Abstract
Generally, long-term waterflooding development often leads to the change of reservoir pore and clay mineral composition, which results in the change of permeability and wettability. In order to explore the relationship between core micropore structure and water cut, based on physical simulation experiments and fractal theory, we proposed a fractal evolutionary model to describe the evolution characteristics of microstructure of long-term water driving reservoirs. In this paper, core pore structure by SEM was first conducted to analyze the change of core pore structure before and after waterflooding under the conditions of magnification of 200 times, 800 times, and 2000 times, respectively. Then, conventional and constant rate mercury injection tests were combined to perform the comparative analysis of core structural parameters before and after waterflooding. Finally, a micropore-throat structure evolution model of core was established. Research shows that the connectivity of larger pores becomes better after long-term water driving, the degree of heterogeneity weakens, and the micro heterogeneity of small pores becomes stronger and stronger. The throat characteristics change in a complex manner, the radius tends to increase, and the sorting becomes better, while the connectivity of small throat changes complex. In general, the heterogeneity of throat increases with the time of water injection in reservoirs with low porosity and permeability. On the basis of fractal theory and variation characteristics of rock pore structure in water driving reservoirs, we have established a micropore-throat structure evolution model of core pore-throat characteristics. This fractal evolution model quantitatively characterized the complexity and evolution law of pore structure and clarified the relationship between fractal dimension of core pore structure and water cut under different stages of water driving.
Highlights
Daqing Oilfield is the largest continental multilayer sandstone reservoir in China
Yin [19] studied through many years the testing data of inspection wells and laboratory core waterflooding experiment of distributary channel sand body of Daqing Oilfield waterflooding process reservoir parameter variation characteristics, and it showed that after long-term water erosion, sample average pore-throat radius and median pore-throat radius, pore throat in reservoir large number, and the permeability contribution rate increase
RT,max pT,min where DP is the fractal dimension of larger pores; DT is the fractal dimension of smaller pores; rP,max is the largest pore of larger pores, μm; rT,max is the smallest pore of smaller pores, μm; pP,min is the minimum capillary pressure corresponding to the larger pores on the capillary pressure curve, MPa; pT,min is the minimum capillary pressure corresponding to the smaller pores on the capillary pressure curve, MPa; rP is the pore radius of larger pores, μm; rT is the pore radius of smaller pores, μm; pP,c is the capillary pressure when the pore radius is rP, MPa; pT,c is the capillary pressure when the pore radius is rT, MPa
Summary
Daqing Oilfield is the largest continental multilayer sandstone reservoir in China. After more than 60 years of longterm water injection development, the comprehensive water cut is over 95%, and it has been fully entered and is a typical representative of ultrahigh water cut oilfield [1,2,3]. Wu et al [18] study the sandstone reservoir physical characteristics and the changing law of the reservoir parameters, and research showed that due to long-term waterflooding, the permeability of sandstone reservoir and moisture content increased, the stratigraphic particles were rushed out, and clay mineral containing basin reduces clever grain of support way and the pore network connectivity, higher permeability, and reservoir wettability by oil into strong hydrophilic. Yin [19] studied through many years the testing data of inspection wells and laboratory core waterflooding experiment of distributary channel sand body of Daqing Oilfield waterflooding process reservoir parameter variation characteristics, and it showed that after long-term water erosion, sample average pore-throat radius and median pore-throat radius, pore throat in reservoir large number, and the permeability contribution rate increase. On that basis, based on the fractal theory and the evolution model of pore structure in different water cut stages during long-term water injection, a micropore-throat structure evolution model of core was established, and the mathematical relationship between water cut and fractal dimension of pore structure of core was obtained, which quantitatively characterized the complexity and evolution law of pore structure
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