36 research outputs found
Gas transfer through clay barriers
Gas transport through clay-rocks can occur by different processes that can be basically subdivided into pressure-driven flow of a bulk gas phase and transport of dissolved gas either by molecular diffusion or advective water flow (Figure 1, Marschall et al., 2005). The relative importance of these transport mechanisms depends on the boundary conditions and the scale of the system. Pressure-driven volume flow (“Darcy flow”) of gas is the most efficient transport mechanism. It requires, however, pressure gradients that are sufficiently large to overcome capillary forces in the typically water-saturated rocks (purely gas-saturated argillaceous rocks are not considered in the present context). These pressure gradients may form as a consequence of the gravity field (buoyancy, compaction) or by gas generation processes (thermogenic, microbial, radiolytic). Dissolved gas may be transported by water flow along a hydraulic gradient. This process is not affected by capillary forces but constrained by the solubility of the gas. It has much lower transport efficiency than bulk gas phase flow. Molecular diffusion of dissolved gas, finally, is occurring essentially without constraints, ubiquitously and perpetually. Effective diffusion distances are, however, proportional to the square root of time, which limits the relevance of this transport process to the range of tens to hundreds of metres on a geological time scale (millions of years).
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Process understanding and the quantification of the controlling parameters, like diffusion coefficients, capillary gas breakthrough pressures and effective gas permeability coefficients, is of great importance for up-scaling purposes in different research disciplines and applications. During the past decades, gas migration through fully water-saturated geological clay-rich barriers has been investigated extensively (Thomas et al., 1968, Pusch and Forsberg, 1983; Horseman et al., 1999; Galle, 2000; Hildenbrand et al., 2002; Marschall et al., 2005; Davy et al., 2009; Harrington et al., 2009, 2012a, 2014). All of these studies aimed at the analysis of experimental data determined for different materials (rocks of different lithotype, composition, compaction state) and pressure/temperature conditions. The clay-rocks investigated in these studies, ranged from unconsolidated to indurated clays and shales, all characterised by small pores (2-100 nm) and very low hydraulic conductivity (K < 10-12 m·s-1) or permeability coefficients (k < 10-19 m²).
Studies concerning radioactive waste disposal include investigations of both the natural host rock formation and synthetic/engineered backfill material at a depth of a few hundred meters (IAEA, 2003, 2009). Within a geological disposal facility, hydrogen is generated by anaerobic corrosion of metals and through radiolysis of water (Rodwell et al., 1999; Yu and Weetjens, 2009). Additionally, methane and carbon dioxide are generated by microbial degradation of organic wastes (Rodwell et al., 1999; Ortiz et al., 2002; Johnson, 2006; Yu and Weetjens, 2009). The focus of carbon capture and storage (CCS) studies is on the analysis of the long-term sealing efficiency of lithologies above depleted reservoirs or saline aquifers, typically at larger depths (hundreds to thousands of meters). During the last decade, several studies were published on the sealing integrity of clay-rocks to carbon dioxide (Hildenbrand et al., 2004; Li et al., 2005; Hangx et al., 2009; Harrington et al., 2009; Skurtveit et al., 2012; Amann-Hildenbrand et al., 2013). In the context of petroleum system analysis, a significant volume of research has been undertaken regarding gas/oil expulsion mechanisms from sources rocks during burial history (Tissot & Pellet, 1971; Appold & Nunn, 2002), secondary migration (Luo et al., 2008) and the capillary sealing capacity of caprocks overlying natural gas accumulations (Berg, 1975; Schowalter, 1979; Krooss, 1992; Schlömer and Kross, 2004; Li et al., 2005; Berne et al., 2010). Recently, more attention has been paid to investigations of the transport efficiency of shales in the context of oil/gas shale production (Bustin et al., 2008; Eseme et al., 2012; Amann-Hildenbrand et al., 2012; Ghanizadeh et al., 2013, 2014). Analysis of the migration mechanisms within partly unlithified strata becomes important when explaining the
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origin of overpressure zones, sub-seafloor gas domes and gas seepages (Hovland & Judd, 1988; Boudreau, 2012).
The conduction of experiments and data evaluation/interpretation requires a profound process understanding and a high level of experience. The acquisition and preparation of adequate samples for laboratory experiments usually constitutes a major challenge and may have serious impact on the representativeness of the experimental results. Information on the success/failure rate of the sample preparation procedure should therefore be provided. Sample specimens “surviving” this procedure are subjected to various experimental protocols to derive information on their gas transport properties.
The present overview first presents the theoretical background of gas diffusion and advective flow, each followed by a literature review (sections 2 and 3). Different experimental methods are described in sections 4.1 and 4.2. Details are provided on selected experiments performed at the Belgian Nuclear Research Centre (SCK-CEN, Belgium), Ecole Centrale de Lille (France), British Geological Survey (UK), and at RWTH-Aachen University (Germany) (section 4.3). Experimental data are discussed with respect to different petrophysical parameters outlined above: i) gas diffusion, ii) evolution of gas breakthrough, iii) dilation-controlled flow, and iv) effective gas permeability after breakthrough. These experiments were conducted under different pressure and temperature conditions, depending on sample type, burial depth and research focus (e.g. radioactive waste disposal, natural gas exploration, or carbon dioxide storage). The interpretation of the experimental results can be difficult and sometimes a clear discrimination between different mechanisms (and the controlling parameters) is not possible. This holds, for instance, for gas breakthrough experiments where the observed transport can be interpreted as intermittent, continuous, capillary- or dilation-controlled flow. Also, low gas flow rates through samples on the length-scale of centimetres can be equally explained by effective two-phase flow or diffusion of dissolved gas
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Experimental study of the membrane behavior of shale during interaction with water-based and oil-based muds
textThree integrated experimental studies were carried out in order to study the membrane behavior of shale when interacting with water-based and oil-based muds. Results confirmed the belief that shales act as leaky semi-permeable membranes. Measured membrane efficiencies were low and ranged from 0.18 % to 4.23 % when shales interacted with water-based muds. Independently, measured ion selectivities (modified diffusion potentials) indicated that shales behaved as ion-selective membranes that restrict the flow of anions. In addition, results showed that both the membrane efficiency and the ion selectivity of shales increase with decreasing shale permeability and increasing cation exchange capacity. Our results also showed a good correlation between the membrane efficiency and the ion selectivity of shales.
A gravimetric test was developed that allows us to measure the flux of water and ions into or out of a shale. Results from this test show that the flux of ions depends on the ionic radii and the shale permeability and CEC. These results are consistent with the ion exclusion and membrane potential measured for the shale.
The membrane efficiency of oil-based muds was high compared to that obtained for water-based muds. However, the measured membrane efficiency was not 100 %. Results obtained from immersion tests also showed that the oil-based mud was not a perfect ionic barrier since it allowed ions to exchange. A capillary threshold pressure was measured which must be overcome before oil-based mud flows through a shale. Results showed that this capillary entry pressure increases as the shale permeability decreases and the interfacial tension between non-wetting fluid and shale pore fluid increases.Petroleum and Geosystems Engineerin
The impact of water content and ionic diffusion on the uniaxial compressive strength of shale
Experimental data showed that water content has a profound influence on the uniaxial compressive strength of shale. Testing has shown a great decrease in the uniaxial compressive strength as the water content increases. Regression analysis was used in this work to develop a general equation for predicting uniaxial compressive strength of shale from the available information on its water content and dry uniaxial compressive strength.
The impact of ionic diffusion on the compressive strength of shale has been investigated under three saturation conditions: wet shale, dry shale and chemically balanced wet shale. A chemically balanced shale has a water activity (chemical potential) which equals that of the test solution. Results show that, except for potassium ions, ionic diffusion has reduced the compressive strength of all studied shales. It has also been confirmed that diffusion osmosis has a detrimental effect on the mechanical stability of shale by reducing its compressive strength. Furthermore, it was found that when the water activity of shale is slightly higher than that of the test solution, chemical osmosis plays a major role in strengthening the shale by extracting water out of the shale. However, when the water activity of the shale is much higher than that of the test solution, diffusion osmosis weakens the shale. In other words, the detrimental impact of diffusion osmosis overtakes the beneficial effect of chemical osmosis.
Moreover, this work shows that compressive strength measurements for completely dried shale could be misleading due to the development of capillary forces that significantly modifies the compressive strength of shale.
Finally, the impact of ionic diffusion on the compressive strength of shale was carried out in the absence of both chemical osmosis and capillary forces. Results show that the invasion of sodium and calcium ions into shale reduced its compressive strength considerably while the invasion of potassium ions enhanced its compressive strength
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Membrane Efficiency Behavior in Shales
An experimental investigation has been initiated in order to understand the fundamental mechanisms that control membrane behavior of shales. We believe that an understanding of this membrane phenomenon will help solve wellbore instability problems in shale formations. Combined hydraulic and electrical
approaches are proposed for this study. The hydraulic approach uses pressure transmission tests to estimate the hydraulic membrane efficiency of shales (o), while electrical measurements are used to estimate the modified diffusion potential of shales (Ep). The effects of salt types and concentration gradients on both the hydraulic membrane efficiency and the modified diffusion potential of shales will be investigated. The combination of hydraulic and electrical tests for the same shale samples will provide us with a better understanding of the membrane efficiency behavior of shales. A complete illustration of the tests equipment and procedure is presented.
We propose to show experimentally that the membrane efficiency and the modified diffusion potential of shales are related. We plan to develop empirical correlations that associate the membrane efficiency and the modified diffusion potential of shales. These correlations could be in the form of charts or empirical equations. Results obtained from existing mathematical models that describe the membrane efficiency and the modified diffusion potential of shales will be compared to our experimental data. The validity and limitations of these models will be studied and analyzed. If necessary, we plan to introduce modifications to these models.Petroleum and Geosystems Engineerin