Heat-Transfer Models for Mitigating Wellbore Solids Deposition
- C.S. Kabir (ChevronTexaco Overseas Petroleum, Inc.) | A.R. Hasan (U. of North Dakota) | D. Lin (U. of North Dakota) | X. Wang (TRW)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- December 2002
- Document Type
- Journal Paper
- 391 - 399
- 2002. Society of Petroleum Engineers
- 4.3.3 Aspaltenes, 2.2.2 Perforating, 4.1.2 Separation and Treating, 3 Production and Well Operations, 5.5 Reservoir Simulation, 5.2 Reservoir Fluid Dynamics, 3.4.1 Inhibition and Remediation of Hydrates, Scale, Paraffin / Wax and Asphaltene, 4.3.1 Hydrates, 4.2 Pipelines, Flowlines and Risers, 1.1.2 Authority for expenditures (AFE), 4.3.4 Scale, 5.6.4 Drillstem/Well Testing, 4.3 Flow Assurance, 5.2.1 Phase Behavior and PVT Measurements, 3.2.2 Downhole intervention and remediation (including wireline and coiled tubing), 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 4.1.5 Processing Equipment, 5.3.2 Multiphase Flow, 4.2.4 Risers, 5.6.11 Reservoir monitoring with permanent sensors, 1.8 Formation Damage
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Tubular flow restrictions caused by solids deposition create serious oilfield problems. Solids normally originate from either asphaltic or paraffinic oil. Hydrates add another dimension to the problem when water production begins. Most solutions center on the use of aromatic solvents for asphaltenes and chemicals or heating for paraffins.
In this study, we seek preventive measures to minimize solids deposition by altering characteristics of fluid flow in the wellbore. The knowledge of fluid thermodynamic behavior, and the application of fluid principles and heat flows, constitute the essence of our approach. Specifically, we present a steady-state model that allows computation of wellbore pressure and temperature profiles for a hot-circulating fluid in the case of parrafins. The same model is used to study the p-T behavior of asphaltic oil while circulating a cold fluid.
A fluid-temperature model is also presented when intermittent or continuous injection of solvent is sought through a chemical-injection line. Both models are general in that flows in offshore wells can be handled rigorously.
Computational results show that preserving energy of paraffinic oil is feasible by circulating hot fluid in the annulus and/or using insulation, thereby keeping solids in solution. The use of an unsteady-state flow model sheds light on the time available for well intervention before the onset of deposition. Similarly, cooling of fluids may help produce asphaltic oils under favorable conditions.
Unwanted flow restrictions frequently occur in the fluid production chain, from perforations to a gathering center. Most flow impediments are caused by the formation of compounds that adhere to the walls of the production string. Asphaltene and paraffin deposition and hydrate formation are primary examples of such sticky problems that cost the industry millions of dollars in lost production and cleaning operations. Ref. 1 presents a comprehensive review of various field experiences arising from asphaltenes. Chemical treatments include the use of aromatic solvents, such as toluene or xylene, or deasphalted oil.2,3 Similar to asphaltenes, deposition of paraffinic substance in tubulars and flowlines is also well documented.4-7 Conventional remedial measures involve hot-oil treatment, mechanical scrapers, and injection of chemical inhibitors and microbes. Conceptual solutions with downhole heaters and electrical heating of tubing showed significant promise.8
Regardless of the method used, these measures are expensive and are undertaken usually after the problem surfaces. In this study, we focus on preventive or proactive measures by examining the phase behavior and fluid and heat flow characteristics in a producing string. In other words, we examine the viability of measures such as well design and altering flow characteristics by exploring the underlying physics. Here, we address issues pertaining to asphaltenes and paraffins; hydrates are not explicitly considered in this study.
The main motivation of this development is to provide simple engineering tools for design engineers before invoking a complex transient simulator. These tools, simple or complex, are not readily accessible to most users.
Understanding the thermodynamic behavior of fluids forms the basis for most remedial measures. This element is particularly true for our proposed solution approach. Most investigators to date focused on separate studies of each solid component; that is, asphaltene, wax or paraffin, and hydrate. However, in many oils, all of these solids can potentially occur and may present problems depending upon the fluids' state (pressure and temperature) in the flow string. Jamaluddin et al.9 presented a unified approach to combining all three solids envelope, as shown in Fig. 1.
Paraffins or waxes are high-molecular-weight, high-carbon- number (30 to 75) n-paraffins that may present significant challenges when transported from reservoir conditions. Wax crystallizes at a higher temperature (WCT) than its deposition temperature. WCT is also known as the cloudpoint. Comparison of extensive laboratory measurements with field data showed10 that the WCT values obtained from dead oils were in good agreement with those measured in flowing wellbores. Hammami et al.10 speculated that the laminar or transitional flow was probably conducive to crystallization of wax and its subsequent deposition.
Recently, several thermodynamic models were tested11 to estimate the cloudpoint temperature and subsequent deposition. Four of the tested models agreed with each other within 3°F and limited comparison with field data suggested moderate success in modeling wax deposition.
Over the years, significant progress has been made in understanding deposition kinetics12-16 of paraffins, unlike asphaltenes. In particular, the recent study of Singh et al.16 provides insights into deposition mechanism and subsequent growth of wax deposit with time. These authors showed that deposition growth is a result of counterdiffusion, wherein the wax molecules diffuse into the deposit while oil molecules diffuse out. The deposition model has also been adopted in a coupled wellbore/reservoir flow simulator8 to seek thermal solution to the deposition problem.
Fig. 2 shows a qualitative paraffin phase envelope (PPE) and a well sketch. Our approach in addressing this issue is to avoid crossing the PPE, while expanding the fluid from downhole (X) to surface (Z), as shown in Fig. 2. As oil expands up the wellbore to reach point Z at the wellhead, it experiences decline in pressure and temperature. In so doing, liquid (L) oil phase gets in the liquid-gas (L+G) region before crossing the solid envelope (L+S+G), thereby triggering flocculation of paraffins. The proposed treatment involves circulating a hot fluid in the annulus and/or insulating the production string. Thermodynamically speaking, paraffins are strongly temperature-dependent; consequently, they are amenable to thermal treatment.
Asphaltenes, defined as n-pentane insoluble fraction of crude oil, are polar molecules, which aggregate through orbital association, hydrogen bonding, and acid-based interactions. The growth of aggregates is limited by the presence of resins in solution. Various asphaltene-solubility studies have been reported, and a number of models to compute asphaltene flocculation or onset condition have appeared in the literature.17-20
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