The Physical Chemistry of Wax, Hydrates, and Asphaltene

Abstract

Abstract Wax, hydrates, and asphaltenes are the three major threats to flow assurancethat must be assessed by design teams. These potential problems can dramatically change system selection and operational procedures. If one of themis discovered after the system is under construction or worse after first oilthen the whole project may be at risk. Each solid must be well understood byengineers so they can neutralize the threats to system uptime that result frompoor solids management. This paper focuses on the problems posed by, and the system implications of, wax, hydrates, and asphaltenes. Introduction Flow assurance in subsea systems focuses on preventing solid deposits fromblocking the flow path. The principle solids of concern are wax and hydrates. Sometimes scale and asphaltenes are also a concern. For a given reservoir fluidthese solids precipitate at certain combinations of pressure and temperature. Precipitated solids are often carried downstream slurried in the fluid; however precipitated solids can also deposit on the walls of the production equipment, which ultimately causes plugging and flow stoppage. Control of this blockage isthe essence of "flow assurance". Solids control strategies involve keeping thesystem pressure and temperature in a region where the solids are unstable(thermodynamic control) or controlling the conditions of solids formation sothat deposits do not form (kinetic control) or allowing solids to deposit, thenperiodically removing them (mechanical control). This paper focuses on the thermodynamic and kinetic behavior of wax, hydrates and asphaltenes in reservoir fluids. The emphasis is on solidsbehavior in subsea systems. So, for each solid we describe the phase envelope, rate of solid formation and system implications. We also describephysicochemical factors important in solids removal. This is not acomprehensive review of the solids but we will describe their behavior wellenough so that the reader can understand why subsea systems are designed andoperated as they are. Wax Paraffinic hydrocarbon fluids can cause a variety of problems in aproduction system ranging from solids stabilized emulsions to a gelledflowline. Problems caused by wax occur when the fluid cools from reservoirconditions and wax crystals begin to form. The temperature at which crystals first begin to form is called the cloud point. At temperatures below the cloudpoint, crystals begin to form and grow. Crystals may form either in the bulkfluid forming particles that are transported along with the fluid or deposit on a cold surface where the crystals will build-up and foul the surface.