Estimation of TOC and Brittleness Volumes and Correlation with Reservoir Production
The Barnett Shale in the Fort Worth Basin is one of the most important resource plays in the USA. TOC and brittleness can help to characterize a resource play to assist in the search for sweet spots. Higher TOC or organic content are generally associated with hydrocarbon storage and with rocks that are ductile in nature. Brittle rocks, however, are more amenable to fracturing with the fractures faces more resistant to proppant embedment. Productive intervals within a resource play should therefore contain a judicious mix of organics and mineralogy that lends to hydraulic fracturing. Identification of these intervals through core acquisition and lab-based petrophysical measurements can be accurate but expensive in comparison to wireline logging. In this work, we estimate TOC from wireline logs using Passey’s method and attain a correlation of 60%. However errors in the base line interpretation can lead to inaccurate estimates of TOC. Using non-linear regression with Passey’s TOC, normalized stratigraphic height and acquired wireline logs the correlation was increased to 80%. This regression can be applied to uncored wells with logs to estimate TOC and thereby provides ground truth within the seismic survey. Core measurements provide relatively more accurate measures of both TOC and mineralogy. Brittleness index (BI) is computed based on mineralogy using Wang and Gale’s formula. The correlation between this mineralogy based BI and BI estimated using elastic logs (lr, mr, VP/ VS, ZP and ZS) and wireline logs is 78%. However, this correlation decreases to 66 % if the BI is estimated using only wireline logs. Therefore, the later serves as a less reliable proxy. We correlate production to volumetric estimate of TOC and brittleness by computing distance weighted averages about assumed perforations in 120 horizontal wells. We obtained a production correlation of 38% on blind wells, which was encouragingly suggesting that the geologic component in completions provides an important contribution to well success.
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