Abstract
AbstractAfter a sandstone oilfield enters the high water-cut period, the viscosity of crude oil has an important influence on remaining oil distribution and waterflooding characteristics under the same factors of, e.g., reservoir quality and development methods. Based on a comprehensive interpretation of the waterflooded layers in new oil wells, physical simulation experiments, and reservoir numerical simulations, we analyzed the waterflooding laws of a high water-cut sandstone reservoir with different oil viscosities in Kazakhstan under the same oil production speed, and we clarified the remaining oil potential of reservoirs with different viscosities and proposed corresponding development measures. The results show that low-viscosity oil reservoirs (1 mPa s) have uniform waterflooding, thick streamlines, small waterflooding areas, and low overall waterflooding degrees because of their homogeneous oil–water viscosities. However, within waterflooded areas, the reservoirs have high oil displacement efficiencies and high waterflooding degrees, and the remaining oil is mainly concentrated in the unwaterflooded areas; therefore, the initial production and water cut in new oil wells vary significantly. High-viscosity oil reservoirs (200 mPa s) have severe waterflooding fingering, large waterflooding areas, and high overall waterflooded degrees because of their high oil–water mobility ratios. However, within waterflooded areas, the reservoirs have low oil displacement efficiencies and low waterflooding degrees, and the remaining oil is mainly concentrated in both the waterflooded areas and the unwaterflooded areas; therefore, the differences in the initial production and water cut of new oil wells are small. Moderate-viscosity oil reservoirs (20 mPa s) are characterized by remaining oil distributions that are somewhere in between those of the former two reservoirs. Therefore, in the high water-cut period, as the viscosity of crude oil increases, the efficiency of waterflooding gradually deteriorates and the remaining oil potential increases. In the later development, it is suggested to implement the local well pattern thickening in the remaining oil enrichment area for reservoirs with low viscosity, whereas a gradual overall well pattern thickening strategy is recommended for whole reservoirs with moderate and high viscosity. The findings of this study can aid better understanding of waterflooding law and the remaining oil potential of reservoirs with different viscosities and proposed corresponding development measures. The research results have important guidance and reference significance for the secondary development of high water-cut sandstone oilfields.
Highlights
After sandstone oilfields enter the stage characterized by high water cut and high recovery degree, their remaining oil distribution patterns become complex, and the controlling factors are diverse [1,2,3,4,5,6]; accurate prediction of the remaining oil distribution is the key to improving oil recovery [7,8,9,10,11,12,13,14]
Based on the oilfield outcrops, the reservoir architecture characterization was carried out [12,13,14,15,16,17,18,19,20,21,22], three-dimensional architecture models of different types of sand bodies were established [23,24,25,26], and the control effect of sand body architecture on waterflooding [27,28,29] and remaining oil distribution pattern [30,31] was clarified, which pointed out the direction for the formulation of reasonable development technical measures during the high water-cut period of sandstone oilfields
The thickness ratio of the unwaterflooded layer decreases to 38.9% and the thickness ratio of the moderate-strong waterflooded layer increases to 48.7%, which shows that most of the new oil wells are distributed in the waterflooded areas, the overall waterflooding degree of the reservoir is high, and the differences in the initial production and water cut gradually become smaller
Summary
Effects of oil viscosity on waterflooding 1737 when the development methods are determined, the internal factors control the remaining oil distribution. Using the comprehensive well logging interpretation method for waterflooded formations, the new oil wells in the waterflooded areas of sandstone oilfields with different viscosities were divided into unwaterflooded layers (water cut of less than 10% and resistivity of greater than 8 Ω m), weak waterflooded layers (water cut of 10–40% and resistivity of 4–8 Ω m), and moderate-strong waterflooded layers (water cut of greater than 40% and resistivity of greater than 4 Ω m) [26] Based on this division, by analyzing the waterflooding characteristics within and between single sand bodies in sandstone reservoirs with different viscosities and by quantitatively determining the thickness ratios of the waterflooded layers of the different grades, the waterflooding laws of oilfields with different crude oil viscosities were revealed. The unwaterflooded, weak waterflooded, and strong waterflooded layers are developed, among which, the development of unwaterflooded layers increases and the development of moderatestrong waterflooded layers decreases (see Figure 3c), indicating that as the viscosity of the crude oil increases, the extent of the vertical waterflooding becomes worse, and the waterflooding efficiency decreases significantly
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