Characteristics of Hydrogen and Oxygen Isotopes in Produced Water and Productivity Response of Coalbed Methane Wells in Western Guizhou

Abstract

The characteristics of hydrogen (H) and oxygen (O) isotopes in produced water of coalbed methane (CBM) wells have abundant geochemical indication information. On the basis of the tests of conventional ions and H and O isotopes of 20 water samples produced from CBM wells in western Guizhou, the characteristics of H and O isotopes and productivity response of the wells in the study area have been analyzed in combination with the geological background. The research shows that, if the H and O isotopic compositions of produced water from a CBM well have obvious D drift (D drift means that, when δ18O takes the same value, the measured value of δD that is higher than the theoretical value of δD is obtained from the regional atmospheric precipitation line equation), the well is likely to obtain high production, and if the H and O isotopic compositions have O drift (O drift means that, when δD takes the same value, the measured value of δ18O that is higher than the theoretical value of δ18O is obtained from the regional atmospheric precipitation line equation), the well is not likely to obtain high production. As drainage continues, the water–rock interaction gradually strengthens and the H and O isotope compositions tend to become heavier, but seasonal rainfall can cause their values to decrease. From shallow to medium-deep formations, δD and δ18O values increase continuously, and the wells with the lowest δD and δ18O values in the shallow formation have the highest average daily gas production. d′, D drift comprehensive index, is proposed, which is of universal significance. With the increase of d′, the average daily gas production decreases, and in combination with the productivity performance, the high production wells and low production wells can be distinguished. d′ of less than 0 indicates high production, which can be used as a geochemical index to evaluate potential productivity. It is proposed that δD = −50‰, d′ = 0, and Cl– = 2000 mg/L can be used as a criterion for judging whether produced water of the CBM well is polluted and δD = −25‰ and Cl– = 4000 mg/L can be used to distinguish the degree of formation of water pollution. Moreover, the identification template of the produced water source based on δD, d′, and Cl– has been established preliminarily, and three ranges, coal seam water, light polluted water, and severe polluted water, can be identified according to the template. Moreover, it is considered that the CBM production will drop as the pollution degree of produced water increases.

Geochemical Characteristics and Productivity Response of Produced Water from Coalbed Methane Wells in the Yuwang Block, Eastern Yunnan, China

“…Generally, a closed groundwater environment is conducive to the enrichment and preservation of CBM while an open groundwater environment is not. Researchers theorize that open hydrological environments are close to oxygenrich water source recharge areas that can enrich Ca 2+ , Mg 2+ , and SO 4 2-, while closed hydrological environments are far from such recharge areas and exhibit Na + , K + , Cl -, and HCO 3 enrichment [22,32,36,37]. With increasing drainage time, the concentrations of K + , Na + , Ca 2+ , Mg 2+ , Cl -, and SO 4 2in the water samples decreased with the gradual discharge of fracturing fluid.…”

“…When groundwater flows through coal-bearing strata, several hydrogenbearing soluble minerals in the coal seams are dissolved continuously. The lighter H atoms in hydrogen-bearing minerals are easily adsorbed by minerals such as clay, while the heavier D atoms are more likely to undergo an isotope exchange with H atoms in the water, thereby continuously enriching D in the formation water and exhibiting D drift characteristics [22,26]. 18 O is enriched in the surrounding rock.…”

“…H and 16 O isotopes in the formation water can be replaced with D and 18 O isotopes in the coal seam and surrounding rock, resulting in the increase of D and 18 O isotopes in the coal seam water in the reduction environment of coal measures [21,22]. In addition, microorganisms can produce HDS, which is soluble in water and can exchange isotopes, leading to D drift characteristics of formation water in a sealed and reduced coal seam environment [21,[23][24][25][26].…”

Hydrogeochemical Characteristics and Water–Rock Interactions of Coalbed-Produced Water Derived from the Dafosi Biogenic Gas Field in the Southern Margin of Ordos Basin, China

“…Guo et al [8] found that coalbed-produced waters in the Bide-Santang basin of western Guizhou exhibit a D drift indicative of hydrogen content in the coalbed that is higher than that of oxygen and established a template for identifying water sources. Wu et al [19] analyzed the hydrogen and oxygen isotopes of the water produced by CBM wells. They showed that it was mainly derived from atmospheric precipitation (i.e., meteoric water) and was subject to water-rock interactions and evaporation.…”

Biogeochemistry and the associated redox signature of co-produced water from coalbed methane wells in the Shizhuangnan block in the southern Qinshui Basin, China

“…Thus, CBM co-produced water serves as an indicator of these processes it has been subjected to, which could provide valuable information on its evolution (Moore, 2012). Additionally, CBM co-produced water may act as a signature to allow an improved understanding of CBM preservation and enrichment, which can be used as a tool to analyze gas production rates in the processes of CBM exploration (Wu et al., 2018).…”