Stratigraphic variations in oil-shale fracture properties. [Colorado and Wyoming

Abstract

The proper design and evaluation of in situ oil shale fracture and retorting experiments require that both the extreme values and spatial distribution of the controlling rock properties be adequately known. Many of the in situ technologies being considered for processing within the Green River Formation in Colorado, Wyoming and Utah depend upon the carefully controlled explosive fracturing of the rock such that suitably uniform permeabilities are achieved. The prediction, control and evaluation of explosive oil shale fracturing require a detailed knowledge of tensile strength behavior as a function of shale grade and stratigraphic position. Direct-pull tensile tests, point-load pinch tests, and four-point-bend fracture toughness tests have been utilized to develop detailed logs of the relevant fracture properties for the 37 m thick Mahogany Zone section of the Green River Formation near Anvil Points, Colorado and for the rich, upper 13 m of the Tipton Member near Rock Springs, Wyoming. For the Mahogany Zone shale tensile strengths ranged up to 15.3 MPa for direct-pull tests and 43.4 MPa for indirect tests. Fracture energy values for this shale ranged from 8 J/m/sup 2/ to 191 J/m/sup 2/. For the Tipton shale tensile strengths ranged up to 3.7 MPa for direct-pull testsmore » and 12.6 MPa for indirect tests. Fracture energy values for the Tipton averaged from 5 J/m/sup 2/ to 91 J/m/sup 2/. Detailed statistical analyses were performed on these data and on Fischer assay oil yield data to establish the correlations between them. Data from both tensile strength and fracture energy tests correlate well with lithologic and oil yield characteristics of the Mahogany Zone shale while poor correlations were found for the Tipton shale. 27 figures, 8 tables.« less