Experimental And Numerical Study On The Characteristics Of Water Blocking By Gas Flooding And Application In Oil Shale In-situ Exploitation

Oil shale resources are abundant in China and have great potential for development and utilization.In-situ pyrolysis of oil shale is one of the technologies for clean and efficient exploitation.In-situ heating is the core of in-situ pyrolysis of oil shale.Due to the high specific heat,groundwater will absorb and take away a lot of heat,which will significantly reduce the heating and energy efficiency if groundwater enters the pyrolysis zone.Meanwhile,if the exploitation zone is connected with the underground environment,the organic components will diffuse to the aquifer,causing irreversible pollution of belowground ecosystem.Therefore,in-situ pyrolysis of oil shale needs to be carried out in a relatively closed space to prevent the inflow of external groundwater and the outward diffusion of pyrolysis products.Based on the existing underground space sealing technologies and related literatures,combined with the oil shale in-situ pyrolysis technology,this paper innovatively put forward the sealing method of gas flooding in order to provide a novel idea for the oil shale in-situ system sealing.Therefore,the characteristics and mechanisms of gas-injection barrier were studied by means of experiment,numerical simulation and reservoir-scale test.（1）According to the reservoir characteristics of oil shale in-situ pyrolysis zone,a two-dimensional visualization experimental system was built,and experimental schemes were designed to simulate the oil shale in-situ conditions.Then,the mechanisms and evolution characteristics of gas-injection barrier were studied.Results showed that with the increase of gas injection flow rate Q（capillary number）,the gas flow pattern gradually changed from capillary fingering to viscous fingering.When the Q was large,the gas flow developed upstream under the action of viscous force,and the desaturated zone covered about 90%of the flow area.In addition,the distribution of relative permeability of water k_ri in capillary fingering and viscous fingering stage was quite different.k_ri in the central subregion during capillary fingering stage was greater than 1,which indicated that gas injection promoted the water flow in this region,while water flow in the adjacent subregions was weakly inhibited(k_ri<1).In the viscous fingering stage,the water flow in all subregions was significantly inhibited.Especially,the k_riof the central subregion decreased from 0.68 to 0.05;the k_ri of the whole desaturated zone presented a V-shaped distribution.With the enhancement of hydrodynamic effect and coarsening of medium particles,the overall relative permeability of water of the desaturated zone increased,that was,the effect of gas-injection barrier was weakening.According to the characteristics of gas/water two-phase flow and pressure differenceΔP,the mechanisms of gas-injection barrier were comprehensively analyzed.When Q is large,the pressure balance state in the front of gas flow is the main mechanism of water blocking.（2）In order to clarify the influence of reservoir characteristics on the characteristics of gas-injection barrier,this paper further studied the influence mechanisms of wettability and fracturing area with high permeability.Results showed that k_ri can be reduced to about 0.1during continuous gas injection in the hydrophobic porous media.The gas flow paths became smoother and had the similar direction;the desaturated zone developed in a cone shape at different gas injection rates.In addition,it was found that the trapped gas caused by temporary gas injection could reduce the relative permeability of water to about 0.4.The trapped gas in the pores presented the following characteristics:In the hydrophilic porous media,floating trapped gas was easier to form,while internal trapped gas was more widely in the hydrophobic porous media.In particular,hydrophobic media was more conducive to trapped gas to play the role of gas-injection barrier.Furthermore,the influences of fracturing area on the characteristics of gas-injection barrier were analyzed experimentally.When the gas injection point was outside the high permeability area,the results showed that with the increase of Q,the range of desaturated zone increased;the high permeability area had an enrichment effect on gas flow.The closer the gas injection point to the fracturing area,the more significant the enrichment effect was.When the gas injection point was in the fracturing area,the gas flow was basically enriched.Due to the enrichment of gas flow in the fracturing area,the blocking effect was basically reflected in the fracturing area,and the relative permeability of water presented a U-shaped distribution.However,regardless of the water flow rates,the k_ri in the fracturing area presented the following rule:Case 3>Case 2>Case 1.From the point of view of water blocking,the gas injection point sets at or outside the boundary can achieve better results.（3）Based on the technical parameters of the Nong’an oil shale in-situ pyrolysis pilot project,this paper studied the implementation process of gas-injection barrier by numerical simulation.Results showed that the desaturated zone with a water blocking radius of 65 m can be formed through high pressure gas injection and pressure relief.The mechanism of water blocking was analyzed quantitatively,the gas resistance in the desaturated zone and pressure balance at the gas/water interface were the internal mechanisms,which was consistent with the experimental results.Furthermore,the feasibility of the method was verified by in-situ test.（4）The inhibition effect of gas flooding on the diffusion of pyrolysis products was studied by numerical simulation and theoretical analysis.Results showed that the diffusion range of dissolved phase methane was up to 25 m,and the concentration was higher than the safety threshold.The migration range of gas phase methane was up to 50 m,and the gas saturation at the horizontal plane was about 0.1.The marginal gas injection with appropriate pressure can effectively inhibit the outward diffusion of the pyrolysis products,the mechanism is that the Fick diffusion controlled by concentration gradient and Darcy flow controlled by pressure gradient can be balanced.It is also found that when marginal gas injection（0.61 MPa）was applied,the water inflow in the pyrolysis zone was also greatly reduced.That is,when marginal gas injection with a pressure slightly higher than the pyrolysis zone pressure is applied,the dual inhibition of groundwater infiltration and product diffusion can be simultaneously achieved,which verifies the effectiveness of this novel technology.By the work of this paper,the theoretical system of sealing technology by gas flooding is basically formed,which provides a new idea and theoretical support for the system sealing of oil shale in-situ pyrolysis exploitation.