Hydraulic fracturing in shale at West Valley, New York: A study of bedding-plane fractures induced in shale for waste disposal

Abstract

Analytical models were developed which indicate that bedding plane fractures can be induced and propagated hydraulically in shale, at least to moderate depths, for example, 3,000 feet (900 metres), even in an area with predominantly vertical principal earth stress, because the tensile strength of shale in the direction parallel to bedding planes is much greater than the tensile strength in the direction normal to bedding planes. Natural joints, if they are intersected by induced bedding-plane fractures, may be extended at certain depths, for example, between 1,000 and 3,000 feet (300-900 metres), however, their extension may cease when they intersect weak bedding planes. Therefore, hydraulically induced bedding-plane fractures in shale are likely to be restricted to a limited vertical zone, which is an essential requirement for the disposal of certain kinds of wastes, such as low- and intermediate-level liquid radioactive wastes, by injection with cement into hydraulically induced fractures in shale. The orientation of induced fractures, the tensile strength of shale and the earth stress in the direction normal to the fracture plane and the cohesive force at the fracture tip can be estimated from injection pressure, pressure-decay and uplift of the ground surface after injections. From 1969 through 1971 at a site near West Valley, New York, five water injections and one grout injection, most of them tagged with radioactive tracers were made in nearly horizontal shale beds of Late Devonian age, at depths ranging from 500 to 1,450 feet (152-442 metres). Bedding-plane fractures had been induced hydraulically by these injections. The orientations of the induced fractures estimated from injection data have been confirmed by the gamma-ray surveys made in four observation wells, each 150 feet (46 metres) distant from the injection well. Bedding-plane fractures seem to have been induced within a 20-foot (6-metre) thick zone. The vertical earth stress at the test site is found to be the equivalent of the overburden pressure. The tensile strengths in the direction normal to bedding planes is estimated, from the injection data, to be 450 to 600 pounds per square inch (3242 kilograms per square centimetre) for shales at depths ranging from 1,010 to 1,450 feet (308-442 metres). At a 500-foot (152metre) depth, however, the tensile strength is about 100 pounds per square inch (7 kilograms per square centimetre). Cohesive force at the fracture tip is estimated to be approximately 20 to 35 percent of the tensile strength. Results from injections made at the same depth at different times are consistent. However, no actual data on the tensile strength of these shales are available. Therefore, there is no way to prove the estimated tensile strengths to be correct.