Philip H. Nelson

Pore-throat sizes in sandstones, tight sandstones, and shales

Pore-throat sizes in siliciclastic rocks form a continuum from the submillimeter to the nanometer scale. That continuum is documented in this article using previously published data on the pore and pore-throat sizes of conventional reservoir rocks, tight-gas sandstones, and shales. For measures of central tendency(mean,mode,median),pore-throatsizes(diameters) are generally greater than2 mm in conventionalreservoir rocks, range from about 2 to 0.03 mm in tight-gas sandstones, and rangefrom0.1to0.005 mminshales.Hydrocarbonmolecules, asphaltenes, ring structures, paraffins, and methane, form another continuum, ranging from 100 A ˚ (0.01 mm) for asphaltenes to 3.8 A ˚ (0.00038 mm) for methane. The pore-throat size continuum provides a useful perspective for considering (1) the emplacement of petroleum in consolidated siliciclastics and (2) fluid flow through fine-grained source rocks now being exploited as reservoirs.

Scientific basis for safely shutting in the Macondo Well after the April 20, 2010 Deepwater Horizon blowout

As part of the government response to the Deepwater Horizon blowout, a Well Integrity Team evaluated the geologic hazards of shutting in the Macondo Well at the seafloor and determined the conditions under which it could safely be undertaken. Of particular concern was the possibility that, under the anticipated high shut-in pressures, oil could leak out of the well casing below the seafloor. Such a leak could lead to new geologic pathways for hydrocarbon release to the Gulf of Mexico. Evaluating this hazard required analyses of 2D and 3D seismic surveys, seafloor bathymetry, sediment properties, geophysical well logs, and drilling data to assess the geological, hydrological, and geomechanical conditions around the Macondo Well. After the well was successfully capped and shut in on July 15, 2010, a variety of monitoring activities were used to assess subsurface well integrity. These activities included acquisition of wellhead pressure data, marine multichannel seismic profiles, seafloor and water-column sonar surveys, and wellhead visual/acoustic monitoring. These data showed that the Macondo Well was not leaking after shut in, and therefore, it could remain safely shut until reservoir pressures were suppressed (killed) with heavy drilling mud and the well was sealed with cement.

Underpressure in Mesozoic and Paleozoic rock units in the Midcontinent of the United States

Potentiometric surfaces for Paleozoic strata, based on water well levels and selected drill-stem tests, reveal the control on hydraulic head exerted by outcrops in eastern Kansas and Oklahoma. From outcrop in the east, the westward climb of hydraulic head is much less than that of the land surface, with heads falling so far below land surface that the pressure:depth ratio in eastern Colorado is less than 5.7 kPa/m (0.25 psi/ft). Permian evaporites separate the Paleozoic hydrogeologic units from a Lower Cretaceous (Dakota Group) aquifer, and a highly saline brine plume pervading Paleozoic units in central Kansas and Oklahoma is attributed to dissolution of Permian halite. Underpressure also exists in the Lower Cretaceous hydrogeologic unit in the Denver Basin, which is hydrologically separate from the Paleozoic units. The data used to construct the seven potentiometric surfaces were also used to construct seven maps of pressure:depth ratio. These latter maps are a function of the differences among hydraulic head, land-surface elevation, and formation elevation. As a consequence, maps of pressure:depth ratio reflect the interplay of three topologies that evolved independently with time. As underpressure developed, gas migrated in response to the changing pressure regime, most notably filling the Hugoton gas field in southwestern Kansas. The timing of underpressure development was determined by the timing of outcrop exposure and tilting of the Great Plains. Explorationists in western Kansas and eastern Colorado should not be surprised if a reservoir is underpressured; rather, they should be surprised if it is not.

Pore-throat sizes in sandstones, siltstones, and shales: Reply

In his discussion of my article (Nelson, 2009), W. K. Camp takes issue with the concept that buoyancy is not the dominant force in forming and maintaining the distribution of gas in tight-gas accumulations (Camp, 2011). I will restrict my response to the issues he raised regarding buoyant versus nonbuoyant drive and to a few comments regarding water saturation and production. I claim that the pressure generated in petroleum source rocks ( P g), instead of the buoyancy pressure ( P b), provides the energy to charge most tight sandstones with gas. The arguments are fourfold: (1) buoyant columns of sufficient height seldom exist in low-permeability sand-shale sequences, (2) tight-gas systems display a pressure profile that declines instead of increases upward, (3) gas is pervasive in overpressured systems, and (4) source rocks can generate pore pressures sufficiently high to charge tight sandstones. As discussed by Berg (1975), the upward migration of gas or oil is driven by buoyant pressure P b but inhibited by capillary (injection) pressure P c. Passage through an individual pore throat by a globule or stringer of gas or oil is accomplished when P b exceeds P c; the term “stringer” is used when the required hydrocarbon height is many times greater than a pore dimension. The buoyant pressure exerted by a stringer of gas of height h is ![Formula][1] where ρ w and ρ g are the specific gravities of water and gas (relative to water), respectively, and 0.433 is the density of water expressed in oil field units of pounds per square inch per foot (Schowalter, 1979). The buoyant pressure increases linearly with the density difference and the height of the stringer. The capillary pressure P c (also called injection pressure or displacement pressure) of a porous medium is ![Formula][2] where c , with a … [1]: /embed/graphic-1.gif [2]: /embed/graphic-2.gif

Permeability-porosity data sets for sandstones

As every geophysicist knows, the reflection amplitude versus angle for reflection from an interface between two elastic rocks is given by the Zoeppritz equation. The Zoeppritz equation in the forward mode predicts the amplitude of the reflection, given the angle, the compressional and shear velocities, and densities of the rocks that form an interface. It is possible to turn the Zoeppritz equation around and ask, given the reflection amplitude and angle, compressional velocities and densities, what is the shear velocity of the lower rock? This turning around of the equation is called the inverse mode of the Zoeppritz equation. A popular procedure for inverting Zoeppritzs equation is to first make the Bortfeld type approximation of the Zoeppritz equation as \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[R\ =\ \mathrm{b}\ {+}\ \mathrm{m\ sin^{2}}{\theta}\] \end{document} Where R is the reflection amplitude, b and m are constants, relative to 𝛉, containing the rock parameters, and 𝛉 is the angle of reflection. Then using this formula, if the amplitude is plotted versus the sin2 𝛉, a straight line is obtained and b and m are determined from the graph since b is the zero angle-intercept and m is the slope. One limitation of this procedure is that the approximation should not be used beyond an angle of 30°. Another limitation of the approximation is that a 70% error in the derived shear- wave velocity can occur (Demirbag and Coruh, 1988). One look at the Zoeppritz equation shows why approximations of its functional dependence on the rock parameters are made. The amplitude is related to the rock parameters via a quite complicated algebraic expression. Until now we have not been able to invert the algebraic expression exactly, but have to first make approximations. Its actually amazing that the Zoeppritz equation with its proliferation of trig functions of the reflection angle and the square root functions of the rock …

Induced polarization research at Kennecott, 1965-1977

Currently there is much concern regarding the viability of research and development efforts within the mining and petroleum industries. This paper reviews the development of induced polarization (IP) technology by the geophysical R&D group at Kennecott over the 10-year period 1965–1975. Although the group worked on electromagnetic methods, magnetic interpretation, and borehole logging, the focus of this paper is on its frequency‐domain IP research. I wish to review not only the technical accomplishments of the group, but also to explore from a historical perspective why the group was successful and to consider the factors surrounding its demise in 1977 when Kennecott terminated its exploration research.

Model Results And Field Checks For A Time-domain, Airborne Em System

The airborne EM system known as Input was calibrated by applying theoretical homogeneous earth response curves to the response obtained on a flight over a large lake of known resistivity. The calibrated response curves for the conductive overburden case agree with field results in that 1) overburden resistivity in excess of 100 ohm‐m produces negligible deflection on the receiver channels, and 2) the maximum channel response occurs between 1 and 10 ohm‐m overburden resistivity. The calibrated response curves for scale model vertical sheets show fair to good agreement with the response to steeply dipping conductors which have been confirmed with ground‐based EM and drilling. The calibrated scale model results also show: 1) The system possesses a “passband” in conductivity‐thickness, with the first channel peaking around 10 mhos and the later channels at progressively higher values, with the sixth channel peaking at 25 mhos. 2) If a conservative detection cutoff is applied, a vertical conductor will not pro...

Geophysical and geochemical logs from a copper oxide deposit, Santa Cruz project, Casa Grande, Arizona

In support of an in‐situ leaching experiment, five holes drilled into a copper oxide deposit have been logged with geophysical and geochemical tools developed for use in the petroleum industry. When combined with geological description, chemical analyses, and mineralogical data from core and cuttings, the logs provide information regarding the alteration, fracturing, copper distribution, porosity, and permeable zones. Correlations among sonic velocity, rock strength from mechanical tests on core, and alteration indicators from neutron and potassium logs demonstrate a close link between the state of alteration and the mechanical state of the rock. Neutron activation, natural gamma‐ray, and density logs, in combination, correlate so well with copper assays that log‐based prediction of copper content is possible; in addition, an estimate of whole‐rock mineralogy is presented in log format. Based on comparisons of flow logs and acoustic logs obtained in the same holes, reductions in acoustic velocity appear t...

Petrophysical properties, mineralogy, fractures, and flow tests in 25 deep boreholes at Yucca Mountain, Nevada

As part of a site investigation for the disposal of radioactive waste, numerous boreholes were drilled into a sequence of Miocene pyroclastic flows and related deposits at Yucca Mountain, Nevada. This report contains displays of data from 25 boreholes drilled during 1979–1984, relatively early in the site investigation program. Geophysical logs and hydrological tests were conducted in the boreholes; core and cuttings analyses yielded data on mineralogy, fractures, and physical properties; and geologic descriptions provided lithology boundaries and the degree of welding of the rock units. Porosity and water content were computed from the geophysical logs, and porosity results were combined with mineralogy from x-ray diffraction to provide whole-rock volume fractions. These data were composited on plates and used by project personnel during the 1990s. Improvements in scanning and computer technology now make it possible to publish these displays.

Gas, oil, and water production from Jonah, Pinedale, Greater Wamsutter, and Stagecoach Draw fields in the Greater Green River Basin, Wyoming

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