Establishing long term production decline in unconventional reservoirs is challenging due to the high degree of uncertainty associated with well and reservoir properties. When Arps’ hyperbolic rate decline method is applied in extremely low permeability reservoirs, we usually obtain b-parameter values higher than 1 which may lead to overestimation of future production. One practical way to constraint future production is to switch from hyperbolic to a terminal exponential rate decline at a specified time (a.k.a. "modified hyperbolic relation"). Since most Montney Shale Gas horizontal wells have not reached stabilized boundary dominated flow due to low matrix permeability, the difficulty with using this technique is that the terminal exponential decline rate cannot be established in advance and typically must be specified from experience.

This paper presents one practical approach to establishing the terminal exponential decline rate for the Montney Shale Gas in Canada. Using the analysis techniques proposed by Blasingame and Lee (1986), important formation characteristics can be estimated analytically from post transient exponential decline. By evaluating long-term production trends of existing vertical wells, reservoir pore volume can be determined along with other naturally-fractured reservoir characteristics such as matrix) fracture permeability ratio and dimesionless fracture storage. Since horizontal wells’ production performance is strongly influence by the same natural fracture reservoir characteristics, this easy to use approach provides a reasonable estimate of terminal exponential decline parameters.

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