Effect of Fracturing Fluid on the Migration of Methane in Shale with Nano-sized Fractures

Spatial fracture-network generated fracturing and the characteristics of shale itself produce large amount of nano-sized fractures in the shale. The effect of water retained in shale on desorption and migration of methane was studied through simulation, experiment and data from filed operations. Calculation with empirical equations showed that in nano-sized environment, methane fills the nano-sized fractures and is dissolved into water to form a water dissolved gas. Simulation with molecular dynamics showed that in nano-sized environment the degree of order of water molecules is increased, the hydrogen bond force between molecular chains is decreased. Low flow resistance in the nano-sized environment led to a coefficient of diffusion that was two orders of magnitude greater than that of the macro environment. When methane was dissolved in the water existed in nano-sized channels, the coefficient of diffusion of water molecules was two orders of magnitude greater than that of the water with no dissolved methane in nano-sized environment, and the hydrogen bond force was much lower. In this condition there is almost no flow resistance. Capillary penetration take place in shale formation drives water into nano-sized fractures, and is therefore adsorbed onto the surfaces of shale. The adsorption of water makes it difficult for the water to flow back, resulting in very low flowback efficiency. Water entering into shale competes with methane for adsorption onto the surfaces of shale and displaces methane, reducing the adsorption quantity of methane and is hence beneficial to increasing production rate. On the other hand, at low water saturation, some small amount of methane dissolved permanently in water. Data from adsorption-desorption experiment with active carbon with pre-absorbed water showed that, when the volume of water is equivalent to the pore adsorption quantity of methane on the surface of dry active carbon, and the amount of residue methane that is unable to desorb because of dissolution in water is 13.5% at most. In this condition there is almost no water flowback. All factors considered, the existence of water is beneficial to hydrocarbon production.