Kristin is a high-pressure, (90 MPa), high-temperature (exceeding 160°C) gas condensate field operated by Statoil ASA. Kvitebjorn is another gas condensate field with slightly lower pressure and temperature around 78MPa and 150°C respectively. In some of the production wells in these fields, a blend of cesium and potassium (CsK) formate was selected as the drilling fluid, to ensure a stable mud density for well control and in order to reduce formation damage.
Formate muds present a serious challenge to well-log interpretation because both the mud and the filtrate have properties significantly different from conventional oil-and water-based muds. The nuclear log properties of CsK formate that are particularly challenging include high density, high photo electric factor (Pe), low hydrogen index (HI), and high thermal neutron capture cross-section (Sigma). To quantify the borehole and invasion effects on logs acquired in this environment, extensive laboratory experiments and mathematical modeling were carried out for relevant wireline (WL) and logging -while- drilling (LWD) tools.
The laboratory measurements confirmed dramatic effects on nuclear tools. These effects increase with increasing formate fluid density and depth of invasion of the formate fluid. The effects of fluid invasion were so severe that porosity estimation from nuclear logs appeared to be unattainable, unless invasion is shallow and precisely known. LWD data acquired in the first well confirmed these large effects and showed significant changes with time. It was observed that the effect of CsK formate is relatively less on wireline density logs when compared to that of LWD density logs. The proposed explanation is that a significant quantity of gas was present in the invaded zone at the time of WL logging. Nuclear modeling demonstrated that such an explanation is plausible. The basic design differences between WL and LWD tools may also be contributing to the effect.
This case study demonstrates that a reasonable log interpretation is possible in wells drilled with heavy formate muds, albeit with reduced accuracy and precision.