Experimental investigation of shale gas production with different pressure depletion schemes

- Gas production in shale with different pressure depletion schemes is investigated.
- Pressure depletion scheme affects not only gas production rate but also ultimate gas recovery.
- Permeability reduction is the major reason for the decrease in ultimate gas recovery.

A matrix pressure depletion scheme can result in pore structure deformation and permeability reduction in shale, which affects the process of gas production in shale. Shale gas production tests were carried out with four core samples and with two crushed samples to investigate the effects of three different pressure depletion schemes on gas production rate and on ultimate gas recovery. The three pressure depletion schemes tested include constant production pressure, linear pressure decline, and step-wise pressure decline. Results of the gas production tests show that, for shale core samples, a production pressure depletion scheme affects not only gas production rate but also ultimate gas recovery. Pressure sensitivity tests of the four shale core samples were also performed, and the results show that permeability reduction of shale cores due to rapid pressure depletion caused the decrease in ultimate gas recovery. The gas production tests with crushed shale samples which were under zero effective stress show that a pressure depletion scheme only affects the gas production rate; that is, the same ultimate gas recovery was obtained for different pressure depletion schemes. The experimental results also show that the linear pressure decline and step-wise pressure decline depletion schemes delay the permeability reduction of the shale matrix, thereby resulting in a greater ultimate gas recovery than the constant production pressure scheme in which the production pressure was dropped immediately to an end value. In a step-wise pressure decline depletion scheme, in which gas diffusion and desorption were allowed to reach equilibrium at each pressure step, the ultimate gas recovery increases with a decrease in the size of the pressure step. However, in a linear pressure decline depletion scheme, the maximum ultimate gas recovery is obtained with that optimum pressure decline rate which causes the best match between the permeability reduction rate and the gas diffusion and desorption rates.