Fluid injection into reservoirs, such as waste disposal, steam or water flooding and well testing, is a common practice in oil and gas industry. The injection of large volumes of fluid into an unconsolidated sands reservoir can result in significant changes to the in-situ stress distributions which may lead to the hydraulic fractures initiation and propagation. Hydraulic fracturing can be broadly defined as a process by which a fracture initiates and propagates due to hydraulic loading (i.e., pressure) applied by a fluid inside the fracture. Although hydraulic fracturing in hard rock has been comprehensively studied both experimentally and numerically, some fundamental mechanisms of hydraulic fracturing in unconsolidated formation have not been well understood.;Several case studies, which include the field produced water re-injection into deep unconsolidated formation and well testing in oil sands formation, are conducted using the developed finite element models. The bottom-hole-pressures predicted by the developed finite element codes are used to history match the field bottom-hole-pressures. The numerical calculations clearly show that the presented numerical model can capture the physical mechanism of hydraulic fracture initiation and propagation in unconsolidated sand formation and matches the field pressure versus time curve very well.;The hydraulic fracture in unconsolidated sand reservoir can be represented as an anisotropic area of dilation zone or a net of micro-cracks, inside which the formation has low effective stresses and high hydraulic conductivity values. In this thesis, we present a 3D finite element model for simulating the hydraulic fracture initiation and propagation in unconsolidated sands reservoir due to large volumes of fluid injection at high injection rate. To simulate this strong anisotropy in mechanical and hydraulic behaviour induced by fluid injection, a poro-elasto-plastic constitutive model together with a strain-induced anisotropic permeability model are formulated and implemented into a 3D finite element simulator, which is used to match the field injection data.