Santos Gladstone Liquefied Natural Gas (GLNG) is a project that will convert coal seam gas (CSG) to liquefied natural gas (LNG) for export to global markets. As part of this project, considerable effort has been undertaken in the assessment, management and minimisation of cross aquifer flow of gas and groundwater. As part of this assessment, EHS Support on behalf of Santos GLNG, undertook a risk assessment which included common mechanism of failure for historic conventional oil and gas wells, CSG wells and private landholder water bores. Their likelihood and consequence (or significance) was also addressed and the methodology used is adaptable to future development areas.
The following failure mechanisms for all well and bore types (Conventional, CSG wells or bore) were included in the assessment:
- The absence of or compromised surface control casing;
- Insufficient cement during installation of the well casing;
- Poor casing selection relative to the corrosivity of the formation and groundwater;
- Insufficient or wrong placement of cement plugs during construction/decommissioning to effectively isolate zones and prevent cross aquifer flow between aquifer systems;
- Age of well and degradation of the cement (cement carbonation) and casing and thereby
providing a mechanism for cross aquifer flow; and - Inadequate driller training, qualification or inadequate regulation.
The risk assessment demonstrated that historical conventional oil and gas wells posed the highest risk for gas migration to surface and inter aquifer interaction. Private landholder water bores were also considered in terms of gas and fluid migration. Whilst the majority of these bores were constructed in an relatively unregulated industry (during the early 20th century), they pose a low risk due to the consequence of cross
aquifer flow being low. These bores are generally located above potential coal seam or conventional gas targets resulting in low risk of gas migration.
Where wells and bores intersected more than one aquifer, the available leakage based on potentiometric surfaces in and around the project development area indicate that >11 fully open penetrations per km2 would be required before significant aquifer interconnectivity (>10% of natural leakage) became an issue.