Cross-formational flow of water into coalbed methane reservoirs: controls on relative permeability curve shape and production profile
Cross-formational flow of water into coalbed methane reservoirs: controls on relative permeability curve shape and production profile
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DOI：
10.1007/s12665-017-6505-0
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发表时间：
2017-02
影响因子：
2.8
通讯作者：
A. Salmachi;C. Karacan
中科院分区：
文献类型：
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作者：
A. Salmachi;C. Karacan
煤层气(CBM)井往往产生大量的水，特别是当煤层与附近地层之间存在水力连通时。产煤层与邻近地层之间的跨层流动可能会对生产和环境产生重大影响，从而影响这些浅层储集层生产的经济可行性。这种流动还可能影响在开采前从煤层中清除多少瓦斯，因此也可能对开采过程中的甲烷控制产生影响。本文的目的是研究外源水流入煤层对生产动态的影响，以及对生产数据分析得出的割理孔隙度和相对渗透率曲线等储层变量的影响。为了研究煤层与上覆地层之间存在交错层流时，煤层气井的生产动态，建立了一个储集层模型。结果表明，通过生产数据分析计算出的割理孔隙度可比实际割理孔隙度高一个数量级以上。由于水力连通，煤层内含水饱和度在一段时间内变化不大，从而推迟了产气高峰。覆岩枯竭后，煤层含水饱和度迅速下降。煤层含水饱和度的快速下降对应着产气量的急剧增加。作为一个重要的结果，当存在错流时，从模拟生产数据得到的气、水相对渗透率曲线与初始相对渗透率曲线相比具有明显的特征。在交错流的情况下，相对渗透率曲线的特征是在一定含水饱和度范围内向下凹陷，气体渗透率较低，然后分别对水和气迅速增加。这项工作的结果和分析有助于从生产数据分析中评估跨层流动对储集层变量的影响，还有助于识别煤层与相邻地层之间的水力连通性。
Coalbed methane (CBM) wells tend to produce large volumes of water, especially when there is hydraulic connectivity between coalbed and nearby formations. Cross-formational flow between producing coal and adjacent formations can have significant production and environmental implications, affecting economic viability of production from these shallow reservoirs. Such flows can also affect how much gas can be removed from a coalbed prior to mining and thus can have implications for methane control in mining as well. The aim of this paper is to investigate the impact of water flow from an external source into coalbed on production performance and also on reservoir variables including cleat porosity and relative permeability curves derived from production data analysis. A reservoir model is constructed to investigate the production performance of a CBM well when cross-formational flow is present between the coalbed and the overlying formation. Results show that cleat porosity calculated by analysis of production data can be more than one order of magnitude higher than actual cleat porosity. Due to hydraulic connectivity, water saturation within coalbed does not considerably change for a period of time, and hence, the peak of gas production is delayed. Upon depletion of the overlying formation, water saturation in coalbed quickly decreases. Rapid decline of water saturation in the coalbed corresponds to a sharp increase in gas production. As an important consequence, when cross-flow is present, gas and water relative permeability curves, derived from simulated production data, have distinctive features compared to the initial relative permeability curves. In the case of cross-flow, signatures of relative permeability curves are concave downward and low gas permeability for a range of water saturation, followed by rapid increase afterward for water and gas, respectively. The results and analyses presented in this work can help to assess the impact of cross-formational flow on reservoir variables derived from production data analysis and can also contribute to identifying hydraulic connectivity between coalbed and adjacent formations.