Full Length ArticleImpacts of clay on pore structure, storage and percolation of tight sandstones from the Songliao Basin, China: Implications for genetic classification of tight sandstone reservoirs
Introduction
Tight sandstone gas is an important unconventional clean fossil fuel with abundant potential [1], and its reserve and production performance are mainly controlled by its pore structure (e.g. size distribution and connectivity) [2], [3], [4]. Tightening sandstone reservoirs usually experience a process of narrowing throats and worsening pore connectivity. Besides the effects of primary sediments (e.g. grain size and matrix content) [5], both mechanical compaction and cementation dominate the formation of tight sandstones [6], [7], [8], [9]. Compared to the compaction that only mechanically damages pore/throat size, clay-dominated cementation usually blocks throats and fills pore voids to tighten the reservoir [10], which commonly leads to a poor correlation between porosity and permeability [11], [12]. For example, when throats are blocked by only a very small amount of clay, there is a slight change in pore volume but a sharp decrease in percolation.
Besides the common negative impacts on permeability, authigenic clay (Aclay), that has precipitated in pore space due to various chemical interactions between fluids and unstable minerals (e.g. dissolution of feldspar), can result in the changes in pore types and connectivities from primary pores [12], [13]. Many studies have paid attention to investigating the structure of pores within or between clay minerals using synthetic materials or natural shale/tight-sand cores [14], [15], [16], [17]. Aclay in grain-supported tight sands is porous and contains multi-scaled pores ranging from several nanometers to several hundred nanometers in size [15], [18]. These pores commonly exhibit larger specific surface area, continuously distributed, well-interconnection [17] and higher withdraw efficiency [3], providing potential storage and percolation channels for tight gas sandstone reservoirs. The clay-related pores and interparticle pores are two major pore types that co-control the storage and matrix-related seepage capability of tight sandstone reservoirs [4], [19], and the relative proportion of clay-related pores affects the microscopic structures (size distribution and pore connectivity) and macroscopic properties (porosity, permeability and production) of tight sandstone reservoirs [20].
Therefore, quantitative evaluation of the impacts of Aclay on pore structure as well as on petrophysical properties (i.e. storage and percolation) will help to understand the genetic mechanism, reservoir types and productivity variation of tight sand reservoirs. The following studies were carried out by combining nuclear magnetic resonance (NMR), rate-controlled mercury porosimetry (RCP), QEMSCAN® (quantitative evaluation of minerals using scanning electron microscopy) [21] and scanning electron microscopy (SEM) on Lower Cretaceous typical tight gas sandstones from the Songliao Basin. First, we used QEMSCAN® and SEM images to identify the content and distribution of Aclay, and then combined RCP and NMR to distinguish the size distribution of clay-related pores and to investigate the impacts of Aclay on pore structure and petrophysical properties; finally, we discussed the genetic classification and features of tight sandstones.
Section snippets
Experimental methods
Thirty-five tight gas sandstone samples were all regular cylinders (about 3.5 cm in length and 2.5 cm in diameter), drilled from a homogeneous section perpendicular to the bedding. All samples were dried at 110 °C for 12 h, and then subjected to both helium porosity and nitrogen permeability measurements under a confining pressure of 30 MPa. A small part with the length of 0.5 cm was cut from the core plugs for the XRD diffusion. Considering the burial depths and the relations between the contents of
Morphology of clay minerals and pores
Pores in tight sand samples comprise the interparticle pores, clay-related pores and dissolution pores, as well as few micro-cracks (Fig. 4). The interparticle pores are the largest (mainly 20–100 µm in diameter), but fewer and scattered (Fig. 4A, B). The dissolution of unstable minerals (i.e. K-feldspar, plagioclase and rock fragments) by acid [36] can both increase the interparticle pores size and produce a few intragranular dissolution pores (mainly <10 µm in diameter) (Fig. 4A, B, C). Aclay
Conclusions
A combination of QEMSCAN®, SEM, RCP, and NMR experiments were conducted on ten typical tight gas sand samples from Songliao Basin to determine the impacts of Aclay on the pore connectivity, PSD, storage and percolation of tight sandstones. These investigations will help to understand the genetic classification of tight sandstone reservoirs. The following observations were derived:
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Interparticle pores dominant and clay-related pores dominant are characterized by the connectivity of “larger pores
Acknowledgements
This paper was financially supported by the National Natural Science Foundation of China (No. 41602141, No. 41330313, No. 41172134) and the National Science and Technology Major Project Foundation of China (No. 2016ZX05061).
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