Effect Of Diffusion On Phase Transition And Flow Mechanism In Condensate Gas Reservoirs

As a special complex gas reservoir,condensate gas reservoir plays an important role in oil and gas exploration and development,as well as national energy strategic reserve.,The special complexity mainly lies in retrograde condensate phenomenon of gas condensate under dew point.The dialytic condensate oil and gas have molecular diffusion phenomena in the gas-liquid interface,the phases of oil and gas will also change accordingly,leading to changes of saturation of oil and gas in the reservoir.Thus it affects the seepage characteristics of the system.Because it takes sometime for the system to reach a new equilibrium,the equilibrium process is not instantaneous.This study is subsidized by the National Natural Science Fund Project(51374179).Firstly,two kinds of fluid samples(water/water free)are formulated to carry out the PVT Test under conditions of equilibrium and non-equilibrium.The influence of gaseous water and non-equilibrium effect on phase equilibrium is analyzed.Simulation experiment for different depleted rate is conducted using actual reservoir cores in the field.Analysis of depleted rate on the impact of the exploitation effect of condensate gas reservoir is conducted.The calculation model of non-equilibrium phase of condensate gas in consideration of diffusion effect and one-dimensional multi-component percolation mathematical model are established on the basis of the double-film theory.Through systematic research,the following important conclusions and understanding is concluded.(1)The nature of the gaseous water is similar to that of heavy hydrocarbon.And the mass transfer phenomenon occurs between the condensate gas and gaseous water,resulting in the loss of the light component and the increase of the heavy in the condensate gas.The presence of vaporous water accelerates the rate of retrograde condensation and retrograde evaporation,which leads to the enhancement of expansion ability of condensate gas system and increase of dew point pressure.The gas/oil ratio and saturation retrograde condensate are increased.(2)The expansion experimental study of constant constitution shows that,the larger the depleted rate is,the more serious the system deviates from the equilibrium state,the more serious the non-equilibrium effect is and the lower the saturation of the retrograde condensate is.(3)The core depletion experiments shows that,with the increase of depleted rate,the heavy component content and condensate recovery rate increases gradually.Before arriving at the maximum of the condensate pressure,the main process of the system is retrograde condensation,the degree of reserve recovery of condensate oil increased greatly and gas/oil ratio increases slowly.After arriving at the maximum of the condensate pressure,the main process of the system is retrograde evaporation.There is a sharp rise in gas/oil ratio and a decrease in recovery degree of condensate oil growth.(4)The results of non-equilibrium phase transition model shows that,the non-equilibrium effect will reduce the precipitation of condensate.With the increase of drawdown rate in the non-equilibrium,the greater the degree a system deviate from equilibrium,the smaller the saturation of condensate is and the more obvious the"unbalanced" phenomenon is.At the same time,in the whole process of reducing pressure,mass transfer flux in the gas phase towards the liquid phase is greater than that in the liquid towards gas phase.In addition,with the increased number of C atoms,the net mass flux becomes smaller and smaller,which presents an enhanced retrograde condensate phenomenon than the retrograde evaporation.In the system,the net mass transfer flux of each component decreases with the decrease of pressure.The greater the pressure drop rate is,the larger the net mass transfer of each component is.(5)The result of non-equilibrium flow model shows that,affected by the molecular diffusion,the faster the depletion is,the greater the heavy component content the condensate gas has and the greater the light component content the condensate oil has.The non-equilibrium effect makes the condensate precipitation lag and it is exploited along with the gas.Under the combined influence of diffusion and non-equilibrium effects,the condensate oil saturation in the formation is lower than the equilibrium value.The recovery efficiency of condensate oil is greatly affected by the depletion rate,irreducible water saturation and reservoir physical property,but the recovery rate of dry gas is affected slightly.