Utilization of natural shale formations for the creation of annular barriers in oil and gas wells is currently discussed as a mean of simplifying cumbersome plugging and abandonment procedures. Shales that are likely to form annular barriers are shales with high content of swelling clays and relatively low content of cementation material (e.g., quartz, carbonates). Shales with large content of quartz and low content of swelling clays will be rather brittle and not easily deformable. In this paper we ask the question whether and to what extent it is possible to modify the mechanical properties of relatively brittle shales by chemically removing some cementation material. To answer this question, we have leached out carbonates from Pierre I shale matrix using hydrochloric acid and we have compared mechanical properties of shale before and after leaching. We have also followed leaching dynamics using X-ray tomography. The results show that removal of around 4–5 wt% of cementation material results in 43% reduction in Pierre I shale shear strength compared to the non-etched shale exposed to sodium chloride solution for the same time. The etching rate was shown to be strongly affected by the volume of fluid staying in direct contact with the shale sample.
Numerous observations have shown that shale formations may creep in and close the annulus between rock and casing in deep wells, thus effectively sealing off the well sections in a natural way. This movement of the rock is partly controlled by strain and partly by stress, and parts of the rock may be in a post-peak state while other parts are in a pre-peak state. The effect of pre-peak versus post-peak conditions on creep characteristics was studied in a set of stress relaxation tests on Pierre II shale. It was found that the stress relaxation process can be described with great precision both by a logarithmic function and a power-law function. However, only the power-law function fulfills the expected asymptotic behavior for infinite time. Temperature fluctuations can be accurately accounted for by an Arrhenius-type factor. The shear stress relaxation appears to be insensitive to pore pressure alterations. There is an indication that the relaxation rate is slightly higher when the rock is in a strain hardening state than in a strain softening state. No significant effect of exposure to NaOH was seen on the relaxation process.
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