AbstractThis research work explored mechanisms that control fluid flow in the natural gas hydrate reservoir and the wellbore to resolve the challenges related to the gas flow assurance and production sustainability. The study was motivated by the fact that natural gas hydrate reservoirs hold the largest deposit of natural gas whose benefit to human being, ranging from energy source to industrial products, cannot be over-emphasised. However, complex nature of the gas hydrate reservoir and factors that influence the flow dynamics in the unconventional resource have been found to correlate with the wellbore complexities to defy viable simulations of the gas hydrate system toward commercial production. Thus, current modelling approaches are limited. This thesis expatiated the transient flow problems in both the unconventional reservoir and the wellbore, incorporated the various convective and diffusive flux phenomena, non-Darcy flow effects, near-wellbore convective mixing processes, reservoir-wellbore dynamics, and developed a new kinetic hydrate reservoir simulation model and a fully implicit fully coupled reservoir-wellbore fluid flow model for natural gas hydrate application. Thus, the thesis consists of two important simulation strategies for the specific gas hydrate reservoir production simulation. First is the new reservoir model that aggregated and simultaneously incorporated the inherent defying features to the gas hydrate fluid flow including threshold pressure, diffusivity flux, Knudsen diffusion, inertia and gas slippage effects and thermal expansion and Joule-Thomson effects. The second is the fully implicit fully coupled reservoir- wellbore model for gas hydrate production optimisation. The solutions of the formulations were obtained using finite different numerical scheme and Newton-Raphson iteration method implemented in MATLAB. The models were verified and validated using CMG STARTS benchmarked with the experimental data of Li et al. (2010a) and analytical model of Selim and Sloan (1989). Simulation results showed excellent agreement with the analyses of the key variables measured, including distributions of phases saturations, pressure, temperature, production rates and cumulative productions of gas and water over time. The models have been used to perform sensitivity analysis of the important unconventional reservoir-wellbore parameters that control the flow system behaviour; and their comparative effects were evaluated using the Monte Carlo simulation technique. Furthermore, near-wellbore conditions related to the macroscopic laws of near wellbore upscaling was modelled to investigate effects of the various functions of the relative permeability and capillary pressure equations and the new optimisation technique. Results indicated that these factors can have severe cumulative negative impact of over estimation of production rate by up to 30%. It was deduced that the gas production process at early stage is controlled by pressure depletion and sensible heat of dissociation dominated by heat advection in the hydrate zone and along the interface of the pore spaces and near wellbore. The temperature dependent model would enable long time production and the later stage gas production could had been dominated by thermal conduction- driven recovery through of the wellbore. It was identified that the thermal diffusion has a strong effect on the performance of the proposed scheme while the pressure diffusion has moderate impact beyond the threshold. The developed fully implicit fully coupled model offered a more efficient and robust flow assurance over a range of the reservoir parameterisation, including dissociation kinetics, spatial heterogeneities, intrinsic permeability and gas saturation; thus, offers a promising solution to the challenging gas hydrate reservoir production. After 1000 days of production, the cumulative gas production of decoupled model began to decrease up to
2.5 times lower than that simulated by the fully coupled approach which also has lower water production. It is concluded that application of the fully implicit fully coupled reservoir- wellbore could guarantee sustainable gas hydrate production.
|Date of Award||10 Mar 2023|
|Supervisor||Amjad Shah (Supervisor), Mohamed Hassan Sayed (Supervisor) & David Sanders (Supervisor)|