An Experimental Study of the Effects of Diagenetic Clay Minerals on Reservoir Sands

Abstract

Abstract--Despite substantial scientific research efforts, accurate determination of the petrophysical effects of clay minerals on reservoir sands remains problematic. Diagenetic clays such as smectite and illite are of particular interest because of the pronounced effects these clays can have on reservoir quality. Here, results are reported from an experimental study based on the hydrothermal growth of smectite in synthetic sands. The sands contained quartz, dolomite and kaolinite, and were reacted at 175-200 ~ for 19-45 d. The hydrothermal reaction can be written as follows:dolomite + kaolinite + quartz --* smectite + calcite + C02 X-ray diffraction (XRD), electron microprobe (EMP) and electron diffraction (ED) analysis show that the synthetic Mg-rich smectite formed is saponite, with a cation exchange capacity (CEC) of about 100 meq/ 100 g. After reaction, brine permeability reductions of up to 98% were observed from the growth of less than 5% smectite. Scanning electron microscopy (SEM) observations of critical-point-dried reacted samples show that the clay behaves as a pervasive microporous cement with a complex pore-bridging texture affecting most of the available pore space. Morphologically, the clay is similar to naturally occurring diagenetic smectite from Gulf Coast sandstone reservoirs. The delicate clay texture collapses during airdrying and forms pore-lining masses. This phenomenon is similar to that observed for air-dried reservoir samples which contain dispersed diagenetic clays. An air-dried sample, then resaturated with brine, showed a marked increase in permeability. This increase is associated with the irreversible collapse of the clay texture. The experimental results indicate that the growth of diagenetic clay can severely reduce formation permeability, even at very low clay contents. The results also demonstrate the utility of hydrothermal experimental petrophysics for investigating the effects of diagenesis on rock properties.

Navajo Sandstone–brine–CO2 interaction: implications for geological carbon sequestration

“…3a-g. This morphology of illite/ smectite has also been observed in Keller et al (1986), Nadeau (1998), Nadeau et al (2002), and Celik et al (1999). Illite, as interwoven ribbons, is intimately associated with the smectite (Fig.…”

CO2–brine–caprock interaction: Reactivity experiments on Eau Claire shale and a review of relevant literature

“…The precipitation of secondary clay minerals as observed in this study could bridge the pores between grains, creating permeability barriers to fluid flow or reduction in the effective permeability. Other studies have proved that minute amounts of diagenetic clay, including illite, smectite as well as other mixed-layered clay, can drastically reduce reservoir permeability, because of the high surface area and fibrous pore-bridging morphology of illite/smectite (McHardy et al, 1982;Pallatt et al, 1984;deWaal et al, 1988;Ehrenberg and Nadeau, 1989;Nadeau, 1998;Rochelle et al, 2004). Experimental study by Nadeau (1998) reported a brine permeability reduction of up to 98% from the growth of no more than approximately 5 wt% smectite.…”

“…Other studies have proved that minute amounts of diagenetic clay, including illite, smectite as well as other mixed-layered clay, can drastically reduce reservoir permeability, because of the high surface area and fibrous pore-bridging morphology of illite/smectite (McHardy et al, 1982;Pallatt et al, 1984;deWaal et al, 1988;Ehrenberg and Nadeau, 1989;Nadeau, 1998;Rochelle et al, 2004). Experimental study by Nadeau (1998) reported a brine permeability reduction of up to 98% from the growth of no more than approximately 5 wt% smectite. The formation of secondary clay minerals is also indicated to physically sorb the CO 2 with capacity as high as natural coals (Busch et al, 2008).…”

Back-reacted saponite in Jurassic mudstones and limestones intruded by a Tertiary sill, Isle of Skye

“…The delicate curved crystal morphologies of the clay aggregates are typical of diagenetic smectites (cf. Nadeau, 1998). The absence of any garnet crystals overgrowing the flakes suggests that the clay post-dates the garnet (Fig.…”