Feasibility of time-lapse gravity monitoring of producing gas fields in the Northern Carnarvon Basin, Australia

Summary

Aquifer influx and pressure depletion are key variables during the production and development of a natural gas field. To obtain an understanding of how aquifer influx and pressure depletion varies across the reservoir, remote geophysical monitoring techniques are commonly used, particularly in offshore environments where well data is geographically sparse. The seafloor time-lapse gravity technique is a candidate technique for remote reservoir monitoring of water influx into producing gas fields in the Northern Carnarvon Basin.

We have developed a method to quickly assess the sensitivity of time lapse gravity measurements to water influx or pressure depletion using a vertical cylinder model for gas reservoirs. In strong water-drive gas reservoirs, a field-wide height change in the gas-water contact greater than 5m may produce a detectable gravity response depending on the reservoir depth and rock quality. In depletion-drive gas reservoirs, large pressure changes between 6MPa (~870psia) throughout the reservoir can produce a detectable response. Applying this technique to Carnarvon Basin gas fields where the primary reservoir is the Mungaroo Formation suggests that gravity monitoring of production related changes may be feasible but needs to be assessed on a field-by-field basis.

The method employed is both flexible and practical. It can be used in a range of applications, and provides a quick assessment of the feasibility of time-lapse monitoring of subsurface density changes.

Key words::

• time lapse gravity
• gas production
• water influx
• reservoir management and monitoring