The Application Of Chemical Flooding Enhanced Oil Recovery Technology In Episodic Displacement

Reservoir oil recovery rate refers to the percentage of recovered oil in comparison to the overall oil amount in a reservoir or the ratio between the recovered oil amount from a reservoir and the reservoir’s overall oil amount. Most of the reservoirs China has already developed are from continental fluvial deposit layers. Affected by rivers and lakes in their formation, these oil reservoirs are characterized of relatively great differentiation in permeability, small oil bearing area in each reservoir, etc. Although a majority of oilfields have entered the later development phase so far, they still have a similarly low recovery rate around 35%. A large amount of crude oil still stay in reservoirs even if measures such as acidification, fracturing, and well distance optimization are taken, so great technological improvement is needed in later development.Nearly a century’s development, our technology has achieved a big leap under the direction of "learning while developing" philosophy. In the earlier phase, we employ water flooding to increase production by means of which the oil in reservoir is displaced. With further learning, we develop the technology of dispersed flooding, cutting flooding, spotted flooding, and edge flooding. In the middle and later development phase, with the aim to maintain crude oil production, we think out measures like polymer flooding, surfactant flooding, miscible gas flooding, immiscible gas flooding and foam flooding, alkaline flooding and steam flooding. Current enhanced oil recovery technologies can be classified as chemical flooding, miscible gas flooding, thermal flooding and microbial flooding according to flooding category. And in terms of mechanism, they can be categorized as oil viscosity reduction, surface or interface tension reduction and flooding fluidity control.Oil reservoirs’formation can be regarded as a process in which oil flooding drives away the water, and water recovery rate in this process is as high as 70%. However, oil recovery rate in the reversed process is only about 35%, which is remarkably lower than the oil flooding water recovery rate. By virtue of simulating these two processes on physical sand filling model, we find that the rate is not only affected by viscosity and humidity but also by fluid’s distribution situation. In reservoir formation process, oil front and water front move episodically with water phase receding from the reservoir orderly and oil phase entering the reservoir continuously. While in development process, producing wells and injecting wells function in the spotted way and reservoirs are split. Oil recovery rate is low due to factors such as poor oil viscosity, irregular oil-water front, water phase’s low sweep efficiency, etc. Based on this research, episodic oil displacement method is invented.Episodic oil displacement method means changing the functioning way of producing wells and injecting wells to make water phase front and oil phase front move episodically. In this method, oil phase is always being connected to the exit point, and the sweep efficiency of displacement phase is raised, which results in the increase of recovery rate. The simulation of oil displacement process in lab perfectly illustrates the practicability of this method. Let us see its application in a five-spot well pattern:In the first stage, we close the producing well, and then inject water and recover oil diagonally with the help of fingering phenomenon till the water-oil front becomes very close to the producing well; In the second stage, we open the middle producing well and turn the diagonally producing well into an injecting well till water appears in the middle producing well.Based on the experiment mentioned above and some reference to previous literature on polymer and surfactant, we attempt to add the combination of polymer for macro sweep efficiency rise and surfactant for water phase’s oil displacement efficiency to episodic oil displacement. This attempt proves feasible, and it indicates that both episodic displacement polymer oil recovery method and episodic displacement polymer-surfactant method can improve the physical model’s recovery rate. This thesis’s main experiments and their findings are as follows:1. Verifying privileged channel theory in reservoir formation process as well as the migrating features of oil-water phase by carrying out visualized oil drop flotation experiments, homogeneous filling sand oil migration experiments and fracture reservoir oil-gas migration experiments. These experiments indicate that the relatively high displacement rate in reservoir formation is attributed to the regular and continuous oil-water contact, water phase’s constant connection to the exit point as well as the orderly recession and entry of oil and water.2. Putting forward episodic oil displacement method through the comparison of formation process and development process.3. Simulating the regular five-spot well pattern in lab and improving this model’s visualization effect as well as the sand filling process. The new model’s feasibility is also tested.4.Employing the new model to compare episodic oil displacement method and regular five-spot method, which shows that episodic oil displacement method improves oil recovery rate by about 10%.5.Introducing the surfactant flooding and the polymer flooding to episodic oil displacement method whose potential gets further development. It demonstrates that episodic displacement polymer oil recovery method and episodic displacement polymer-surfactant method achieve higher recovery rate than episodic oil displacement method. Moreover, it enriches episodic displacement theory, integrating chemical flooding enhanced oil recovery technology into episodic displacement oil recovery theory.