The operator's strategy is to maximise economic upstream gas production by drilling technically-mature sustaining wells in depleted areas of Surat basin in Queensland, Australia. To date the operator has drilled more than 2800 Coal seam gas wells with minimal well control issues.

Historically wells drilled in Surat basin were normally pressured vertical wells with depths ranging from 600-1000m TVD. The wells are drilled with water + KCL (weighting agent) with mud weight in range of 8-6-9 ppg. However, few wells drilled in late 2016/17 in Central Surat area showed signs of significant depletion in terms of drilling fluid losses and multiple well control events which suggested that prolonged production from these coal seams have reduced the reservoir pressure & correspondingly fracture gradient. Those wells were controlled with conventional well control methods. But this had an impact on the well duration and the wells were drilled at considerable cost. This prompted the Well Engineering team to look for alternative mud systems which can reduce the losses to coals and any reservoir productivity impact.

This paper represents the well planning and design process undertaken to drill these depleted wells with a Near balanced fluid system – Aerated mud (a combination of water and air). The author also has included operational results from four trial coal seam gas wells drilled with Aerated mud. The objective of the trial was to understand the impact of drilling the depleted coal seams with lighter fluid and any time/cost impact.

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