Experimental investigation of the wellbore strengthening phenomenon
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An experimental approach was employed to study the Wellbore Strengthening (WBS) phenomenon. A state-of-the-art experimental set-up was designed to carry out high-pressure borehole fracturing tests on cylindrical rock samples. The experimental set-up offers full control over borehole, confining, and pore pressures. Fracturing experiments were conducted on three different rock types, namely Berea sandstone, Castlegate sandstone, and Mancos shale. Several injections were performed on each sample to characterize the values of the fracture initiation pressure (FIP) and the fracture propagation pressure (FPP) and thereby characterize the WBS phenomenon. Typical experimental variables include the applied confining pressure, type of base fluid (water-based or synthetic-based), and concentration, type, and particle size distribution (PSD) of the lost circulation material (LCM) used to achieve WBS benefits. Post-fracturing analysis was conducted by using techniques such as computerized axial tomography (CAT) scan and petrographic imaging to investigate the geometry of induced fractures and formed seals. The experimental results show that the FIP is mainly a function of the rock fracture toughness and stress concentration around the borehole, and independent of the drilling fluid used. The FPP, however, is mainly affected by the formulation of the drilling fluid and can be significantly enhanced by adding LCM. The obtained FPP values are compared with the large-scale fracturing experiments conducted at the Drilling Engineering Association (DEA) 13 investigations. Excellent agreement was observed between the DEA 13 and UT MudFrac experimental results. Furthermore, it is shown that FPP changes linearly with the minimum horizontal stress (Shmin), and the results of fracturing experiments using a relatively small borehole size at low confining pressures can be extrapolated to predict the FPP of large-scale fracturing experiments, and possibly field applications. The effect of LCM concentration on strengthening effects is investigated. It was found that although a minimum concentration of LCMs is required for effective WBS, FPP does not increase significantly for concentrations above a certain upper threshold value. Moreover, for any rock with a given set of rock strength and failure parameters, there exists an optimum PSD to maximize WBS benefits. Optimum PSD appears to be of primary importance for WBS, almost independent of LCM type. The experimental results presented in this dissertation are in clear disagreement with wellbore stress augmentation (WSA) mechanisms such as stress caging (SC) and fracture closure stress (FCS) which were previously proposed to explain the WBS phenomenon. Furthermore, they clearly favor the fracture propagation resistance (FPR) explanation to WBS. Existing guidelines to design WBS treatments such as the one-third rule, the Vickers criteria, and the ideal packing theory are evaluated. It is shown that none of these theories properly represents the physics of fracture sealing. To remedy this situation, a new family of design curves is introduced to determine the optimum PSD for WBS applications.