SHALE PORE STRUCTURE ANALYSIS USING KLINKENBERG GAS SLIPPAGE MEASUREMENTS

The challenges of economically exploiting shale gas and oil reservoirs necessitate better understanding of pore structures and flow paths in fine grained rocks. The nano- to micrometre scale of pore systems in shale render them technically and theoretically difficult to characterize, and new techniques must be developed for proper characterization. This study moves forward such ends by investigating the utility of Klinkenberg gas slippage analysis for evaluating pore size and sensitivity of pore size to stress. Gas slippage causes permeability to vary with pore pressure when the mean free path of a gas approaches the size of the pores in which it is flowing. Therefore permeability is not constant as predicted by Darcy’s law and the magnitude of gas slippage is a proxy for the size of the pores in a pore system. In contrast to pore size analysis using scanning electron microscopy or mercury intrusion capillary pressure (MICP), gas slippage analysis measures the pores most responsible for fluid flow and can be applied at stresses comparable to those experienced during reservoir production. In this study slippage was measured on plugs oriented both parallel and perpendicular to bedding from three Eagle Ford Shale samples before and after conditioning by pre-stressing and at a range of effective stresses from 1000 to 5000 psi (6.9 to 34.5 MPa). It would be expected that increasing the effective stress would decrease the average pore size and therefore increase gas slippage. However, in some instances slippage decreased, meaning the average size of the pores most responsible for flow increased. This counterintuitive result is not attributed to pores increasing in size but rather smaller pores most responsible for slippage being cut off from the effective flow paths.

Theoretical calculations of average pore diameter derived from slippage data collected in this study yielded larger diameters than those estimates from MICP data. This is attributed to higher effective stresses during MICP measurements and/or the volumetrically disproportionate contribution of larger pores to flow than smaller pores. The ability of slippage measurements to characterize the portion of the pore system responsible for flow at a variety of stress states sets it apart from other techniques currently employed to characterize pore structures in shale.