From Darcy's law of two phases (oil and water), an analytic expression for oil and water vertical cross flow including gravity cross flow, capillary cross flow and additional cross flow under the effect of cyclic waterflooding is developed. Based on function J, this paper analyzes the vertical cross flow characteristics of capillary and its influence on production, explains the essential distinction between cyclic waterflooding and conventional waterflooding. Introduction At the end of 1950's, a Former Soviet Union expert, M.R. Surkuchef, advanced the concept of cyclic waterflooding for the first time. He thought that production process could be improved by changing the type of water-injection and establishing non-steady state artificially in the reservoir. Based on this theory, the Former Soviet Union has conducted cyclic waterflooding in Pokorove oilfield and some other oil fields for field test and commercial production since 1964. All obtained better production results. The field test showed that cyclic waterflooding can improve recovery rate by 3 -10% compared with conventional waterflooding. Since 1965, the Former Soviet Union focused on cyclic waterflooding theory research. Among those theories the most influential one was the mathematical model set up by O.I. Chenkowa et al. of the Former Soviet petroleum Scientific Institute in 1970's. The problem was that when cyclic waterflooding was conducted, this model could only predict the degree and influential factor of its improvement, by estimating the flow rate through porous medium between high and low permeability layers in vertical heterogeneous reservoir on the base of water stagnant coefficient concept; but could not explain the mechanism of cyclic waterflooding. Through laboratory experiments, W.G. Ogengianiyanches et al. believes that the physical nature of improving production by cyclic waterflooding is that when injection is halted, capillary cross flow becomes important so more crude oil is displaced from layers with low permeability and the water saturation of high permeability watered-out layers is reduced. Thus, the conclusion that cylic waterflooding is more effective in water-wet reservoirs. Field test and laboratory numerical simulation data show that cyclic waterflooding in oil-wet reservoirs can also be effective. This paper discusses the mechanism of cyclic waterflooding on the basis of seepage flow dynamics. The Way of Oil & Water Vertical Cross Flow Under Cyclic Waterflooding Set up a coordinate system, with the vertical downward direction as Z-axis. Suppose there are only two phases (oil and water) existing in the reservoir. Based on Darcy's Law, oil and water vertical movement equations can be written as: (1) (2) Analyses of seepage flow and numerical simulation show that during the half cycle when injection is halted or reduced, another pressure differential between the high- and low-permeability layers besides capillary pressure and gravity potential is created. This kind of pressure differential inherent in unconventional water injection is called as additional pressure differential. In contrast, during the half cycle of reinjection or greater injection, the pressure buildup is quick in high permeability layers and slow in low permeability layers, creating a reverse pressure differential (Fig. 1). Then (3) Table (1) (2) and (3) as simultaneous equations and let Then we have: (4)
An experimental method for measuring three-phase relative permeability of water, oil, and gas in reservoir core samples has been developed. Water, oil, and gas saturation measurement by microwave-weight technique, relative permeability measurement apparatus and experimental procedures are involved in the method. Three-phase relative permeability of water, oil, and gas for a number of water-wet sandstone core samples has been measured by this method in steady state condition. Experimental results indicate that this method is valid to measure three-phase relative permeability of water, oil and gas. Introduction The situation of simultaneous flow for water-oil-gas mixture in reservoir is encountered sometime during oil development. Relative permeability and fluid saturation are required to describe the behavior of water, oil and gas, to predict production performance in the reservoir and to adjust the mode of production. Thus, this paper gives an experimental method for measuring water, oil and gas relative permeability. An experimental investigation of three-phase fluid flow in porous media was made early in 1940's. So far, there are eleven methods for conducting experimental studies with different conditions and methods. The experimental methods were divided into two types, one was steady-state, the other was unsteady-state. However, for saturation measurement, the investigative keys are end effects and hysteresis effects. Some of the references, do not investigate end effects and hysteresis effects, while other authors consider their impact. There are some problems. In fact, experimental results of water, oil, and gas three-phase relative permeability are influenced by different apparatus, testing technology, and multiphase saturation history. Microwave Attenuation-Weight Technology An experimental method using microwave attenuation-weight technology has been developed to measure saturation of water, oil, and gas in cylindrical reservoir core samples. It is known as the ‘Microwave-weight’ method. A microwave attenuation testing system was designed for measuring multiphase saturation of water, oil, and gas in cylindrical samples. This system consists of a microwave element, power, singlecard micro-computer and control board. Testing Principal and Procedure. The typical curve between microwave attenuation voltage (V) and water weight in cores must be measured. Take the core sample, weight dry and vacuum saturate it. Then, calculate pore volume and porosity of the core sample. The saturated core sample is mounted in a three-piece core holder. The water/gas two-phase flow test is conducted after the manner of reference [13]. After flow is steady, the core sample is removed from the holder. By weight the water content in core sample is measured immediately. Microwave attenuation voltage and water content in core sample is obtained. We can change injection water proportion and repeat process obtaining many correlative data points. A typical curve for the relationship between the microwave attenuation voltage and the water content in the core sample is given in Figure 1. It should be noted that experiments are repeated after obtaining curve (V vs Gw). If the error between experimental is within 1 percent, the curve can be a typical curve for three-phase relative permeability measurement. Testing Accuracy. Accuracy test has been conducted to check the test effect. The experimental method is the same as above. After getting the first curve relating microwave attenuation voltage and water saturation on water/gas system, the curve will be a typical curve. Then the core sample is resaturated with distilled water. The flow test of the water/gas system is conducted for the second time. This time if flow state is stable, we use the weight method to determine water saturation. Then attenuation voltage is measured by microwave instrument. After obtaining the voltage, we can use the typical curve to determine water saturation. Finally, water saturations obtained with the two different methods are compared.
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