Geochemical identification of the source and environment of produced water from CBM wells and its productivity significance: examples from typical CBM wells in eastern Yunnan and western Guizhou

Abstract

The geochemical information of produced water from coalbed methane (CBM) wells is abundant in geological significance. Based on the conventional ions, hydrogen and oxygen isotopes (δD and δ18O) and trace element tests of 27 produced water samples from CBM wells, the geochemical identification of the source and environment of produced water from CBM wells and its productivity significance have been analysed. The following conclusions have been drawn: CBM produced water can be divided into three categories, namely, polluted water from frac fluids, polluted surface water and formation water. The formation water has low Cl- concentration, low total dissolved solids (TDS) concentration, and light δD and δ18O. The water polluted by frac fluid has high Cl- and TDS concentrationt, and heavy δD and δ18O. The polluted surface water has high SO42- and light δD and δ18O. CBM groun dwate r environments can be divided into confined and unconfined systems. The confined system is characterized by low Na+/Cl- value, high (Cl- - Na+)/Mg2+ value, a general absence of SO42-, high Sr and Ba and low F. The unconfined system is characterized by high Na+/Cl- value, low (Cl- - Na+)/Mg2+ value, low SO42-, low Sr and Ba and high F. Constructing a cross plot with the new D drift index (d') value and Cl- concentration can further identify four types of source-water environments: an unconfined formation water system, an unconfined surface water system, a confined system heavily polluted by frac fluids, and a confined system only slightly polluted by frac fluids. The unconfined system often produces more formation water and has high CBM production. The confined system often produces water more heavily polluted by frac fluids and is low in CBM production. The fitting formulas of hydrochemical productivity have been established for daily CBM and water production of CBM wells.