延川南煤层气田动态特征和SEC动态储量评估方法研究

Dynamic characteristics and SEC dynamic reserve assessment of CBM gas field in South Yanchuan

煤层气的SEC储量评估难点是:①煤层气的产量变化有其不同于常规油气的特点;②不同煤阶煤层气的开发规律存在差异。研究表明:深层高阶煤层气与浅层低阶煤层气在评估参数上差异很大。粉河盆地浅层低阶煤层气在评估参数上表现出上产快、峰值产量高和递减大的特点,而延川南深层、高阶煤层气则表现为排水达产时间较长、峰值产量较低和递减小的特点。总结出“三段式”产量预测模式的3种类型和4个亚类。对于I类,采取完整三段式预测,上产时间为28个月,稳产气产量850 m^3/d,稳产期一年,递减率25%。对于IIa亚类,产量已超过封顶产量,目前产量稳定,采取两段式预测,稳产期半年,递减率25%;对于IIb亚类,排采时间长,目前产量稳定,虽未达到封顶产量,采取两段式预测,稳产期半年,递减率25%。对于IIIa亚类,产量已超过封顶产量,目前产量在下降,采取一段式预测,递减率25%;对于IIIb亚类,整体上产量在上升,但在评估基准日前产量出现下降,采取一段式预测,递减率25%。利用该方法和评估参数进行的自评估结果与外方评估结果误差在±10%,证明该方法和评估参数适用于延川南深层、高阶煤层气SEC储量评估。

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.