Review of MEOR Technology Application in Dagang Oilfield for Last Decade

Abstract

Abstract MEOR, particularly indigenous MEOR technology, has been studied extensively in Dagang Oilfield, China, since early 1990s. Based on lab study results, pilot tests were conducted in four different reservoirs (in the Gangxi, Gangdong, Wangguantun and Kongdian oilfields). In total24injection wells and 55 production wells were involved in tests with responses observed in 47 wells. For MEOR, a series of microbial species, which were suitable to target reservoirs, were developed using produced fluids. The process included separation, culture, and special domestication, contributing to improved oil properties and producing surfactants, emulsion and gas. Core experiments in lab showed that, comparing to waterflooding, microbial flooding could increase the recovery rate by 6–8%. Field tests indicated the cumulative incremental oil production 28,000 tons from the reservoir at 54 to 73°C. One in the tested fields had already been flooded by polymer solution. Consideringthe limitation of microbial species variability, match of culture with formation water, numerous of fermentation surface and injections, it was determined to develop indigenous MEOR. This process provided better effectiveness at 60°C. The test results showed improved ecosystem and increased oil production of over 31,000 tons with water cut decreased by 5% at average. Our study and field tests formicrobial flooding showed that reservoir temperature was the most important factor and then microbial nutrients and production pattern follow. Introduction In MEOR technology, two types of microbes, the exgenous and the indigenous, have been studied and applied for flooding so far. For exgenous microbial flooding, displacing bacteria was derived fromreservoirs. Field tests indicated a significant incremental oil recovery. The selected species usually came from reservoir or polluted soil related to oil spill. Cultured to be facultative in lab, the bacteria were fermented and injected into the reservoir. They had limited adaptability to the pay zones. Furthermore, the bacterial effect was restrained by staying time of fermentation fluid in anaerobic environment with a certain temperature/pressure window and long fluidcycle time from injectors to producers. As a result, flooding response was measeured to be in stages, which reflected bacterial deterioration. For indigenous microbial flooding, the microbes were added in injection water, injected into the reservoir. They were later activated to be effective through periodic replenishing of necessary nutrient in the reservoir. It gave the same oil incremental production as that achieved in exgenous flooding. The indigenous microbes were also capable of anaerobic fermentation and could distinctly change reservoir ecological setting and produce polysaccharides, which not only expanded the swept volume and enhanced oil recovery rate but also improved the level of developed reserves. In addition, channeling of fermenting fluid wouldlikely take palce along pathways where the strong washout by injection water had appeared. It was necessary to plug these channeling zones before performing MEOR. Therefore, shutting-off operation was incorporated in field tests. Field Tests From 1996 to 2006, exgenous microbial flooding tests in four blocks and indigenous tests in the other two blocks were carried out, respectively. Reservoir Geology and Performance Fig. 1 shows the well locations in the tests. As shown in Table 1, the reservoir and fluid properties vary widely. Before MEOR application, these blocks had been flooded by water or polymer with water cuts at 65.4 to 94.4 percent. The Gangxi 4 block had reached such a high recovery that sufficient productivity replacement was really difficult. Except for the Zao 35 block in which the Sha 3 layer is composed of fractured volcanics and the Guan 69 block where limestone occurs in the same layer, most reservoirs are sandstones. Generally, the oil is heavy but less waxy, 85% oil has a density over 0.9g/cm3, with one exception -- the Guan 69 block (27.4% higher). The reservoir and fluid property meets basic requirement for microbial flooding [3,11–13].