Abstract
The special sedimentary environments of conglomerate reservoir lead to pore structure characteristics of complex modal, and the reservoir seepage system is mainly in the “sparse reticular-non reticular” flow pattern. As a result, the study on microscopic seepage mechanism of water flooding and polymer flooding and their differences becomes the complex part and key to enhance oil recovery. In this paper, the actual core samples from conglomerate reservoir in Karamay oilfield are selected as research objects to explore microscopic seepage mechanisms of water flooding and polymer flooding for hydrophilic rock as well as lipophilic rock by applying the Computed Tomography (CT) scanning technology. After that, the final oil recovery models of conglomerate reservoir are established in two displacement methods based on the influence analysis of oil displacement efficiency. Experimental results show that the seepage mechanisms of water flooding and polymer flooding for hydrophilic rock are all mainly “crawling” displacement along the rock surface while the weak lipophilic rocks are all mainly “inrushing” displacement along pore central. Due to the different seepage mechanisms among the water flooding and the polymer flooding, the residual oil remains in hydrophilic rock after water flooding process is mainly distributed in fine throats and pore interchange. These residual oil are cut into small droplets under the influence of polymer solution with stronger shearing drag effect. Then, those small droplets pass well through narrow throats and move forward along with the polymer solution flow, which makes enhancing oil recovery to be possible. The residual oil in weak lipophilic rock after water flooding mainly distributed on the rock particle surface and formed oil film and fine pore-throat. The polymer solution with stronger shear stress makes these oil films to carry away from particle surface in two ways such as bridge connection and forming oil silk. Because of the essential attributes differences between polymer solution and injection water solution, the impact of Complex Modal Pore Structure (CMPS) on the polymer solution displacement and seepage is much smaller than on water flooding solution. Therefore, for the two types of conglomerate rocks with different wettability, the pore structure is the main controlling factor of water flooding efficiency, while reservoir properties oil saturation, and other factors have smaller influence on flooding efficiency although the polymer flooding efficiency has a good correlation with remaining oil saturation after water flooding. Based on the analysis on oil displacement efficiency factors, the parameters of water flooding index and remaining oil saturation after water flooding are used to establish respectively calculation models of oil recovery in water flooding stage and polymer flooding stage for conglomerate reservoir. These models are able to calculate the oil recovery values of this area controlled by single well control, and further to determine the oil recovery of whole reservoir in different displacement stages by leveraging interpolation simulation methods, thereby providing more accurate geological parameters for the fine design of displacement oil program.
Highlights
Based on the 3D images of different pore volume from Computed Tomography (CT) scan results of two samples, combined with the porosity axial distribution information, it can be found that the hydrophilic sample has severe heterogeneity features of pore structure and poor pore connection in different porosity range, and the change width of porosity axial distribution curves is large, above characteristics indicate that the pore structure of hydrophilic sample is more complex
The main reason caused above rule is what the polymer solution change shear stress of different fluid interfaces to displace effectively remaining oil after water flooding which exist in film throats, pore intercross, large pores by surrounding small pores and attaching particle surface with oil film, but there is no residual oil in all the advanced seepage channels
The residual oil distributed fine throats and pore interchange after water flooding are cut into a lot of small droplets by the stronger shearing drag effect of polymer solution, pass well narrow throats to move forward along with polymer solution flowing to achieve the enhanced oil recovery purpose
Summary
The Computed Tomography (CT) scan technology makes it possible to observe seepage status of the different type of flooding liquid in different displacement volume dynamically without changing outer shape and inner structure of the core sample This technology with its unique advantages in acquiring fluid distribution information along axial direction within rock core, directly revealing the microscopic seepage mechanism, and locating the remaining oil distribution features inside the rock core has effectively reduced result errors from other experiments (Wang et al, 1985; Wellington and Vinegar, 1987; Xu et al, 2005). By selecting conglomerate oil reservoir in Karamay oilfield as the research subject and leveraging CT scan technology, the authors try to analyze the differences of seepage mechanisms for two displacement types and locate remaining oil distribution features based on axial changing information of oil saturation, establish respectively two final oil recovery forecasting models for polymer flooding and water flooding, and further to provide technical support for polymer flooding project design
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