Determination of Clay Bound Water in Shales from NMR Signals: The Fractal Theory

Abstract

Clay bound water (CBW) is an essential parameter to calculate not only water saturation but also other petrophysical parameters in shaly formations. Hence, in order to accurately obtain the information pertaining the CBW, nuclear magnetic resonance (NMR) is commonly used, which is sensitive to hydrogen that is adsorbed by clay minerals. Knowing that temperature can remove CBW, we combined thermal treatment (dehydration of the samples under a series of temperatures), low field NMR in the laboratory, and fractal theory to acquire better insights on how NMR signals can reveal the critical dehydration temperature to determine the CBW. For this purpose, a shale sample is exposed to a series of elevated temperatures and then a NMR transverse relaxation signal T 2 was acquired and analyzed with the fractal theory. The fractal dimensions of the T 2 revealed a sharp decrease as the hydration temperature exceeds a certain critical value. Furthermore, the T 1/T 2 from the 2D NMR map demonstrated that at this critical temperature, the signal intensity representing the hydroxyl group decreases as well, indicating the loss of the absorbed water. Based on the result, we were able to confirm that fractal dimension analysis can reveal the critical hydration temperature to ultimately determine the CBW, most importantly independent of running extra unnecessary experiments to save costs.

Storage Capacity and Microdistribution of Pore Water in Gas-Producing Shales: A Collaborative Evaluation by Centrifugation and Nuclear Magnetic Resonance

“…A NMR technique was often used to analyze the distribution of water in gas shales under different water-bearing conditions using the T 2 or T 1 – T 2 spectrum and to reveal the storage characteristics of clay-bound water, capillary-bound water, adsorbed water, free water, etc. for the different purposes of reservoir evaluation. ,− Although the NMR technique provides an effective means to distinguish pore water from hydrocarbon fluids, it is difficult to separate adsorbed and free water in matrix pores. Moreover, strong heterogeneity in the microscopic structure of shale pores is still a large challenge for the determination of the microdistributions of adsorbed and free water.…”