Computing the Danger of Hydrate Formation Using a Modified Dynamic Kick Simulator

Abstract

Abstract Planning offshore wells in deep water depths needs special tools to optimize operation and safety procedures. One of the many technical problems in deep-water operations is the possible formation of hydrates in the BOP or Kill/Choke line. Hydrates are ice type substances consisting of frozen mixtures of water and hydrocarbons which may plug the BOP stack and well circulation path and will be very difficult and time consuming to remove. An advanced dynamic kick simulator, developed at Rogaland Research, has recently been expanded to include the determination of the potential for hydrate formation. Using dynamic temperature simulation, detailed PVT computations of the hydrocarbon influx on the component level, and an advanced hydrate formation program it is possible to obtain the ‘distance’ in temperature from hydrate formation throughout the well at any time during a simulated operation. The simulator includes code that takes into consideration the effect of hydrate inhibitor chemicals such as salts and alcohols. Thus it is possible to make several trial runs with differently inhibited muds to compare the danger of running into hydrates. The paper briefly discusses the physical and chemical models used, and points out how to circulate in a way to minimize the chance of hydrate formation. Simulating kick with hydrate formation is a novel and significant contribution. This will enable the industry to perform planning and engineering of complicated well control situations. In particular it will allow the planning engineer to evaluate the risk of hydrate formation versus the cost of mud inhibition - this will be very useful for low mud weight situations where providing hydrate inhibition is very expensive. Introduction In recent years more challenging drilling conditions are being encountered. As operations move into deeper water we are moving into an environment where the well pressure and the ambient seabed temperature are in the region where influx gases produce hydrates. Hydrates are ice-like crystalline compounds formed by a hydrogen-bonded network of water molecules.1 In the oil industry hydrates formation is often viewed as a hazard since it can plug flowlines and severely interfere with well control operations. Advanced drilling simulators that are able to correctly predict the well conditions are recognized as critical tools required for planning such wells. Rogaland Research Kick Simulator (RF-Kick)2 is one such simulator which has continuously improved with both new features and accuracy due to extensive experimental work and improved numerical modeling. In recent years RF-Kick has incorporated deep water, horizontal well, advanced PVT, HPHT and lost circulation features3. The hydrate formation module adds the thermodynamical computations needed to determine the conditions of the influx gas in the well that is based upon the composition of hydrocarbon components as well as the temperature and the pressure in the well. The module also includes the contribution of salts and glycerol in the drilling mud to the determination of the hydrate formation condition. The main purpose of the hydrate formation module is to combine many effects of well control in order to determine the chance of hydrate formation during well operations. Most hydrate reports are investigations into hydrates formation under carefully controlled conditions. We saw a need to add this information into dynamic oil/gas well operations where pressures, temperatures and hydrocarbon compositions can vary dramatically within hours of operation. This paper briefly discusses the basic equations of the kick simulator, the dynamic temperature model, the hydrate model and the thermodynamic elements used in the simulator.