Parameters that control the development of clay smear at low stress states: an experimental study using ring-shear apparatus

Abstract

Clay smear involves entrapment of clay or shale within the fault zone, thereby giving the fault a high capillary entry pressure and a low permeability. We examine the coherence of a clay smear as a function of normal stress, deformation rate, and water content in clay and sand. The work is based on drained ring-shear experiments. The samples used in the experiments consist of mixed sectors of sand (Baskarp Sand No. 15) and clay. The tested clays include clay from Trondheimsfjorden (Norway) and Drammensfjorden (Drammen Clay; Norway), London Clay (UK) and three different clays from the Utsira High (offshore Norway). Three stages are recognised in the development of a clay membrane. In the first stage, utilising low normal stress of 6 kPa, there is a complete absence of any semi-continuous to continuous clay smears for all clays used. Only occasional clay fragments occur on the fault plane. In the second stage, a mixture of clay and sand or patchy clay in a sand matrix developed (normal stress >25 kPa dependent upon clay type), whereas a semi-continuous to continuous clay membrane is typical for the third stage (normal stress >100 kPa dependent upon clay type). Experiments that reflect a transitional stage between the second and third stages are frequently observed. Comparing experiments with dilation and compaction, which had the same deformation conditions, it is observed that those samples that compacted during deformation became clay-covered to a larger extent than those that dilated. Differences of 2 and 9% were observed in the present study. The potential for developing a continuous clay membrane increases as: (1) normal stress increases, (2) water content of sand and clay increases, and (3) shear strength of the clay decreases. A complex relationship between strain-rate, clay types and normal stress is observed.