Pore Connectivity and Wettability of Typical Shales in South China: Small Angle Neutron Scattering Study

Abstract

With the development of shale gas, the evaluation and reformation of shale reservoirs have attracted the worldwide interest. Current studies are limited to geometrical aspects of pore structure, but lacks effective approaches to characterizing pore connectivity and wettability of shales. Three typical shale formations in south China (Longmaxi, Niutitang, Longtan shales) were examined using small-angle neutron scattering (SANS), contrast matching small-angle neutron scattering (CM-SANS), mercury intrusion capillary pressure (MICP), gas (CO2 and N2) sorption isotherm, field emission scanning electron microscopy (FE-SEM), as well as spontaneous imbibition. The polydisperse spheres model (PDSP) was used for SANS data analysis to obtain porosity and pore size distribution (body/throat), and the results were compared with those from MICP and gas (CO2 and N2) sorption isotherm. By evaluating the difference of porosities determined from these methods, the closed fraction of shale pores are derived and discussed. Combining these pore size distribution (body/throat) and FE-SEM observations, it was concluded that the closed pores were rarely present in illite but were mainly within organic matter. A pore may be inaccessible to a fluid because it is physically closed or because the pore surface is non-wettable to a given fluid due to the surface tension of the fluid. Contrast matching compositions of H2O/D2O (hydrophilic fluid) and toluene/deuterated toluene (hydrophobic fluid) were used to probe open and closed pores of these shales. Combining the resultant spontaneous imbibition slopes of water and toluene fluids, the wettability of pore network in shales was derived. Overall, an improved understanding of the pore connectivity and wettability of shale will lead to a better understanding of shale gas preservation and production.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018