Research on stress sensitivity of coal reservoir and a new method for its evaluation

The stress sensitivity of the coal reservoir is one of the key geological factors affecting coalbed methane well productivity, so how to reduce or avoid the effect of stress sensitivity on permeability is a question worth considering in CBM wells production process. Through the experiment of coal samples on permeability under different stress condition, the change law of permeability affected by effective stress has been studied. Based on the analysis of the pre-existing stress sensitivity evaluation parameters of coal reservoir, new stress sensitivity coefficients S1 and S2 were proposed, and the impact law of the effective stress on the permeability of coal reservoir was revealed. It is shown that the permeability of coal reservoir reduces with the increasing effective pressure by the negative exponential law and coal reservoir exhibits obvious stress sensitivity during the development of CBM wells. The permeability of coal reservoir cooperatively responses to coal strain under different stress condition. The stress sensitivity regression coefficient of tested coal samples is 0.099~0.115 MPa^-1 with an average value of 0.108MPa^-1, and is consistent with the well testing analysis results. The stress sensitivity coefficient S₁ is 0.383~0.436 with an average value of 0.414, and the stress sensitivity coefficient S₂ is 0.572~0.666 with an average value of 0.625. The defined stress sensitivity coefficients S₁ and S₂ in this paper exhibit integrality and uniqueness, and can be used to evaluate the coal reservoir stress sensitivity when combining with the stress sensitivity regression coefficient a.