This paper presents an experimental study on the effect of water/oil saturation on sandstone strength. Unconfined compressive strength and triaxial tests were conducted on outcrop and downhole sandstone saturated with oil and water. It is found that, in general, strength reduction is more significant for water saturation than for oil saturation. The degree of strength reduction due to water saturation is related to total clay volume; the greater the clay volume the more significant is the strength reduction. The results indicate that effect of water cut on sand production depends on the clay content of the formation. Significant strength reduction is expected for clay rich sandstones whilst this may be insignificant for clean sandstones.
Sand production in petroleum industry is a phenomenon of solid particles being produced together with reservoir fluids. Conceptually, this process may be divided into two stages the loss of mechanical integrity of the rocks surrounding an open hole or periforation, and transportation of the failed rock particles to the surface. Prediction for sand production involves comparing the stresses around the opening with the rock strength using a sand production prediction model. It is well known that the effective stresses increase as the reservoir pressure is depleted. Furthermore, increase in water-cut in the reservoir formation during late life of the field is inevitable, be it due to water injection or water coning. On average, the oil companies today produce three barrels of water for each barrel of oil (Bailey et. al. 2000). Increase in water-cut can reduces capillary pressure that exists between the water and hydrocarbon fluid, and rock strength. Although the effect associated with reservoir depletion is, in general, incorporated in sand production prediction, the effect of water-cut on the sandstone strength has rarely been considered. This is due to a lack of understanding on the effect of water/oil saturation on rock strength, particularly for weakly/poorly consolidated sandstones.
The effect of water saturation on rock mechanical properties has been a subject of intensive research in the past (e.g. Baud et al. 2000, Colback & Wiid 1965, Dyke & Dobereriner 1991, Hawkins & McConnell 1992). It is well known that water saturation of specimens prior to testing has a strength reduction effective for all types of rock. For example, the influence of moisture content on the strength and deformation of sedimentary rocks was described in detail by Colback & Wiid (1965). For shale lithologies and a quartzitic sandstone, they reported a 50% loss in unconfined compressive strength when the rocks were fully saturated with water from their dry state.
It has been found that the effect of water on rock strength is highly variable for different types of rock. Dyke & Dobereiner (1991) discussed variation of unconfined compressive strength of three sandstones with moisture content. They found that the reduction of unconfined compressive strength from dry to saturated varied from 24 to 34%. They concluded that, in general, the weaker the rock the more sensitive it is to changes in moisture content. Hawkins & McConnell (1992) compared the loss in unconfined compresslye strength between dry and saturated specimens for 35 British sandstones. They found that the difference between the unconfined compressive strengths at dry and saturated states was 78% for the clay-rich Greensand while for the siliceous sandstone the strength decreased by only 8%. They concluded that the degree of strength sensitivity to water saturation is primarily controlled by the mineral composition and to a lesser extent by the rock microfabrics.