Rock cuttings are often collected and characterized during a drilling operation for reservoir monitoring. The depth origin of the sampled cuttings could be determined by calculating the lag time required for the cuttings to travel from the bit to the surface, and the accuracies in depth of the cuttings play an important role in describing the subsurface. Traditional mud logging methods may give inaccuracies of ±20 feet in the depth correlation due to the shuffling and settling of cuttings as they travel with drilling fluid to the surface, especially for horizontal wells. Aiming to remedy these inaccuracies in mud logging, more efficient mud tracers and rapid detection techniques in mud logging for improved cutting depth correlation are highly desired. Nanoclays are nanoparticles of layered minerals, such as montmorillonite, bentonite, kaolinite, hectorite, and halloysite. In this work, we report the functionalization of nanoclays by simultaneously incorporating superparamagnetic dots and fluorescent dye components into bentonite, so that the resulting composite nanoclays can be use as tracers to tag drilling muds. Multiple “barcode” nano-clays can be generated by embedding different fluorescent dyes which can be identified by fluorescence spectroscopic or microscopic imaging measurement, while the superparamagnetic property allows the functionalized nan-clays being separated and collected by a magnet from the drilling fluid with cuttings. The synthesized nano-clay tracers have been tested in simulated mud formulation and cutting samples. The functionalized nano-clays are readily compatible with water-based mud (WBM), and when formulated with of surfactants or surface functionalization, can be miscible with oil-based mud (OBM). Stability test demonstrated that the tracers are stable in simulated reservoir condition with high-salinity water and crude oil at elevated temperature (~95°C). Experimental procedures are developed to analyze the cutting samples. We demonstrate that each of the tracer materials can be detected by orthogonal analysis using different methods. The multifunctional nano-clays as tracers can tag cuttings precisely at the drill bit during mud logging operations, and the higher depth assignment accuracy enabled by these new types of tracers may reduce the occurrence of some systematic errors as compared with the traditional mud logging depth calculations.

Functionalization of Nano-Clays for mud logging application in drilling fluids

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