The Effect of Oil Saturation On the Mechanism of Compaction In Granular Materials: Higher Oil Saturations Lead To More Grain Fracturing and Less Pressure Solution

Abstract

Whether addition of oil to sandstones slows, stops, or has no influence upon pressure solution and quartz cementation has long been disputed, despite having major implications for petroleum exploration and appraisal strategies. To elucidate the effect of addition of oil to compaction, pressure solution, and cementation processes, this study utilizes an experimental approach simulating isochemical, volumetric compaction using granular halite in the presence of variable brineoil mixtures. Each experiment, at 500 kPa effective stress, lasted 48 hours and involved repeated measurements of volumetric strain. The lithified products of experiments were examined using SEM techniques. After 30 hours, approximately 4% volumetric strain occurs when the brine-oil ratio is 100:0 (pure brine). Fractures are rare in samples from experiments undertaken with pure brine so that pressure solution, and concomitant cementation, must be the compaction mechanism. When the brine-oil ratio is decreased from 100:0 to 40:60, the volumetric strain remains about 4% but there are more fractures, as quantified from SEM image analysis. These observations suggest that, although pressure solution has occurred, some of the volumetric strain is the result of fracturing, implying that pressure solution has been less effective in the presence of some oil than it is in the presence of pure brine. When the brine-oil ratio is decreased further from 40:60 to 5:95, the volumetric strain increases up to 7.3%; SEM image analysis reveals that the strain is predominantly due to grain fracturing. This change from pressure solution to grain fracturing is likely due to heterogeneous pressure solution resulting from heterogeneous distribution of stresses. Brine-coated grain contacts undergo pressure solution but oil-coated grain contacts experience increasing stress up to the point of failure, as the surrounding grain pack compacts. When the brine-oil ratio is decreased to less than 5:95, volumetric strain decreased to about 1% with few induced fractures and little pressure solution. Compaction is negligible when the brine-oil ratio is 0:100 (only oil in the pores) and fractures are negligible. Halite is more water-soluble and more hygroscopic than quartz so that water will cling to halite surfaces more tenaciously than to quartz. Since even halite has its mechanism of volumetric strain affected when the brine-oil ratio is less than 40:60, quartz pressure solution and subsequent cementation is even more likely to be affected by oil filling. This experimental study suggests that the mechanism and extent of compaction and subsequent cementation in sandstones is strongly affected by the addition of oil.

Petroleum reservoir quality prediction: overview and contrasting approaches from sandstone and carbonate communities

“…There are a variety of ways to speed up processes, including using higher temperatures than found in diagenetic systems; an example of this are routine rock-eval pyrolysis measurements of source rock maturity and richness (Hunt 1995), simulation of dolomitization (Kaczmarek & Sibley 2011) and measurements of the rate of redox reactions during thermochemical sulphate reduction (Cross et al 2004). Reaction rates can also be increased by using analogue materials that are more soluble than silicates or carbonates (Sathar et al 2012) and fluid compositions that are exceptionally far from equilibrium with the host rock (Heald & Renton 1966;Chermak & Rimstidt 1990;Lander et al 2008a).…”

“…The question of the role of petroleum emplacement on sandstone diagenesis (and the possible preservation of porosity) has fired up much excitement with numerous papers seeking to address the problem from a combination of theoretical, oil-field data and experimental approaches (Marchand et al 2001;Bloch et al 2002;Taylor et al 2010;Sathar et al 2012;Wells et al 2015). Some have concluded that petroleum emplacement has no influence on sandstone diagenesis (Taylor et al 2010).…”

Exceptional reservoir quality in HPHT reservoir settings: Examples from the Skagerrak Formation of the Heron Cluster, North Sea, UK

“…Field-based (Marchand et al, 2002) and experimental (Sathar et al, 2012) studies have shown that quartz cementation is inhibited at high oil saturations. However, other studies (e.g Aase and Walderhaug, 2005;Molenaar et al, 2008;Saigal et al, 1992) have suggested that there is no correlation between hydrocarbon emplacement and porosity preservation.…”

Fluid overpressure as a control on sandstone reservoir quality in a mechanical compaction dominated setting: Magnolia Field, Gulf of Mexico

“…Factors such as the presence of clay mineral coats or microcrystalline quartz coats, early emplacement of hydrocarbon, presence of salt‐related thermal anomalies, mineral dissolution and fluid overpressures can all play a crucial role in preserving anomalous high porosity in sandstones (e.g. Spotl et al ., ; Worden and Morad, ; Taylor et al ., ; Wilkinson and Haszeldine, ; Sathar et al ., ; Nguyen et al ., ). Fluid overpressure, defined as the excess pore pressure above the hydrostatic pressure for a given depth, is commonly encountered in deep High Pressure High Temperature (HPHT) reservoirs (Osborne and Swarbrick, ).…”