A Devonian shale - specific geophysical log interpretation model was developed to extract as much geological and reservoir data as possible from a suite of air- or gas-filled-hole logs. Development of the methodology and evaluation of the geological and reservoir properties was supplemented with core data, formation tests, mud logging data, temperature logs and borehole TV logs acquired in Devonian shale wells.
This new analysis technique improves the identification of hydrocarbon bearing intervals and the types of formations associated with production. As a result, the analysis when production. As a result, the analysis when combined with other data, allows a more effective selection and treatment of the hydrocarbon productive intervals. productive intervals. Examples are shown which compare the computed lithology and porosity with core measurements. The computed results are also compared to mud logs; noise logs; temperature logs; borehole television data; production logs; and production tests.
The existing models in common use for interpretation of logs were developed to evaluate data collected in conventional reservoir rocks such as sandstones and carbonates. Such models are not well adapted to the evaluation of shales due to the complexity of the lithology and the typically low porosities. Thus, a new interpretation scheme was developed to take these factors into account.
The development and evaluation of this interpretation methodology was aided by the extensive use of core measurements which were integrated with the geophysical well logs. Close control of the quality of data as it was acquired in the field and during the analysis aided in obtaining consistent and reliable results.
A volumetric Devonian shale constituents model, calibrated with thin-section descriptions and X-ray diffraction measurements, has been developed to evaluate lithology. The model. considers variable amounts clay minerals (predominantly illite, micaceous illite, and kaolinite), quartz, kerogen, and heavy minerals (predominantly pyrite).
A Devonian shale-specific porosity model has been developed to take into account bound-water porosity, free-water-filled porosity, porosity, free-water-filled porosity, free-hydrocarbon-filled porosity, and kerogen "porosity".
The resulting computations yield improved values of porosity, hydrocarbon content and the constituents of reservoir. These data integrated with other available data allow an evaluation of the different types of zones contributing to production. production.
Because Devonian shale wells are commonly drilled using air or foam as the drilling fluid, the variety of geophysical well logging measurements that can be recorded is limited. A typical set of geophysical well logs used in the development of the Devonian shale-specific interpretation method includes nine basic measurements.