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Unconventional Reservoirs: Advances and Challenges

Behzad Ghanbarian1, Feng Liang2, and Hui-Hai Liu2

1 Porous Media Research Lab, Department of Geology, Kansas State University, Manhattan, KS, USA

2 Aramco Americas: Aramco Research Center-Houston, Houston, TX, USA

1.1 Background

Energy is one of the most important components in the world. Primary sources of energy take various forms, such as fossil energy, nuclear energy, and renewable energy sources. Fossil energy resources (e.g. coal, oil, and natural gas) were formed when plants and animals died and were buried underground. The quality of hydrocarbon accordingly depends on organic content as well as temperature and pressure conditions. Although there are limited reserves of fossil energy resources and despite recent advances in renewable energy, global economy still depends on fossil fuels to a great extent (Figure 1.1). Statistics reported by the British Petroleum (BP) company based on data from 1994 to 2019 show that the world primary energy consumption growth in 2019 slowed to 1.3%. This is less than half the growth rate i.e. 2.8% in 2018. Three-quarters of the energy consumption increase was driven by natural gas and renewable resources in 2019.

Based on analyses reported by the BP company, oil has contributed to the share of global primary energy more than others since 1994 (Figure 1.2) with 33.1% contribution in 2019. After oil, coal and natural gas are the second and third largest contributors. Although coal lost its share to account for nearly 27%, the contribution of natural gas increased to 24% in 2019. The share of renewable resources rose to record highs of 5% in 2019, and they overtook nuclear energy with about 4% contribution. Figure 1.2 shows the share of hydroelectricity has been nearly constant and about 6%.

Unconventional reservoirs, including oil and gas shales and tight sandstones, are distributed around the world (Figure 1.3) with an estimated endowment of several thousand trillion cubic feet (Kim et al. 2000). Since shale reservoirs have been successfully explored and produced in the United States (Figure 1.3), they recently became one of the major contributors to energy supplies.

There exist three general types of unconventional reservoirs, i.e. (i) organic-rich source rocks, (ii) tight oil reservoirs, and (iii) hybrid plays in which production occurs from source rocks and conventional reservoirs (Zoback and Kohli 2019). These types of unconventional reservoirs are different in geologic formations and, therefore, should be optimally exploited using different and appropriate approaches.

Despite numerous practical applications in oil/gas exploration and production as well as recent progress, we are still far from fully understanding all mechanisms of flow and transport in shales and tight sandstones across scales, particularly from pore to reservoir. In the following, we briefly address recent advances in unconventional reservoirs and discuss current challenges in oil and gas exploration and production.

Figure 1.1 World total energy consumption between 1994 and 2019.

Source: BP Statistical Review of World Energy (2020)/BP International Limited.

1.2 Advances

Since 2005, the beginning of shale gas revolution in the United States, unconventional oil and gas resources as well as their developments and productions have received a remarkable amount of attention around the world (Zoback and Kohli 2019). Despite various challenges that still exist, the petroleum engineering community made tremendous progress, particularly in the past decade. In what follows, we briefly address several notable achievements. For further details and comprehensive recent advances, see e.g. Barati and Alhubail (2020), Rezaee (2021), and Moghanloo (2022).

1.2.1 Wettability

Characterizing the contact angle of fluids (e.g. water, oil, and gas) and its spatial variability within unconventional reservoirs and under in situ conditions are essential not only to understand the trapping phenomenon and enhance oil and gas recovery but also to improve greenhouse gas (e.g. carbon dioxide and hydrogen) sequestration underground. In the literature, various methods, such as contact angle measurements (Iglauer et al. 2015; Roshan et al. 2016), spontaneous imbibition (Liu et al. 2019; Siddiqui et al. 2019), and nuclear magnetic resonance (Odusina et al. 2011; Su et al. 2018) were proposed to determine wettability in unconventional reservoir rocks. Recently, Arif et al. (2021) collected published data on shale contact angle measurements and developed a repository. They concluded that the oil-brine mixture in shales behaved in terms of wettability over a wide range from water-wet to strongly oil-wet. Although the CO2-brine mixture typically showed weakly water-wet to CO2-wet behavior, the CH4-brine mixture in shales was weakly water-wet. Arif et al. (2021) also investigated what causes high variabilities in shale wettability and found that the main factors were pressure, temperature, thermal maturity, total organic content, and mineralogy of shales.

Figure 1.2 Shares of global primary energy between 1994 and 2019 (BP Statistical Review of World Energy 2020/BP International Limited).

Although our knowledge of shale wettability has improved, further investigations are still needed to study the solid-fluid and fluid-fluid contact angles under realistic reservoir conditions more comprehensively. This would help enhance oil and gas recovery and exploit unconventional reservoirs even more successfully.

1.2.2 Permeability

Liquid and gas transports in shales and tight porous rocks were widely studied, particularly at the pore and core levels. The literature on gas permeability and its modeling is indeed vast and extensive (Javadpour et al. 2021; Liu 2017; Tahmasebi et al. 2020; Zhang et al. 2019). Numerous models were developed to address gas flow in nanostructures of shales by taking the effect of different transport mechanisms, such as slip flow, Knudsen diffusion, surface diffusion, and sorption into account. For example, Beskok and Karniadakis (1999) incorporated the effect of slip flow and modified the Poiseuille equation to describe gas flow in a cylindrical tube. Civan (2010) later applied the Beskok-Karniadakis model to scale up gas permeability in a network of pores. Using concepts from first-order slip flow and Knudsen diffusion, Javadpour (2009) developed another model for gas transport in a nano-scale cylindrical tube.

Figure 1.3 World shale gas resources (top) and shale gas and oil plays in the United States (bottom).

Source: Both maps are from US Energy Information Administration.

Recently, in addition to slip flow and Knudsen diffusion, more complex mechanisms such as sorption and surface diffusion were incorporated to model gas flow in shales. For instance, Wu et al. (2016) proposed a unified theoretical model by coupling various mechanisms including slip flow, Knudsen diffusion, sorption, and surface diffusion. Those authors stated that ". surface diffusion is an important transport mechanism, and its contribution cannot be negligible and even dominates in nanopores with less than 2 nm in diameter." Jia et al. (2018) found that surface diffusion might increase gas flow significantly at low pore pressures (e.g. < 2000?psi) depending on the value of surface diffusivity.

In the literature, most theoretic models developed to study gas permeability are based on the bundle of tortuous capillary tubes approach. However, it is an oversimplifying idealization in which a porous medium with actual interconnected pores is replaced with non-interconnected tortuous tubes of equal length (Purcell 1949). An important inconsistency between the bundle of tubes model and real porous media was noted by Fatt (1956). Fatt's main objection to the parallel-tubes model was that it has no connections between the individual tubes, in contrast to the network models that he developed. An additional problem is that, in a bundle-of-tubes model, individual pores span the entire sample or problem domain, regardless of its size.

Instead of the bundle of tortuous tubes approach, one may theoretically model gas transport in shales using more appropriate upscaling techniques e.g. percolation theory, critical path analysis (CPA), and effective-medium approximation (EMA) from statistical physics (Ghanbarian et al. 2020; Hunt et al. 2014; Sahimi 2011). Such approaches take into account the effect of pore connectivity. For example, Zhang and Scherer (2012) applied the CPA approach to estimate Klinkenberg-corrected permeability in shales from mercury intrusion capillary pressure curve and formation factor. They also used the Kozeny-Carman model, compared their estimations with experiments, and found that measured permeability values matched with CPA estimations more accurately. In another study, Ghanbarian and Javadpour (2017) assumed that slip flow and Knudsen diffusion are dominant...