Changes in the structures of mixed-layer illite-smectite during flooding of terrigenous oil reservoirs

Abstract

A difference-spectrum method is proposed for the qualitative assessment of changes of illite-smectite structures accompanying the flooding of oil reservoirs. The method permits one to get an open system and reduce the application of procedures based on Markov’s chain formalism. A computer simulation is made to obtain spectra by subtracting the spectrum of an ethylene glycol-saturated sample from the spectrum of an air-dried preparation throughout the entire range of concentrations of illite and smectite components with a short-range order factor R = 0 or R = 3.It has been established that only in the presence of filtration are the maximum and minimum of the spectra in the range of 12.5-9.4 A complicated by a number of local extrema, whose position is specified by the structure of intermediate phases. The flooding process first involves mixed-layer phases with R = 0, leading to a partial segregation of the structures into phase with one and two networks of interlayer H2O. When the secondary mica particles break, phases with R = 3 appear along the boundaries of nanoblocks, first only with 1 H2O and then only with 2 H2O in labile interspaces. Their coexistence with the phases R = 0 in the sample proves the existence of percolation effects due to two-phase filtration in the porous medium. The fully flooded reservoir is always dominated by a mechanical mixture of illite-smectite phases of different nature with R = 0 and with different ratios of components. Transformation of mica that can drastically reduce oil production begins long before the appearance of flooding zones, which are revealed by standard logging methods.