Impacts of pore- and petro-fabrics, mineral composition and diagenetic history on the bulk thermal conductivity of sandstones

Abstract

The present study aims to model the bulk thermal fabric of the highly porous (26.5 øHesiliceous Nubia sandstones in south Egypt, as well as their pore-and petroanisotropy. The thermal fabric concept is used in the present study to describe the magnitude and direction of the thermal foliation 'F', lineation 'L' and anisotropy 'l'. Cementation, pressure solution, compaction and the authigenic clay content are the main pore volume-controlling factors, whereas the cement dissolution and fracturing are the most important porosity-enhancing factors.The bulk thermal fabric of the Nubia sandstone is raised mostly from the contribution of the mineral composition and the pore volume. The kaolinite content and pore volume are the main reducing factors for the measured bulk thermal conductivity 'k', whereas the quartz content is the most important enhancing factors. The optical scanning technique, which is one of the most accurate and precise techniques, was applied for measuring the bulk thermal conductivity 'k' of the studied samples. For the dry state, the average thermal condutivity 'kav' in the NEeSW, NWeSE and vertical directions, varies from 1.53 to 2.40, 1.54 to 2.36 and from 1.31 to 2.20 W/(mK), respectively. On other hand, 'kav' for the saline water-saturated state for the NEeSW, NWeSE and vertical directions varies between 2. 94 & 4.42, 2.90 & 4.31 and between 2.39 & 3.65 W/(mK), respectively. The present thermal pore fabric is slightly anisotropic, 'l' varies from 1.10 to 1.41, refers mostly to the NWeSE direction (kmax direction, elongation direction), whereas the petro-fabric refers to NEeSW di-rection (kmax direction, elongation direction). This gives rise to a conclusion that the pore-and petro-fabrics have two different origins. Therefore, studying the thermal conductivity of the Nubia sandstone in 3-D indicates a pore fabric elongation fluctuating around the NeS direction.

Analyses of the factors influencing sandstone thermal conductivity

“…The effect of the mineral composition of dry rocks on their thermal conductivity is minor (Duchkov et al, 2014). The thermal conductivity under the saturated condition is larger than the dry condition because the thermal conductivity of saline is higher than that of the air (Nabawy and Géraud, 2016).…”

“…(2), respectively and in Figure 1. Comparison between the results under the dry and saturated conditions show that the correlation between the thermal conductivity and porosity for water-saturated sandstones is much weaker than those under the dry condition, because the orientation of the pore and grain fabrics are different and not aligned parallel (Nabawy and Géraud, 2016). 1 0.295 0.018…”

“…The porosity of sandstone sometimes can be up to 40 % (Nabawy and Géraud, 2016). It has an important influence on the thermal conductivity of rock.…”

“…The test methods of rock thermal conductivity are mainly divided into two mineral particles with various chemical compositions and different degrees of crystallization. Therefore, the thermal conductivity of rocks depend not only on the pressure and temperature but also on their mineralogical compositions, the structure and geometry of pores, grain size, crack shape, and their concentration (Nabawy and Géraud, 2016).…”

Controls on Reservoirs Quality of the Upper Jurassic Mengyin Formation Sandstones in Dongying Depression, Bohai Bay Basin, Eastern China

“…Diagenetic alterations are believed to enhance the heterogeneity to various extents in sandstones [15]. Diagenetic processes of compaction, cementation and dissolution are the dominated porosity reducing/preserving/enhancing factors, respectively, which significantly alter the petrophysical properties of the reservoir rocks to various extents [16]. The coexistence of multi-type and multi-scale pores in sandstones are largely the results of various diagenetic modifications [15,17,18].…”