Recycling redundant coreholes

Abstract

In 2009, QGC (a BG Group business), began developing coal seam gas (CSG) feedstock for its new Liquefied Natural Gas (LNG) plant and required a subsurface model and field development plan to underpin the project's investment case. Fundamental to the modeling process was acquiring appropriate data to calibrate subsurface models and enhance confidence in their predictive quality. As part of an integrated reservoir surveillance strategy, specific pressure and temperature monitoring wells were required. This paper describes the successful lab and field implementation of an innovative completion technique that converts suspended coreholes into reservoir and aquifer monitoring wells. The coreholes are re-entered and vibrating wire piezometers (VWP) attached to capillary coil tubing, are positioned and cemented in place alongside selected reservoir intervals. The well is then "plugged and abandoned" and the lease remediated to leave only a small surface footprint for the data logger and telemetry. The cement is effectively impermeable to fluid movement but possesses sufficient permeability to transmit reservoir pore pressure to an adjacent piezometer. Recompletion of coreholes provides a cost-effective means of acquiring valuable downhole data while eliminating the need to drill a bespoke monitoring well in the same vicinity. Data over the reservoir interval (the Walloon Coal Measures) can inform and help resolve reservoir properties on a regional scale, understand contribution to flow and provide data points to pressure-match a reservoir model. Data over the aquifers within Springbok Sandstone and Gubbermunda Sandstone enables estimates of any CSG production effects on groundwater.