Abstract

The oil and gas industry relies primarily on locally calibrated shale velocity/sonic, electrical resistivity trends, and exponential loss of porosity from high-porosity shallow mud to model compaction. These methods do not properly identify and isolate the variable shale mineralogy and the specific components of compaction which control shale porosity. The variable shale mineralogy is captured by the shale bulk cation exchange capacity (CEC) (meq/g). CEC is a measurement of the amount of exchangeable charge per unit mass of dry sample and is used as a petrophysical surrogate for mineralogy and grain size. The components of compaction are mechanical, governed by effective overburden stress; thermal, associated with increasing burial temperature, and chemical, such as the smectite to illite transformation. Desorption water vapor isotherms of Na-exchanged pure clays, clay mixtures, and shales help to quantify the properties and components that ultimately control shale porosity. The isotherms are a measurement of the gravimetric water content (WC) as the relative humidity p/po, is decreased from 100%. Plotting p/po versus WC normalized by CEC (water-g/meq) results in the data collapsing to a single general trend. With knowledge of the bulk CEC of the shale, a protomechanical compaction curve results from this trend when a simple thermodynamic equation converts p/po to effective overburden stress. Additionally, water vapor sorption isotherms measurements at different temperatures calibrate the thermal compaction component associated with increasing temperature with burial. The effects of diagenesis and variable shale mineralogy are incorporated in the compaction model via the bulk CEC parameter, calculated from petrophysical well logs. After accounting for temperature and CEC, the shale porosity is dependent only on the effective overburden stress state, i.e., pore pressure. There is excellent agreement between the model’s pore pressure with pore pressure inferred from drilled mud weights and pressures measured in interbedded sands.

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