Modified low field NMR method for in-depth pore space analysis and Fe-containing minerals influence mitigation in tight siliciclastic and extrusive rocks

Description

Precise evaluation of rock filtration and capacity properties is challenging, particularly in unconventional reservoir formations containing para- and ferromagnetic minerals. This data set contains T2 relaxation time data of two different Low-Field Nuclear Magnetic resonance (LF-NMR) approaches, standard and modified differential, along with the Mercury Injection Capillary Pressure (MICP) and X-Ray Diffraction (XRD) data. The modified differential approach employs the subtraction of the dried sample signal from the signal of the 100 % water-saturated sample before taking the Laplace transform, giving a differential distribution devoid of the unwanted signals from rock matrix, closed porosity and clay-bound water. This method skips the background subtraction step characteristic of the standard approach, achieving a higher Signal-to-noise ratio. In the proposed PSD conversion workflow based on the percolation theory, a power function relationship between differential LF-NMR T2 relaxation time and MICP pore-throat diameter can be established, enabling the determination of LF-NMR absolute pore size distribution and pseudo capillary pressure (Pc) curves for samples with Fe-containing minerals.

Steps to reproduce

To conduct the LF-NMR analysis, core samples in a dried and saturated state were prepared. Dry samples were obtained by vacuum oven drying at 110°C and under the pressure of 0.007 bar for 12 h to remove free and capillary-bound water. Samples in a saturated state were obtained by saturation in demineralized water in vacuum conditions (pressure of 0.007 bar) for 12 h. The LF-NMR measurements were performed using a 2 MHz Magritek Rock Core Analyzer (Magritek, Germany) with a 29 mm probe. Experiments of the T2CPMG (The Carr-Purcell-Meiboom-Gill) sequence were carried out using the following acquisition parameters: repetition time, RT = 2000 ms; echo time, TE = 60 μs; number of scans, NoS = 512; and number of echoes, NoE = 20000. The LF-NMR output data were analyzed using the Prospa software package. 1D distributions of T2 relaxation times for all samples in both dry and saturated states as well as differential T2 distributions were obtained using standard and modified, differential T2 protocols. MICP measurements were performed using an Auto Pore IV 9520 (Micromeritics, USA) mercury porosimeter. Before the analysis, all samples were crushed and dried at 105°C for 24 h to remove moisture from the pore space. The pressure was measured at 120 points in the 0.003 – 414 MPa range. The XRD measurements were performed using the Panalytical X’Pert Pro apparatus equipped with an advanced ultra-fast detector (real-time ultra strip X’Celerator). The operating conditions were set at a voltage of 40 kV, a current of 40 mA, and a step width of 0.02° 2Θ. The scanning range was from 5° to 65° 2Θ. Before the XRD measurements, all samples were ground using a McCrone micronizing mill to achieve a grain size below 20 µm. Measurements were taken on randomly prepared samples by side-loading. A quantitative mineral composition analysis was performed using the Rietveld method, facilitated by the SIROQUANT program.

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