Gas Solubility in Oil-Based Drilling Fluids: Effects on Kick Detection
Authors: Thomas David C. , Lea James F. , Turek E.A. 
Published: December 2007

Summary
This paper describes the effects of gas solubility on the properties of oil-based drilling fluids. Prediction methods for gas solubility in oil-based muds were tested by experimental work and found acceptable. Expansion of discrete mixtures of mud and gas are predicted. A blowout simulation program was written and used to predict the effects of a kick on the surface-observable indicators (pit gain, well flow). While the bulk of this paper emphasizes the properties of oil-based muds, a comparison with water-based muds under similar conditions is made. The general conclusions are applicable to both types of muds. It is concluded that pit gain is the most reliable indicator of a kick during drilling in either oil- or water-based muds. It is recommended that a pitlevel measurement system be designed that will detect a pit gain of less than 5 bbl [0.795 m3] in the whole mud system.

Introduction
Normally, gas or gas-condensate mixtures enter the wellbore only through production equipment under controlled conditions. However, during the drilling of a well, an unexpected over pressured zone may be encountered such that a gas or oil influx can occur. This condition, known as a "kick," can be very dangerous to the crew, rig, and ancillary equipment and can produce a blowout. The events surrounding a kick or a blowout are usually poorly documented because of the highly charged atmosphere and lack of time to record the event. Detailed data on a blowout are very rare.
Gas and gas-condensate solubilities in oil and in water are topics that have been studied extensively by reservoir and production engineers. Their work has produced equations of state (EOS's) that will adequately describe reservoir fluid properties under differing pressure and temperature regimes. One such equation of state is called the Amoco Redlich-Kwong equation of state (ARKES).1 Results of this development were used heavily in this paper. Multiphase flow pressure drop models have been developed in the past.2 These models commonly use slightly simpler EOS's or data correlations to predict the PVT behavior of a fluid. Since production is a dynamic phenomenon, they account for viscosity effects in the fluid and migration of gas bubbles in the wellbore fluid. The work reported here is an application of multiphase flow and PVT models to the interaction of drilling fluids with produced gas or gas condensate during a kick.

Gas Solubility in Oil and Water

Gas Solubility in Oil.
Calculation of gas solubility in complex hydrocarbon mixtures at elevated temperatures and pressures requires an advanced EOS such as the Redlich-Kwong EOS3:Equation 1
For pure components, parameters a and b can be transformed into dimensionless parameters Oa and Ob:Equation 2 and 3
Generalized correlations were developed for Oa and Ob by Yarborough.1 These are incorporated into ARKES, which is applicable to a large variety of components over a wide temperature range. As discussed by Yarborough,1 ARKES parameters for C7+ boiling-point cuts are assigned by use of these correlations together with API nomographs and knowledge of the average C7+ molecular weight and specific gravity.
To apply the EOS to mixtures, a and b parameters characteristic of particular mixture compositions are derived from pure component parameters through use of the following mixing rules.Equations 4 and 5
Nonzero values are employed for the unlike-pair interaction parameter, Cij, for nonhydrocarbon/hydrocarbon pairs and nonhydrocarbon/nonhydrocarbon pairs.

Gas Solubility in Water.
The solubilities of hydrocarbon gases in water are very small compared with the solubilities of the same gases in oil. In most of the work discussed here, gas solubility in water could be safely ignored; however, it was not. In all the calculations described, gas solubility in water has been estimated from literature values.4 The amount soluble is less than 1% of the amount soluble in oil at the same temperature.

Gas Solubility in Oil.
Calculation of gas solubility in complex hydrocarbon mixtures at elevated temperatures and pressures requires an advanced EOS such as the Redlich-Kwong EOS3:Equation 1
For pure components, parameters a and b can be transformed into dimensionless parameters Oa and Ob:Equation 2 and 3
Generalized correlations were developed for Oa and Ob by Yarborough.1 These are incorporated into ARKES, which is applicable to a large variety of components over a wide temperature range. As discussed by Yarborough,1 ARKES parameters for C7+ boiling-point cuts are assigned by use of these correlations together with API nomographs and knowledge of the average C7+ molecular weight and specific gravity.
To apply the EOS to mixtures, a and b parameters characteristic of particular mixture compositions are derived from pure component parameters through use of the following mixing rules.Equations 4 and 5
Nonzero values are employed for the unlike-pair interaction parameter, Cij, for nonhydrocarbon/hydrocarbon pairs and nonhydrocarbon/nonhydrocarbon pairs.

Gas Solubility in Water.
The solubilities of hydrocarbon gases in water are very small compared with the solubilities of the same gases in oil. In most of the work discussed here, gas solubility in water could be safely ignored; however, it was not. In all the calculations described, gas solubility in water has been estimated from literature values.4 The amount soluble is less than 1% of the amount soluble in oil at the same temperature.