Molecular simulation of the influence of foam fracturing fluid additives on coalbed methane diffusion

Materials Studio(MS) software was used to establish a slit pore molecular model which considers the in-teractions among water-bearing coal, foam agent and foam stabilizer. Based on molecular diffusion theory, the diffusion ability of methane molecule respectively affected by four common foam agents and four foam stabilizers was calculated. Compared to original water-bearing coal, self-diffusion coefficient of coalbed methane(CBM) is reduced by 71.3%, 74.7%, 56.3%, 54.0% after adding foaming agents including sodium dodecyl sulfonate(SDS), sodium lauryl sulfate(SLS), sodium dodecyl benzene sulfonate(SDBS) and lycine respectively. Based on adding SDBS into water-bearing coal, the self-diffusion coefficient of CBM is further reduced by 63.2%, 57.9%, 55.3% and 71.1% after adding PAM, PVA, PEG and CMC respectively. It shows that foaming agent plays the most important role in reducing diffusion ability of CBM, and the use of foam stabilizer can further decrease the diffusion ability. Influenced by foam agent and foam stabilizer simultaneously, the self-diffusion coefficient of CBM can even drop more than 80%, which is adverse to CBM exploitation. Consequently, foaming and stabilizing performance of foam fracturing fluid should be guaranteed and the formation damage should be reduced through improving the molecular structure of foam agent and controlling its usage amount. On the other hand, foam stabilizer is not recommended to be used in coalbed.