SPE-165825-MS

SPE 165825

An Overview of Polymer Broken Down for Increasing Injectability in Polymer Flooding

Liu He, SPE, Gao Yang, RIPED, Petrochina, Liu Fangchao, Daqing Oilfield Company, Petrochina

Copyright 2013, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Asia Pacific Oil & Gas Conference and Exhibition held in Jakarta, Indonesia, 22–24 October 2013. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract

There are many reasons for the low fluid volume of oil wells and decreasing injectability of the polymer injection wells, such as the unreasonable process, the variations of the formation conditions, imperfect injection-production pattern, and improper production management, etc., and moreover, another important factor is the injected polymer and cross-linked polymer jamming in the near wellbore area and deep oil reservoir. According to the Statistical analysis of the 289 wells in Daqing Oilfield, there are two situations existing in polymer jamming which caused decreasing injectability, one is the shallow blocking, blockage radius within 3 meters, mainly caused by the polymer micelles, polymer floc, mechanical impurities, salt scale and iron sulfide, etc.; the second is the deep formation block, blockage radius around 10 meters, there are two factors resulting in this situation, which are, high viscosity caused near wellbore area jamming and polymer retention and adsorption in far wellbore zone. Aiming at these two situations, we have developed the polymer blockage remover and its supporting technology, which can efficiently remove blockage from polymer / cross-linked polymer, through this technology, the injection stop or under-injection can be solved to some extent, and the development effects of the polymer injection area have been improved.

Introduction

Polymer flooding technology is one of the effective ways to enhance oil recovery. It was widely used in Daqing oilfield and Shengli oilfield in China and achieved good results. However, the injection pressure was higher than the bursting pressure in the development process of polymer flooding. As a result, the injection production cannot be realized in such polymer flooding area. These wells are about 22.9% of the whole polymer flooding development wells. (Fig. 1)

Reasons of reservoir damage caused by polymer flooding

There are many reasons for the low liquid loading and decrease of water injectability in the polymer flooding area. In the process of polymer flooding, because of the change of the injected medium, the reservoir damage is different from what may cause by water flooding. Theoretical analysis and experimental results show that polymer can be retained in porous media by chemisorptions and mechanical trapping, which lead to decrease of pore cross section and increase of flow resistance. In addition, the molecular weight, structure, injecting concentration, injecting rate and injecting volume of the polymer can all greatly affect the permeability. Among all these factors, the molecular weight and structure of the polymer have greater impact on the flow property of the polymer solution. When the size of the polymer molecule, namely the coil gyration radius, matches the median radius of the rock pores, no blockage would happen for the polymer inside the porous media. Besides, the sewage salinity, bacterial and other factors can exacerbate the blockage of polymer to some extent. The blockage caused during the polymer injection is the result of the combined effect of various factors. For different damage factors the mechanism of blockage vary; core damage is mainly concentrated in the front of the core, namely the near well zone. Reasons of injection well blockage for different oil field differ from each other, but the blockage of the injected polymer and linked polymer in the hole and the near wellbore area is also an important factor. Fig. 2 shows the downhole blockage and Fig. 3 shows the filter blockage.

Additionally, the polymer blockages of near-wellbore and far-wellbore are caused by two factors together: retention and absorption. The blockage radius is about 10 metres.

The test results of metal ions of the injected and flowback fluids of some blocked wells in Daqing Oilfield demonstrate that when the Fe3+ concentration is higher than 1mg/L, significant blockage can happen, causing the injection pressure to

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rise; the iron concentration of flowback fluid is generally greater than 1mg/L, which is three times greater than that of the injected fluid. Table 1 shows the test results.

The principle of well selection

Stop injected or underinjected water wells and liquid production decreased oil wells caused by polymer and linked polymer blockage. The principle to select well is as follows.

Criteria of water wells: (1) when injecting water into the well, the injection pressure should be stable and water be absorbed well; when turning to injection, the injectability is good in the beginning; after the injection pressure becomes stable, the pressure will then increase rapidly over a period of time, finally leads to injection difficulties; (2) the regurgitation of polymers resulted from pressure fluctuation, which is caused by operation and pipeline damage, can also lead to injection difficulties.

Criteria of oil wells: (1) polymer produced in oil well. The early days of production is normal, but as the polymer injection continues, the liquid volume decreases gradually; (2) blockage of sand control strings downhole by polymers can lead to liquid volume decrease.

1. Blockages in shallow layers

Ammonium persulfate polymer flooding underinjection plug removal technology is employed. Polyacrylamide (HPAM) and its crosslinking system are used in polymer flooding; the main chain of HPAM is C-C bond and the side chain is amide or carboxyl. According to organic chemistry, a compound whose main chain is C-C bond shows high stability and is not easily degradable; only under the combined action of high temperature and strong oxidants will it degrade. As a result, to deal with the blockages caused by HPAM, a strong oxidant or several oxidants mix should be chosen to achieve the goal of degradation. By screen evaluating the plug removal performance of several common strong oxidants, we determine the main agent and additives of the plug removal formula; and by checking the various factors affecting the plug removal performance, we finally get the preferred formula of plug remover for polymer and its crosslinking system on Gudao Oilfield by means of core dynamic simulation optimization and adjustment of plug removal formula. As the oxidant, ammonium persulfate reacts with the polymer flooding blockage in the stratum, destroying the three-dimension network structure, thereby make the blockage detached from the rock surface and finally achieve the goal of plug removal.

The chemical reaction formula of ammonium persulfate degradable polymer is as follows.

Construction technology: (1) Injected blocking remover into the layers. (2) Shut in the well for 12 hours. (3) Open the well for normal polymer injection.

2. Blockages in deep layers

The blocking radius is about 10m, and the polymer blockages of near-wellbore and far-wellbore are caused by high viscosity. So the plug removal technology which contains sodium hypochlorite as the surface active agent is used. In order to realize the purpose of decompression and augmented injection, sodium hypochlorite is used to remove the plug, and the surface active agent is injected to increase the permeability of water. Surface active agent is ion surfactant FLZB composed of ethyl alcohol amine and polyoxyethylene oxygen poly propylene copolymer. Construction technology:

(1) String: the same as conventional broken down augmented injection with the characteristics of general plug removal and separate layer plug removal.

(2) Construction equipment: a mixing liquid pool, a cementing truck, a mixing funnel.

(3) Water for madding solution: waste water from water injection station or water for injection water. (4) Operation pressure: lower than the formation fracture pressure. (5) Density of liquid: 1%. (6) Response time: 24 hours.

Conclusion

(1) Field tests showed that DOC-8 plug removal +JNC-600 composite plug removal technology can break the blocking made by the polymer adsorption effect and the insoluble substance. It also can remove the blocking made by the clay organic impurities.

(2)This technology has the characteristics of high success rate, good effect and easy to operate. It has been used in 15 wells.

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References

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Fig.1 Relationship between injection pressure and injection rate when the viscosity is constant.

Fig. 2 Polymer floc, mechanical impurities, salt scale, iron sulfide, etc.

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Fig.3 The wellhead filter is blocked by a large number of polymer gels and flocs (micelles)

Table 1 Test results of metal ions of the injected and flowback fluid of blocked wells

Sampling sites and water types

Injected polymer

Well North x-x-P45 Well North x-x-P36 Well North x-xx-P34

iron（mg/L）

0.44 1.83 1.66 5.17 0.19 0.56

Flowback Injected polymer Flowback Injected polymer Flowback