Most coal seam gas (CSG) wells in Queensland require some form of artificial lift to remove water and initiate the liberation of gas adsorbed onto the coal matrix. Careful management of the flowing bottom hole pressure (FBHP) is a standard practice in CSG operations. Inducing rapid pressure swings can cause plastic deformation to coals, rapid two-phase flow in the near-wellbore region, fines production and/or wellbore break out as well as the associated premature failure of mechanical lift systems.
The high-density compressed natural gas (HDCNG) well unloading system coupled with a novel gas lift technology removes all downhole moving mechanical components. Initial well unloading is managed through HDCNG stored on-site, which is injected through a capillary string. Low-pressure gas is then recirculated through a compressor unit to maintain a stable FBHP. HDCNG removes the need for oversized compressor equipment during the initial start-up, which ultimately transforms the economics of conventional gas lift systems deployed in low-pressure environments, as it considerably reduces the upfront infrastructure and maintenance costs.
As there are no moving mechanical components downhole there is reduced limitation on well design (i.e., can be deployed in vertical and non-vertical wells). The system is designed for autonomous operation and complete drawdown control. The removal of moving parts will also result in a longer mean time between failures for well uptime.
The availability of the compressor system on-site also results in lower wellhead suction pressures, which will ultimately lead to a lower FBHP. The recovery factor is a function of the abandonment pressure; thus, this system is capable of a higher recovery and associated recoverable reserves.
The design of this gas lift system utilises a workflow to accurately quantify reservoir production over time, such that the optimal FBHP is achieved.
In early 2021, the first coal seam gas well with HDCNG gas lift technology was installed and put on production in the Surat Basin. The well was previously shut-in for an extended period due to premature artificial lift failure. Currently, with HDCNG gas lift technology the FBHP is maintained at less than 210 kPa(g). Additionally, an enhancement of reservoir inflow performance is expected with a lower FBHP than previously achieved.
Therefore, this paper details a description of the HDCNG well unloading system, the methodology implemented for modelling and achieving the FBHP with proprietary gas lift technology along with first observations and evidence from actual production data in the Surat Basin.