非常规天然气经济评价对策思考

Study on unconventional natural gas economic evaluation

以煤层气、页岩气、致密砂岩气为代表的非常规天然气资源是重要的接替能源,但我国非常规天然气工业的发展严重受制于经济评价研究的滞后。结合我国非常规天然气勘探开发及经济评价特点,提出了非常规天然气经济评价的对策思考。根据非常规天然气赋存连续性强、单井生产表现差异大、工业产能边界难以确定等特点,提出"先解剖再整合"的经济评价思路以及"甜点接续评价"的经济评价流程,侧重推荐了微分积分、概率统计、模糊评价及情景分析等4种评价方法,以突出不确定性参数及随机过程。采用上述思路、方法和流程构成的评价模式,在某煤层气区块进行了应用,突出了对不确定性和风险的控制,说明了该模式的适用性和可推广性。不同类型天然气的形成和分布具有相互关联性和序列性,因此提出天然气经济评价序列理论,建议按照从常规到非常规、从致密砂岩气到页岩气再到煤层气的顺序,逐步增强经济评价不确定性和风险性的预期,递进开展针对性天然气经济评价。

Research on unconventional natural gas economic evaluation is still in its infancy in China.Most approaches are directly borrowed from conventional natural gas models without proper adaptations or adjustments,which causes errors in field practice.To provide better tools for investment decision-making,principles,processes,methods,and coping-strategies of unconventional natural gas economic evaluation were proposed based on the resource properties,and were illustrated in a case study of a coalbed methane project.Theories of calculus and probability were applied to the evaluation principle due to the strong continuity in resources and vast differences in well performances.The principle required dividing the unconventional gas field into 3D-subunits based on diverse reservoir properties in different regions and different formations,and the economic evaluation of the entire field was grounded on the analysis of individual subunits.In each subunit,the production performances of different wells were further studied statistically based on probabilities.As unconventional reservoirs are commonly characterized by strong heterogeneity and commercial gas flow is usually obtained in the＂sweet spots＂,a chain evaluation process with the sweet spots as succeeding units was suggested in order to achieve a more accurate and timely evaluation,and to minimize risk.Four methods,including integration,probability statistics,fuzzy assessment and scenario analysis,were used in the evaluation,which put more emphasis on the uncertain parameters and stochastic processes.Based on the theory of natural gas accumulation and distribution spectra in clastic basins,natural gas economic evaluation spectra were proposed as the coping strategies for the economic assessment of different natural gas types,which suggested a gradual increase of the uncertainty expectations and risk appetites from conventional to unconventional,from tight gas to shale gas,and to coalbed methane.