Optimization of Gas Production from Hydrate-Bearing Sediments with Fluctuation Characteristics

As an important source of low-carbon,clean fossil energy,natural gas hydrate plays an important role in improving the global energy consumption structure.Developing the hydrate industry in the South China Sea is important to achieving‘carbon peak and carbon neutrality’goals as soon as possible.Deep-water areas subjected to the action of long-term stress and tectonic movement have developed complex and volatile terrains,and as such,the morphologies of hydrate-bearing sediments(HBSs)fluctuate correspondingly.The key to numerically simulating HBS morphologies is the establishment of the conceptual model,which represents the objective and real description of the actual geological body.However,current numerical simulation models have characterized HBSs into horizontal strata without considering the fluctuation characteristics.Simply representing the HBS as a horizontal element reduces simulation accuracy.Therefore,the commonly used horizontal HBS model and a model considering the HBS’s fluctuation characteristics with the data of the SH2 site in the Shenhu Sea area were first constructed in this paper.Then,their production behaviors were compared,and the huge impact of the fluctuation characteristics on HBS production was determined.On this basis,the key parameters affecting the depressurization production of the fluctuating HBSs were studied and optimized.The research results show that the fluctuation characteristics have an obvious influence on the hydrate production of HBSs by affecting their temperatures and pressure distributions,as well as the transmission of the pressure drop and methane gas discharge.Furthermore,the results show that the gas productivity of fluctuating HBSs was about 5%less than that of horizontal HBSs.By optimizing the depressurization amplitude,well length,and layout location of vertical wells,the productivity of fluctuating HBSs increased by about 56.6%.

Numerical Investigation of Combined Production of Natural Gas Hydrate and Conventional Gas

Natural gas hydrate(NGH)is generally produced and accumulated together with the underlying conventional gas.Therefore,optimizing the production technology of these two gases should be seen as a relevant way to effectively reduce the exploitation cost of the gas hydrate.In this study,three types of models accounting for the coexistence of these gases are considered.Type A considers the upper hydrate-bearing layer(HBL)adjacent to the lower conventional gas layer(CGL);with the Type B a permeable interlayer exists between the upper HBL and the lower CGL;with the type C there is an impermeable interlayer between the upper HBL and the lower CGL.The production performances associated with the above three models are calculated under different conditions,including only a depressurized HBL(only HBL DP);only a depressurized CGL(only CGL DP);and both the HBL and the CGL being depressurized(HBL+CGL DP).The results show that for Type A and Type B coexistence accumulation models,when only HBL or CGL is depressurized,the gas from the other layer will flow into the production layer due to the pressure difference between the two layers.In the coexistence accumulation model of type C,the cumulative gas production is much lower than that of Type A and Type B,regardless of whether only HBL DP,only CGL DP,or HBL+CGL DP are considered.This indicates that the impermeable interlayer restricts the cross-flow of gas between HBL and CGL.For three different coexistence accumulation models,CGL DP has the largest gas-to-water ratio.

Numerical studies of hydrate dissociation and gas production behavior in porous media during depressurization process

In this study, a numerical model is developed to investigate the hydrate dissociation and gas production in porous media by depressurization. A series of simulation runs are conducted to study the impacts of permeability characteristics, including permeability reduction exponent, absolute permeability, hydrate accumulation habits and hydrate saturation, sand average grain size and irreducible water saturation. The effects of the distribution of hydrate in porous media are examined by adapting conceptual models of hydrate accumulation habits into simulations to govern the evolution of permeability with hydrate decomposition, which is also compared with the conventional reservoir permeability model, i.e. Corey model. The simulations show that the hydrate dissociation rate increases with the decrease of permeability reduction exponent, hydrate saturation and the sand average grain size. Compared with the conceptual models of hydrate accumulation habits, our simulations indicate that Corey model overpredicts the gas production and the performance of hydrate coating models is superior to that of hydrate filling models in gas production, which behavior does follow by the order of capillary coating〉pore coating〉pore filling〉capillary filling. From the analysis of tl/2, some interesting results are suggested as follows： （1） there is a ＂switch＂ value （the ＂switch＂ absolute permeability） for laboratory-scale hydrate dissociation in porous media, the absolute permeability has almost no influence on the gas production behavior when the permeability exceeds the ＂switch＂ value. In this study, the ＂switch＂ value of absolute permeability can be estimated to be between 10 and 50 md. （2） An optimum value of initial effective water saturation Sw,e exists where hydrate dissociation rate reaches the maximum and the optimum value largely coincides with the value of irreducible water saturation Swr,e. For the case of Sw,e〈,Swr,e, or Sw,e〉Swr,e, there are different control mechanisms dominating the process of hydrate dissociation and gas production.

Mechanical Modeling and Analysis of Stability Deterioration of Production Well During Marine Hydrate Depressurization Production

Different from oil and gas production,hydrate reservoirs are shallow and unconsolidated,whose mechanical properties deteriorate with hydrate decomposition.Therefore,the formations will undergo significant subsidence during depressurization,which will destroy the original force state of the production well.However,existing research on the stability of oil and gas production wells assumes the formation to be stable,and lacks consideration of the force exerted on the hydrate production well by formation subsidence caused by hydrate decomposition during production.To fill this gap,this paper proposes an analytical method for the dynamic evolution of the stability of hydrate production well considering the effects of hydrate decomposition.Based on the mechanical model of the production well,the basis for stability analysis has been proposed.A multi-field coupling model of the force state of the production well considering the effect of hydrate decomposition and formation subsidence is established,and a solver is developed.The analytical approach is verified by its good agreement with the results from the numerical method.A case study found that the decomposition of hydrate will increase the pulling-down force and reduce the supporting force,which is the main reason for the stability deterioration.The higher the initial hydrate saturation,the larger the reservoir thickness,and the lower the production pressure,the worse the stability or even instability.This work can provide a theoretical reference for the stability maintaining of the production well.

Gas Production from Offshore Methane Hydrate Layer and Seabed Subsidence by Depressurization Method

Numerical simulations on consolidation effects have been carried out for gas production from offshore methane hydrates (MH) layers and subsidence at seafloor. MH dissociation is affected by not only MH equilibrium line but also consolidation (mechanical compaction) depended on depressurization in the MH reservoir. Firstly, to confirm present model on consolidation with effective stress, the history matching on gas production and consolidation has been done to the experimental results using with synthetic sand MH core presented by Sakamoto et al. (2009). In addition, the comparisons of numerical simulation results of present and Kurihara et al. (2009) were carried out to check applicability of present models for gas production from MH reservoir in field scale by depressurization method. The delays of pressure propagation in the MH reservoir and elapsed time at peak gas production rate were predicted by considering consolidation effects by depressurization method. Finally, seabed subsidence during gas production from MH reservoirs was numerically simulated. The maximum seabed subsidence has been predicted to be roughly 0.5 to 2 m after 50 days of gas production from MH reservoirs that elastic modulus is 400 to 100 MPa at MH saturation = 0.

Influence of depressurization rate on gas production capacity of high-rank coal in the south of Qinshui Basin, China

A desorption simulation experiment with the condition of simulated strata was designed. The experiment, under different depressurizing rates and the same fluid saturation, was conducted on the sample from 3# coal of Daning coal mine in Jincheng, Shanxi Province. The gas production rate and pressure change at both ends of the sample were studied systematically, and the mechanisms of some phenomena in the experiment were discussed. The experimental results show that, whether at fast or slow depressurizing rate, the methane adsorbed to high-rank coal can effectively desorb and the desorption efficiency can reach above 90%. There is an obvious inflection point on the gas yield curve during the desorption process and it appears after the pressure on the lump of coal reduces below the desorption pressure. The desorption of methane from high-rank coal is mainly driven by differential pressure, and high pressure difference is conducive to fast desorption. In the scenario of fast depressurization, the desorption inflection appears earlier and the gas production rate in the stage of rapid desorption is higher. It is experimentally concluded that the originally recognized strategy of long-term slow CBM production is doubtful and the economic benefit of CBM exploitation from high-rank coal can be effectively improved by rapid drainage and pressure reduction. The field experiment results in pilot blocks of Fanzhuang and Zhengzhuang show that by increasing the drainage depressurization rate, the peak production of gas well would increase greatly, the time of gas well to reach the economic production shortened, the average time for a gas well to reach expected production reduced by half, and the peak gas production is higher.

Numerical Simulation on Production Trials by Using Depressurization for Typical Marine Hydrate Reservoirs:Well Type and Formation Dip

Natural gas hydrate has huge reserves and is widely distributed in marine environment.Its commercial development is of great significance for alleviating the contradiction between energy supply and demand.As an efficient research method,numerical simulation can provide valuable insights for the design and optimization of hydrate development.However,most of the current production models simplify the reservoir as a two-dimensional(2D)horizontal layered model,often ignoring the impact of formation dip angle.To improve the accuracy of production prediction and provide theoretical support for the optimization of production well design,two three-dimensional(3D)geological models with different dip angles based on the geological data from two typical sites are constructed.The vertical well,horizontal well and multilateral wells are deployed in these reservoirs with different permeabilities to perform production trial,and the sensitivity analysis of dip angles is also carried out.The short-term production behaviors in high and low permeability reservoirs with different dip angles are exhibited.The simulation results show that 1)the gas and water production behaviors for different well types in the two typical reservoirs show obviously different variation laws when the short-term depressurization is conducted in the inclined formation;2)the inclined formation will reduce the gas production and increase the water extraction,and the phenomena becomes pronounced as the dip angle increases,particularly in the low-permeability reservoirs;3)and the impact of formation dip on hydrate recovery does not change significantly with the variation of well type.

The status of exploitation techniques of natural gas hydrate

Natural gas hydrate(NGH)has been widely considered as an alternative form of energy with huge potential,due to its tremendous reserves,cleanness and high energy density.Several countries involving Japan,Canada,India and China have launched national projects on the exploration and exploitation of gas hydrate resources.At the beginning of this century,an early trial production of hydrate resources was carried out in Mallik permafrost region,Canada.Japan has conducted the first field test from marine hydrates in 2013,followed by another trial in 2017.China also made its first trial production from marine hydrate sediments in 2017.Yet the low production efficiency,ice/hydrate regeneration,and sand problems are still commonly encountered;the worldwide progress is far before commercialization.Up to now,many gas production techniques have been proposed,and a few of them have been adopted in the field production tests.Nevertheless,hardly any method appears really promising;each of them shows limitations at certain conditions.Therefore,further efforts should be made on the economic efficiency as well as sustainability and environmental impacts.In this paper,the investigations on NGH exploitation techniques are comprehensively reviewed,involving depressurization,thermal stimulation,chemical inhibitor injection,CO2–CH4 exchange,their combinations,and some novel techniques.The behavior of each method and its further potential in the field test are discussed.The advantages and limitations of laboratory studies are also analyzed.The work could give some guidance in the future formulation of exploitation scheme and evaluation of gas production behavior from hydrate reservoirs.

压缩机降压采气系统压损分析与适应性研究

降压采气是气田开发后期稳产和提高采收率的重要途径,效果主要取决于其能够形成的地层压力降幅。但受气藏物性不同和天然气可压缩性的影响,降压采气形成的地层压降与压缩机形成的地面压降有明显的差异。文中从降压采气基本原理出发,结合天然气地层渗流、井筒与地面管流基本理论,以降压前后气井生产系统沿程压力损失特征分析为基础,对影响降压采气效果的各种因素进行了剖析,提出应该以经济极限产量或井筒临界携液气量为依据,从而合理确定降压采气起始与失效地层压力,取得了无须苛求极限降低压缩机进口压力的认识,得出了压缩机降压采气更适用于弹性产率大、高渗不产液或低渗产液气藏的结论。

Experimental study on sand production and coupling response of silty hydrate reservoir with different contents of fine clay during depressurization

To further understand the characteristics of clay and sand production(hereafter collectively referred to as sand production)and to provide optimization designs of sand control schemes are critical for gas production from clayey silt natural gas hydrate reservoirs in the South China Sea.Thus,gas-water-sand production behavoirs and coupling reservoir subsidence characteristics before,during,and after hydrate dissociation of the clayey silt hydrate reservoirs with different clay contents(5%,10%,15%,20%,25%,and 30%)have been studied through a self-developed experimental system.The results show that with the increase of clay content,the total mass of sand production first increases and then decreases,and it reaches maximum when the clayey content is 20%.The sand production is the lowest before hydrate dissociation and increases significantly during hydrate dissociation,which mainly occurs in the high-speed gas and water production stage at the beginning of hydrate dissociation.After hydrate dissociation,the sand production decreases significantly.During the whole depressurization process,the clay and free sand particles generally move to the sand outlet due to the fluid driving force and overlying stress extrusion.However,for conditions of high clay contents,those particles fail to pass through the sand control screen and gradually accumulate and block the screen by forming a mud cake,which greatly reduce the permeability of the screen and limite sand production as well as gas and water production.Our research lays a foundation for sand production prediction and sand control scheme selection during gas recovery from clayey silty hydrate reservoirs that greatly need to consider a balance between sand control and gas productivity.