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Permeability is one of the most important petrophysical parameters for reservoir characterization, completion design and production prediction, etc. However, due to the strong heterogeneity, poor pore structure, extremely low porosity and complicated minerals types in shale gas reservoirs, it is challenges for estimating the permeability accurately.
In this paper, we present a method for predicting permeability from porosity and mineral contents data. 30 samples drilled from the Lower Silurian Longmaxi shale formations, Sichuan Basin, China, were conducted to porosity, permeability, TOC contents and XRD analysis. Several samples were also used for nitrogen adsorption experiments to get the rock specific surface. We used the Kozeny’s equation to calculate the specific surface of the solid phase. The specific surface derived from the Kozeny’s equation, which is lower than that obtained from nitrogen adsorption, is view as an effective specific surface related to micro and macro pores. The specific surface obtained from the nitrogen adsorption is mainly attributed to the micro pores of shale rocks. The relationship between the effective specific surface and TOC and mineral contents have been analyzed. The effective specific surface are positively correlated with clay contents, but negatively correlated with other non-clay minerals. However, there is no correlation between the TOC and effective specific surface. This may be because the Kozeny’s equation method removes the TOC effect on porosity. Based on the analysis, an empirical equation between the effective specific surface and clay and several main non-clay minerals was established using weighted least square method to process the heteroscedasticity. Thus, the permeability can be predicted from porosity and mineral contents via the Kozeny’s equation. The results show that the predicted and measured permeability have a good agreement.
The proposed permeability prediction method was applied to the field wells. The mineral contents could be obtained from geochemical log data or calculated using conventional well logs with optimization method. Good applications were achieved, indicating the proposed method is effective and reliable.
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