Showing papers on "Effective porosity published in 1984"

A Model for Investigating Mechanical Transport in Fracture Networks

Abstract: A technique is presented to determine when anisotropic fracture systems can be modeled as equivalent porous media (continua) for transport. In order to use the continuum approach, one must demonstrate that the fracture system has the same transport behavior as an equivalent porous medium. Hydraulic effective porosity is calculated as the product of specific discharge and mean travel time, divided by linear length of travel. Specific discharge and hydraulic effective porosity are measured in different directions of flow in regions of varying size with constant hydraulic gradients. If the fracture system behaves like an equivalent porous medium, directional flow has the following properties: (1) specific discharge can be predicted from a permeability tensor and (2) hydraulic effective porosity is independent of direction of flow. A numerical model has been developed to simulate mechanical transport under steady flow in a discrete fracture network. The model is used to determine the distribution of travel times from inlet to outlet for fluid traveling in stream tubes. We have examined only systems with parallel fracture sets in which all fractures are long compared to the region under study. These systems satisfy criterion 1 in that flux can be calculated using a porous medium equivalent. However, these systems do not satisfy criterion 2 because hydraulic effective porosity is shown to be directionally dependent. Thus, even though flux can be accurately predicted using porous medium assumptions for some fracture systems, it may not be possible to accurately predict mechanical transport using these same assumptions.

The Chemistry of Secondary Porosity: Part 2. Aspects of Porosity Modification

Abstract: The development of secondary porosity (porosity enhancement) in many sandstones is the result of aluminosilicate and/or carbonate dissolution. The dissolution of aluminosilicate minerals and subsequent porosity enhancement is a problem of aluminum mobility. Our experimental data demonstrate that it is possible to increase significantly the mobility of aluminum and to transport it as an organic complex in carboxylic acid solutions. These same carboxylic acid solutions have the capability of destroying carbonate grains and cements. Carothers and Kharaka have shown that concentrations of carboxylic acid anions range up to 5000 ppm over a temperature range of 80-200°C in some oil field formation waters. Our experiments show that acetic acid solutions at the same concentrations and over the same temperature range can increase the solubility of aluminum by one order of magnitude, whereas oxalic acid solutions increase the solubility of aluminum by three orders of magnitude. The textural relations observed in the experiments are identical to those observed in sandstones containing porosity enhancement as a result of aluminosilicate dissolution. A natural consequence of the burial of sedimentary prisms is the maturation of organic material. These maturation reactions result in the evolution of significant amounts of organic acids and carbon dioxide. The experiments suggest that the enhancement of porosity in a sandstone as a result of aluminosilicate or carbonate dissolution is the natural consequence of the interaction of organic and inorganic reactions during progressive diagenesis. The degree to which porosity enhancement develops depends on the ratio of organic to inorganic matter, the initial composition of the organics, the sequences, rates and magnitude of diagenetic reactions, fluid flux, and sand/shale geometry.

Probability encoding of hydrologic parameters for basalt. Elicitation of expert opinions from a panel of three basalt waste isolation project staff hydrologists

Abstract: The present study implemented a probability encoding method to estimate the probability distributions of selected hydrologic variables for the Cohassett basalt flow top and flow interior, and the anisotropy ratio of the interior of the Cohassett basalt flow beneath the Hanford Site. Site-speciic data for these hydrologic parameters are currently inadequate for the purpose of preliminary assessment of candidate repository performance. However, this information is required to complete preliminary performance assessment studies. Rockwell chose a probability encoding method developed by SRI International to generate credible and auditable estimates of the probability distributions of effective porosity and hydraulic conductivity anisotropy. The results indicate significant differences of opinion among the experts. This was especially true of the values of the effective porosity of the Cohassett basalt flow interior for which estimates differ by more than five orders of magnitude. The experts are in greater agreement about the values of effective porosity of the Cohassett basalt flow top; their estimates for this variable are generally within one to two orders of magnitiude of each other. For anisotropy ratio, the expert estimates are generally within two or three orders of magnitude of each other. Based on this study, the Rockwell hydrologists estimate the effectivemore » porosity of the Cohassett basalt flow top to be generally higher than do the independent experts. For the effective porosity of the Cohassett basalt flow top, the estimates of the Rockwell hydrologists indicate a smaller uncertainty than do the estimates of the independent experts. On the other hand, for the effective porosity and anisotropy ratio of the Cohassett basalt flow interior, the estimates of the Rockwell hydrologists indicate a larger uncertainty than do the estimates of the independent experts.« less

Mechanical transport in two-dimensional networks of fractures

Abstract: The objectives of this research are to evaluate directional mechanical transport parameters for anisotropic fracture systems, and to determine if fracture systems behave like equivalent porous media. The tracer experiments used to measure directional tortuosity, longitudinal geometric dispersivity, and hydraulic effective porosity are conducted with a uniform flow field and measurements are made from the fluid flowing within a test section where linear length of travel is constant. Since fluid flow and mechanical transport are coupled processes, the directional variations of specific discharge and hydraulic effective porosity are measured in regions with constant hydraulic gradients to evaluate porous medium equivalence for the two processes, respectively. If the fracture region behaves like an equivalent porous medium, the system has the following stable properties: (1) specific discharge is uniform in any direction and can be predicted from a permeability tensor; and (2) hydraulic effective porosity is directionally stable. Fracture systems with two parallel sets of continuous fractures satisfy criterion 1. However, in these systems hydraulic effective porosity is directionally dependent, and thus, criterion 2 is violated. Thus, for some fracture systems, fluid flow can be predicted using porous media assumptions, but it may not be possible to predict transport using porous media more » assumptions. Two discontinuous fracture systems were studied which satisfied both criteria. Hydraulic effective porosity for both systems has a value between rock effective porosity and total porosity. A length-density analysis (LDS) of Canadian fracture data shows that porous media equivalence for fluid flow and transport is likely when systems have narrow aperture distributions. 54 references, 90 figures, 7 tables. « less

The pore geometries of reservoir rocks

Abstract: A ternary pore geometry classification scheme for reservoir rocks places intergranular and intercrystalline porosity at a common pole. These porosity types tend to have well interconnected pores and usually are good reservoirs. Intragranular, moldic, and vuggy porosity are grouped at another pole of the classification triangle. Rocks with these porosity types usually have poorly interconnected pores and low permeability. Rocks with microporosity are grouped at the final pole of the triangle. These may be argillaceous sandstones, finely textured carbonates, diatomites,or tripolitic cherts. Water‐wet microporous rocks hold bound water. Rocks with significant amounts of microporosity and/or intragranular, moldic, and vuggy porosity need fractures, either natural or induced, to make an attractive reservoir. Fracture porosity may occur by itself or combined with any other porosity type. Reservoirs typically contain multiple pore types, although one type often predominates. Large scale cavernous features in carbonates occasionally contain oil.

The dual-porosity model, A newly developed interpretation method for shaly sands

Abstract: The dual-porosity model has recently been developed by Total-Compagnie Francaise des Petroles for the determination of water-saturation in hydrocarbons-bearing shaly reservoirs form wholly log-derived data. It is operated by use of an equation whereby the influence of the clay content of the reservoir rock onto the formation resistivity is interpreted through modern theories of cation-exchange conductance. By taking advantage of the concepts applied to the elaboration of the dual-water model, the equation relates the amount of clay-bound water in the reservoir to the total and effective porosities determined by interpretation of conventional logs. On the other hand the equation applies to the bound-water specific volume a geometrical factor which is different from that affecting the effective porosity. Hence the appelation of dual-porosity model.

Effective porosities of basalt: A technical basis for values and probability distributions used in preliminary performance assessments

Abstract: The Basalt Waste Isolation Project in its role of evaluating the feasibility of siting a high-level nuclear waste repository in the deep basalts of the Columbia Plateau is gathering information on basalt parameters that define waste isolation capability. Effective porosity is an important parameter in fluid transport analysis on repository performance assessment. This report presents and discusses the technical basis for values and probability distributions of effective porosity for the candidate site in basalts of the Hanford Site in Washington State. The appropriateness of different types of probability distributions of values of effective porosity for the major basalt hydrogeologic units (i.e., flow tops and flow interiors) of candidate flows are considered with respect to their applicability in performance assessment models. 36 refs., 23 figs., 9 tabs.

Carbonate pore systems: porosity/permeability relationships and geologic analysis

Abstract: The porosity/permeability relationships of the common carbonate rock types have been studied, with emphasis on the variety of pore types in upward-shoaling grainstone sequences including: Smackover, Lansing, Salem, and San Andres Formations. A result of these studies is an improved conceptual understanding of permeability gained from the cross-plotting of porosity and permeability data from plugs and whole cores accompanied by textural and fabric analyses of rock samples, thin sections, serial sections, and pore casts. Once the depositional texture and fabric of the rock are defined in terms of porosity and permeability, the evaluation of fractures and secondary porosity can be addressed. The secondary porosity is observed to be as high as 14% of the rock volume in the Smackover example and 21% of an oolitic sample from the Lansing Formation. Pore casts and serial sections reveal that the grain-moldic porosity is poorly connected to the intragranular pore system and contributes little to the permeability of the rock. This insight allows quantitative estimates of this type of secondary porosity using standard porosity and permeability data. The geologic and quantitative analysis of the various pore types and porosity/permeability relationships also aided in the interpretation of the log data from the reservoirs studied.

Reservoir properties and ore structure of tight gas sands

Abstract: Thin section and SEM observations indicate that tight gas sands may be grouped into four broad categories based on pore geometry. These consist of (1) primary interparticle porosity; (2) primary interparticle porosity filled with authigenic minerals; (3) primary porosity reduced to narrow cracks with secondary honeycombed grains; and (4) intercrystalline porosity within a fine grained, elongate matrix. Type 1 porosity is common in conventional sands, types 2 and 3 are prevalent in tight sands, and type 4 is a rare class found in extremely tight rocks. Reservoir property data on 51 sandstone core samples from the Mesaverde, Spirit River, and Frontier Formations led to an attempt to correlate reservoir parameters with pore geometry. The reservoir properties measured on these rocks under net confining stress include dry permeability, relative permeability, porosity to gas, and pore volume compressibility. Results of the core analysis were combined with petrographic information.

Reservoir Property Implications of Pore Geometry Modification Accompanying Sand Diagenesis: Anahuac Formation, Louisiana: Part 3. Applications in Exploration and Production

Abstract: This paper represents a study of reservoir pore modification accompanying diagenetic secondary porosity development within a deep (13,400 ft) overpressured Anahuac Formation sandstone in southern Louisiana. Secondary porosity formed by dissolution of carbonate cement, detrital grains, and other soluble minerals comprises a significant portion of porosity formed in U.S. Gulf Coast Tertiary reservoir sands. The primary pore system within this reservoir is believed to have been significantly enlarged (by up to 32% porosity) by acidic fluids generated during hydrocarbon maturation and dewatering of adjacent shales. Subsurface secondary porosity development within sandstones is significant in influencing the development of potential reservoir porosity after much of the primary porosity has been destroyed by mechanical and chemical compaction. Properties of the reservoir pore system that affect fluid flow and mechanical resistance of the reservoir to compaction accompanying production will also be influenced. Characteristics of the reservoir pore system were established by study of whole core samples using scanning electron microscopy, petrographic examination, mercury injection, and simulated in-situ reservoir condition core testing. Secondary pore size and distribution was found to be influenced by sandstone mineralogy, grain size, sorting and angularity, the pore matrix content, and by sedimentary structures and resulting textural components that may hinder fluid flow. Changes in the mechanical resistance to compaction caused by the development of secondary porosity in sandstone reservoirs is important when considering reservoir stress sensitivity. Keystone bridging relationships between grains can be established during the initial phases of compaction so that when leaching of cement and soluble grains occurs, a less soluble quartz grain matrix is left the support porosity development. Special core tests were performed at simulated in-situ reservoir conditions of pressure and temperature to examine porosity and permeability reduction as a function of effective stress generated by pore pressure reduction (simulated fluid production). Observed volumetric strain to uniaxial compaction at reservoir conditions was determined within portions of the sand c ntaining high (25-30%) porosity. Test results exhibited less than 1% reduction in total bulk volumes accompanying a 60% reduction in pore pressure. Permeabilities measured at in-situ conditions were commonly an order of magnitude less than those measured at ambient conditions. However, with increased effective stress applied the rock fabric, data suggest that permeabilities decrease at a much slower rate, reflecting constriction of pore throats rather than constriction of stress-induced microfactures thought to exist in core samples at ambient conditions.

Log Evaluation Of Clastic Shaly Formations Using Corrected Rwa-Ratio Techniques

Abstract: This Rwa-Ratio technique compensates simultaneously for the effects of shaliness on porosity and resistivity. The shale correction factor which aids in defining the effective porosity limit of hydrocarbon production can also be used to determine the magnitude of these shale effects. The ability to select the water saturation exponent permits flexibility in formations with varying physical characteristics.