地层水对凝析气藏注CO_2相态的影响

常规的凝析气藏衰竭开发和注CO_2开发研究中均忽略了地层水的影响,这与真实情况存在偏差,有可能导致研究结果的不确定性加大。为此,基于CO_2—烃—水相平衡热力学模型,以一个实际近临界凝析气藏为例,通过相态模拟研究了地层水存在对凝析气藏反凝析相态特征和注CO_2相态的影响规律;计算了考虑地层水存在的凝析气定容衰竭反凝析液饱和度和剩余流体组成,以及注CO_2过程中凝析油气相体积分数和CO_2在凝析油气相中体积分数的变化规律。结果表明:1考虑地层水时定容衰竭的反凝析油饱和度更大,剩余流体重组分含量更高;2近临界凝析气藏压力衰竭过程中,由凝析气转变为挥发油的相变发生得更早;3在注CO_2过程中,地层水的存在使得CO_2对凝析油的反蒸发作用降低;4考虑地层水存在时凝析油相体积分数高约14%,CO_2在凝析油中溶解量比不考虑地层水大6%,CO_2含量高和压力较高时差异更明显,同时,地层水的存在也增强了CO_2的溶解封存能力。该研究成果对凝析气藏注CO_2提高采收率和温室气体CO_2埋存评价具有指导意义。

Impacts of formation water on the phase behavior of CO2 injected into gas condensate reservoirs

The impacts of formation water are generally ignored when gas condensate reservoirs are developed by means of conventional depletion and CO₂ injection, but this is not exactly the actual situations, possibly making the research results more uncertain. In this paper, a real near-critical gas condensate reservoir was taken as an example for study. Based on the CO₂–hydrocarbon–water phase equilibrium thermodynamic model, the impacts of formation water on retrograde condensation phase characteristics and phase behavior of CO₂ injected into gas condensate reservoirs were analyzed by means of phase simulation. The compositions of the remaining fluid and the retrograde condensate saturation of the constant volume depletion were calculated by taking such impacts into consideration. Besides, volume fraction of condensate oil/gas phase during CO₂ injection and the volume fraction changes of CO₂ in the condensate oil/gas phase were studied. It is indicated that the content of heavy constituents in remaining fluid and the retrograde condensate saturation of constant volume depletion are higher when the impacts of formation water are taken into account. During the pressure depletion of near-critical gas condensate reservoirs, the phase transition from gas condensate to volatile oil occurs earlier. In the process of CO₂ injection, the retrograde evaporation of CO₂ on condensate oil is weakened due to the presence of formation water. If the impacts of formation are considered, the volume fraction of condensate oil is 14% higher and the amount of CO₂ dissolved in the condensate oil increases by 6%. The difference is larger with the increase of pressure and CO₂ content. It is revealed that the dissolution and sequestration capacity of CO₂ is enhanced due to the presence of formation water. These research results play a guiding role in evaluating the CO₂ injection for enhancing the recovery of gas condensate reservoirs and CO₂ (greenhouse gas) sequestration.