Abstract

The understanding of kick and gas - oil-based drilling fluid (OBDF) interaction is essential for the planning and operational safety of drilling. The capability to detect gas kick at an early stage and to estimate the gas kick severity is critical.

In this respect the two classes of OBDFs are investigated, one with normal mineral base oil, and the other with linear paraffin base oil. The latter is often used in HPHT drilling applications.

The gas kick detection characteristics is determined by experimental studies of the methane-OBDF equilibrium conditions at pressure and temperature conditions ranging from STP conditions to conditions relevant for HPHT; 200 °C/1000 bar. The OBDF gas loading characteristics is derived from the determination of the two-phase envelope of methane and OBDF. The two-phase envelope towards single phase liquid is determined for selected temperatures and gas mass fraction of methane in the gas-liquid mixture. The equilibrium conditions are studied for both the two OBDFs and the associated classes of base oils; normal mineral base oil and linear paraffin base oil.

It is observed that the maximum methane gas loading capabilities for the two OBDFs are quite similar and follow a linear relation between max gas mass loading in the liquid vs. pressure at pressure below 400 bar. However at higher pressures the performance of the OBDFs differs distinctively, where far more gas can be absorbed in the OBDF with linear paraffin base oil. This OBDF enters into the dense phase region at pressures exceeding 400-450 bar, for which the solubility of gas into the liquid goes to infinity. The dense phase region is encountered at somewhat 100 bar higher with the normal mineral base oil, reflecting fare more attractive characteristics when it comes to the capability for gas kick detection and well control.

The findings illustrate that the selection of base oil for OBDF has an impact on the kick detection capability at HPHT as the gas loading capability and the dense phase region depends on the class of base oil selected for the OBDF. The results also indicate that the operational procedures should be adjusted according to which type of OBDF is used.

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