Numerical Simulation of Extended Leak-Off Tests
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In-situ stresses are key information for well and formation integrity assessments, especially in difficult drilling operations such as high pressure high temperature wells, extended reach wells and in-fill drilling. During drilling, the minimum horizontal stress is best obtained by dedicated tests. In particular the extended leak-off test. Challenges associated with such tests are the quality of stress data obtained in complex stress regimes, difficult formations (plasticity, ductility, thermal effects) and difficult geometries. These conditions present the need for more reliable determination of in-situ stresses during drilling. This thesis was part of the full development of an extended leak-off test simulator for deep wells based on a modified discrete element method (MDEM) at SINTEF Petroleum. The overlying objective of MDEM as a fracturing simulator is modelling of dynamic fracture initiation and propagation in 3D. XLOT in deviated wells may be evaluated for information, and phenomena such as fracture twisting may be captured. The process of refining MDEM to become an XLOT simulator in low-permeable rock in 2D is described with results and discussion. In low permeability formations, the only volume available for the well fluid to flow into is the induced fracture volume. With single phase water as fracturing fluid in an open hole segment of unit length (modelled by MDEM, in 2D), this corresponds to a highly stiff and non-linear system, where fracture mechanics and fluid flow are intimately coupled. Change in flow rate immediately translates to fracturing rate. A full well volume attached to the fracturing simulator implies that decompressed volume will flow at high rates into the fracture at formation breakdown, inducing a large fracture. A sequentially coupled geomechanical and fluid flow model requires limited fracture extension per simulation timestep to ensure a stable hydromechanical coupling, which makes coupling of third model, the well, a non-trivial problem.Two schemes to couple a deep petroleum well to the MDEM fracturing simulator were developed in MATLAB, and applied to simulate XLOT in a 2000 mTVD well. Simulation results are presented. Flow-back through a fixed choke was implemented in the coupling algorithms in MATLAB.