A Gas-Risking Workflow for Top-Hole Drilling Studies

The most effective method of mitigating the risk of shallow gas in top-hole drilling is avoidance, but in cases where well locations are tightly constrained a more rigorous and informative workflow is required. Gas probability assessments are only the initial step in devising risk mitigation strategies so a workflow is adopted in which the physical characteristics of a shallow gas reservoir are estimated, likely gas volumes and pressure regimes are calculated and a prediction is made of the effects of drilling into the reservoir. Gas probability is initially assessed by reference to the attributes of reflection data and AVO characteristics are used to refine the probability assessment. Analysis of wireline log data along with seismic inversion, fluid substitution and pore-pressure gradient studies are used to construct a geological model of the potential gas reservoir in which questions surrounding sand content, porosity and likely gas concentration are addressed, resulting in an upper-case assessment of the likely Hydrocarbon Pore Volume (HCPV). A formation-factor calculation is then used to predict maximum potential gas production, in the event of a gas-control incident (‘gas-kick’ or ‘blow-out’).