Abstract:

The difficulty of SEC reserve assessment of CBM is: ①the yield change of CBM is different from conventional oil and gas; ②there are differences in the development rules of CBM with different coal ranks. The research shows that deep high-rank and shallow low-rank CBM vary greatly in the evaluation parameters: the shallow low-rank CBM in the Powder River Basin shows the characteristics of quickly entering of high yield period, high peak yield and large production decline, while the high-rank coal seam gas in the deep parts of South Yanchuan are characterized by longer drainage time, low peak yield and low decline. The three types and four sub-classes of the “three-segment” yield forecasting model are summarized. For type I, a complete three-stage forecast is adopted. The yield time is 28 months, the stable production output is 850 m ³/d with the cycle for one year, and the decline rate is 25 %. For the IIa subcategory whose production has exceeded the capped production and is stable at present, or the IIb sub-class whose discharge time is long and the current production of which is stable, although the production hasn’t reached the capped production, two-stage forecast is adopted, the stable production period is half a year, and the decline rate is 25 %. For the IIIa subcategory whose production has exceeded the capped production and the production is declining at present, or the IIIb sub-category whose overall production is on the rise but goes down before the base date of assets evaluation, single-stage forecast is adopted, and the decline rate is 25 %. The error between the self-assessment result and the external assessment result by using this method and the assessment parameters is ±10 %, which proves that this method and the assessment parameters are suitable for the SEC reserve assessment of deep and high-order coalbed methane in south Yanchuan.

Key words: CBM development, dynamic assessment, SEC reserves, Yanchuannan coalbed methane field, Powder River Basin