Abstract:

Evaluation of “double sweet spot” in shale gas reservoirs， i.e. shale gas sweet spot that are both geologically and engineeringly feasible， is challenging due to insufficient accuracy of gas saturation calculation， limitations of fracability assessment methods （though quite a few）， and inadequate integration between the geological and engineering assessments. In view of these， a gas abundance curve representing the geological “sweet spot” is established to tackle the accuracy problem through a sequential reconstruction of the sensitivity relationship between logging curves and gas bearing property based on gas bearing principles. A comprehensive fracability index representing the engineering “sweet spot” is developed to enhance the usability of the fracability assessment methods based on the correlation of rock brittleness index， mechanic parameters and in-situ stress difference coefficients to fracturing. And a geology-engineering evaluation chart is also proposed to better integrate the two aspects based on the production data and productivity classification of layers tested. This study improves the accuracy of qualitative identification and fracability assessment of shale gas sweet spots. The chart has been proven to be effective in recognizing high- medium- and low-potential gas pay zones as well as dry layers. Practices show that the technology is applicable and reliable in assessing shallow/deep layers in the Sichuan Basin and its periphery.

Key words: gas abundance, compressibility index, sweet spot, geology-engineering integration, shale gas, reservoir assessment, Sichuan Basin