Higher-Resolution Monitoring of Saturation Distribution in Carbonate Plug Core by Micro Computed Tomography Technology—Proper Core Restoration For EOR Laboratory Experiments
Authors: Yonebayashi Hideharu , Uetani Takaaki , Kaido Hiromi 
Published: March 2018

It is ultimately important to restore core to the original reservoir wettability condition for evaluating low salinity water EOR properly because wettability should be a critical parameter for carbonate reservoir in particular. For achieving the appropriate initial water saturation (Swi) in preparatory stage prior to the main examinations such as Amott and/or coreflood tests, a new saturation method: i.e. vacuum displacement was suggested to apply in stead of the common pumping displacement method. The vacuum displacement method can achieve the Swi within convenient time, in spite of the conventional method of pumping displacement which usually requires much amount of oil injection. Its advantage is obvious; however, little discussion of how fluid saturations distributed in plug core scale. For checking validity of this new saturation method, a study was planned to monitor saturation distribution in a plug core scale.

First, the recent digital rock techniques (NMR) were considered as candidates; however, suspended due to their time-consuming and inconvenient usability even higher resolution. Second, we decided a conventional computed tomography (CT) scanner from a viewpoint of convenient handling. CT method, originally developed for medical, has been used in many industries. Its resolution is satisfied level for simple and/or small object to be scanned. However, it was still a challenge for separate monitoring of two liquids (oil and water) distribution in plug core size. Thus, a recent industry-use micro-CT, which has more powerful X-ray generator, was applied because of ten-time higher resolution than medical one limiting its generator voltage to avoid influence to human body. An identical core was separately scanned for three conditions: dry, water-saturated, and oil-and-water-saturated with contrast-enhanced technique.

In general, the micro-CT can easily recognize solid and liquids separately because of clear variation of their densities whereas difficult to distinguish oil and water which have close densities. To improve low oil/water separating-quality of normal scanned picture, sodium iodide (NaI) was added into water to increase density variation as contrast-enhanced agent. The core was set at the same location in each scanning work to get identical-positioned comparative slice data. Saturation distribution was interpreted by comprehensively taking all the data into account. Based on resolution of the industry-use micro-CT, the minimum pixel size was 38 μm for 1.5 inch diameter core in this study, while resolution of 350–450 μm in the traditional medical-use CT. The observation result show clear oil and water saturation distribution for all sliced pictures. Those distributions were never unnatural and did not include any artificial bias. This concluded that the application of industry-use micro-CT could monitor high resolution saturation distribution with better usability than other techniques.