特低渗油藏CO2混相驱注采压力系统保压设计方法

为实现特低渗油藏注CO_2后形成混相驱,通过渗流力学和油藏工程方法研究特低渗油藏CO_2混相驱注采压力系统保压设计方法。首先,利用等值渗流阻力法,考虑特低渗油藏压裂后储层渗透率分布的"四台阶"特性,建立特低渗油藏CO_2混相驱渗流阻力数学模型;其次,考虑CO_2注入地层后溶解于原油所引起的驱替滞后效应,引入CO_2驱迟滞因子,对传统前缘推进方程进行修正。在此基础上,建立特低渗油藏CO_2混相驱注采压力系统保压设计方法,并从注采压差、注采井距、水井近井渗透率、油井近井渗透率四个方面进行单因素影响程度分析;最后,绘制M油藏在不同注采压差条件下注入速度与CO_2混相驱驱替前缘位置的关系图版。结果表明:注采压差对各项保压设计参数影响最大;纯油相区渗流阻力与混相区渗流阻力的比值仅与注采压差有关,与其他因素和驱替前缘位置无关;注入速度是CO_2混相驱保压设计的关键,在合理施工参数范围内,注CO_2无法实现全程混相驱,驱替前缘极限位置约在油水井距的3/4位置处。研究结果为特低渗油藏CO_2混相驱注采压力系统保压设计提供了理论依据和技术指导。

Pressure maintaining design method of injection-production pressure system in CO2 miscible driving of extra-low permeability reservoirs

In order to form miscible flooding after CO2 injection in extra-low permeability reservoir, the pressure maintaining design method of injection-production pressure system in CO2 miscible driving of extra-low permeability reservoir was studied through seepage mechanics and reservoir engineering methods. Firstly, the seepage resistance mathematical model of CO2 miscible flooding in extra-low permeability reservoirs was established by using equivalent seepage resistance method with considering the "four steps" characteristic of reservoir permeability distribution after fracturing in extra-low permeability reservoirs. Secondly, considering the displacement hysteresis effect caused by the dissolution of CO2 in crude oil after injection into the formation, the CO2 displacement hysteresis factor was introduced to modify the traditional front-edge displacement equation. On this basis, the pressure maintaining design method of injection-production pressure system in CO2 miscible driving of extra-low permeability reservoir was established. The influence degree of single factor was analyzed, including injection-production pressure difference, injection-production well spacing, near-well permeability of injection well and near-well permeability of production well. The results showed that the injection-production pressure difference had the greatest influence on the pressure maintaining design parameters. The ratio of seepage resistance in the pure oil zone and the CO2 miscible zone was only related to the injection-production pressure difference, and had no relationship with the other factors and the displacement front. The CO2 injection speed was the key parameter for the pressure maintaining design of CO2 miscible driving. And within the reasonable construction parameters, it was unable to achieve miscible driving in the whole distance. The limit position of displacement front is about 3/4 of the distance between oil and water wells. The relationships between the CO2 injection speed and displacement front position in M reservoir under different injection-production pressure differential conditions were drawn, which can provide theoretical basis and technical guidance for the pressure maintaining design of injection-production pressure system in CO2 miscible driving of extra-low permeability reservoir.