Mechanical characteristics of laminated sand–shale sequences identified from sonic velocity and density correlations

Abstract

In the past, numerous relations between sonic velocity (i.e. an inverse of sonic travel time DT) and rock density were developed that were suitable for certain fields or certain rocks only. In this paper, the correlations between sonic and density log measurements were investigated again using the data information from multiple fields of Gulf of Mexico (GOM) and North Sea (NS). For sandstone-shale sequences, the dominant linear relationships between sonic travel time (DT) and formation density (RHOB) were observed, and compared with the popular Gardner's method (Gardner et al. in Geographics 39(6):770-780, 1974) along with other known methods. The implications of data clusters distinguished by formation lithologies and rock mechanical strengths were revealed from cross-plot analysis. The probabilities and uncertainties of the developed correlations were determined using the actual histograms from the collected data of GOM and NS fields coupled with Monte Carlo simulations.Keywords Sonic velocity and density correlation Á Mechanical properties Á Sandstone-shale formation Sonic velocity and density correlationsIt is recognized that the relationships between the rock sonic velocity and rock density depend on numerous factors such as rock strength and integrity, effective stress, mineral composition, granular structure, cementation, porosity, lithology, fluid content, saturation, micro-cracks, fluid pressure, depth of burial, geological age, and many other environmental conditions and factors (Horsfall et al. 2013).Recognizing the significant variability between the sonic velocity and rock density, Gardner et al. (1974) derived an empirical relation between P-wave velocity and density of various saturated sedimentary rocks from a series of controlled field and laboratory measurements as:where q is the density (g/cm 3 or g/cc), V p is the P-wave velocity (ft/s), 0.23 is the empirical magnitude constant, and 0.25 is the empirical shape constant.Gardner's method has been widely used in oil/gas industry for sedimentary formations (e.g. offshore wells in Gulf of Mexico) with the primary purpose of deriving the rock density from P-wave velocity. With the increasing needs to interpret the formation mechanical properties from sonic velocity, Gardner's method has been also used to derive the P-wave velocity from density data, i.e.

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