Pore Network Simulation of Low-Field NMR Relaxometry under Conditions of Drainage and Imbibition: Effects of Pore Structure and Saturation History

摘要

Nuclear magnetic resonance (NMR) relaxometry tests, carried out on partially saturated porous media, convey information on the distribution of fluids within the pore space. Proper interpretation of this information requires a means to isolate the effects of pore structure (pore and throat shape and size distributions, spatial correlations, etc.) wettability and saturation history. A pore network simulator of NMR relaxation that can clarify the effect of these factors on observed T_2 distributions is developed in this work. The pore network model can be calibrated with pore structure parameters (pore volume and throat size distributions, pore connectivity) determined from analysis of stochastic 3D replicas of the pore space generated from thin-section image information. The presence of irregular, and possibly, fractal pore surfaces is accounted for through a "late pore filling" model calibrated to match mercury porosimetry data. The network model is then used to simulate the water distribution and proton magnetization decay under conditions of drainage and imbibition. The model can be used to examine: (a) conditions under which diffusive coupling between pores has a significant effect on the decay spectra; (b) contributions to the magnetization decay spectra from water trapped at the irregular pore surfaces; and (c) contributions to the magnetization decay spectra from water in large pores which are inaccessible by the non-wetting phase during drainage, and water in pores containing oil trapped during imbibition.