Coalbed Methane Reserve Evaluation Methods Based on “Sweet Spot”-Case Study of Australia Surat Basin Coalbed Methane Block

The well spacing based reserve estimation methods are widely used for unconventional gas reserve evaluation. The PUD reserve area usually extrapolate one well spacing to the PDP reserve area. The probable reserve area usually extrapolates one or two well spacing to the proved reserve area and possible reserve area usually extrapolated two or three well spacing to the probable reserve area. But there are two disadvantages of this methods: first, the well type is not considered and well spacing exploration is a little conservatively in “Sweet Spot” area. In fact, due to the continuous accumulation of coalbed methane and the large-area distribution, the evaluation methods of coalbed methane reserves are different from those of conventional gas. Since the accumulation of CBM is not controlled by structures and traps, the core of CBM reserves evaluation is to look for “sweet spots” and carry out reserves evaluation on the “sweet spots”. First, based on the physical characteristics of the CBM reservoir, the “sweet spot” of CBM is determined. Since the “sweet spot” has greater certainty, a larger well spacing can be appropriately extrapolated to determine the classification of reserves in the “sweet spot”. A new coalbed methane reserve evaluation method based on “sweet spot” is putting forward. In the “sweet area”, For vertical well, the PUD reserve area could be two well spacing extrapolated to the PDP reserve area. The probable reserve area could be four well spacing extrapolated to the proved reserve area. The possible reserve could be extrapolated from the probable reserve area to the boundary of the “Sweet Spot” area. For the horizontal well, the PUD reserve area could be extrapolated two well spacing to the PDP reserve area in directions perpendicular to horizontal intervals and extrapolated one spacing to PDP reserve area in directions parallel to the horizontal intervals. The probable reserve area could be extrapolated four well spacing to the proved reserve area in directions perpendicular to horizontal intervals and extrapolated one well spacing to the proved reserve area in directions parallel to the horizontal intervals. The possible reserve could be extrapolated from the probable reserve area to the boundary of the “sweet spot” area. On the other hand, regards to contingent resources, if the continuity of the coal seam is confirmed in the “sweet spot” area, the principles for determining the C1 should be consistent with the P1 reserves in the “sweet spot” area, and the C2 could be extrapolated from C1 boundary to “sweet spot” area. In the CBM research block in the Surat Basin, Australia. First, coal reservoir evaluation system was set up, and then the “sweet spot” of the CBM block based on the main control factors of CBM production capacity was determined. Using the above reserve classification method based on the “sweet spot”, a case study of reserve and contingent resource classification was carried out based on the “sweet spot” block in the central gas field and the pilot “sweet spot” block in the north. The above-mentioned reserve evaluation method based on the “sweet spot” successfully supports the current and future development planning of the Surat CBM project, and also has important guiding significance for the reservoir evaluation of similar unconventional plays.