Beam Centrifuge Modelling of Caprock Integrity
Creators: Jia, Shenglong
Publisher: name: University of Alberta Library
Year: 2022

<p>This study, which adopts the physical modelling technique to investigate caprock deformation behaviour, intends to elucidate the impact of steam chamber expansion rate on caprock integrity.<br>
This study develops a synthetic material to replicate the behaviour of Clearwater Formation shale for the physical modelling tests. Initial experimental results with consolidated Speswhite kaolin clay show that the material is too ductile to represent the Clearwater shales. Subsequently, a mixture of Speswhite kaolin clay, Sil325 (38-75µm), cement and water is assessed but again, unconfined compressive strength of the mixture material indicates that this mixture material is relatively soft when compared with the Clearwater shale. An experimental program, which includes 160 separate formulations and considers the effect of water content, cement content, Atterberg limits and soil particle size distribution, is developed for the establishment of the database for the synthetic Clearwater shales. The results of this program provide the most suitable formulation (Speswhite kaolin clay: Sil325= 40:60, total soil-water content= 2.5 times the liquid limit, total soil-water to cement ratio= 3), which is used for the geotechnical centrifuge modelling tests. The mechanical properties of the synthetic Clearwater shale are quite close to those of the Clearwater shales. <br>
Three centrifuge modelling tests are performed by using the synthetic Clearwater shales. An appropriate thickness of lead bars is used to simulate the overburden pressure applied to the prototype. A Mariotte bottle controls water level in the plane strain box (PSB). The geomechanical caprock deflection mechanism, or GeoCDM, mimics the expansion of the steam chamber. A DIC camera records soil particle displacement at constant time intervals as the GeoCDM shears the sample. These images analyzed with GeoPIV_RG demonstrate the caprock failure evolution at different stages. <br>
Experimental results of the geotechnical centrifuge modelling test reveal that vertical displacement of the soil particles decreases as the distance from the uplifting table increases. On any horizontal plane above the uplifting table, vertical displacement decreases as the horizontal distance between the soil particle and the centreline of the model increases. The model can be divided into four regions based on the horizontal displacement contours. Intuitively, the soil particles at the left side of the centreline of the model should move towards the –X direction, while those at the right side should move towards the +X direction. However, at the bottom of the model, soil particles at the left side of the centreline move towards the +X direction while those at the right side move towards the –X direction. The top surface of the model heaves as the uplifting table moves up. The geotechnical centrifuge modelling results not only show the surface heave but also the development of vertical fractures at the top surface of the model as the uplifting table moves up. Based on the analysis of the horizontal displacement contours, the vertical fractures at the top of the model are the combined result of uplift movement of the model and the low tensile strength of the model material. However, vertical fractures of the three tests only propagate towards the mid-height of the model for a few centimeters and then stops propagating as the uplifting table still moves up. Moreover, with the increase of model depth, the width of these vertical fractures decreases, indicating that high horizontal stress can effectively prohibit the development of vertical fractures. <br>
A comparison is made between results from the geotechnical centrifuge modelling tests and those from the finite element model in “Summary of investigations into the Joslyn May 18th 2006 Steam Release”. Both geotechnical centrifuge modelling test and numerical simulation test reveal that vertices of the pressurized zone are the highly strained area. The finite element model points out that the sheared zone has turned into wide fractures providing conduit for injected steam as the SAGD process continues. Similarly, geotechnical centrifuge modelling tests reveal that the inclined fractures originate from the vertices of the uplifting table and then propagate towards from the model surface. <br>
This study uses the maximum total shear strains as the indication of the failure planes. The failure planes emerge from the uplifting table and propagate towards the model surface. This study finds that it is difficult to find the relationship between uplifting velocity and dip angles of these highly strain areas. </p>