Water Disposal Management in a Sour Gas Field Development in Sichuan

Abstract

Sour gas fields usually contain high H₂S and/or high CO₂, of which their development brings challenges to fluid separation, processing, and disposal due to concerns of safety, the environmental impact, and operations. Based on the Overall Development Plan approved, the operator is to inject all the produced water and water generated as a by-product of the purification plant into the subsurface formation. Prior to injection, both sources of water (produced water and by-product water) will have to be treated at the gas processing facilities to remove most impurities via a treatment and filtration process. More specifically, the concentration and grain size of suspended particles, oil content, and pH of the treated water after the treatment and filtration process are required to meet the regulatory requirements (i.e., these requirements for disposal water quality are specified as quality indices in the method for injection of produced water [1]). The treated water is then injected into a water disposal well, which has an excellent injectivity and capacity. In addition, there are six monitoring wells (so-called “observation” wells) surrounding the disposal well, to provide important monitoring information about the injected water movement and the well injection performance (via wellhead data analysis), and the change in chemical composition (via frequent water sample analysis). The data from both the disposal well and the observation wells are critical to ensure a sustainable long-term water disposal performance and in compliance with the environmental regulations. In brief, regular surveillance is a key to get data, such as injection water volume, pressure, temperature, composition, etc. and this is very critical to monitor the well injectivity performance and the quality of the disposal water. Cross-functional efforts including earth scientists, petroleum engineers, production engineers, operators, facility engineers, environmental specialists, and lab technicians are important to achieve technical and operational excellence, and hence to fulfill the regulatory requirements. Third party involvement in sampling, chemical treatment program, and quality assurance quality control of surveillance data analysis strengthens confidence in the operator’s effort to maintain and protect the environment and compliance to the regulations.