Gas/liquid equivalence evaluation method for the sensitivity of coal reservoir to flow velocity

The sensitivity evaluation on oil and gas reservoirs is the foundation for reasonably formulating the drilling & production technologies and measures and the reservoir protection principles to prevent reservoir damage, and the determination of the critical flow velocity in the reservoir is the prerequisite to the other sensitivity evaluations. The sensitivity to flow velocity is commonly evaluated using the simulated formation water. As for coalbed methane (CBM) especially, however, its production is different from that of conventional oil and gas reservoirs. It is mostly of low permeability and its production has two links, i.e., water drainage and gas recovery. Therefore, existing evaluation method of liquid measuring core flow experiment cannot provide comprehensive and effective evaluation on its sensitivity to flow velocity. A new method for evaluating the sensitivity to flow velocity based on gas/liquid equivalence was established according to the kinetic energy theorem. In this method, the critical flow velocity obtained from gas measuring method and that from liquid measuring method can be substituted equivalently. Then, a method for calculating the critical flow velocity based on gas measuring method was derived by taking full consideration of gas slippage effect. In this way, a gas measuring experiment method on the sensitivity of CBM reservoir to flow velocity was developed. It is experimentally indicated that gas measuring method is applicable to evaluate the sensitivity to flow velocity and the error between its derivative result of critical flow velocity and the result of liquid measuring method is within 20%, so it can provide valuable reference and is suitable for evaluating the sensitivity of coal reservoir to flow velocity.