Main controlling factors of coalbed methane well productivity and enrichment–high-yield model of the Xishanyao Formation in the Midong Block, southern Junggar Basin
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摘要:
【目的】准噶尔盆地南缘西山窑组中低阶煤层气资源丰富,但米东区块煤层气井产能差异显著,产能主控因素与富集高产模式认识不足制约了勘探部署。本文旨在查清米东区块西山窑组煤层气井产能差异的控制机理,建立煤层气富集高产模式。【方法】以米东区块中侏罗统西山窑组42-45号煤层为研究对象,综合煤层地质参数与生产动态资料,分析不同产能类型煤层气井的排采特征及产气阶段演化规律;选取煤层有效厚度、含气量、临储比及压裂规模等关键参数,采用Spearman秩相关分析方法,定量识别平均日产气量与最大日产气量的主控因素;结合构造分区与典型井剖面对比,构建煤层气富集高产模式。【结果】构造上,研究区西山窑组煤层气井产能由八道湾向斜向北单斜整体呈现逐渐升高的空间分异特征。其中,北单斜煤层气井产能明显受地质因素控制,平均日产气量与煤层有效厚度相关性最强,最大日产气量受临储比与含气量共同控制;此外,八道湾向斜煤层气井平均日产气量与压裂规模显著正相关,工程改造对稳产能力具有关键影响,最大日产气量明显受临储比与含气量制约。研究区西山窑组煤层气可动气富集带主要发育于北单斜与八道湾向斜北翼之间垂深约600~900 m的中等埋深区间,临储比与煤层有效厚度的空间耦合决定了煤层气高产井的产生与分布。【结论】临储比与煤层有效厚度是控制米东区块西山窑组煤层气井产能差异的关键参数,构造背景通过调控气体再分配与可动气比例,对富集高产区的形成具有基础性作用;中等埋深区间为水动力与构造作用的优势叠加带,是煤层气富集高产的有利区带。研究成果为米东及其类似区块煤层气有利区预测与开发部署提供了理论依据。
Abstract:【Objective】The Middle–low rank coalbed methane (CBM) resources of the Xishanyao Formation in the southern Junggar Basin are abundant. However, significant productivity differences among CBM wells in the Midong Block, coupled with an insufficient understanding of the main controlling factors and enrichment–high-yield model, restrict exploration deployment. This study aims to clarify the controlling mechanism of productivity differences in Xishanyao Formation CBM wells and establish an enrichment–high-yield model for the study area. 【Methods】Taking the No. 42–45 coal seams of the Middle Jurassic Xishanyao Formation in the Midong Block as the research object, geological parameters and production dynamic data were integrated to analyze the drainage–production characteristics and gas production stage evolution of wells with different productivity levels. Key parameters including effective coal thickness, gas content, critical desorption–storage ratio, and fracturing scale were selected. The Spearman rank correlation method was used to quantitatively identify the main controlling factors of average daily gas production and maximum daily gas production. Combined with structural zonation and typical well profile comparison, a CBM enrichment–high-yield model was constructed. 【Results】Structurally, the productivity of Xishanyao Formation CBM wells in the study area shows an overall increasing trend from the Badaowan syncline to the northern monocline. In the northern monocline, CBM well productivity is significantly controlled by geological factors: average daily gas production has the strongest correlation with effective coal thickness, while maximum daily gas production is jointly controlled by the critical desorption–storage ratio and gas content. In the Badaowan syncline, average daily gas production is significantly positively correlated with fracturing scale, indicating that engineering stimulation has a key impact on stable production capacity, whereas maximum daily gas production is obviously constrained by the critical desorption–storage ratio and gas content. The movable gas enrichment zone of the Xishanyao Formation in the study area is mainly developed in the intermediate burial depth interval (600–900 m) between the northern monocline and the northern wing of the Badaowan syncline. The spatial coupling of the critical desorption–storage ratio and effective coal thickness determines the occurrence and distribution of high-yield CBM wells. 【Conclusion】The critical desorption–storage ratio and effective coal thickness are the key parameters controlling productivity differences of Xishanyao Formation CBM wells in the Midong Block. The structural background plays a fundamental role in the formation of enrichment–high-yield zones by regulating gas redistribution and the proportion of movable gas. The intermediate burial depth interval, as a superimposed zone of hydrodynamic and structural effects, represents a favorable belt for CBM enrichment and high yield. These research results provide a theoretical basis for favorable area prediction and development deployment in the Midong Block and similar areas.
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