This paper presents the results of a laboratory study on the effect of different fracture fluid systems on permeability impairment of a typical coalbed methane (CBM) reservoir. These fluid systems include conventional gel fluids (linear and cross-linked gel), gel fluid with surfactant and a viscoelastic fluid system. A series of flow tests on coal plugs were conducted under simulated reservoir conditions to assess permeability reduction due to matrix swelling and cleat plugging by gel fluids. Tests included surface behavior of different fracture fluids and surfactants on coal surface, degrees of matrix swelling and plugging of fractures and cleat systems by fracture fluids.
The results of these tests have shown that permeability impairment induced by matrix swelling is highly irreversible. This irreversible damage can be prevented to a certain extent by conventional practices of adding certain types of salt (such as KCl) into fracture fluids. Both linear gel and cross-linked gel fluids cause a significant reduction (around 70%) in permeability of CBM reservoirs. Addition of KCl along with certain types of surfactants to gel fluid can marginally improve gel clean up. However, the permeability impairment could be as high as 60%. With the use of viscoelastic fluid system, on the other hand, permeability impairment can be as little as 20 to 30%. This means the viscoelastic fluid system has a great potential in reducing permeability impairment which in turn can help rapid dewatering from CBM reservoirs and increase production.