Effect of hydraulic fracture extension into sandstone on coalbed methane production

Hydraulic fracturing is applied to stimulate low permeability coals by producing a conductive fracture that enhances the connectivity between a wellbore and coal reservoir. The extant literature suggests that hydraulic fractures (HFs) can be vertical or horizontal, and may sometimes extend to neighbouring aquifers. This paper presents a quantitative study, using numerical simulations with a compositional simulator, into the effect of HF direction and its connectivity with aquifer on coalbed methane (CBM) production. Results show that vertical HFs yield higher peak gas rates than horizontal HFs, assuming that the permeability and volume of HF zones are the same. The extension of a HF into an aquifer results in high water production; however, the cumulative gas production is similar to models where the HF is constrained in a coalbed and the volume of the aquifer is less than 1.79 × 106 m3. An increase in aquifer volume causes decreases in gas production, but increases in water production, in fact, the peak gas rate and cumulative gas production decreases logarithmically with the increase in aquifer volume. When HFs extend into aquifers, with a volume of at least 34.8 × 106 m3, both peak gas rates and cumulative gas production yields decrease, and there is 10 times more cumulative water production than is seen in models without HFs. The results of the numerical simulations used in the study are justified by the production performance of a well from a producing CBM field.