Carbonate Petrophysics in Wells drilled with Oil Base Mud
by Austin Boyd, Schlumberger, SPWLA Distinguished Speaker
was presented on the 3 rd of October. A PDF of the presentation is here
Recent advances in NMR log and core analysis, complemented by more quantitative use of borehole image logs has led to the application of log based porosity partitioning to aid carbonate petrophysical interpretation. The foundation of this approach is the link between NMR T2 distributions and pore size distributions obtained from special core analysis. Several case studies have now been presented where this approach was successfully used in carbonate formations drilled with water base mud, where the NMR response is well characterized and has been validated by core analysis.
The recently discovered carbonate reservoirs offshore Brazil are typically drilled with oil base mud to avoid problems in the 2000 meters of salt overlying the reservoir. NMR logs are routinely run in the reservoir section and the interpretation methods developed for carbonates drilled with water base mud have now been shown to be valid for evaluating reservoir quality, even when oil base mud is used. The tendency of these carbonates to be oil wet to both the 28-30 API reservoir oil and the oil base mud filtrate, ensures that surface relaxivity is the dominant relaxation mechanism in the NMR response, which enables the correlation of T2 distributions with the variety of pore sizes encountered in the reservoir zones. Interpreting NMR logs in such conditions requires detailed knowledge of the oil-base mud filtrate properties, the reservoir oil properties and the wettability of the formation at down-hole conditions. Performing lab NMR measurements on these fluids and representative core samples at PVT conditions is one of the key requirements for correctly interpreting the NMR log data.
Examples of lab measurements, performed on native state, restored state, brine saturated, and partially saturated cores at PVT conditions when compared with down-hole NMR log data show that log based porosity partitioning can be applied in the pre-salt carbonates. Options for analyzing borehole image logs in carbonates drilled with oil-base mud will also be presented as an aid to porosity typing with NMR. The key considerations will then be summarized for core analysis and also for acquiring, processing and analyzing NMR and borehole image logs in carbonates drilled with oil based mud.
Austin Boyd, CV
Austin Boyd is the Petrophysics Program manager at Schlumberger Brazil Research and Geoengineering Center in Rio de Janeiro. Prior to moving to Brazil, he was Petrophysics Program manager at Schlumberger-Doll Research in Cambridge, Massachusetts. He joined Schlumberger in 1981 as a field engineer after receiving a degree in Electrical Engineering from Dalhousie University, in Nova Scotia, Canada, and has held a variety of positions in operations, research and engineering in the US, Europe, Brazil and the Middle East.