High Pressure Gas Microflow Characteristics And Its Effect On Gas Seepage In Tight-low Permeability Reservoir

In order to reveal gaseous abnormal seepage characteristics and mechanism in tight-low permeability,in this paper,we carried out gas high pressure microflow experiment and studied the microflow characteristics of high-pressure gas and its influence on the seepage.Due to the extensive investigations on high temperature and high pressure?HPHT?gas microflow and gas seepage behaviors,requirements on high accuracy gas viscosity measurement is urgent.However,when measuring gas viscosity with the capillary tube viscometer,it is difficult to determine reasonable correction coefficients for the end effect,the slippage effect and the gas compressibility effect.And the uncertainty of those corrections brings considerable inaccuracy and reduces the reliability of measurement results.In order to solve this problem,we developed a HPHT gas microtube viscometry.By adopting the use of a high pressure micro differential pressure gauge and a high pressure micro flow meter,high-pressure gas viscosity measurement was conducted at low flow velocity.Nitrogen was used as the test fluid.With the novel experimental apparatus,we studied the main factors affecting the above effects and the associated rules of behavior.The experimental results indicated that the threshold tube diameter enabling negligible slippage effects in microtube viscosity measurement d_sc,the threshold pressure gradient enabling negligible gas compressibility effects in microtube viscosity measurement Dp_c,threshold flow velocity v_c,tube length L_c and tube diameter d_ec enabling negligible end effects in microtube viscosity measurement.The general experimental conditions are:d_sc?d?d_ec,v?v_c,L?L_c and Dp?Dp_c,where gas viscosity measurement can be conducted by applications of the Hagen-Poiseuille?H-P?equation without corrections.As a calibration of this novel setup and method,nitrogen viscosity measurement at 25?60?and 0.3?20MPa were taken and the results all falls within 1.2%error of the reference from National Institute of Standards and Technology?NIST?database.In this paper,we improved the microscale flow experiment which was widely used in the field of MEMS to adapt for high-pressure conditions.By using microtubes with inner diameter ranging from 1 to 100?m,we investigated the low velocity nonlinear flow characteristic of nitrogen especially under high pressures.The results indicated that gas flow behavior in microtube under high outlet pressures was opposite to gaseous slippage effect under low outlet pressures,which presented"negative slip"phenomenon.We define the ratio of measured flow rate to H-P theoretical flow rate as flow rate coefficient?C_f?.It has been observed that,under high outlet pressures,the C_f increased in the region with smaller pressure gradients and varied inversely with the pressure gradient.This phenomenon became more significant with the increase in the outlet pressure and the decrease in the tube inner diameter.In addition,we also found that the influences of the end effect and the gas compressibility effect on the flow in microtubes are quite different from those in conventional tubes.The end effect induced pressure drop?p_end increased monotonically with flow velocity v and decreased monotonically with tube diameter d.At high flow velocity,the influence of pressure became significant and?p_end became larger with the increase of pressure.While at low flow velocity,pressure had little effect on?p_end.The influence of temperature on?p_end was negligible.And the criterion enabling negligible gas compressibility effect in conventional scale flow?Ma<0.3?is not applicable to the flow of gas in microtubes.In this paper,we took pore-throat as the basic flow unit and studied gas flow behavior in a single pore-throat experimental model.The results indicated that under high in-situ pressure the lower the flow velocity,the less the influence of pore-throat local resistance on the flow and with the increase of pressure,the pore-throat local resistance changed rarely.Based on above regulations,an empirical equation for calculating the additional pressure drop induced by the pore-throat local resistance?p_pore-throat was obtained.Combining the results of microflow experiment and pore-throat model experiment,the seepage model of tight-low permeability pore-throat bundle was established.The simulation results indicated that under different in-situ conditions,the factors affecting gas seepage resistance are different.When the seepage velocity is low,the“negative slip”effect of high-pressure gas plays a dominant role.While at high seepage velocity,the effect of pore-throat local resistance is more significant.