The issue of pipeline scaling and oil-water separation caused by treating produced water in Alkali/Surfactant/Polymer (ASP) flooding greatly limits the wide use of ASP flooding technology. Therefore, this study of the demulsification-flocculation mechanism of oil-water emulsion in ASP flooding produced water is of great importance for ASP produced water treatment and its application. In this paper, the demulsification-flocculation mechanism of produced water is studied by simulating the changes in oil-water interfacial tension, Zeta potential and the size of oil droplets of produced water with an added demulsifier or flocculent by laboratory experiments. The results show that the demulsifier molecules can be adsorbed onto the oil droplets and replace the surfactant absorbed on the surface of oil droplets, reducing interfacial tension and weakening interfacial film strength, resulting in decreased stability of the oil droplets. The demulsifier can also neutralize the negative charge on the surface of oil droplets and reduce the electrostatic repulsion between them which will be beneficial for the accumulation of oil droplets. The flocculent after demulsification of oil droplets by charge neutralization, adsorption bridging, and sweeping all functions together. Thus, the oil droplets form aggregates and the synthetic action by the demulsifier and the flocculent causes the oil drop film to break up and oil droplet coalescence occurs to separate oil water.

Previous studies showed the difficulty during polymer flooding and the low producing degree for the low permeability layer. To solve the problem, Daqing, the first oil company, puts forward the polymer-separate-layer-injection-technology which separates mass and pressure in a single pipe. This technology mainly increases the control range of injection pressure of fluid by using the annular de-pressure tool, and reasonably distributes the molecular weight of the polymer injected into the thin and poor layers through the shearing of the different-medium-injection-tools. This occurs, in order to take advantage of the shearing thinning property of polymer solution and avoid the energy loss caused by the turbulent flow of polymer solution due to excessive injection rate in different injection tools. Combining rheological property of polymer and local perturbation theory, a rheological model of polymer solution in different-medium-injection-tools is derived and the maximum injection velocity is determined. The ranges of polymer viscosity in different injection tools are mainly determined by the structures of the different injection tools. However, the value of polymer viscosity is mainly determined by the concentration of polymer solution. So, the relation between the molecular weight of polymer and the permeability of layers should be firstly determined, and then the structural parameter combination of the different-medium-injection-tool should be optimized. The results of the study are important for regulating polymer injection parameters in the oilfield which enhances the oil recovery with reduced the cost.

The water produced by alkaline–surfactant–polymer (ASP) flooding is difficult to treat due to the presence of residual chemicals. Therefore, research and development of efficient and low-cost methods for the treatment of ASP flooding produced water is necessary. Chemical destabilization is the most common and effective way to treat the produced water. This paper describes an optimization method for demulsification and flocculation. Some demulsifiers and flocculants commonly used in oilfields were screened, compounded, and optimized. Since the effect of treatment using only demulsifier or flocculant to treat the produced water is often not enough to meet the reinjection standard, an orthogonal experiment was carried out to study the demulsification–flocculation method to treat produced water. Five main influencing factors of the oil concentration were investigated. Based on the results of the range analysis and the relationship between the five factors and oil concentration, the order of significant factors was found to be demulsifier dosage > flocculant dosage > settling time > stirring time > stirring intensity, and the optimal demulsification–flocculation treatment conditions were successfully optimized.

Many factors influence the evaluation process of thief zones. The evaluation index contains very complex information. How to quickly obtain effective information is the key to improve the evaluation quality for thief zones. Considering that the correlation and information redundancy among the evaluation indexes will seriously affect the evaluation results for the thief zone, based on the principal component analysis (PCA) method, this paper proposes a multi-layer weighted principal component analysis method (MLWPCA). Firstly, factor analysis is performed on the original data to obtain the plurality subsystems of the evaluation index. Then, a principal component is analyzed through the subsystems of the evaluation index PCA to obtain the principal component score. Finally, the subsystem is weighted by the factor score and the comprehensive thief zone score is obtained by combining the subsystem weight and the subsystem score. A case study on the Daqing oilfield shows the effectiveness of the method, verified by tracer tests when applying the MLWPCA method to evaluate the thief zone. The thief zone of the Daqing oilfield is obviously affected by effective thickness, coefficient of permeability variation and interwell connectivity. At present, there are 10 well developed thief zones and eight medium developed thief zones in Daqing oilfield. The accuracy of this method is 94.44%. Compared with PCA, this method has better pertinence in evaluating thief zones, and is more effective in determining the principle influencing factors.

With the wide application of ASP (alkali/surfactant/polymer) flooding oil recovery technology, the produced water from ASP flooding has increased greatly. The clay particles carried by crude oil in the process of flow have a synergetic effect with oil displacement agents in the produced water, which increases the treatment difficulty of produced water. The stability of produced water is decided by the stability of oil droplets in the ASP-flooding-produced water system. The oil content, Zeta potential, interfacial tension and oil droplet size are important parameters to characterize the stability of produced water. In this paper, the changes of the oil content, Zeta potential, interfacial tension and oil droplet size of ASP flooding oily wastewater under the synergetic effect of different concentrations of quartz sand particles and oil displacement agents were studied by laboratory experiments. The experimental results show that the negatively charged quartz sand particles can absorb active substances in crude oil and surfactant molecules in the water phase and migrate to the oil–water interface, which increases the repulsion between quartz sand particles, decreasing the oil–water interfacial tension. Thus, the stability of oil droplets is enhanced, and the aggregation difficulty between oil droplets and quartz sand particles is increased. With the continually increasing quartz sand concentration, quartz sand particles combine with surfactant molecules adsorbed on the oil–water interface to form an aggregate. Meanwhile, the polymer molecules crimp from the stretching state, and the number of them surrounding the surface of the flocculation structure is close to saturation, which makes the oil droplets and quartz sand particles prone to aggregation, and the carried active substances desorb from the interface, resulting in the instability of the produced water system. The research on the synergetic effect between quartz sand particles and oil displacement agents is of great significance for deepening the treatment of ASP-produced water.

In order to improve the oil displacement effect of alkali/surfactant/polymer (ASP) solution in low-permeability oil layers, Daqing Oilfield has proposed a separate injection technology. The objective of separate injection technology is to reduce the viscosity of ASP solution through a different medium injection tool and increase the injection amount of ASP solution in low permeability oil layer, thus improving the oil displacement effect. In order to study the effect of the different medium injection tool on ASP solution, SEM is used to observe the changes in molecular micromorphology before and after the ASP solution flows through the tool. Then, the influence of the tool on viscosity and the first normal stress difference of the solution are studied through static shear experiments. Finally, the storage and loss modulus of the solution are measured through dynamic mechanical experiments and the relaxation time and zero shear viscosity of the solution are verified and compared. The results show that molecular chains are obviously broken and the grid structure is destroyed after the ASP solution is acted on by the different medium injection tool. The viscosity and elasticity of ASP solution decrease, and the influence degree of the different medium injection tool on viscosity is greater than elasticity. The results of the steady shear experiment and dynamic mechanics experiment are consistent. Therefore, the different medium injection tool can achieve the purpose of use, which is conducive to the injection of displacement fluid into low-permeability oil layers and enhance the recovery ratio.