Gas lift is being utilized to enhance production of heavy crude from the Campos field offshore Brazil. Downhole chemical injection is implemented to treat the crude and assist with chemical inhibitor delivery to protect the reservoir fluids in the well tubing from corrosion due to elevated concentrations of hydrogen sulfide (H2S). Due to the absence of dedicated chemical injection lines in the field, H2S scavenger is diluted in ethanol and transported with the gas lift, through the gas lift line (GLL), to the well casing. The gas lift and aqueous chemical phase flow together in the inner annulus between the casing and tubing as a multiphase mixture. The mixture is then injected into the well tubing through a gas lift valve (GLV). Field measurements have shown the crude is not always uniformly treated with the chemicals due to unsteady discharge of the chemicals from the casing into the production tubing through the GLV.
Advanced steady state and transient simulations were carried out to analyze the supercritical / multiphase flow in the gas lift system. Moreover, the complex phenomena associated with the hydraulic instability upstream of the GLV, and subsequent irregular injection of the multiphase mixture into the tubing, were well-characterized.
Simulation results are qualitatively accurate and descriptive as they duplicate the phenomena observed in the field. It was found that the dynamic stability of the wellbore was disturbed by either insufficient pressure gradients at the GLV or by localized slugging initiated in the casing. As such, chemical accumulation in the casing and intermittent pressure build-up upstream of the GLV were responsible for the non-uniform injection into the well tubing. The system’s dynamic stability can be restored by either increasing the casing pressure to a level high enough so the GLV tolerates normal variations in the casing pressure or by manipulating the flow pattern in the casing to avoid slug flow. Simulation results proved both techniques to be effective.
Field data and simulation results show that GLL may be substituted for chemical injection lines provided the GLV is designed based on a sound understanding of the system’s hydraulics and a reasonable prediction of the operating pressures on its sides throughout the well life.
Findings from this study provide guidelines for proper flow assurance practice and safe design and operation in artificial gas lift and chemical injection applications