低渗透油藏CO_2吞吐压力响应曲线分析

Pressure transient analysis on CO_2 huff and puff in low permeability reservoir

CO_2吞吐是低渗透油藏提高采收率的一种重要方式,但是目前对吞吐过程中的压力响应认识不足。低渗透油藏注入CO_2后,黏度等流体性质随CO_2浓度分布变化而产生平面非均质现象。为了表征低渗透条件下的压力动态,基于Fick定律求解CO_2浓度分布来反映流体的性质差异,在达西渗流方程中引入渗透率模数和启动压力梯度来反映油藏的低渗透特性,通过黏度耦合浓度方程和渗流方程,建立了低渗透油藏CO_2吞吐压力响应模型,采用数值方法对模型进行求解,并通过解析方法和数值模拟对模型可靠性进行了验证,最后分析了低渗透特性参数以及CO_2浓度相关参数对压力响应曲线的影响。研究结果表明:启动压力梯度越大,CO_2注入压力越大,压力及压力导数曲线斜率也越大;渗透率模数越大,压力及压力导数曲线末期斜率越小;CO_2注入速度越快,其注入压力越大,浓度扩散越快,波及区与未波及区的界限越明显;CO_2扩散系数越大,储层流体性质的非均质性越小,波及区与未波及区界限越不明显。该模型可应用于低渗透油藏CO_2吞吐注入过程试井分析、地层参数预测以及CO_2浓度分布动态分析。

CO_2 huff and puff are important way to improve oil recovery in low permeability reservoir. However,the recognition of the pressure response performance is not clear. After the injection of CO_2 in low permeability reservoir,the fluid properties such as viscosity vary with the CO_2 concentration,resulting in the phenomenon of plane heterogeneity. In order to characterize the pressure transient,the CO_2 concentration distribution was obtained based on Fick＇s law to characterize the fluid property. The permeability modulus and starting pressure gradient were introduced in Darcy flow equation to characterize the low permeability characteristics. The pressure response model for CO_2 huff and puff in low permeability reservoir was established by viscosity coupling concentration equation and flow equation,and it was solved by numerical method. The reliability of the model was verified byanalytical method and numerical simulation,and finally the influences of low permeability properties and CO_2 concentration on the pressure response curve were analyzed. The results show that the increase of starting-up pressure gradient leads to higher CO_2 injection pressure and the rise of pressure and pressure derivative curve slope. The pressure and pressure derivative curve slope become smaller with the increase of the permeability modulus. The diffusion of CO_2 becomes faster while increasing the injection rate of CO_2,and the injection pressure becomes larger,at the same time,the boundary between swept area and unswept area is clearer. As the diffusion efficient of CO_2 increases,the difference of fluid properties in difference place becomes smaller and the boundary between swept area and unswept area is not clear. This model can be applied to well testing of CO_2 huff and puff process in low permeability reservoir,the prediction of reservoir parameter and analysis of CO_2 concentration distribution.