Leak-Off Pressures In Relation To Lithological Properties And Attributes In The Northern Carnarvon Basin
Murray, Craig W.
Geoscience Honours Degree, 2011
University of Adelaide
Knowledge of the minimum horizontal stress (Shmin) is vital to the drilling of and production from a wellbore. Shmin is repeatedly estimated by perforating the borehole wall. Significant variation in leak-off pressure values are observed globally, including within the Northern Carnarvon Basin. It has been repeatedly suggested that lithology is a major influence on the variation encountered in leak off pressure. The aim of this project was to collate all leak off pressure data within the Exmouth, Dampier and Barrow Sub basins and examine this variation in leak-off pressure in the Northern Carnarvon Basin, as well as to investigate any possible geological or geographical controls on this variation.
Leak-off pressure data was collected from 114 wells located within the three defined Sub basins and plotted against lithology, sonic compressional and shear wave velocities, Possion’s Ratio and the uniaxially-derived minimal horizontal stress estimate. In all four cases no specific trend or correlation was observed and consistent scatter dominated throughout. No normal compaction curve was identified and there was a significant lack of correlation between depth and leak-off pressure.
Due to the lack of correlation observed from two dimensional linear analyses, principal component analysis (PCA) was applied. This method is used to understand the structure of multi-dimensional data, effectively summarising it in a more simplistic manner. The PCA resulted in the definition of lithology as a second order control on leak-off pressure, with depth as the primary control. The result of this study also highlights the need for a more complex analysis than simple cross plotting in the geomechanics field.
Overall, lithology has been shown to influence the variation of leak-off pressure, especially when principal component analysis was utilised. However the variation observed is more likely to have been controlled by a combination of different factors rather than one single variable. Depth, lithology, local in-situ stress regime and testing conditions are all considered to have a control on the variation in leak-off pressure estimates.