Resistivity Characterization and Modeling of Synthetic Based Drilling Fluids and Corrosion Detection in Steel

With deeper drilling concern (high in situ pressure and temperature) and increased environmental concerns, there is interest in developing synthetic based drilling fluids. In this study, synthetic based drilling fluids (SBF) with very high insulation properties were developed for high pressure and high temperature (HPHT) applications and modified using nano materials to improve the sensing properties for monitoring. The major constituents of drilling fluids were fatty acid methyl esters (Insulator), surfactant and water. The drilling fluids were also characterized based on the electrical properties. Based on the electrical impedance and frequency relationship, the SBF was characterized as a resistive material. The rheological properties, sensing properties and contamination were correlated to the resistivity. The test results showed that 1% nano material decreased the electrical resistivity of the drilling fluids by over 25.5%. The nano particles addition increased the yield stress by over 100%. The rheological properties of drilling fluids have been correlated to the electrical resistivity of the drilling fluids using nonlinear power and hyperbolic relationship. The model predictions agree well with the experiments results. Also, the effects of various types of contamination on the SBF were investigated.

The effects of synthetic based drilling fluids (SBF) contamination on the smart cement with a water-to-cement ratio of 0.38 were investigated. The test results showed that the 1% SBF contaminated reduced the density of cement by 1%. The initial resistivity of SBF contamination cement was 1.12 and 1.56 $\Omega \cdot m$. with 0.1% and 1% SBF contamination. Hence the initial resistivity of smart cement changed by 8.7% and 51.5% for 0.1 and 1% SBF contamination. The compressive strength of the cement was reduced by contamination. Also, SBF contamination affects the rheological properties and the piezoresistive properties of smart cement.

In order to demonstrate the real-time monitoring of the oil well, small model, large model and field test were developed to monitor the displacement of drilling fluid, spacer and oil well cement. Resistance was used to monitoring the displacement of drilling fluid and cementing with time and level (depth). Change in resistance of smart cement was measured over 400 days to determine the long term behavior of cementing. Also resistivity of cement was measured to verify the contamination. During a model test spacer fluid contamination of cement happened during the displacement process in one model study and the resistance was a good indicator to check the quality of drilling fluid and hardening cement.

Corrosion of steel will affect the durability and performance of the oil wells and other infrastructures. In this study, detection method for steel corrosion was developed for room and HPHT conditions. Electrical impedance measurements were used with the two probe method to characterize corroded steel. With corrosion, the resistivity of the steel corroded under HPHT condition increased. Also the contact parameters changed with corrosion of the steel. The changes in the bulk and contact of electrical properties of corroded steel samples were much more sensitive than the classical methods used to detect corrosion.