Modelling of Gas-water Flow with Capillary Trapping and Adsorption in Coal Beds (CBM)

I. Panfilova and L. Tien Dung and C. Moyne and R. Sadykov and U. Zhapbasbayev and Jean-Jacques Royer. ( 2016 )
, Amsterdam, Netherlands,

Abstract

The gas of coal seam represents the non-conventional resources of energy of big industrial importance, which exist in considerable volumes worldwide, but they are confined in low-permeable coal rocks (matrix permeability is often less than 1 mD). Often the coal seams are naturally fractured due to their mechanical fragility to different deformations caused by geodynamic movements or mechanical stress. This allows to exploit them without damaging the environment, in contrast to the shale gas, which requires to stimulate the production by hydraulic fracturing. The coal seam represents highly heterogeneous multi-scale fractured media, which often proves a similarity to different scales. The big fractures (face cleats) are alternated by the small fractures (butt cleats) and a network of microfissures, which are in contact with the microporous coal matrix. The matrix porosity is usually high, while the connection between blocks of coal is assured only by network of fractures. Methane is adsorbed onto the internal surfaces of coal micro-pores. At initial state the coal seams are filled with water. The water occupies all fractures and fissures and also enters into the micro-pores by spontaneous capillary imbibition, which leads to gas trapping. The coal is usually classified as non-wetting rock, but experiments show that wettability of coal is related to the coal moisture content. The coal is also wetted in the presence of water and some gases (CO2, CH4, nitrogen). The first stage of production is dewatering. The progressive reduction of pressure during this stage leads to desorption of methane. The desorbed gas arrives into the micro-fissures. Then, due to the good connectivity of fracture network, it can be produced. The problem is that desorbed gas cannot flow out from micro-fissures, being trapped by the water, because the wetting phase prefers the small pores. The capillary trapping does not allow to reduce the pressure in blocks, therefore it stops the gas desorption. The capillary trapping can be reduced by solvent injection (CO2). Having more strong chemical affinity with the coal surface, CO2 also helps to desorb the methane. To describe the mentioned phenomena we use the numerical model of two-phase flow with binary adsorption and capillary trapping on the scale of micro-pores. The modeling on the scale of one block was performed with the two-phase capillary network simulator, developed by authors. The objective of this study is to determine the qualitative mechanisms of the capillary trapping reduction and the desorbed gas liberation.

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    BibTeX Reference

    @PROCEEDINGS{,
        author = { Panfilova, I. and Tien Dung, L. and Moyne, C. and Sadykov, R. and Zhapbasbayev, U. and Royer, Jean-Jacques },
         title = { Modelling of Gas-water Flow with Capillary Trapping and Adsorption in Coal Beds (CBM) },
         month = { "aug" },
     booktitle = { ECMOR XV – 15th European Conf. on the Mathematics of Oil Recovery, Amsterdam, Netherlands, },
          year = { 2016 },
     publisher = { Amsterdam, Netherlands, },
    organization = { EAGE },
      abstract = { The gas of coal seam represents the non-conventional resources of energy of big industrial importance,
    which exist in considerable volumes worldwide, but they are confined in low-permeable coal rocks (matrix
    permeability is often less than 1 mD). Often the coal seams are naturally fractured due to their mechanical
    fragility to different deformations caused by geodynamic movements or mechanical stress. This allows to
    exploit them without damaging the environment, in contrast to the shale gas, which requires to stimulate
    the production by hydraulic fracturing. The coal seam represents highly heterogeneous multi-scale
    fractured media, which often proves a similarity to different scales.
    The big fractures (face cleats) are alternated by the small fractures (butt cleats) and a network of microfissures,
    which are in contact with the microporous coal matrix. The matrix porosity is usually high, while
    the connection between blocks of coal is assured only by network of fractures. Methane is adsorbed onto
    the internal surfaces of coal micro-pores. At initial state the coal seams are filled with water. The water
    occupies all fractures and fissures and also enters into the micro-pores by spontaneous capillary
    imbibition, which leads to gas trapping. The coal is usually classified as non-wetting rock, but experiments
    show that wettability of coal is related to the coal moisture content. The coal is also wetted in the presence
    of water and some gases (CO2, CH4, nitrogen).
    The first stage of production is dewatering. The progressive reduction of pressure during this stage leads to
    desorption of methane. The desorbed gas arrives into the micro-fissures. Then, due to the good
    connectivity of fracture network, it can be produced. The problem is that desorbed gas cannot flow out
    from micro-fissures, being trapped by the water, because the wetting phase prefers the small pores. The
    capillary trapping does not allow to reduce the pressure in blocks, therefore it stops the gas desorption. The
    capillary trapping can be reduced by solvent injection (CO2). Having more strong chemical affinity with
    the coal surface, CO2 also helps to desorb the methane.
    To describe the mentioned phenomena we use the numerical model of two-phase flow with binary
    adsorption and capillary trapping on the scale of micro-pores. The modeling on the scale of one block was
    performed with the two-phase capillary network simulator, developed by authors. The objective of this
    study is to determine the qualitative mechanisms of the capillary trapping reduction and the desorbed gas
    liberation. }
    }