A Novel Method to Map Heavy Oil Viscosity From Standard Mud Gas – A Field Case From the Peregrino Field

Peregrino is a heavy oil field in the Campos Basin offshore Brazil. The field started production in 201, and Phase 2 was on stream in 2022. More development drilling is ongoing. The oil viscosity in Phase 2 has large variations from limited exploration wells, ranging from 30 to 180 cp. Any changes in the viscosity in the reservoirs can lead to a large difference in oil recovery. Therefore, updating the viscosity distribution in the reservoirs along with the development drilling is important. Different methods can acquire reservoir oil viscosity, including downhole logging and sampling, mud gas, extracts from cuttings, and surface oil sampling. Our previous studies demonstrate that mud gas provides near real-time and continuous reservoir oil properties. Due to the use of water-based mud (WBM) in the development drilling and relatively high degasser temperature, the standard mud gas (SMG) C1 to C3 data have been of good quality. SMG logging is part of the mud-logging service, and there is no additional cost to acquire the data. The challenge is to develop a simple approach to predict the oil viscosity from the available SMG data. A study of the Peregrino Field fluid database shows that methane/ethane and methane/propane ratios are the best parameters that correlate to reservoir oil viscosity. Before adopting the new method using SMG, we compared results with the measurement of pressure-volume-temperature (PVT) samples and pressure points. The comparison shows that the simple approach based on SMG provides an oil viscosity classification that distinguishes between high- and low-viscosity fluids along a given well. The threshold for the gas ratios division between the two classifications is identified from the reservoir fluid database, and the results from the mud gas method are in excellent agreement with the experimental results from the PVT analysis. The PVT viscosities are regarded as the ground truth. Therefore, we deployed a simple approach based on SMG data to map oil viscosity for future wells. The new approach using SMG data provides a real-time method to identify the continuous reservoir oil viscosity following the well path without additional data acquisition cost. Along with drilling more wells, we will soon achieve a detailed and accurate reservoir oil viscosity distribution in different reservoirs. The viscosity mapping of the reservoirs lays the ground for further optimizing the drilling target and well placement and improving the oil recovery.