Research On In-depth Fluid Diverting Technology In Polymer Flooding Of Offshore Oilfield

In this paper, two types of in-depth fluid diverting agents are developed by using hydrophobically associating polymer that can be applied in polymer flooding of offshore oilfield, one is gel typed in-depth fluid diverting agent, another is gel dispersion typed in-depth fluid diverting agent.The gel typed in-depth fluid diverting agent consists of chromium gel which is made of oxidation-reduction chromium system, organic acid chromium system and phenol formaldehyde resin gel which is made of the prepolymer of phenol and formaldehyde. Based on the conditions of offshore oilfield, the gelation time and gel strength are studied. The optimized combination formula of gel typed in-depth fluid diverting agent can be applied for shutoff different locations from the wellbore. The phenol formaldehyde resin gel is environmental with a long gelation time, and suitable for in-depth fluid diversion in polymer flooding of offshore oilfield. As a result, this kind of gel is focused on studying with the Anton Paar rheometer. The rheological behavior in gelation process and after gelation are researched, the gelation mechanism is analysised. It is confirmed that not only chemical crosslinking but also physical crosslinking take effect in gel formation, which indicates that hydrophobically associating polymer is more suitable to prepare in-depth fluid diverting agent. It is also demonstrated in the physical simulation experiments that the phenol formaldehyde resin gel has a good injectivity, plugging performance, thermal stability, flushing stability, shear performance and capacity to enhance oil recovery.The gel dispersion typed in-depth fluid diverting agent is prepared by tube shearing crosslinking method with hydrophobically associating polymer and chromium acetate. With the Anton Paar rheometer, the crosslinking process under shear conditions can be divided into four stages, that is induction period, gelation period, confining particle period and steady period. The gelation extent, micromorphology, particle diameter and viscosity of the system are also studied. According to the results of starch-cadmium iodide method and vacuum drying method, over 90% polymer can form gel dispersion. The atomic force microscope(AFM) shows that dispersion gel is made of small pseudo-spheral particles. The ratio of major and minor axis is almost 1, the larger the shear rate, the more closely the ratio is to 1. Salinity has a minor effect on micromorphology, and there is ageing aggregation phenomenon. The results of dynamic light scattering also supports the conclusions above. The rotary viscosimeter shows that gel dispersion has a low viscosity and salinity has a minor effect on viscosity of the system, besides, the system has a good shear stability. The research above shows that gel dispersion is an ideal in-depth fluid diverting agent with a good injectivity, capacity of in-depth shutoff and insensitivity to salinity and shear. It is also demonstrated in the physical simulation experiments that gel dispersion has a good performance in injectivity, blocking ability and enhancing oil recovery. The particle diameters of gel dispersion have a matching relationship with the sizes of core pores. The relationship of shutoff capacity(residual resistance factor) of gel dispersion with core sizes and mass concentration is as follows: . The gel dispersion can easily aggregate after high temperature ageing, which causes its good ability to shutoff high permeability zones. The iso-gradient placement technology of in-depth fluid diverting agent is also researched. The in-depth fluid diverting agent will stop moving when its breakthrough gradient is equal to pressure gradient of reservoir. With this concept, the in-depth fluid diverting agent can be placed at different locations from the wellbore, and optimal fluid diversion performance can be acquired. In order to establish this placement technology, formation pressure gradient curve is derived by using mathematical approach in the case of Suizhong 36-1 oilfield which has a five-spot well pattern. Meanwhile, the relationship between breakthrough gradient of in-depth fluid diverting agent and core permeability is studied, and the description of excessive water is proposed to analysis large channel in polymer flooding of offshore oilfield. These studies above lay a foundation for practical application of this placement technology. The matching technologies of in-depth fluid diversion are also investigated, including depressurization augmented injection technology, temporary plugging technology and over displacing technology. These technologies make sure that the in-depth fluid diverting process can be carried on successfully.