Nanopore-Structure Analysis and Permeability Predictions for a Tight Gas Siltstone Reservoir by Use of Low-Pressure Adsorption and Mercury-Intrusion Techniques

Abstract

The pore structure of unconventional gas reservoirs, despite having a significant impact on hydrocarbon storage and transport, has historically been difficult to characterize because of a wide poresize distribution (PSD), with a significant pore volume (PV) in the nanopore range. A variety of methods is typically required to characterize the full pore spectrum, with each individual technique limited to a certain pore size range.In this work, we investigate the use of nondestructive, low-pressure adsorption methods, in particular low-pressure N 2 adsorption analysis, to infer pore shape and to determine PSDs of a tight gas siltstone reservoir in western Canada. Unlike previous studies, core-plug samples, not crushed samples, are used for isotherm analysis, allowing an undisturbed pore structure (i.e., uncrushed) to be analyzed. Furthermore, the core plugs used for isotherm analysis are subsamples (end pieces) of cores for which mercury-injection capillary pressure (MICP) and permeability measurements were previously performed, allowing a more direct comparison with these techniques. PSDs, determined from two isotherm interpretation methods [Barrett-Joyner-Halenda (BJH) theory and density functional theory (DFT)], are in reasonable agreement with MICP data for the portion of the PSD sampled by both. The pore geometry is interpreted as slot-shaped, as inferred from isotherm hysteresis loop shape, the agreement between adsorption-and MICP-derived dominant pore sizes, scanning-electron-microscope (SEM) imaging, and the character of measured permeability stress dependence. Although correlations between inorganic composition and total organic carbon (TOC) and between dominant pore-throat size and permeability are weak, the sample with the lowest illite clay and TOC content has the largest dominant pore-throat size and highest permeability, as estimated from MICP. The presence of stress relief-induced microfractures, however, appears to affect laboratory-derived (pressure-decay and pulse-decay) estimates of permeability for some samples, even after application of confining pressure.On the basis of the premise of slot-shaped pore geometry, fractured rock models (matchstick and cube) were used to predict absolute permeability, by use of dominant pore-throat size from MICP/adsorption analysis and porosity measured under confining pressure. The predictions are reasonable, although permeability is mostly overpredicted for samples that are unaffected by stressrelease fractures. The conceptual model used to justify the application of these models is slot pores at grain boundaries or between organic matter and framework grains.

A preliminary study on the pore characterization of Lower Silurian black shales in the Chuandong Thrust Fold Belt, southwestern China using low pressure N2 adsorption and FE-SEM methods

“…For the evaluation of pore concentrations in any pore size, the plot of dV/ dW versus W (Fig. 10b), however, is better than the plot of dV/ dlog(W) versus W, and the latter will amplify the concentrations of larger pores because dV/dlog(W) ¼ 2.303 Â dV/dW Â W (Meyer and Klobes, 1999;Clarkson et al, 2012c).…”

Pore structure characterization of different rank coals using gas adsorption and scanning electron microscopy

“…Over the past few years, pore classification standards have been proposed by a number of researchers, according to these parameters [7][8][9]. Based on physical absorption capability and capillary condensation theory [10], pores are classified into three categories by the International Union of Pure and Applied Chemistry (IUPAC): macropores (P50 nm in diameter); mesopores or transitional pores (2-50 nm in diameter); and micropores or adsorption pores (62 nm in diameter) [11,12].…”

Necessary conditions and methodology for fitting pore size distribution curves

“…3. The course of the cumulative curve is not only dependent on pore volume as a function of pore bodies, because of various flow mechanism which changes with the pore size [8,12,18].…”

“…Moreover, additional factors like mixed wettability and destruction of pore space by mercury could also affect final results [1,8].…”

“…Darcy flow regime changes into slip flow regime and K App increases [8,12,13,15]. Additionally another factor starts to dominate in the flow phenomena: both Knudsen diffusion and self difussion.…”