URTeC: 2203 Novel Techniques to Measure Oil-Gas Diffusion at High Pressure & High Temperature Conditions: Application for Huff-n-Puff EOR in Shale Sanchay Mukherjee, Son Thai Dang, Chandra Rai and Carl Sondergeld. Mewbourne School of Petroleum and Geological Engineering, The University of Oklahoma. Copyright 2020, Unconventional Resources Technology Conference (URTeC) DOI 10.15530/urtec-2020-2203 This paper was prepared for presentation at the Unconventional Resources Technology Conference held in Austin, Texas, USA, 20- 22 July 2020. The URTeC Technical Program Committee accepted this presentation on the basis of information contained in an abstract submitted by the author(s). The contents of this paper have not been reviewed by URTeC and URTeC does not warrant the accuracy, reliability, or timeliness of any information herein. All information is the responsibility of, and, is subject to corrections by the author(s). Any person or entity that relies on any information obtained from this paper does so at their own risk. The information herein does not necessarily reflect any position of URTeC. Any reproduction, distribution, or storage of any part of this paper by anyone other than the author without the written consent of URTeC is prohibited. Abstract Reservoir modeling and experimental results have shown that diffusion is a dominant mass transport mechanism in tight reservoirs. Knowing diffusion rates allows engineers to optimize field parameters such as injection time and production time for enhanced recovery. In huff-n-puff EOR, oil swelling, vaporization, and condensation are significant to the recovery (Hoffman et al., 2019). In the context of nanoporous media, these physical phenomena are the result of mutual bulk diffusion between the gas and liquid phases. This study presents two different techniques for measurement of bulk diffusion at reservoir conditions, using Nuclear Magnetic Resonance (NMR) 1-D gradient and real time oil swelling tests. All of the measurements were done for a Meramec oil sample (API-42.7) and different methane-ethane mixtures as the injectates. In the first method, we use a Daedalus® cell, made of NMR transparent ZrO 2 ; the cell can be operated up to 10,000 psi internal pressure. NMR 1-D gradient profiles were continuously acquired using an Oxford 2MHz GeoSpec™ spectrometer with Green Imaging acquisition and processing software. Typical experiments ran for 8 days to monitor the dynamic change of hydrogen index (HI) profiles across the oil-gas interface during the diffusion process. This allows extraction of bulk diffusion parameters by directly calculating the concentration of the injectate in the liquid phase. However, due to temperature limitation of 95 o F in the above setup, we designed another experiment which can be utilized up to 10,000 psi and 350 o F. This design includes a visual cell with a transparent window to record oil swelling over a time span of 8 days. The data is fitted to the Fick’s second law to derive the diffusion coefficient with rate restricted boundary condition. At 95 o F and 6000 psi, both techniques provide a similar diffusion rate for the oil sample and methane as the injectate, in the range of 1*10 -10 to 3.7*10 -10 m 2 /s. When measured at reservoir temperature of 175 o F, the diffusion coefficient varies between 10 -9 to 10 -10 m 2 /s for the pressure range 2500 – 7000 psi using three different gas mixtures, including C1, C1:C2 (95:5 mole%) and C1:C2 (72:28 mole%). Using pure methane