With the advancement of horizontal drilling and hydraulic fracturing technologies, the unconventional reservoir resources (e.g., tight oil and shale gas) have received a growing attention; however, it is a challenging task to accurately simulate the transient pressure response and single/two-phase flow behaviour due to the reservoir boundary, fracture geometry, fracture network, and flow dynamics including stress-sensitivity, slippage effect, non-Darcy flow, and gas adsorption/desorption. Therefore, it is of a fundamental and practical importance to evaluate the performance of a multifractured horizontal well (MFHW) in an unconventional reservoir conditioned to an arbitrary boundary, fracture geometry, and complex fracture networks with the consideration of pressure-dependent permeability, non-Darcy flow, slippage, and/or gas adsorption/ desorption. By taking the arbitrarily-shaped reservoir boundaries into account, the boundary element method has been proposed to accurately describe the boundary-dominated flow during the late time period for an MFHW in an unconventional oil reservoir. The stresssensitive effect of the hydraulic fracture subsystem is semi-analytically evaluated in the Laplace domain with the iteration method, while the Pedrosa's transform formulation can be incorporated into the governing equations in the matrix and fracture subsystems in order to couple the matrix-fracture flow models. As for a shale gas reservoir, the dual reciprocity boundary element method is applied to deal with the nonlinearity resulted from more complex conditions (i.e., slippage, stress-sensitivity, and gas adsorption/desorption). In addition to its flexibility, the newly proposed model can be used to simultaneously obtain solutions at multiple locations inside the matrix domain. As for the two-phase flow, a skin factor on a fracture face is defined and introduced to represent the change in relative permeability in the matrix domain at each timestep. A two-phase flow model coupled with geomechanics has been employed to capture transient flow behaviour during the flowback period in an unconventional reservoir by considering fracture geometry and capillary pressure. Different from the traditional treatment by assuming it as a constant, a function of interfacial tension (IFT) between gas and water as well as fracture aperture is employed to obtain the capillary pressure within a fracture, during which the gas/water saturation and the fracture aperture in each fracture segment with an equal length can be iteratively obtained and updated. All the proposed theoretical models have been validated and then extended to field cases. As for the single-phase flow, type curves are generated and beneficial to examine the effect of each factor on the transient pressure behaviour of an MFHW in an unconventional reservoir conditioned to different fracture networks and flow dynamics. In the two-phase flow model coupled with geomechanics, dynamic fracture properties, and capillary pressure are found to exert a considerable influence on the gas/water production rates and cumulative production. The discrepancy between the field pressure/production data and transient type curves without geometrical structure (i.e., reservoir boundaries and proppant characteristics) and flow dynamics can be adopted to evaluate the contribution of stress-sensitive effect and gas adsorption/desorption.