Methods of estimating shale gas resources – Comparison, evaluation and implications

Estimates of technically recoverable shale gas resources remain highly uncertain, even in regions with a relatively long history of shale gas production. This paper examines the reasons for these uncertainties, focussing in particular on the methods used to derive resource estimates. Such estimates can be based upon the extrapolation of previous production experience in developed areas, or from the geological appraisal of undeveloped areas. The paper assesses the strengths and weaknesses of these methods, the level of uncertainty in the results and the implications of this for current policy debates. We conclude that there are substantial difficulties in assessing the recoverable volumes of shale gas and that current resource estimates should be treated with considerable caution. Most existing studies lack transparency or a rigorous approach to assessing uncertainty and provide estimates that are highly sensitive to key variables that are poorly defined - such as the assumed ratio of gas-in-place to recovered gas (the ‘recovery factor’) and the assumed ultimate recovery from individual wells. To illustrate the uncertainties both within and between different methodological approaches, we provide case studies of resource estimates for the Marcellus shale in the US and three basins in India.