Nowadays, most oilfields have entered the high water cut stage of waterflood development. The importance of oil–water separation technology becomes more obvious. Gravity separation is one of the most commonly used treatment techniques for produced fluid. The gravitational separator has a large processing capacity and a wide application range, but its structure is relatively simple and the separation efficiency gradually falls behind to meet current production needs. The key difficulties to improve the separation efficiency are to analyze the flow field and coalescing components inside the separator. Aiming at these difficulties, this paper reports an innovatively designed series-parallel multistage multiphase separation system (MMSS). A horizontal separator is connected in series with a vertical separator, and the vertical separator consists of five discrete pipes connected in parallel. Different coalescing components are then set inside the vertical separator. The separation effect of the MMSS is studied by numerical simulation and laboratory experiments. The oil phase volume distribution cloud diagrams of coalescing components are analyzed by numerical simulation, including semicircle baffle, spiral track plate, four-hole plate and seven-hole plate. Laboratory experiments show that MMSS has a high separation efficiency, and the water content at the oil outlet is 3.0% less than that of the horizontal separator. By observing the shape of oil droplets at the outlet and measuring the oil cut and water cut at the sampling outlet, the separation effect of four coalescent plates is obtained. According to the statistics, when the volumetric flow at the inlet of the separator is 1.5 m3/h, the average particle size of oil drops in the blank pipe, semicircular baffle, four-hole plate, spiral track and seven-hole plate increases in turn. A continuous oil layer appears at the outlet of the vertical separator in the fully open state. The water content at the oil outlet of the semicircular baffle coalescing component is always at a high level under different flow rates. When the inlet volumetric flow rate is less than 1.6 m3/h, the performance of the spiral track coalescing component is better. With the increase of the inlet volumetric flow rate, the separation efficiency of the spiral track is lower than that of the orifice. The results show that the semicircular coalescing component has the worst performance, the spiral track coalescing component is superior at small volumetric flow rates, and the orifice coalescing component is superior at large volumetric flow rates.
Downhole pressure is an important parameter for evaluating the production status of production wells. A wireless monitoring system was designed for downhole pressure which consists of two parts, i.e., the downhole tool and the wellhead tool. The tubing was used as the information transmission medium between the two parts. The downhole tool includes four pressure sensors, and the measured results will be applied to the tubing in the form of an alternating voltage with a frequency of 2.5 Hz. The amplitude of the alternating voltage output by the downhole tool is related to the magnitude of the downhole pressure. The wellhead tool measures and decodes the signal uploaded through the tubing, and the decoded result will be transmitted to the host computer through the ground cable. The wireless transmission model of downhole pressure in the production well was equivalent to a resistance network. Simulation analysis indicates that the amplitude of the signal received by the wellhead tool is negatively correlated with the downhole depth of the downhole tool and positively correlated with the formation resistivity. The designed wireless monitoring system for downhole pressure was tested in gas-producing wells in southwestern China. The test results verified the stability and reliability of the system for downhole pressure monitoring. It also confirmed that the strength of the received signal at the wellhead is negatively correlated with the downhole depth of the downhole tool for the first time and is not affected by the structure of the wellbore. The wireless monitoring scheme of downhole pressure introduced in this paper can be extended and applied to the monitoring of other downhole parameters such as temperature.
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