Asphaltene and Other Heavy-Organic Particle Deposition During Transfer and Production Operations

Abstract

SPE Members Abstract The production and transportation of petroleum fluids could be severely affected by deposition of suspended particles (i.e. asphaltene, paraffin/wax, sand, and/or diamondoid) in the production wells and/or transfer pipelines. In many instances the amount of precipitation is rather large causing complete plugging of these conduits. Therefore, it is important to understand the behavior of suspended particles during flow conditions. In this paper we present an overview of the heavy organic deposition problem, its causes, effects and preventive techniques. We also present an analysis of the diffusional effects on the rate of solid particle deposition during turbulent flow conditions (crude oil production generally falls within this regime). We utilize the turbulent boundary layer theory and the concepts of mass transfer to explain the particle deposition rates on the walls of the flowing conduits. The developed model accounts for the Brownian and eddy diffusivities as well as for inertial effects and other forces acting acting upon the particles. The analysis presented in this paper shows that rates of particle deposition (asphaltene, paraffin/wax, sand, and/or diamondoid) on the walls of the flowing channel, due solely to diffusional effects, are negligible. It is also shown that deposition rates decrease with with increasing particle size. However, when the deposition process is momentum controlled (large particles) higher deposition rates are predicted. It is shown a decrease in deposition rates with increasing crude oil kinematic viscosity. An increase in deposition rates with increasing production rates is also observed. Introduction Numerous experimental works have revealed the colloidal nature of the heavy asphaltene fraction of a crude oil. We consider the asphaltenes to exist in crude oil as both dissolved and suspended particles. Dispersed asphaltenes are sterically stabilized by neutral resins, they are electrically Charged, and have a diameter of 30–40 A. The stability of these particles can be disrupted by addition of solvents (i.e. n-heptane), it could also be disrupted during flow conditions due to shear stresses, or by counterbalancing the weak asphaltene particle charge. The latter is an important phenomenon since during crude oil production a streaming potential is generated which causes asphaltene aggregation. When solvents are used to precipitate asphaltenes, the resulting aggregates may have a diameter as large as 300. In addition to asphaltenes there may be other types of particles suspended in the crude oil as well. For instance, sand particles swept from the reservoir matrix, paraffin crystals if the temperature falls below the cloud point of the crude, and/or diamondoids. HEAVY ORGANIC DEPOSITION (CAUSES, EFFECTS, AND PREVENTIVE TECHNIQUES) The parameters that govern the precipitation of heavy organics from petroleum fluids appear to be composition of crude and injection fluid (if any), pressure, temperature, flow characteristics, and the properties of the conduit (pipeline, well, etc.) in which the reservoir fluid is flowing. With alterations in these parameters the nature of organic substances which precipitate will vary. Also, precipitation of some of the families of organic compounds (asphaltenes) is generally followed with polymerization or flocculation of the resulting precipitate, which produces an insoluble material in the original reservoir fluid. Because of the complexity of the nature of heavy organics in petroleum fluids the phenomena of precipitation and flocculation of these substances are not well understood. Also in view of the complexity of the petroleum reservoirs, study and understanding of the in situ precipitation of heavy organics seems to be a challenging and timely task. Such an understanding will help to design a more profitable route for petroleum production and processing systems. In order to model the phenomena of organic deposition from petroleum fluids under the influence of a miscible solvent, or in the process of its blending with another petroleum fluid one has to consider the following:The nature of organic compounds being deposited.;The nature of the petroleum fluid as a whole.;The role of temperature, pressure, and composition.;The role of flow regime and the structure of the conduit (pipeline, well, etc.) through which the flow is underway. Miscible flooding of Petroleum Reservoirs Secondary recovery practices usually involve injection of either water or gas into the reservoir to increase the insufficient pressure and keep the reservoir fluids flowing. Oftentimes the injection of miscible solvents is preferred because significantly less residual oil is left in the swept reservoir. Miscible flooding may be divided into three major categories: First-Contact Miscible Drive: The basis of this process is the injection of hydrocarbons which are completely soluble in residual oil. A typical fluid used for this purpose is propane/butane (or a mixture of both). P. 343