Stress State And Hydraulic Fracturing In the Athabasca Oil Sands

Abstract

Abstract The geological history of the Athabasca Oil Sand (with respect to historical overburden or glacial load) suggests that the minor principal compressive stress is vertical to a depth of at least 350m. Features of natural slopes, hydraulic fracturing operations, and computer stress-strain modeling confirm the existence of higher lateral stresses in the Athabasca Oil Sand, Hydraulic fracture planes are created normal to the minor principal compressive stress direction, and are therefore horizontal above a depth of 850m, and probably vertical below a depth of 850m, Change of orientation of the minor principal stress may be brought about by fracturing operations, and in turn, this may result in an ability to control inter-well communication in areas of vertical hydraulic fracturing. INTRODUCTION Development of the Athabasca Oil Sands deposit, and other similar deposits in Alberta, eventually will require hydraulic fracturing or the opening of underground cavities. A crucial design parameter for both of these procedures is the orientation and magnitudes of the principal compressive stresses. Complete stress state data in oil sands are not available at present, but methods have been developed (e.g, Haimson, 1973) to assess the state of stress at depth by hydraulic fracturing. Because of the extremely low permeabilities of the oil sand, these methods may be successful in the oil sands despite their porous, uncemented nature. Qualitative data on the state of stress are available from natural slope observations, hydraulic fracturing data, and computer slope monitoring, Quantitative data may be obtained by computer back-analysis of prototype tunnels, and by carefully controlled hydraulic fracturing. GEOLOGICAL HISTORY OF THE OIL SANDS The Athabasca Oil Sands are contained largely (95 %) within the McMurray Formation. The McMurray Formation is of Lower Cretaceous age, is largely orthoquartzitic, and was deposited upon an undulating paleoerosion surface during regional transgression of an epicontinental sea (e,g, Carrigy, 1973). The McMurray Formation has many similarities to other "blanket sands" (Krumbein and Sloss,1963); for example, an argillaceous basal sequence, overlain by a conglomeratic channel facies, a general quartzose nature, a fining-upwards of grain size, and a distinct upwards progression to more marine facies. The basal argillaceous strata have been removed locally by erosion, but underlie at least 50% of the deposit in the surface-mineable area (Dusseault, 1977). The strata usually consist of argillaceous beds with associated lignites, occasional arenaceous strata, and zones of siderite- and pyrite-cemented materials. Paleosols have been observed upon paJeotopographic highs. The argillaceous strata, commonly called the "basal clays", have been noted in thicknesses of up to 30 m (Scotland and Benthin, 1954) and have a distinctive mineralogy. Illite, kaolinite, vermiculite and mixed-layer clays comprise the clay-mineral portion of the -2fJ- fraction (Dusseault, 1977). Smectite and chlorite are absent although smectite has been identified at higher elevations within the McMurray Formation (Carrigy, 1973). Oil-bearing strata may be found directly on and somewhat interspersed with the basal deposits. The lower beds are quartz-pebble conglomerates to coarse-grained sands, often with chert pebbles and rip-up clasts.