Can laboratory ultrasonic measurements on core plugs reproduce results obtained from sonic well logging in carbonates?

In the absence of wireline data in the open literature, the use of core plugs extracted from outcrops or subsurface reservoirs is significant for establishing the link between geology, petrophysical and elastic properties. Establishing such a link, known as rock physics transform, is an essential task in rock physics studies. The use of core plugs raises however questions about representation and upscaling, especially in heterogeneous rocks like carbonates. This study aims to investigate whether or not ultrasonic and petrophysical measurements on core plugs can reproduce results obtained from wireline logs in carbonates. The comparison between core plugs and downhole data is conducted in terms of values and trends. Specifically, we compare the data obtained from core plugs with wireline data in terms of 1) the magnitude of velocity and porosity at the same corresponding depth, and 2) the velocity-porosity relationship establishing a rock physics transform. Laboratory ultrasonic and porosity measurements were conducted on 128 carbonate core plugs extracted from subsurface oil field in the Middle East and results were compared with wireline data from the same well. Laboratory measurements were done under reservoir pressure and at dry conditions, while the fluid impact was incorporated using Gassmann's fluid substitution theory. The results show that laboratory-derived porosity and velocity failed significantly (error percentage of 81% and 10% respectively) to reproduce the wireline values at the same corresponding depth. Nevertheless, the velocity-porosity relationship and the resultant rock physics transform established based on core plugs were consistent with those obtained from wireline data. Such a result confirms, using experimental and field data, the hypothesis about rock physics transform scale independence which was only theoretically proposed in earlier studies. One main point here is that such scale independence is valid even in rocks with heterogeneous microstructure like the studied carbonates. The results suggest that one approach for upscaling cores-derived results is to establish and use relationships (trends) between different measurable properties rather than directly using (or attempting to upscale) their magnitude at the corresponding downhole depths. Moreover, the results show the validity of using Gassmann's theory to incorporate the impact of fluids based on dry ultrasonic measurements conducted at reservoir pressure, and thus providing a valuable tool for establishing rock physics transforms in carbonates. The conclusions presented here should be valid in carbonate reservoirs as long as no large-scale features such as fractures or karsts are dominant.