Temperature and Pressure in Subsurface
This on the one hand, improves their sealing capacity due to increased plasticity. On the other, non-uniform mineral composition and lamination result in the increased anisotropy and deterioration of their sealing capacity. Experiments conducted by Savchenko and Bereto (1977) showed increased plasticity of evaporites under hydrostatic and, even, uniaxial pressure (Fig. 3.4). Much higher plasticity of salt compared to anhydrite was recorded. Evaporite plasticity increases as temperature increases. An example of the effect of temperature on salt plasticity is presented in Fig. 3.5.
The effective stress (pe) determines the degree of compaction. Of importance here is the escape of water occupying the pore space (mainly upward). This process is determined by the permeability that changes with degree of lithification. Naturally, during burial different rocks compact differently. Fig. 3.2 shows compaction for different rock types. Sometimes, argillaceous rocks are more compactable than sandstones (also see Rieke and Chilingarian, 1974; Chilingarian and Wolf, 1975, 1976). Shale compaction is significantly affected by mineralogy. Fig. 3.3 shows changes occurring in montmorillonite, illite, and kaolinite clays upon compactio. Numerous experiments established that the most rapid water loss (hence compaction) occurs during the initial burial of sediments to a depth of 25–30 m. During this stage, the sediments lose 50–60% of their originally contained water. Consequently, the compaction rate significantly slows down. Most of the sediment compaction occurs at a depth of 600–800 m. After that, the process becomes almost imperceptible. Moreover, reservoir pressure increases due to the decrease in permeability and consequent decrease in the rate of water escape. Thus, under- compacted rocks form in some areas. Their major characteristic is the anomalously high pressure. The change in the clay’s water content causes modifications in the claytexture. Mutual orientation of mineral microblocks and microaggregates changes. Also, the deflocculated clays are capable of plugging pores, por e throats, and canals, thereby significantly decreasing the reservoir–rock permeability. Carbonates in the process of compaction also lose water, and rapidly change from
the domain of plastic deformations into the domain of disruptive deformations. This results in microfracturing, and the rock that was a seal may become a reservoir. The carbonates with argillaceous and organic matter components usually have laminated
This on the one hand, improves their sealing capacity due to increased plasticity. On the other, non-uniform mineral composition and lamination result in the increased anisotropy and deterioration of their sealing capacity. Experiments conducted by Savchenko and Bereto (1977) showed increased plasticity of evaporites under hydrostatic and, even, uniaxial pressure (Fig. 3.4). Much higher plasticity of salt compared to anhydrite was recorded. Evaporite plasticity increases as temperature increases. An example of the effect of temperature on salt plasticity is presented in Fig. 3.5.
Category: Petroleum Engineering
