| In its depletion development of abnormal high pressure gas reservoir,significant changes will take place in the reservoir's physical properties and fluid physical properties due to its dramatic pressure drop,thus having great impacts on gas reservoir seepage mechanism,gas well productivity,and recovery ratio,etc.The impacts include the increase of net effective stress and the reduction of pore-throat radius and porosity caused by the decrease of formation pressure after putting into production,which has an irreversible effect on reservoir absolute permeability.Meanwhile,water-soluble gas content is high in boundary and bottom water of abnormal high pressure gas reservoir and the water-soluble gas gives off and displaces boundary and bottom water after pressure drop development,which has an obvious impact on the water invasion performance and recovery ratio of gas reservoir.The middle-porosity and middle & low permeability reservoir of the main sandstone in Huangliu Formation of Dongfang X Gas Field in Yinggehai Basin,with the gas reservoir's initial pressure coefficient approaching 2.0 and actually measured water-soluble gas content reaching 25.6 m3/m3,is a typical gas reservoir with abnormal high pressure as well as boundary and bottom water.At present,the emphases concerned on the exploitation of abnormal high pressure gas reservoir at home and abroad include the technology of analyzing stress sensitivity feature,the solubility of water-soluble gas and its impact on exploitation,along with productivity calculation method,production optimization and reasonable gas production speed design method when the stress sensitivity of abnormal high pressure gas reservoir is considered.This study,I choose the abnormal high pressure gas reservoir of Dongfang X Gas Field in Yinggehai Basin Dongfang X Gas Field abnormal high pressure gas reservoir as the research object.Through various methods such as the experimental research,theoretical analysis and numerical simulation,the research(author)establishes the relational model of the reservoir stress and its influence on reservoir physical properties,the relationship between the gas reservoir's fluid high pressure physical properties and pressure,fluid-solid coupling seepage mechanism and technology of oil reservoir numerical simulation technology.Meanwhile,it carries outdoes quantitative and qualitative analysis of the size of pressure sensitivity related to various types of reservoir seepage and its influence on single well productivity and gas reservoir recovery ratio,and hereby guide guides the development strategies formulation and development program optimization of water-containing high pressure gas reservoir.(1)The cause of formation high pressure and depletion development law of Dongfang X Gas Field are studied.Since the neogene fault of the basin does not develop,with a lack of channels for fluid to move and release in vertical direction,and high rate of the basin's subsidence and sedimentary,it is mainly filled with fine-grain sediments based on mud shale and fine sandstone.Therefore,under the joint action between the particular regional geological background and the sedimentary environment,it results in the extreme unbalance between mud shale compaction and discharge of fluid in neogene and quaternary of huge depth in the region.At the same time,the basin's high earth heat flow leads to strong hydrothermal pressurization and hydrocarbon generation,thus forming strong large-scale uncompaction overpressure in the central depression zone of Yinggehai Basin with a pressure around 55 MPa.In terms of changes after gas field development,material balance methods and numerical simulation methods are mainly used to analyze the pressure changes.Single-well numerical simulation model is established with stress sensitivity effect taken into account.Through the analysis of dynamic change process of pressure drop funnel nearby the bottom hole,it indicates that,at the initial production stage of gas well,the production drawdown pressure is around3-5 MPa,and local pressure nearby of the gas well will drop synchronously with formation pressure,and that pressure drop funnel is relatively deeper with a large difference in influence caused by stress sensitivity between the area nearby the bottom hole and the area far from borehole;in the late production period,the wellhead pressure will reach the minimum limit,the gas well enters into constant pressure drop stage,production differential pressure will drop gradually,and pressure drop funnel will become shallow slowly,and then the average pressure of formation will drop by up to 35 MPa,while local pressure nearby gas well will drop by up to 40 MPa.(2)Change of the reservoir's physical properties in the pressure-droping development process of gas reservoir is studied.Based on Anderson's Relationship Theory Between Fault Morphology and Ground Stress,magnitude and position of ground stress in Dongfang X Gas Field is analyzed,through which the conclusion can be drawn that the maximum ground stress of the gas field lies at N120°E,and that the overburden pressure is the maximum ground stress of the gas field,as well as that certain difference value exists between horizontal ground stresses.As abnormal high pressure exists,horizontal ground stress has a relatively high value.Based on predicted value of gas reservoir pressure drop,variation law of ground stress in reservoir is analyzed,as well as influence of weakening gas reservoir pressure on porosity,permeability and reservoir strength.When gas reservoir pore pressure drops,the vertical principal stress will not change,while the horizontal principal stress will decrease and the formation strength will increase.Influence on the development well caused by the changes in formation stress as well as mechanism and influence factors of stress sensitivity are analyzed.Fluid and rocks in the reservoir and the formations around form a tight coupling system,accordingly,gas reservoir seepage mechanism change caused by the change in formation stress will influence productivity,water breakthrough time and recovery ratio of gas well,and may influence the stability of wellbore and lead to casing damage and sand production,etc.(3)Reservoir stress sensitivity experiment is conducted.This research makes some improvements in traditional experimental methods and equipment.Through the experimental method of fixing confining pressure and changing fluid pressure,it can factually simulate and study the reservoir sensitivity when the underground reservoir stress changes.Through carrying out reservoir stress sensitivity experiment under a close underground actual conditions,representative observations are obtained: as for the medium-porosity and medium-permeability Gas Reservoir II,with the increase of effective stress,permeability decreases rapidly in the early stage while slowly in the late stage,which is especially evident in the process of net effective overlying pressure increasing from 15 MPa to 45 MPa.Permeability decreases maximally compared with the condition of initial formation when net effective overlying pressure rises to 65 MPa,with a loss rate of 10%~40%,and low to medium stress sensitivity.Even though pressure recovers to initial formation pressure(the process of net effective overlying pressure decreases),permeability shall not recover to initial formation permeability values;the rock's pore volume compressibility decreases with the increase of net effective overlying pressure.Rock compressibility coefficen changes greatly during the early pressure buildup.However,with rock compressibility reducing,it decreases during the late pressure buildup.Reduction rate of porosity is only a few thousandths with a slight change.At the same time,rate of porosity decrease gets slower with the continuous increase of net effective overlying pressure.The research shows that the main mineral components influencing the stress sensitivity of the core reservoir are quartz,feldspar and clay mineral.With more low hardness minerals content,deformation comes more easily in rock core when external stress changes.And with more notable deformation,clay mineral content has a deeper influence on stress sensitivity of rock.(4)The change of fluid physical properties during the pressure-droping development process of gas reservoir is studied.The change of condensate water content in natural gas during the process of pressure drop is analyzed.Dongfang X Gas Field is affected by the HTHP environment,where the natural gas is saturated with a certain amount of water.With the decline in natural gas pressure during the depletion development process,the saturated water vapor content in the natural gas will change and condensate water will be produced,leading to effusion in borehole and productivity reduction or even shut-in of gas well.The quantitative analysis of variation law of water-soluble gas in formation water shows that the water-soluble gas content is mainly related to formation pressure,temperature,mineralization and gas composition.With temperature rising,it decreases first and then increases with a turning point around 80-90?.After the temperature is higher than 100?,the amount of dissolved gas increases with the increase of pressure.Under the condition of formation temperature and pressure,the water-soluble gas content in??X gas reservoir formation water is about 22.5 m3/m3.The two-phase volume coefficient of formation water is analyzed.A large amount of natural gas will be dissolved in high pressure gas reservoir formation water,and the water-soluble gas will increase the volume coefficient of formation water.The two-phase volume coefficient of gas reservoir formation water in Dongfang X Gas Field II can be 22.8 through analysis and calculation,which is 22 times higher than that of the formation water compared with the formation water without water-soluble gas and greatly increases the energy of water invasion.(5)The fluid-solid coupling seepage mechanism during the pressure-droping development process of gas reservoir is studied.The effect of pressure drop on irreducible water saturation is analyzed.During the process of depletion exploitation of gas reservoir,the net effective pressure increases continuously and the rocks are compressed constantly,which results in the changes of irreducible water saturation and its occurrence.The influence of pore volume shrinkage after pressure drop on the water film of irreducible water inside pore is analyzed.As the difference between pore-throat radius and water film thickness reduce,the reservoir's gas phase effective permeability will reduce significantly.According to the ideas from Carman-Kozeny formula,a new model to calculate the effective permeability of the low permeability reservoir is established,which calculates that the water film thickness of pores inside gas reservoir is 0.05?m~0.09?m and the ideal irreducible water saturation is 25.54%~36.11%.On this basis,the change of gas phase effective permeability during pressure drop is calculated.The experimental method of “water-soluble gas displacement” mechanism is established.Through the design of visualization experiment of pressure drop release of water-soluble gas in porous media,the percolation mechanism of dissolved gas driving inside boundary and bottom water in porous media is observed.The expansion of formation water,the release of dissolved gas,and the flow of dissolved gas,as well as the variation law for flooding the flow of pore water and moving the gas-water interface during boundary and bottom water pressure drop are observed.Based on the experiment,combined with numerical simulation technology,the evaluation technology about the influence of the water-soluble gas release of water-bearing gas reservoir with abnormal high pressure on the gas reservoir development is established.It describes the influence of water-soluble gas release on exploitation of gas reservoir.And it holds that the water invasion amount has been significantly increased compared with the one caused by pressure drop and expansion of slightly compressible formation water without considering water-soluble gas,and predicts that the water breakthrough time of gas wells will be greatly advanced,which establishes theoretical basis for waterproof strategy of HTHP gas reservoir with boundary and bottom water.The influence of reservoir's stress sensitivity on single well productivity is studied.With the increase of stress sensitivity coefficient,the productivity of gas well will decrease.(6)The influence of the pressure sensitivity on the development index is studied.The numerical simulation research by considering reservoir's stress sensitivity shows that,Gas Reservoir permeability reduces by 10% and recovery ratio reduces by2.6% when considering the effect of stress sensitivity.Research on the effect of the water-soluble gas release on the gas reservoir recovery ratio shows: due to the expansion produced by pressure drop and the release of the dissolved gas in the water,when the gas does not form the continuous phase,the discharged dissolved gas floods the pore water,and carries the boundary and bottom water flowing to only produce low pressure area,which causes the gas and water interface of the gas reservoir rises fast.With the increase of dissolution coefficient of the water soluble gas,the gas and water interface near the wellbore rises fastest and the water inside it reaches the gas well first.Under the condition that the volume multiplier of boundary and bottom water is 4 and the solubility of water-soluble gas is 22.5 m3/m3,compared with the condition when the solubility of water-soluble gas is not considered,the downhole gas-water interface will increase greatly,the time of water breakthrough will advance,different gas wells shift to an earlier date about 90-1710 days.(7)The strategies and methods of improving gas reservoir recovery ratio are studied.Improve recovery techniques by establishing the "natural-flow drainage" of abnormal high pressure gas reservoir to reach water-control and gas-stabilization.According to the simulation results of physical experiment on the displacement mechanism of gas reservoir boundary and bottom water by releasing high water-soluble gas from water-bearing gas reservoirs,combined with material equilibrium calculation and numerical simulation,it is proposed that the drainage well shall be drilled in the gas reservoir edge water area,the strategy of "natural-flow drainage" and water-control and gas-stabilization is implemented in the early development by using the natural-flow drainage with natural energy of abnormal high pressure gas reservoir,which can economically and effectively prevent the intrusion of gas reservoir boundary and bottom water,and improve gas reservoir recovery ratio.The simulation results of the newly established mechanism model show that drilling a natural-flow drainage well in the edge water area prosposed in the early development can prolong the effective production time of gas wells and increase gas reservoir recovery ratio by 5%.(8)The development direction of HTHP gas field development technology is put forward.In line with the condition that abnormal high pressure gas reservoir development is facing many challenges,the key research direction in the future will be HTHP lithomechanics core experiment,1D lithomechanics fine modeling,3D lithomechanics modeling and 4D lithomechanics coupling simulation.The interaction between the stress field and the pressure field of the rock in the production process is analyzed synthetically,the numerical simulation of gas reservoir is embedded in the stress field simulation of gas field in large scale,which can improve the accuracy and reliability of the fluid-solid coupling prediction result,avoid gas field development risk,and optimize gas field development plan and safe production management. |
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