitic packstones and skeletal/peloidal wackestones formed in restricted lagoonal environments. Geochemical data and crosscutting relationships between porosity and unconformities, pressure solution features, fractures, and faults provide evidence of three distinct episodes of porosity formation. Earliest porosity is probably due to periodic, localized exposure during the Pennsylvanian. A second stage of porosity is associated with shallow-burial dolomitization that probably began during Permian-Triassic subaerial exposure. Reservoir quality in dolomites varies with the degree of neomorphic recrystallization. The third stage began in the Cretaceous and lasted in o the Tertiary, and is associated with final burial and hydrocarbon maturation. This burial dissolution event also opened existing fault systems, creating a complex reservoir. Most Lisburne field effective porosity is of late-burial origin due to either dolomitization or dissolution. Faults and the SAZ act as giant collectors of oil from low-permeability matrix, and they have reduced the number of wells needed for field development. Faults also complicate waterflood implementation and maintenance of uniform reservoir pressure. Field studies such as these demonstrate the importance understanding the diagenetic history of a reservoir can have for field management and development planning.

Diffusion of14C in dense saturated bentonite under steady-state conditions

Abstract: Diffusion coefficients are critical parameters for predicting migration rates and fluxes of contaminants through clay-based barrier materials used in many waste containment strategies. Cabon-14 is present in high-level nuclear fuel waste and also in many low-level wastes such as those generated from some medical research activities. Diffusion coefficients were measured for14C (in the form of carbonate) in bentonite compacted to a series of dry bulk densities,ρb, ranging from about 0.9 to 1.6 Mg/m3. The clay was saturated with a Na-Ca-Cl-dominated groundwater solution typical of those found deep in plutonic rock on the Canadian Shield. Both effective,De, and apparent,Da, diffusion coefficients were determined.De is defined asD0Τane, where D0 is the diffusion coefficient in pure bulk water,Τa the apparent tortuosity factor, andne the effective porosity available for diffusion; andDa is defined asD0Τane/(ne+ρbKd), where Kd is the solid/liquid distribution coefficient. BothDe andDa decrease with increasingρb:De values range from about 10×10−12 m2/s atρb≃0.9 Mg/m3 to 0.6×10−12 m2/s at 1.6 Mg/m3, andDa values vary from approximately 40×10−12 to 4×10−12 m2/s over the same density range. The decrease inDe andDa is attributed to a decrease in bothΤa andne asρb increases. The data indicate thatne is <10% of the total solution-filled porosity of the clay at all densities.Kd values for14C with the clay range from about 0.3 to <0.1 m3/Mg; this indicates there is a small amount of14C sorbed on the clay and/or some14C is isotopically exchanged with12C in carbonate phases present in the clay. Finally, theDe values for14C are lower than those of other diffusants — I−, Cl−, TcO4−, and Cs+ — that have been measured in this clay and pore-water solution. This is attributed to lower values for bothne andD0 for14C species relative to those of the other diffusants.

Validity of the porosity/effective-stress concept in sedimentary basin modelling

Abstract: When modelling sedimentary basins, the aim is to simulate the geometric deformations caused by compaction during geological time by coupling them with fluid flows (water and hydrocarbons). Compaction of sediment is the result of various mechanisms that are not yet well understood. In actual basins it is analysed almost exclusively through field measurements of the porosity, the only 'strain' parameter which can be measured. The standard approach to simulating compaction in basin modelling is to couple a porosity/depth or a porosity/effective-stress relationship with a fluid flow law in porous media (Darcy's law).

Computation of porosity redistribution resulting from thermal convection in slanted porous layers

Abstract: Unlike fluid displacement due to regional hydraulic head, thermoconvective motions are generally slow. The thermal impacts of such movements are very weak, whereas their chemical impacts may be significant because of their cumulated effects over geologic time. For nonhorizontal thick sedimentary reservoirs, the fluid velocity due to thermal convection can be accurately approximated by an explicit function of the dip of the reservoir, the permeability and the difference in thermal conductivity between the aquifer and the confining beds. The latter parameter controls the rotation direction of the flow and, for clastic reservoirs bounded by impervious clayey media, fluid moves up the slope along the caprock layer. As the fluid velocity is small, the major rock-forming minerals control the fluid composition by thermodynamic equilibrium. Thus, whereas the volume of redistributed minerals depends on the volume of water circulated, the localization of porosity enhancement is strongly controlled by the reservoir mineralogy. With realistic values of permeability and layer thickness, several per cent of secondary porosity per million years can be created or lost at shallow depth (<2 km), depending on the chlorinity, the set of representative minerals and the temperature. In sandstone reservoirs and high-chlorinity calcarenite reservoirs, the porosity decreases under the caprock where hydrocarbons can accumulate. In chloride-depleted carbonate aquifers, the simultaneous control by carbonates, silica and aluminosilicates can produce a decrease of porosity above the bedrock and an enhancement of porosity under the caprock. However, computations show that the quality of the upper part of the reservoir is mainly reduced by the precipitation of silica and clays.

Determination From Well Logs of Porosity and Permeability in a Heterogeneous Reservoir

Abstract: A technique has been developed for the petrophysical evaluation of well logs from lithologically complex sandstone reservoirs located in the Barrow Sub-basin in Western Australia. The reservoirs are sands of fairly high porosity and low permeability with a significant amount of clay minerals, among them glauconite. Principal component and cluster analysis were applied to classify the electric facies associated with lithofacies. Bayes discriminant analysis was used to identify several lithofacies in the reservoirs. The different lithofacies are characterized by distinct relationships between porosity and acoustic interval transit times as well as between porosity and permeability. This paper presents mathematical models for each lithofacies for the determination of porosity and permeability from well logs.

Effective porosity of unconsolidated sand; Estimation and impact on capture zone geometry

Abstract: Effective and total porosity were measured for three sand samples, and a computer model was used to evaluate the impact of effective porosity (ne) on the geometry of a groundwater capture zone. Estimates of effective porosity ranged from 90 to 94 percent of values obtained for total porosity. While the magnitude of effective porosity is inversely proportional to capture zone area, the upgradient radius of capture is most sensitive to changes in the magnitude ofne. Incremental changes in the magnitude ofne have the greatest impact on capture zone radii for low values ofne. The results of this study suggest that: (1) a simple permeameter and vacuum pump apparatus can facilitate the estimation of effective porosity for unconsolidated sands, (2) the magnitude of effective porosity is close to that of total porosity for unconsolidated sand, and (3) accurate estimates of effective porosity are important for modeling the remediation of contaminated groundwater with capture zones.

Sedimentologic and Diagenetic Controls on Aquifer Properties, Lower Cretaceous Edwards Carbonate Aquifer, Texas: Implications for Aquifer Management

Abstract: The distribution of water in the Edwards aquifer was assessed using a core and log-based study. Porosity distribution reflects both depositional fabric and subsequent diagenesis. Vertical facies stacking patterns influence the depositional porosity as well as dolomitization and diagenetic porosity modification. Subtidal facies deposited during sea-level highstands are generally undolomitized and exhibit low porosity (5 to 10 percent); platform grainstones commonly have high depositional porosity and significant solution enhancement (20- to 42-percent porosity). Dolomitized subtidal facies in tidal-flat-capped cycles have very high porosity (20 to 40 percent) because of selective dolomite dissolution in the fresh-water aquifer. Porosity in former evaporite beds is high in some areas because of dissolution and collapse, but it is low where gypsum was replaced by calcite cement. Low-energy subtidal and evaporitic units in the Maverick Basin have porosities that are generally less than 15 percent. The overlying basinal packstones and grainstones have solution-enhanced porosities of 25 to 35 percent. Mapped average porosity also shows nonstratigraphically controlled variations. Diagenesis associated with fluctuations in water chemistry near the saline/fresh-water interface may be one cause. Other complex patterns of high and low porosity are attributed to structurally and hydrologically controlled porosity enhancement and cementation. Three-dimensional mapping of porosity trends provides data for improved aquifer management. Only about 3 percent of the water stored in the aquifer during a year of average water level lies above the water table at which natural spring flow is diminished. An average specific yield of 42 percent in the unconfined aquifer is determined from total porosity, changes in the water-table elevation, and changes in estimated recharge and discharge. Average storativity of 2.6 10-4 in the confined Edwards is estimated using average porosity and barometric efficiency calculated from comparing water-level hydrographs and atmospheric-pressure changes.

Single-well tracer methods for hydrogeologic evaluation of target aquifers

Abstract: Designing an efficient well field for an aquifer thermal energy storage (ATES) project requires measuring local groundwater flow parameters as well as estimating horizontal and vertical inhomogeneity. Effective porosity determines the volume of aquifer needed to store a given volume of heated or chilled water. Ground-water flow velocity governs the migration of the thermal plume, and dispersion and heat exchange along the flow path reduces the thermal intensity of the recovered plume. Stratigraphic variations in the aquifer will affect plume dispersion, may bias the apparent rate of migration of the plume, and can prevent efficient hydraulic communication between wells. Single-well tracer methods using a conservative flow tracer such as bromide, along with pumping tests and water-level measurements, provide a rapid and cost-effective means for estimating flow parameters. A drift-and-pumpback tracer test yields effective porosity and flow velocity. Point-dilution tracer testing, using new instrumentation for downhole tracer measurement and a new method for calibrating the point-dilution test itself, yields depth-discrete hydraulic conductivity as it is affected by stratigraphy, and can be used to estimate well transmissivity. Experience in conducting both drift-and-pumpback and point-dilution tests at three different test sites has yielded important information that highlights both the power and the limitations of the single-well tracer methods. These sites are the University of Alabama Student Recreation Center (UASRC) ATES well field and the VA Medical Center (VA) ATES well field, both located in Tuscaloosa, Alabama, and the Hanford bioremediation test site north of Richland, Washington.

Hydraulic conductivity of shaly sands

Abstract: THEORETICAL BASIS The effects of clays on the hydraulic conductivity of a sandstone are analyzed by considering a simple clay coating structure for the sand grains. In the model, silicate insulating nuclei are uniformly surrounded by charged clay particles. The total charge on the clays is compensated by a counterion density Assuming a capillary Aow regime inside this granular model a Kozeny-Caxman type equation has been derived, expressing its intrinsic permeability k in terms of a porosity-tortuosity factor and of the parameter The power-law derived expression shows that k decreases with the amount of clay, not only because a high implies a narrowing of the pore channels, but also because it modifies the hydraulic tortuosity of the medium. This new equation has been statistically tested with extensive petrophysical laboratory data for different types of shaly sandstones. In the worked model a sand grain is represented as a coated sphere of radius having an insulating nucleus of radius and a shale shell encompassing a volume fraction of the solids. A shaly sand is a self-similar mixture of such composite particles and water, having an effective porosity The shale shells are also treated as self-similar mixtures of clays and water with a porosity On the other side, a clay particle is taken as a charged sphere of radius and charge density that, when immersed in an aqueous solution, develops around itself a purely diffuse counterionic atmosphere. Such representation has been shown adequate to simulate the electromagnetic properties of shaly sands (de Lima and Sharma, 1990; 1992), as well on interpret log measurements (de Lima and Dalcin 1994). From the hydraulic point of view the above model can be approximated by an intricate system of sinuous capillaries (Wyllie and Spangler, 1952) having an intrinsic permeability given by INTRODUCTION A fundamental petrophysical parameter in evaluating oil and water reservoirs is the hydraulic conductivity or permeability of the medium. Several empirical procedures of limited validity have been proposed to estimate the intrinsic permeability of sandstones based on petrophysical data derived from well logs (Raiga-Clemenceau, 1987; Scott and Purdy, 1988; Sen et al., 1990). They are mostly supported by a relation originally due to Kozeni (Wyllie and Spangler, 1952) that was derived for a porous model represented as a bundle of capillaries. The Kozeni equation relates the permeability k to the effective porosity the hydraulic tortuosity and the average of the capillary radius of the medium. In this work, based on a simple geometrical representation for the electric and hydraulic behaviors of grain-coated sands, following the treatment proposed by Wyllie and Spangler (1952), a new Kozeny-Carman type equation is derived, expressing the intrinsic permeability of these rocks in terms of parameters that are easily measured or computed from geophysical log measurements. The correlation of computed values and data effectively measured on core samples attest the feasibility of the proposed relation. ing well