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Volume 211, 1 January 2018, Pages 390-404
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Full Length Article
Impacts of clay on pore structure, storage and percolation of tight sandstones from the Songliao Basin, China: Implications for genetic classification of tight sandstone reservoirs

https://doi.org/10.1016/j.fuel.2017.09.084Get rights and content

Abstract

Authigenic clay (Aclay) is common in grain-supported tight reservoirs and exerts a primary control on reservoir’s percolation properties. In this study, we investigated the impacts of Aclay on pore connectivity, pore size distribution (PSD), and rock compressibility, to the resulting impacts on storage and percolation in order to arrive at a genetic classification of tight sandstones. QEMSCAN® combined with scanning electron microscopy was employed to identify the distribution and content of Aclay in a set of the Cretaceous tight gas sandstones, China. The size distribution of clay-related pores and their contributions to storage and percolation were distinguished from the interparticle pores by combining rate-controlled mercury porosimetry (RCP) and nuclear magnetic resonance (NMR). We found that Aclay evolved from predominantly pore-lining clays to predominantly pore-filling types as its content increased. Along this transition, clay-related pores gradually play a significant role in controlling the storage and percolation of tight sandstone reservoirs. There is very little change in pore volume and a sharp decrease in percolation threshold (PT) and permeability as Aclay content increases, which is mainly attributed to the dissolution that leads to the clay precipitation. On the other hand, compaction results in the reduction in PSD, porosity, PT and permeability together, however, the reduction rate caused by compaction become smaller as Aclay content increases, due to the limited compressibility of massive Aclay in grain-supported rocks. The hybrid effects of Aclay and compaction result in the formation of tight sandstones with various pore structures, corresponding to distinct pore types and reservoirs performances. We propose a threefold genetic classifications of tight sandstones: either from compaction alone, from Aclay cementation that plugs the original interparticle pores, or from a hybrid effect of these two processes.

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:

  • (1)

    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).

References (47)

  • M. Saidian et al.

    Effect of mineralogy on nuclear magnetic resonance surface relaxivity: a case study of Middle Bakken and Three Forks fromations

    Fuel

    (2015)
  • J. Li et al.

    A comparison of experimental methods for describing shale pore features—a case study in the Bohai Bay Basin of eastern China

    Int J Coal Geol

    (2015)
  • E. Rosenbrand et al.

    Permeability in Rotliegend gas sandstones to gas and brine as predicted from NMR, mercury injection and image analysis

    Mar Pet Geo

    (2015)
  • C.F.J. Colón et al.

    Experimental investigation of the effect of dissolution on sandstone permeability, porosity, and reactive surface area

    Geochim Cosmochim Acta

    (2004)
  • S. Kulesza et al.

    A comparative study of correlation methods for determination of fractal parameters in surface characterization

    Appl Surf Sci

    (2014)
  • J. Lai et al.

    Fractal analysis of tight gas sandstones using high-pressure mercury intrusion techniques

    J Nat Gas Sci Eng

    (2015)
  • R. Rezaee et al.

    Tight gas sands permeability estimation from mercury injection capillary pressure and nuclear magnetic resonance data

    J Petro Sci Eng

    (2012)
  • A. Sakhaee-Pour et al.

    Effect of pore structure on the producibility of tight-gas sandstones

    AAPG Bull

    (2014)
  • R.H. Lander et al.

    Predicting porosity through simulating sandstone compaction and quartz cementation

    AAPG Bull

    (1999)
  • A. Berger et al.

    Porosity-preserving chlorite cements in shallow-marine volcaniclastic sandstones: evidence from cretaceous sandstones of the sawan gas field

    Pakistan AAPG Bull

    (2009)
  • P.J. Hill et al.

    The Kapuni sandstones from Inglewood-1 well, Taranaki-petrology and the effect of diagenesis on reservoir characteristics

    N Z J Geol Geophys

    (1978)
  • K.A. Morris et al.

    The role of clay minerals in influencing porosity and permeability characteristics in the Birdport sands of Wytch Farm

    Dorset Clay Miner

    (1982)
  • M.R. Giles et al.

    Origin and significance of redistributional secondary porosity

    Mar Pet Geol

    (1990)
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