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Laboratory studies simulating thermal maturation of source rocks to generate and expel oil/gas differ from natural conditions. Amounts and compositions of products deviate between experiments and those found in natural petroleum source, carrier and reservoir rocks. Experimental data used in numerical models simulating oil/gas generation/expulsion thus seem to require adaption, causing significant uncertainty in present petroleum systems analysis. We designed and tested a procedure to simulate laboratory generation and expulsion of hydrocarbons under conditions most similar to natural conditions.
The critical factors in combined petroleum generation and expulsion simulation studies are: i) the pore and kerogen network of a sediment must remain as closely as possible in its natural state, as destroyed pore-systems are unsuitable for interpretation due to artificial reaction sites and migration avenues created upon experiment; ii) kerogen maturation must occur in the presence of water, as water acts as hydrogen donor for oil/gas formation and via hydrostatic pressure and associated flow stimulates migration of oil/gas; consequently, dry pyrolysis often used in petroleum generation experiments for kinetics calculation will not reflect near-natural oil/gas expulsion; iii) pressure regimes applied upon experiment must mimic differentially the lithostatic and hydrostatic pressures, as lithostatic squeezing will cause collapse of clay mineral aggregates but opposes kerogen swelling in non-lithified rocks; consequently experiments under all-directional identical confining pressures are unsuitable to reflect oil/gas expulsion; iv) the generated and then expelled oil/gas fluids must be allowed to migrate in the given pressure and permeability regime in order to avoid artificial secondary reactions, including condensation as well as cracking, hence rendering closed pyrolysis unsuitable for many petroleum expulsion studies. Exceptional cases may include tight shales, where expulsion is hindered or burial of reservoirs associated with oil to gas cracking.
Berest, P. (Laboratoire de Mecanique des Solides, Ecole Polytechnique, Palaiseau) | Bergues, J. (Laboratoire de Mecanique des Solides, Ecole Polytechnique, Palaiseau) | Brouard, B. (Laboratoire de Mecanique des Solides, Ecole Polytechnique, Palaiseau) | Durup, J.G. (Gaz de France, La Plaine Saint Denis) | Guerber, B. (Gaz de France, La Plaine Saint Denis)
Khristianovich, S.A. (Member of the USSR Academy of Sciences, Head of Laboratory Institute for Problems in Mechanics, the USSR Academy of Sciences) | Salganik, R.L. (Institute for Problems in Mechanics, the USSR Academy of Sciences) | Kovalenko, Yu.F. (Institute for Problems in Mechanics, the USSR Academy of Sciences)
American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
The operability of in-situ oil-shale retorting has been established by retorting beds of crushed shale under lithostatic loads simulating the column loads to be encountered in practical operations. The resulting compaction practical operations. The resulting compaction and reduction in permeability were correlated to predict the final permeability behavior of an actual fragmented in-situ bed with particles averaging 6 in. in diameter.
Two compaction cells, 1.25 and 5 in. in diameter, were used for the retorting experiments in which the permeability of the compacted bed was determined for average shale particle size from 0.046 to 1.0 in., shale richness from 18 to 58 gal/ton, lithostatic load from 70 to 900 psi and heating rate from 5 to 100 deg. F per hour. A correlation was developed that predicts the permeability using the lithostatic pressure, shale richness, and particle size for a shale heating rate of 10 deg. F per hour. The modified Kozeny equation was used as the theoretical basis for the correlation and the measured permeabilities could be predicted with an permeabilities could be predicted with an average error of 21 percent.
The initial permeability of an in-situ bed of broken shale will be more than adequate for the injection of heating fluids, but retorting drastically reduces the permeability as a result of compaction caused by heating the oil shale under an overburden load. To estimate the permeability of the fragmented shale bed during permeability of the fragmented shale bed during retorting, the influence of shale particle size, lithostatic pressure, temperature history, and shale richness was determined by laboratory experiments. These experimental data were correlated and then extrapolated to a larger average particle size to predict the permeability of fragmented shale beds anticipated for commercial in-situ retorting. Green River oil shale from the Piceance Creek Basin in Colorado was utilized throughout these studies.
EXPERIMENTAL APPARATUS AND PROCEDURE
Large-Scale Compaction Cell
A simplified sectional view of the large-scale compaction cell is shown in Fig. 1. The cell is cylindrical and holds about 5,000 to 7,000 gm of crushed shale, depending upon the shale richness.
The review by Professor J. C. Jaeger (Chapter 1) briefly summarizes and discusses a select group of papers by various workers dealing primarily with aspects of fracture in brittle rock materials. Professor Jaeger's treatment is primarily concerned with the correlation of phenomenological and theoretical failure criteria with empirical results from short duration tests at ambient temperatures and generally at low lithostatic and interstitial fluid pressures.
As engineering rock mechanics becomes more concerned with new techniques of mineral exploration and production at increasingly greater depths in the crust, it is well to be aware of the effect of the environment --that of the superposed stress field upon the lithostatic and interstitial fluid pressure, the temperature and the duration of the loading upon the failure of rocks either by fracture or flow. All results to be discussed here are from laboratory tests on small homogeneous samples, thus caution should be used in the direct extrapolation of these results to large inhomogeneous rock masses having quite different boundary conditions in the earth. The purpose of this supplement is to briefly point out the major effects of these parameters as examined singly, with all others held constant, and to cite some recent results which apply to the main theme on the brittle failure of rocks.
The total stress field imposed upon a small volume of porous and permeable rock in the earth's crust may be thought of as the sum of an applied differential stress, plus a lithostatic pressure, plus an interstitial fluid pressure. The differential stress portion may be completely specified by nine components, six shear and three normal. If the small volume element is in rotational equilibrium, by suitably choosing our coordinate axes we may resolve these nine components in to the three principal components. Since neither the lithostatic pressure nor the interstitial fluid pressure* contain shear components, the fluid pressure may be subtracted from the lithostatic to give the effective lithostatic pressure (see Terzaghi 1) and this in turn added to principal components from the differential stress to give the three principal effective stresses.
Brittle failure of rock as evidenced by either extension (tension) or shear fracture may be enhanced or suppressed in all rock types by suitably altering either the differential stress, or the lithostatic or the interstitial fluid pressure. This is reflected in the absolute values of the three principal effective stresses at failure as well. Typically, at the lower values of differential stress, low lithostatic and high interstitial pressures (equaling relatively low principal effective stresses), the rock is weak, being nearly elastic up to its rupture point and failure is by extension fracture normal to the least principal effective stress, s3. As the differential stresses and the lithostatic pressure are increased or as the interstitial pressure is decreased, the rock becomes stronger and more ductile. Failure at these conditions takes place by a combination of the extension fracture (normal to s3) and shear fracture which is inclined at an angle from a few degrees to less than 45 degrees to s1.