页岩油藏油气两相流动模型及其对油藏开发的启示

The oil-gas two-phase flow model for shale reservoirs and its enlightenment to reservoir development

页岩油藏开采中若井底压力低于饱和压力,储层压力下降会导致溶解气变为游离气,使储层流体流动规律复杂,因此油井产能预测需要考虑早期溶解气驱的影响。国内外相关研究尚未阐明原油依靠游离气泡膨胀流动的本质规律。基于流体自扩散模型,考虑储层压力衰减导致溶解气析出建立自扩散两相流动数学模型,初步表征了储层压力衰减与溶解气驱油的耦合扩散流动过程。通过与已有实验数据对比,论证了模型的合理性。在自扩散主导的致密页岩油藏一次开采流动中,随着溶解气析出变为游离气且原油含气饱和度增大,游离气泡膨胀驱动原油流动的能力不断增强。重点分析了原油泡点压力与溶解气油比对页岩油流动的影响,发现在相同生产条件下更高的原油泡点压力与溶解气油比可显著提高原油中游离气的饱和度,流动中更多原油被膨胀的气泡“挤”出,原油产量更高。最后探讨了研究结论对页岩油藏开采的启示。

When the bottom hole pressure is lower than the saturation pressure in the exploitation of shale reservoir,the dissolved gas turns into free gas due to reservoir pressure depletion,and the flow law of reservoir fluids becomes more complex.Thus,the influence of dissolved gas drive in the early stage should be considered in oil production prediction.The essential law of oil flow relying on the expansion of free bubbles has not been clarified in previous domestic and foreign researches.Based on the fluid self-diffusion model,a mathematical model of self-diffused two-phase flow has been established considering the release of dissolved gas due to reservoir pressure depletion,by which the coupling diffusion-flow process jointly controlled by reservoir pressure depletion and dissolved oil drive is characterized.Then the model is verified by comparing the results with existing experimental data.In the process of oil flow during the primary exploitation of tight shale reservoirs dominated by self-diffusion,the flow capacity of oil driven by gas expansion is increased with the formation of free gas released from dissolved gas and the increase of gas saturation in crude oil.Through analyzing the influence of bubble point pressure and dissolved gas-oil ratio on oil flow,it is found that under the same production conditions,the higher bubble point pressure or dissolved gas-oil ratio can significantly improve the gas saturation in crude oil,so that a larger amount of oil can be squeezed out,thus increasing oil output.Finally,according to research results,reasonable suggestions are proposed for exploitation of shale reservoir.