Compaction and Permeability of Oil-Shale Aggregates at High Temperatures

We have measured the change in bulk volume and permeability of an oil-shale aggregate subjected to high temperatures and pressures in a one-dimensional strain loading configuration. At stresses of 0.69 to 6.2 MPa and temperatures as high as 700 deg. C, the 6-liter sample may compact by as much as one-third, and permeability may decrease by factors of 10 to 100. Then is no simple relationship between strain or porosity and permeability. Attempts to analyze the data in terms of a Kozeny-type equation were unsuccessful. Introduction Most proposed methods for extracting hydrocarbons from oil shale involve either injection of hot gases or in-situ combustion of air/oxygen in the shale to produce reaction temperatures in excess of 300 deg. C. Unfortunately, undisturbed oil shale is not permeable enough to permit the hot gases to circulate freely. Thus, before these processing methods can become technically and economically feasible, some satisfactory fracturing treatment must be developed.The advocates of modified in-situ processes for recovering oil from oil shale propose to solve the permeability problem by extracting about 20% of the shale and blasting in slots or raises to form a cylindrical or prismatic rubble column. In the Rubble In-Situ Extraction (RISE) process, a modified sublevel caving operation is used to form a rubble column 50 to 150 m wide and 100 to 300 m high. However, when such a rubble column is retorted by either in-situ combustion or heat transfer from hot, circulating gases, the mechanical properties of both the shale particles and the wall rock will deteriorate, and the initial permeability (and porosity) may decrease as a result of compaction in the aggregate. Compaction occurs because of body forces (product of density times acceleration) acting within the rubble column itself.Although it is not yet clear what permeability or porosity values are adequate for retort operation or even what the optimum rubble column sizes and aspect ratios are, it is clear that certain quantitative data will be needed to design a successful in-situ oilshale retort. For example, measurements of temperature- and time-dependent compaction and permeability in different grades of oil shale over the range of stresses and temperatures to be expected in a retort column must be made. Data also are needed on the effect of particle-size distribution on compaction and permeability for differing grades of shale. Finally, the temperature- and time-dependent strength and frictional behavior of the wall rock and the aggregate must be studied.In this paper, we present laboratory measurements of the permeability and compaction behavior of aggregates formed from 103-L/t oil shale during retorting to high temperatures under several constant stresses. Further work at other heating rates, particle-size distributions, and oil-shale grades remains to be done. Data on the mechanical behavior of solid samples and aggregates from a spectrum of oil-shale grades are reported elsewhere. Previous Work Data on the creep/permeability behavior of oil-shale aggregates of several grades and particle-size distributions have been reported by Tisot and Sohns, Burwell el al., and Needham. Those data relevant to this study are summarized in Table 1.Tisot and Sohns report creep/permeability data for four grades of oil-shale aggregate (113, 144, 190, and 265 L/t) tested at axial stresses of 155, 138, and 2.24 MPa. These tests were conducted on samples 5.08 cm long and 1.90 cm in diameter comprising oil-shale particles 0.48 to 0.95 cm in diameter. The samples were heated from ambient temperatures to 385 or 440 deg. C at 67 deg. C/hr and then maintained at that value for about 10 hours. SPEJ P. 95＾