保德地区煤层气井产出水化学特征及其控气作用

地层水的化学组成反映地下水交替和径流特征,对煤层气的富集条件具有一定的指示作用。以山西省保德地区煤层气井产出水测试数据为依据,系统研究了该区煤层气井产出水的离子组成、pH值、矿化度分布及氘氧同位素组成等特征。结果表明:1该区煤层气井产出水呈弱碱性,北部产出水以K+、Na+、Cl-和HCO-3为主,Ca2+、Mg2+含量较低,SO2-4含量极少,而东南部产出水Ca2+、Mg2+、SO2-4则相对富集;2产出水的δDHO和分布在鄂尔多斯盆地大气降水线之下,表明存在地表水2δ18 OHO值均2的渗入;3地层水矿化度由东向西、由南向北逐渐增加。在此基础上,分析了该区地下水的动力环境及其控气作用。结论认为:东南部靠近补给区,水动力条件活跃,不利于煤层气的保存,其煤层含气量一般小于2m3/t,甲烷含量通常低于70%;而北部处于弱径流—滞留区水动力条件,煤层气藏保存条件较好,煤层含气量一般大于4m3/t,甲烷含量通常高于80%。此外,北部地区煤层气井的产气效果明显好于东南部地区,也说明北部地区煤层气的富集条件更优。

Chemical behaviors of produced water from CBM wells in the Baode area,Shanxi, China,and their control on gas accumulation

Chemical components of groundwater can reflect the characteristics of groundwater replacement and runoff and also reveal, to some extent, the enrichment conditions of CBM. The test results of the produced water from CBM wells in the Baode area, Shanxi, China, were used to systematically study the ion components, pH value, mineralization and isotopic compositions of deuteron and oxygen. The following results were obtained. (1) The produced water of this area is slightly alkaline. In the northern part of the study area, ions are mainly K⁺, Na⁺, Cl^- and HCO₃, followed by Ca²⁺ and Mg²⁺, and very few SO₄². In contrast, in the southeastern part, the Ca²⁺, Mg²⁺ and SO₄² are relatively enriched. (2) The δD_H2O and δ¹⁸O_H2O values of the produced water are both below the meteoric water line in the Ordos Basin, which indicates the infiltration of surface water. (3) The salinity of formation water increases from south to north and from east to west. Based on these results, the hydrodynamic conditions and their control on the CBM enrichment were analyzed. In the southeastern part of the study area close to the recharge area with relatively active hydrodynamic conditions unfavorable for the CBM preservation, the gas content is ＜2 m³/t and the methane content is generally ＜70%, whereas in the northern part with weak runoff or retention conditions favorable for the CBM preservation, the gas content is ＞4 m³/t and the methane content is generally ＞80%. In addition, the performance of CBM gas wells in the northern part is significantly better than that in the southeastern part, also indicating that the CBM enrichment conditions in the north is better than that in the south.