1-D Modeling of Hydrate Decomposition in Porous Media

This paper describes a one-dimensional numerical model for natural gas production from the dissociation of methane hydrate in hydrate-capped gas reservoir under depressurization and thermal stimulation. Some of the hydrate reservoirs discovered are overlying a free-gas layer, known as hydrate-capped gas reservoirs. These reservoirs are thought to be easiest and probably the first type of hydrate reservoirs to be produced. The mathematical equations that can be described this type of reservoir include mass balance, heat balance and kinetics of hydrate decomposition. These non-linear partial differential equations are solved using finite-difference fully implicit scheme. In the model, the effect of convection and conduction heat transfer, variation change of formation porosity, the effect of using different equations of state such as PR and ER and steam or hot water injection are considered. In addition distributions of pressure, temperature, saturation of gas, hydrate and water in the reservoir are evaluated. It is shown that the gas production rate is a sensitive function of well pressure. Keywords—Hydrate reservoir, numerical modeling, depressurization, thermal stimulation, gas generation. NOMENCLATURE Adec = specific surface area per unit bulk volume, m AHS = specific area of hydrate particles, m E = activation energy, J/mol fe = fugacity of gas at T and pe, kPa fg = fugacity of gas at T and pg, kPa l g . = generation rate of phase l per unit volume, kg/m3s hl = specific enthalpy of phase l, J/kg K = absolute permeability, md Kc = thermal conductivity, w/m·K Kd = intrinsic decomposition rate constant, kmol/m2kPa·s kd = decomposition rate constant, kmol/m2kPa·s krl = relative permeability to phase l Mc = molar mass of component c, kg/kmol NH = hydrate number (= 5.75) Pl= pressure of phase l, kPa Pc = capillary pressure between gas and water, kPa Pe = H-V-Lw equilibrium pressure, kPa qml = mass production rate of phase l per unit volume, kg/m3s H Q . = heat of hydrate decomposition per unit volume, J/m3s F. Esmaeilzadeh (phone: +987112343833; fax: +987116287294; e-mail: [email protected]), M. E. Zeighami (e-mail: [email protected]), and J. Fathi are with Chemical and Petrochemical Engineering Department, Shiraz University, Shiraz, Iran. in Q . = direct heat input per unit volume, J/m3s R = gas constant (= 8.314 J/mol·K) Swr = irreducible water saturation Sgr = residual gas saturation Sl = saturation of phase l l S = normalized saturation of phase l t = time, s T = temperature, K vl = velocity of phase l, m/s Ul = specific internal energy of phase l, J/kg = porosity l = viscosity of phase l, Pa·s l = density of phase l, kg/m3 SUBSCRIPT g = gas w = water H = hydrate R = rock i = initial condition