NMR Investigation of Pore Structure in Gas Shales

Abstract

In shales, the wettability contrast between water wet pores located between clay particles, oil wet organic pores and mixed wettability pores located at the interface between clays and organic matter, can be exploited to quantify the porosity associated with each pore system. In order to quantify the different types of porosity, the NMR T 2 spectra of 5 Barnett shale samples have been acquired, both in their native state, after dodecane saturation at 3000 psia, and after immersion in a 65% MnCl 2 aqueous solution. These NMR spectra were acquired with Tau ϭ 57 sec on a 2MHz instrument. 65% MnCl 2 aqueous solution has been found to exhibit no NMR signal and it does not mix with dodecane.All the samples displayed a unimodal T 2 distribution in their native state with a peak around 0.2 ms. This NMR peak is generally associated with clay pores. After dodecane saturation the development of a second peak around 5 ms forms. These two NMR peaks decreased after immersion in the MnCl 2 solution. MnCl 2 imbibition decreases the NMR water signal as it diffuses into the existing water in clay-dominated pores, and decreases the mixed pore system signal by a displacement process. The true organic pores are not affected by the MnCl 2 solution because an aqueous, water-based solution cannot displace dodecane by simple imbibition in an oil-wet pore. After 30 days of MnCl 2 imbibition the signal remaining on the peak at 5 ms is due to true organic pores. Also, this organic pore system displayed a positive relationship with the TOC of each sample.Upon placing samples in both a vacuum ϭ 4x10 -6 mTorr and 150°C for 2 weeks, the remaining NMR peak around 0.2 ms reached a threshold beyond which it could not be reduced. These NMR results mimicked the results achieved chemically by using a 65% MnCl 2 aqueous solution for the same NMR peak. These two experimental results, in conjunction with an understanding that the NMR peak around 0.2 ms represents clay dominated pores, yield support to the notion that the remaining peak around 0.2 ms (after 1 month of MnCl 2 aqueous solution imbibition) represents clay-bound water.Ultimately, this study allows for the quantification of effective porosity, inorganic porosity, organic porosity, the porosity at the inorganic-organic interface and clay-bound water.

New Insight into the Petrophysical Characterization of Shales with Different Fluid Saturation States Based on Nuclear Magnetic Resonance Experiments

“…Because the inorganic pores are usually water-wet, there is no doubt that inorganic pores and microfractures absorb water. However, the water could also fill the oil-wet organic pores under high pressure, according to the NMR results of these shale samples, and this agreed with other studies . From this perspective, it is proved that it is much more difficult for oil to enter inorganic pores even under high pressure.…”

“…However, the water could also fill the oil-wet organic pores under high pressure, according to the NMR results of these shale samples, and this agreed with other studies. 17 From this perspective, it is proved that it is much more difficult for oil to enter inorganic pores even under high pressure. Two peaks in the T 2 distributions of the watersaturated shale samples were observed at approximately 1 and 90 ms.…”

“…The method of measuring NMR of shale samples in different saturation states has been studied by several researchers. Gannaway 17 obtained shale samples in different saturation states by using the MnCl 2 solvent to separate oil and water in the T 2 distributions. Fleury and Romero-Sarmiento 18 investigated shale samples in gas-saturated states and in watersaturated states to understand the patterns of different fluids in the NMR responses.…”

Characterization of pore structures of hydrated cements and natural shales by 129 Xe NMR spectroscopy

“…Diffusion, relaxation, NMR cryoporometry (NMRC) and magnetic resonance imaging (MRI) measurements of fluids adsorbed in porous materials provide detailed information about the pore size distribution, morphology, transport and adsorption phenomena, as well as chemical exchange [11,12,13,14,15,16,17]. NMR relaxometry has been exploited to characterize porous structures of cements and shale [18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33]. Variabletemperature MRI of freezing water was also successfully employed to observe the spatially resolved pore size distribution in cements [34].…”

Methods for Petrological and Petrophysical Characterization of Gas Shales

“…carried out combined experiments of NMR, heat treatment, and high-speed centrifugation, and they suggested the shale pores can be classified into unrecoverable fluid pores, capillary-bound fluid pores, and movable fluid pores (Figure a) . Gannaway performed a series of NMR experiments under the sample conditions of the native state, after dodecane saturation at 3000 psia, and after immersion in a 65% MnCl 2 aqueous solution, and they classified the shale pores into organic and inorganic pores (Figure b) . NMR has great potential in disclosing the pore fluid information, but further studies are needed to expand the 2D NMR method to investigate the distribution of multiphase fluids in shale pores.…”