Assessing the Alterations In Berea Sandstone Mechanical Properties Induced by the Permeation of Bentonite Gels of Varying NaCl Concentrations: A Wellbore Strengthening Perspective

Abstract

Lost circulation, which is the loss of drilling fluid into a formation during drilling, is commonly induced by wellbore pressure exceeding that of the fracture initiation pressure (FIP) and fracture propagation pressure (FPP) of the formation. This expensive occurrence can be mitigated by increasing the FIP and FPP by altering the drilling fluid composition, which has a large effect on the mechanical behaviour of rock and subsequently on the FIP and FPP. Bentonite is used both as a base fluid, as well as a lost circulation material. It is therefore imperative to understand the effect that bentonite, at different swelling capacities, has on the mechanical properties of rock to optimise its use in drilling operations to better attenuate lost circulation. In this study, Berea sandstone cores, of four permeability ranges, were permeated with bentonite gels of four NaCl concentrations, using a novel apparatus. Two batches of gel-permeated cores were prepared, one set was allowed to dry, to observe an ageing effect, and the other was kept at 4°C to preserve moisture. The cores were then indented to obtain stress-strain curves. The results of the indentation testing showed a statistically significant increase in peak strength and Young’s modulus in the wet gel-permeated cores relative to the non-permeated samples, whereas the cores with the dried gel displayed a decrease in these properties compared to the control, The dry gel-permeated cores, however, exhibited a significantly longer displacement distance compared to the control, implying these cores take longer to fully fracture apart. In addition to indentation, viscometry experiments were carried out to assess the rheological properties of the gels, this, alongside SEM and CT imaging of the samples, was done in an attempt to understand potential mechanisms behind the alterations in mechanical properties caused by gel permeation. The research carried out shows an initial set of promising results for the use of bentonite gels for wellbore-strengthening applications during drilling. The work undertaken also highlights the need for future research into interactions between non-Newtonian fluids and solids, and the potential that these interactions provide in altering the physicochemical properties of materials.