Due to the decline in conventional hydrocarbon reservoir production, particularly in North America, the energy industry has focused on the development of low- to ultra-low permeability unconventional reservoirs. A technology that has enabled commercial development of unconventional reservoirs is horizontal wells combined with multiple stages of hydraulic fracturing along the well (multi-fractured horizontal wells, MFHWs). MFHWs are extremely costly to implement, and there are challenges associated with their use in developing unconventional reservoirs, such as inter-well communication. Consequently, reservoir engineering methods are required to characterize hydraulic fractures and quantify well interference, particularly during the early stages of the MFHW life cycle, in order to optimize hydraulic fracture design and field development. Some researchers have suggested that analyzing carefully gathered and high-frequency flowback data is an effective method for these purposes. Flowback and early-time production data analysis for some unconventional reservoirs may be affected by complexities such as fracturing fluid imbibition into the reservoir matrix and the dynamic behavior of the created hydraulic fractures during the early flowback period. However, previously published flowback models do not account for these complex physics and therefore their application for characterizing hydraulic fractures could lead to significant errors in estimated fracture properties. Therefore, in this dissertation, new analytical and semi-analytical models are proposed and developed to account for capillary imbibition and the dynamic behavior of the hydraulic fractures during the flowback and early-time production period. The models are validated using rigorous numerical simulation that account for the physics, and their practical application demonstrated with field cases. Importantly, the impact of ignoring the effects of imbibition and dynamic fracture properties, when they exist, is quantified and demonstrated to be significant. In addition to fracture characterization, flowback data can be used to quantify inter-well communication between parent, child and co-developed wells. Incidences of communicating wells have increased over the past decade due to larger hydraulic fracture stimulation treatments, more perforation clusters per stage, and a reduction in the spacing of horizontal wells (both laterally and vertically, for the development of multi-layered reservoirs). Inter-well communication poses a significant challenge in the development of unconventional reservoirs, leading potentially to decreases in well productivity. Therefore, in this dissertation, a rate-transient analysis (RTA) approach is proposed to quantify inter-well communication during flowback and early-time production data when multiphase flow is occurring. The method is validated using rigorous numerical simulation, and its practical application demonstrated with field cases. Analysis of flowback and early-time production data from MFHWs (used for injection and production) applied to enhanced geothermal systems (EGS) can also be used to provide estimates of subsurface properties which are generally unknown or highly uncertain. The evaluation of these properties is critical for determining resource potential, optimizing geothermal energy production, and evaluating the risk of induced seismicity, amongst other applications. Therefore, in this dissertation, analytical and semi-analytical models are developed to enable subsurface reservoir and hydraulic fracture characterization of enhanced geothermal systems (EGS) using early-time flowback data from the injection well, and early-time production data from the production well. These models are similarly validated using rigorous numerical simulation, and their practical application demonstrated with data from the Utah FORGE EGS pilot site. The analytical and semi-analytical techniques developed in this dissertation will be useful to petroleum and geothermal engineers who are responsible for optimizing energy production from unconventional hydrocarbon and geothermal reservoirs, respectively.