In coal seam gas (CSG) fields, where single wells tap multiple seams, it is likely that some of the individual seams hardly contribute to gas recovery. This study aims to examine the contribution of individual seams to the total gas and water production considering that each seam may have different properties and dimensions. A sensitivity analysis using reservoir simulation investigates the effects of individual seam properties on production profiles.
A radial model simulates the production of a single CSG well consisting of a stack of 2 seams with a range of properties for permeability, thickness, seam extent, initial reservoir pressure, compressibility and porosity. The stress-dependency of permeability obeys the Palmer and Mansoori (1998) model. A coefficient (Α) relates seam radius, thickness, porosity, compressibility, permeability, and initial pressure. It is used to aid interpretation of the sensitivity study. Finally, a case study is modelled with 5 seams of different thicknesses and depths obtained from a producing well. The range in properties represents conditions found in the Walloon Coal Measures of the Surat Basin, relevant to the Australian CSG industry.
The sequence in which peak of gas production rate of each seam is achieved can be estimated using Α. For Αtop/Αbottom > 1, the bottom seam peaks first but achieves lower gas recovery than the top seam. For Αtop/Αbottom < 1, the top seam experiences fast depletion and total gas production rates decrease drastically. The peak gas rate of each seam may be identified on gas production profiles depending on Α. When 1 < Αtop/Αbottom < 10, individual peaks merge. For 10 < Αtop/Αbottom < 27, individual seams can be clearly identified as dual-peaks on production curves. For Αtop/Αbottom > 27, the contrast between maximum rate and time to peak increases and the top seam’s contribution is significantly reduced in early production time. A more realistic case based on a section of an actual Surat Basin well with 5 seams confirmed that when the Αtop/Αbottom of seams of greater permeability-thickness (kh) is higher than 27, gas recovery decreases. Even with higher total kh, seams with Α ratio = 100 produced less gas than seams with Αtop/Αbottom = 10. An increasing Α ratio is associated with inhibition of less permeable seams and reduced overall well productivity.