No abstract

The irreducible water saturation, the saturation of water when it begins to flow, is not a fixed value in the development process, whose numerical value is related to specific reservoir production conditions. Based on the integration of relative permeability curve and mercury intrusion curve, the author studies the change regularity of the irreducible water saturation under different production conditions (drawdown pressure), and puts forward the concept of dynamic irreducible water saturation and its determination method. Taking west Sulige gas field for application example, the actual drawdown pressure is 13.7 MPa and its dynamic irreducible water saturation is 30%. However, the conventional irreducible water saturation is 38%, which is getting from relative permeability curve. By analyzing the differences between two kinds of irreducible water saturation, the author thinks that the dynamic irreducible water saturation is closer to the actual production condition and it can be used in the correction of relative permeability curve and the study of lower limit of reservoir physical property.

Based on the oil-water two-phase liquid flow model, with equal-viscosity and water-wet unequal-diameter parallel channels, the previous researches on the retention mechanism of “residual oil in the form of oil droplets or oil column” during water flooding, and they stated that “residual oil in the form of oil droplets (or oil column)”can be retained in both large and small channels. Re-examination of the model shows that “residual oil in the form of oil droplets (or oil column)” cannot be retained in large channels, but only in small channels. By changing the wettability and oil-water viscosity (variable viscosity) of this model, the small channel retention characteristics of “residual oil in the form of oil droplets (or oil column)” remain unchanged. This research shows that: (1) When the oil-water phase flows in the unequal-diameter parallel channel model, the flow velocity of the large channel is always greater than the small channel, and the “residual oil in the form of oil droplets (or oil column)” is always retained in the small channel. (2) The retention of small channels of “residual oil in the form of oil droplets (or oil column)” is general, and is not affected by changes in factors such as pressure difference, capillary force, total flow, wettability, and oil-water viscosity ratio.

The minimum flow pore throat radius of the reservoir is an important indicator to characterize the seepage capacity of the reservoir pore throat. At present, the type of minimum flow pore radius is single, and there are few quantitative methods to determine it. To this end, through the analysis of the seepage capacity at different scale pores and the experimental method of mercury intrusion, the minimum flow pore throat radius and the determination method of the reservoir under the three seepage states of theoretical seepage, production seepage and filling seepage were studied and an example is applied. The analysis shows that the theoretical minimum flow pore throat radius corresponds to the lower limit of the capillary pore radius, and its size is related to the reservoir pore structure, which can be directly determined according to the mercury intrusion curve. The production minimum flow pore throat radius is the lower limit of the flowing pore radius of the reservoir. Its size is related to the reservoir pore structure and the production pressure difference. It can be determined by combining the mercury intrusion curve and the production pressure difference. The filling minimum flow pore throat radius is the lower limit of the filling pore radius. Its size is related to both the pore structure and the filling dynamics. It can be determined by combining the mercury intrusion curve and the original oil saturation. The above method was used to determine the minimum flow pore throat radius of Chang 63 reservoir in X Oilfield, Ordos Basin: the theoretical minimum flow pore throat radius is 0.015 μm, the production minimum flow pore throat radius is 0.017 μm, and the filling minimum flow pore throat radius is 0.085 μm. This study deepened the understanding of the minimum flow pore throat radius of the reservoir, and clarified the determination method of the minimum flow pore throat radius under different seepage states, which has reference significance for the calibration of the lower limit of the reservoir physical properties and the study of the reservoir utilization status.

The lower limit of reservoir physical properties is an important parameter for identifying reservoirs and determining effective thickness in reserves evaluation, and is also an important basis for selecting perforated test intervals in oilfield exploration and development. There are many methods to determine the lower limit of reservoir physical properties, and the minimum flow pore throat radius method is one of the commonly used methods. The method uses 0.1μm as the minimum flow pore-throat radius, and uses this to calibrate the lower limit of reservoir physical properties. However, according to the water film theory, the minimum radius of the reservoir's flowing pore throat is not a definite value, but varies with the displacement dynamics. Therefore, there is no exact basis for using 0.1μm as the minimum flow pore-throat radius, so it needs to be corrected. To this end, a new method for determining the lower limit of reservoir physical properties—the corrected minimum flow pore-throat radius method is proposed. The correction method comprehensively considers the factors of oil and gas accumulation dynamics, and determines the lower limit of reservoir physical properties by obtaining the minimum flow pore-throat radius value suitable for oil and gas accumulation dynamics. A case study of Chang 63 reservoir in A Oilfield shows that the minimum flow pore radius of oil and gas determined by the correction method is 0.08 μm, and the lower limit of reservoir physical properties (porosity 9.1%, permeability 0.117 × 10-3 μm2). The traditional method has a minimum flow pore-throat radius of 0.1 μm and a lower limit of reservoir physical properties (porosity of 9.8% and permeability of 0.133 × 10-3 μm2). Due to full consideration of the impact of oil and gas accumulation dynamics, the minimum flow pore-throat radius determined by the correction method is more reliable than the traditional method, and the lower limit of the reservoir physical property calibrated by it has practical significance.