Investigation Of Replacement Combined With In-situ Heating Enhanced Exploitation Of Natural Gas Hydrates And The Inverse

Replacement exploitation is the most promising application technique among the current exploitation methods of natural gas hydrates,which is commonly recognized for methane?CH₄?recovery,carbon dioxide?CO₂?sequestration and formation stabilization.However,low exploitation efficiency,low energy efficiency and poor exploitation economy hinder replacement from industrial application.To tackle these problems,this dissertation combined replacement with thermal stimulation,proposing a novel technique of intermittent in situ heating?IISH?enhanced replacement exploitation.The aim was to improve exploitation efficiency and energy efficiency.After replacement exploitation,free CH₄ gas generated within formation would re-enrich.This may induce instability,decomposition and leakage of the sequestrated CO₂hydrates.Thus,this dissertation further investigated the inverse replacement of CO₂hydrates caused by CH₄ re-enrichment to assess the effect of greenhouse gas leakage on environment.In this dissertation,CO₂ or CO₂/N₂ mixed gas was injected into CH₄ hydrate layers to investigate the process of replacement exploitation and IISH enhanced exploitation.The effects of gas composition like CO₂ phase and N₂ composition and initial exploitation temperature on CH₄ recovery efficiency and production-injection ratio were tested.The experimental results showed that the CH₄recovery percentage was 11.79%?35.64%and the production-injection ratio was 0.05?0.18 for replacement exploitation;the CH₄recovery percentage was19.56%?64.80%and the production-injection ratio was 0.09?0.33 for IISH enhanced exploitation.Overall,the exploitation efficiency of IISH enhancing exploitation was higher than that of replacement exploitation.The exploitation efficiency was improved with the increase of replacement temperature or initial exploitation temperature.At the experimental condition of 280.15 K and 8.00 MPa in this dissertation,the CH₄recovery percentage was 35.64%and the corresponding production-injection ratio was 0.18 for CO₂/N₂replacement exploitation.Furthermore,the CH₄recovery percentage was improved to 64.80%and the corresponding production-injection ratio was 0.30 for IISH enhanced exploitation with CO₂/N₂,indicating an obvious enhanced effect.Based on the experimental results,the processes of replacement exploitation and IISH enhanced exploitation were proposed.Energy efficiency of exploitation was calculated using Aspen Plus.The values of Energy Return on Investment?EROI value?for the two exploitation schemes were compared.According to the energy efficiency results,the EROI value was improved from 2.27?10.50 for replacement exploitation to 2.80?13.54 for IISH enhanced exploitation.The economy of IISH enhanced exploitation was higher than that of single replacement exploitation.The lowest unit energy consumption was 0.76 k Wh/m³·CH₄ for IISH enhanced exploitation at 280.15 K and 3.90 MPa.The unit energy consumption can be reduced efficienctly by decreasing the energy consumption for gas compression,which is the largest proportion among exoloitation stages.Higher efficienct heating modes needs to be investigated to further imrove energy efficiency.In addition,this dissertation also simulated the inverse replacement of CO₂ hydrates with CH₄ gas.CH₄ gas was injected from the bottom of CO₂ hydrates to simulate the process of inverse replacement of CO₂ hydrates with CH₄ gas.The effects of different initial pressures and temperatures on CO₂ release efficiency,CH₄ hydration efficiency and CH₄/CO₂ in-out ratio during the inverse replacement of CO₂ hydrates with CH₄ gas were tested.The experimental results denoted that lower initial pressure and higher temperature provided larger driving force for CO₂ hydrate decomposition with higher CO₂ release efficiency and lower CH₄/CO₂ in-out ratio.The highest CO₂ release efficiency 27.65%and the largest CH₄ hydration efficiency 58.11%occurred at 279.15 K and 3.00 MPa,with the corresponding CH₄/CO₂ in-out ratio was 0.83.Considering the greenhouse effect of CH₄ was stronger than that of CO₂,the inverse replacement of CO₂ hydrates with CH₄ gas mitigated greenhouse effect to some degree.