Petrophysical evaluation of capillary pressure for a naturally fractured tight gas sandstone reservoir: a case study, A

Abstract

The evaluation of capillary pressure is critical to assess and model fluid flow through porous media. The main objective of this research is to improve the understanding of capillary pressure for Field M. The field of study is a quarzitic sandstone gas condensate naturally fractured reservoir located in Bolivia. The particularity of Field M is that the pore space is composed of micro to macro scale fractures from, where the matrix porosity is mainly composed of microfractures. Capillary pressure is a fluid-rock property that depends on interfacial tension, contact angle between fluid and solids and capillary pore throat radius. A literature review of the theory and models/correlations to estimate capillary pressure indicate that the predominant parameter to determine capillary pressure profiles is the pore throat radius. Following this theory, capillary pressure is evaluated using pore scale simulation and petrophysical evaluation at the log scale while studying the pore throat radius variation. To investigate the validity of core capillary pressure profiles, pore scale simulation from a modified Berea sandstone network of pores and throats to resemble Field M rock conditions is used. The results from simulation show that the main property affecting capillary pressure profiles, as pointed out from literature review, is the pore throat size distribution. Comparing simulated and experimental capillary pressure curves resulted in different permeability values. The helium core permeability compared to the permeability values from simulation is always lower. Further inspection exhibit that the core samples with higher clay content do not show great disparity between simulated and core permeability. These observations lead to the conclusion that the pore system of the less brittle core samples, samples with less presence of microfractures, are less affected. Moreover sensitivities run in the simulator show that a variation in fracture density and pore throat size have a significant effect of one to two orders of magnitude on permeability hence capillary pressure values. Further work included testing the feasibility of using core capillary pressure to calibrate a saturation height function to obtain a water saturation profile at the log scale. A full petrophysical evaluation is performed to obtain a water saturation profile from resistivity logs. As a quality assurance, the water saturation profile derived from capillary pressure is compared to the log derived water saturation. The water saturation profiles obtained from these two techniques are comparable to each other. To optimize the core calibrating process, a methodology founded on the hydraulic flow unit concepts is proposed. The application of this methodology improves the comparison between water saturation profiles from capillary pressure and resistivity logs. Despite the uncertainties associated with capillary pressure core measurements in this field, this research shows that these measurements integrated with other data are valuable to characterize rock properties at different scales