Study On Production Characteristics And Influence Mechanisms Of Muddy Hydrate Reservoir By Depressurization

Natural gas hydrate resources have large reserves,wide distribution,high energy density,and clean combustion.Achieving safe and efficient exploitation of hydrates is a global research hotspot and scientific frontier.South China Sea is an important natural gas hydrate resource enrichment area.The commercial production of hydrate resources will play an important role in optimizing the national energy structure and ensuring energy security.However,the natural gas hydrate reservoir in South China Sea is a low-permeability muddy silt type.This type of hydrate reservoir has complex geological characteristics and is difficult to control during exploitation.At present,there is a lack of systematic research on muddy hydrate reservoirs.In particular,there are few studies on the seepage characteristics of muddy hydrate reservoirs under effective stress.In addition,the characteristics of joint-exploitation of hydrate and underlying gas are not yet clear.Therefore,experiments and model studies were conducted using the muddy hydrate reservoirs which remolded by real marine sediments sampled in South China Sea.The control factors and influence mechanisms of gas production in muddy hydrate reservoir by depressurization were systematically analyzed.The results of this study will provide a theoretical basis for the safe and efficient exploitation and utilization of marine hydrate resources.In order to study the permeability characteristics of muddy hydrate reservoirs under the effective stress,an experimental platform of core holder loaded with effective stress was selfdesigned and self-built.The low-porosity muddy reservoir samples were reshaped using real marine sediments.The gas permeability of muddy reservoirs with and without hydrate was measured using methane gas.The effects of initial water saturation,hydrate saturation,and effective stress on the permeability of muddy reservoir were revealed.The results show that the permeability of muddy reservoir suddenly changed during the increase of effective stress.The causes of irreversible damage to the permeability of muddy reservoir caused by increasing effective stress were analyzed.Based on the experimental results,a modified Masuda permeability model considering the effect of effective stress was proposed.On the premise of clarifying the seepage characteristics of muddy hydrate reservoir,the gas-saturated and water-saturated muddy reservoir samples were constructed,respectively.The formation and decomposition characteristics of natural gas hydrates in muddy reservoirs were systematically analyzed.The results indicate that the squeezing effect of effective stress on muddy hydrate reservoir during depressurization is the key to affecting gas production efficiency.The residual amount of hydrate and pore gas bound by 3 MPa effective stress was quantified.The homogeneous decomposition stage of hydrate in water-saturated muddy reservoir during depressurization was found.The inducing factors and the responses of reservoir temperature and pressure during hydrate homogeneous decomposition stage were explored.The mechanism of liquid water in water-saturated reservoir enhancing the heat transfer performance of hydrate reservoir is revealed,which provides a new idea to improve the gas production efficiency of muddy hydrate reservoir.Based on clarifying the seepage and gas production characteristics of muddy hydrate reservoir during depressurization,the experimental and model study on joint-exploitation from hydrate and underlying gas was conducted.The results presented the phenomenon of watersaturated hydrate reservoir capping high-pressure underlying gas during depressurization.The effects of underlying gas breaking through the cap and flowing into the hydrate reservoir on temperature and pressure,hydrate decomposition rate,and gas production rate were revealed.Additionally,the relationships between hydrate decomposition rate,gas production rate,water production and underlying gas pressure were analyzed.Through the comprehensive evaluation of multiple parameters of the hydrate reservoir,the gas production enhancement effect of underlying gas on muddy hydrate reservoir was preliminarily demonstrated.Besides,the interdependence relationship between hydrate decomposition and underlying gas production during joint-exploitation by depressurization was illustrated.The comprehensive influence mechanisms of intrinsic permeability,effective stress,and hydrate saturation on the seepage and production characteristics of hydrate reservoir during depressurization were clarified.Based on the experimental results of this work,the gas production prediction model of joint exploitation from hydrate reservoir and underlying gas and the trajectory prediction model of hydrate decomposition front were proposed.Dynamic permeability calculated by the modified Masuda permeability model was applied to the prediction models.The precise trajectory of the hydrate decomposition front in the muddy reservoir and the evolution of underlying gas pressure were accurately predicted.The reliability of the gas production prediction model was verified by self-designed experiments.The results of this study will provide significance guidance for the safe and efficient exploitation of marine hydrate resources.