Thermal Magnetic Properties Variation of Rock During In-Situ Combustion Process

Abstract

In-situ combustion (ISC) has been proven as a promising technique for the extraction of heavy oils. It has been used in oil fields since the 1920s; however, it is still not as widely used as steam injection. One of the difficulties limiting its wide application is monitoring and controlling the movement of the combustion front. This work is aimed at studying the change in the properties of rock during the ISC process, which is expected to be used for developing an effective monitoring method of the combustion front movement. Rock samples before and after the ISC process were obtained from the Xinjiang Oil field (China) where an ISC industrial pilot has been implemented. In the temperature range of lower than 500℃, the minerals may only alter slightly. Therefore, it is difficult to determine whether the rock was heated or not during the ISC processes using general mineralogical or geochemical methods, for example, X-ray diffraction. This work takes a comprehensive approach to study the variation of rock properties. Magnetic analysis was chosen as the primary method since a very tiny change in the mineral composition during heating leads to profound changes in the magnetic properties. We analyzed magnetic susceptibility (MS), natural remanent magnetization (NRM), hysteresis parameters and thermomagnetic data. In addition, we performed differential thermomagnetic analysis (DTMA) for tracing magnetic minerals based on their Curie temperatures as well as for monitoring transformations in magnetic minerals during heating. Simultaneously, X-ray diffractometer (XRD), optical microscope for thin-sections, and organic content measurements were used as assistive methods to get a comprehensive evaluation on the variation of rock. We found that there is a big difference in magnetic minerals between the initial samples (not subjected to the ISC process) and burned samples from different wells and depths in the ISC pilot. Several magnetic clusters with different coercive force and domain structure were found in these samples. Based on the difference in magnetic properties, we found that the burned samples were heated to different temperatures during the ISC process. In addition, for some rock samples, the heating temperature during the ISC process was determined, and an analysis was made of the propagation of the combustion front. The thermal magnetic properties variation of rock during the ISC process is obvious, which makes it promising to be used for monitoring the propagation direction of the combustion front. Theoretical calculations of magnetic anomalies that occur due to changes in the magnetic properties of rocks during the ISC process indicate the possibility of the detection of such anomalies from the Earth’s surface through high-precision magnetic surveys. The findings in this work provide a theoretical base and direction for developing combustion front monitoring technologies.