Simulation Study On Various Types Of Hydrate Reservoirs Exploitation By Depressurization Using Horizontal Well Configuration

With the depletion of traditional fossil energy,increasing energy and environmental issues,and the demand of "dual carbon" policy,it is urgent to develop new clean energy to replace traditional fossil energy.Natural gas hydrate,which is an unconventional natural gas resource with huge reserves,high energy density and environmental friendliness,is considered as the most potential new clean energy.However,due to the low recovery and economic efficiency,the development of natural gas hydrate is still in the stage of trial production.Therefore,it is important to carry out research on the gas production characteristics of various types of hydrate reservoirs,and to clarify the exploitation efficiency of hydrate reservoir under different well spacing strategies and production strategies.Based on mass,energy and hydrate phase balance equations,a numerical simulation model for hydrate decomposition was established.And the simulation research on gas production behavior of various classes hydrate reservoirs by depressurization in horizontal well configuration was conducted.First,the gas production behavior of various types of hydrate reservoirs under different well configuration was analyzed,and the related evaluation indexes such as average gas production rate,gas-water ratio and Ste number were introduced to compare the exploitation efficiency and economy.Subsequently,based on the well configuration with better production performance,the influence of depressurization strategy on gas production behavior of various classes hydrate reservoirs was studied,and a production strategy with better comprehensive exploitation efficiency was obtained.Finally,the effects of reservoir physical properties on gas production behavior of various types of hydrate reservoirs were studied and their degree of influence on exploitation performance was quantified by sensitivity analysis.The results show that the well spacing strategy has a significant effect on gas production behavior of various types of hydrate reservoirs.For Class Ⅰ hydrate reservoir,gas production can be increased by about 47% by placing production well near the free gas layer,compared to placing production well in the hydrate layer.For Class Ⅱ and Ⅲ hydrate reservoirs,water production can be effectively reduced by placing the wellbore on the upper side of the hydrate layer.Stepwise depressurization method shows good exploitation performance in exploiting all types of hydrate reservoirs: the free gas production is significantly increased when stepwise depressurization is used in Class Ⅰ hydrate reservoir,and the cumulative gas production is increased by 68%.Secondly,the use of stepwise depressurization reduces 53% of the water production for Class Ⅱ hydrate reservoirs,14% of the water production for Class Ⅲ hydrate reservoirs,resulting in a significant increase in the gas-water ratio.In addition,sufficient time is provided for external heat input and the contribution of reservoir sensible heat to hydrate decomposition is reduced,by using stepwise depressurization,which enables the reservoir temperature to remain at a higher level.Sensitivity analysis results of reservoir physical property parameters of various types of hydrate reservoirs indicate that,due to the existence of free gas layer,the physical parameters related to seepage and thermal properties,including specific heat capacity,thermal conductivity,intrinsic permeability,porosity and relative permeability of gas phase,have significant influence on gas production behavior of Class Ⅰ hydrate reservoir.For Class Ⅱ and Class Ⅲ hydrate reservoirs,thermal conductivity is the main factor affecting gas production,followed by specific heat capacity and porosity,while intrinsic permeability and relative permeability of gas\liquid have weak influence.