强CO_2-原油传质作用的化学改性CO_2驱油体系构筑及其驱油特征研究

Because of high ability to displace the crude oil, CO_2 flooding has become one of the most researched methods to enhance oil recovery. But in the injection process, CO_2 channels easily along the high permeability layers or the fracture, making a large mount of crude oil stranded in reservoirs. After the formation of CO_2 channeling, it can not drive the oil directly anymore, but the mass transfer between CO_2 and the crude oil still exists, and the strength of this mass transfer becomes the key factor affecting the recovery of the stranded oil. So, focusing on the stranded oil, the project plans to enhance the mass transfer between CO_2 and oil by adding chemical modifiers to CO_2 to improve recovery of this oil. The specific research content includes: (1) Research of the effects of mass transfer between CO_2 and oil on recovery of the stranded oil. (2) Construction of the oil displacing system with high ability of mass transfer between CO_2 and oil using the chemical modifiers and research of the modified mechanism. （3)Research of the oil displacement characteristics of the modified CO_2 system. By the research of this project, effects of mass transfer between CO_2 and oil on recovery of the stranded oil are supposed to be revealed. The system, having the high ability of mass transfer with crude oil, is hoped to be got. And its displacement characteristics would be figured out. Results of this project can establish the technical foundations for enhancing oil recovery of CO_2 flooding in the heterogeneous and fractured reservoirs, and especially for the low permeability heterogeneous reservoirs.

由于具有较高的驱油能力，CO_2驱逐步成为提高原油采收率研究的重点。但是注入中，CO_2极易沿高渗或裂缝窜流，在油藏中形成大量的滞留原油。窜流后，CO_2不能直接驱动原油，但是其与原油间仍存在质量传递，这种传质作用的强弱成为影响滞留原油采收率的关键。本项目针对油藏中的大量滞留原油，拟通过对CO_2进行化学改性的方法，构筑具有强CO_2-原油传质作用的改性CO_2驱油体系，提高滞留原油采收率。具体研究内容包括：（1）CO_2-原油传质作用对滞留原油采收率的影响规律研究；（2）强CO_2-原油传质作用的化学改性CO_2驱油体系构筑及改性机制探索；（3）改性CO_2体系驱油特征研究。通过研究，拟揭示CO_2-原油传质作用对滞留原油采收率的影响规律，得到具有强CO_2-原油传质作用的改性CO_2驱油体系，明确体系的驱油特征。研究结果可为低渗、特低渗非均质或裂缝性油藏，提高原油采率奠定技术基础。

CO2驱油过程中，极易沿高渗或裂缝窜流，在油藏中形成大量的绕流原油。窜流后，CO2不能直接驱动原油，但是其与原油间仍存在质量传递，这种传质作用的强弱成为影响绕流原油采收率的关键。本项目针对油藏中的大量绕流原油，拟通过对CO2进行化学强化的方法，构筑具有强CO2-原油传质作用的CO2驱油体系，提高绕流原油采收率。首先开展了CO2-原油传质作用及其提高绕流原油采收率特征研究，同时考察了相态、绕流原油尺寸等因素对绕流原油采收率的影响；然后，研究了一些极性小分子（水、乙醇）助剂、碳氢类表面活性剂等对CO2-原油传质作用的强化效果，以及该型强化CO2体系驱替绕流非绕流和绕流原油效果。结果表明，1）根据原油与CO2接触程度的不同（模拟其在绕流区域分布位置的差异），CO2与原油的主要传质形式不同：对于接触较充分的浅层绕流原油，主要表现为萃取导致的体积收缩，并且这种体积收缩通常伴随残余油重质组分的富集、黏度的增大甚至是胶质沥青质沉积。对于无法直接接触的深层绕流原油，CO2的间接萃取作用较弱，主要表现为溶解导致的体积膨胀。2）仅仅通过CO2浸泡过程中的传质作用很难显著动用绕流原油，且其采收率随相态的转变（由一次接触混相到非混相转变）和绕流原油尺寸的增大明显降低；CO2吞吐能够在浸泡基础上显著提高绕流原油采收率，并且吞吐驱油效果在研究范围内（岩心长度：6.0cm-20.0cm），受绕流原油尺寸影响较小。3）选用的强化剂能够增强CO2-原油的传质作用，其效果由弱到强依次为：水、乙醇、TX100、TX100/乙醇、TX45和TX45/乙醇；强化CO2体系较纯CO2具有更强的驱替非绕流原油和绕流原油的能力。4）对比改变注气方式（浸泡vs.吞吐）和增强CO2-原油传质作用（CO2vs.CO2/TX100），提高绕流原油采收率的效果发现：注气方式的调整对改善绕流原油动用效果优于强化传质。但是，考虑到吞吐注气是一种单井注气措施，对整个油藏的控制程度低，而且强化体系（CO2/TX100）在浸泡和吞吐驱油效果均优于纯CO2，建议化学强化CO2体系与注气方式调整联合应用。