Showing papers on "Effective porosity published in 1986"

Porosity Reduction in Sandstone by Quartz Overgrowth

Abstract: A mathematical model for predicting porosity histories due to quartz overgrowth in quartz-rich sandstone reservoirs has been developed. The model neglects secondary dissolution porosity and other diagenetic processes such as compaction and precipitation of carbonate and clay minerals. Nevertheless, the model is straightforward and easy to use to calculate porosity. The calculated porosity corresponds to measured porosity of simple quartz-cemented sandstones in 27 wells in North and South America. The model is based on fluid flow through initially porous, unlithified sands in which the fluid phase is saturated with Si(OH)4 and is always in equilibrium with quartz. As the continuously circulating fluid migrates updip toward the basin edge, it cools, and quartz precipitates into the pore spaces causing a loss of porosity. Basinal fluid velocities may be calculated assuming (1) thermal convection and (2) hydrostatic pressure due to recharge at the edge of the basin. Porosity diagrams relating porosity to geothermal gradient, burial rate, and depth of burial are compared to thermal maturation models of source rocks, fluid flow history, and grain-size distribution. The rate of porosity reduction depends on the following variables in decreasing order of significance: buria rate, age, initial porosity, basin size (dip angle), fluid dynamics, initial permeability, and geothermal gradient.

Dependence of Effective Porosity on Fracture Continuity in Fractured Media

Abstract: Fractured geologic media are typically considered to behave as equivalent porous-media continua for the purposes of hydrologic testing and modeling. The equivalent-continuum effective porosity of a fractured medium is frequently measured using tracer tests performed over distances that are small relative to the scale of interest in hydrologic and transport modeling. The larger-scale value of effective porosity can differ from the smaller-scale measured value by several orders of magnitude, depending in part on the relative degree of fracture continuity between inflow and outflow zones at the different scales. Based on the results of a simplified numerical modeling study for fractured media with poor continuity at the location and scale of interest, flow is dominated by the intact rock matrix, resulting in a higher effective porosity. For fractured media with uninterrupted fracture continuity at the location and scale of interest, the effective porosity may be substantially lower due to the fracture-dominated nature of the flow.

Organic-Inorganic Interactions as a Mechanism for Porosity Enhancement in the Upper Cretaceous Ericson Sandstone, Green River Basin, Wyoming

Abstract: Geochemical and petrographic studies of core and cuttings samples from the Upper Cretaceous Ericson Sandstone in the Forest Oil, Jonah Gulch well, 9-26n-l07w, reveal the presence of two distinct enhanced porosity zones that can be differentiated by their position relative to a paleo-oil/water contact. The evolution of enhanced porosity in these two zones can best be explained by experimentally predicted organic-inorganic chemical interactions. Both intragranular carbonate dissolution within chert clasts in the upper diagenetic zone and mixed-layer clay dissolution in the lower zone are apparently the result of organic-inorganic interactions. These interactions are the natural consequence of progressive burial of a sedimentary prism containing sand and shale. The sequences of inorganic diagenesis for the Ericson Sandstone reservoir and organic maturation in the adjacent shales were de- termined independently. Then, spatial, temporal, and textural relationships in conjunction with reaction temperatures were used to integrate the two sequences. The synthesis of the organic and inorganic systems shows that the two major porosity enhancement events can be directly tied to organic solvents produced by progressive maturation reactions. Dissolution of mixed-layer clay in the lower zone was associated with a pulse of organic acids generated prior to the formation and migration of most liquid hydrocarbons. These organic solvents-primarily carboxylic acids and phenols-complexed aluminum in the mixed-layer clay. This complexing action caused the destabilization and dissolution of the clay resulting in porosity enhancement. In the upper diagenetic zone, porosity enhancement occurred after hydrocarbon migration. The carbonic acid that dissolved carbonate within the chert grains was produced by thermal destruction of reservoired organic fluids in the upper Ericson. Thus, both porosity enhancement events can be related to organic solvents generated during progressive maturation. The interpreted organic-inorganic interactions and associated porosity enhancement events can be diagrammed using a clastic reaction pathway flow chart. The flow chart uses the four component system of COz, organic acids, carbonates and aluminosilicates to illustrate the evolution of enhanced porosity through time. Use of such flow charts represents the initial step in predicting regions of maximum enhanced and preserved porosity in the subsurface.

Porosity and permeability of reservoirs and caprocks in the eromanga basin, south australia

Abstract: When effective porosity and permeability are measured at simulated overburden pressure, and grain size variation is taken into account, two distinct relationships are evident for Eromanga Basin reservoirs. Reservoirs in the Hutton Sandstone and Namur Sandstone Member behave such that significant porosity reduction can be sustained with retention of high permeability, whereas permeability of reservoirs in the Birkhead Formation and Murta Member is critically dependent on slight porosity variations. Logging tool responses are compared with core-derived data to show in particular the effects of grain size and clay content on the gamma ray, sonic, and density tools, where clay content is assessed from cation exchange capacity measurements. Sonic and density crossplots, constructed to provide comparison with a water-saturated 'reference' reservoir, are advantageous in comparing measured effective porosity from core plugs at overburden pressure with porosity calculated from logs. Gamma ray and sonic log responses of the Murta Member in the Murteree Horst area are clearly distinct from those of all other reservoirs, perhaps partly due to differences in mineralogy and shallower depth of burial compared with other formations.

The Preservation of Primary Porosity Through Hydrocarbon Entrapment During Burial

Abstract: The authors examine the manner in which trapped pore fluids can inhibit sediment compaction during burial and, hence, can enhance primary porosity at depth. In model studies, they find that primary porosity can be enhanced through fluid entrapment by a factor of two to three at a depth of 30,000 ft (9144 m). Preservation of porosity is accompanied by the development of an overpressured condition. The magnitude of these effects depends on the pore fluid, its compressibility, the depth at which the reservoir is sealed, the mechanical properties of the rock matrix, and-in the case of a gas reservoir-the thermal gradient. These results indicate that reservoir formations of significant porosity may be preserved to depths greater than 30,000 ft (9144 m) and that, under favorable conditions, it may be possible to identify such formations seismically.

Method and apparatus for differentiating low porosity limestones from high porosity gas sands

Abstract: METHOD AND APPARATUS FOR DIFFERENTIATING LOW POROSITY LIMESTONES FROM HIGH POROSITY GAS SANDS Abstract of the Disclosure By taking the ratio of silicon to calcium pulsed neutron capture gamma radiations together with a formation capture cross section measurement, in formations (20) having low hydrogen index, a meas-urement indicator is provided for differentiating low porosity limestones from high porosity gas sands.