Detection of Asphaltene Deposition by Capillary Flow Measurements

Abstract

Abstract This paper presents an experimental methodology that allows a sensitive determination of the onset and rate of deposition in asphaltene-containing fluids when the pressure, temperature, or fluid composition are varied. This determination is inferred from pressure drop measurements across a capillary tube in which the fluid is injected at a constant flow rate and at specified conditions of temperature, pressure and composition. When the conditions for deposition are met the pressure drop across the capillary tube is no longer stationary but increases with time as a result of the buildup of a deposition layer on the tube walls. In the experimental set-up that has been constructed, the fluid composition is controlled by co-injecting the various fluid components into the capillary by means of different pumps. This setup has been utilized to determine, at some given temperature and pressure, the deposition envelope of systems made of crude oil, solvent (xylene) and precipitant (heptane). The measured deposition points coincided with bulk flocculation thresholds determined by a conventional light transmission method. The light transmission method was, however, difficult to for fluids with a very low asphaltene content, whereas the proposed method was efficient for fluids containing as low as 0.04 wt.% asphaltenes. The rate of deposition was observed to increase with the distance from the deposition onset. Reversibility of deposition was significant only when the asphaltene-containing fluid was returned to conditions remote from those of the deposition onset. Introduction Asphaltene deposition near the wellbore of a formation or in production and surface handling facilities may have devastating economic consequences by severely reducing well productivity.1 Usually, this is a consequence of the pressure drawdown and/or temperature variations that occur near the wellbore and in the pipes. Asphaltene deposition may also occur deeper in the formation as a result, for instance, of mixing the oil with solvents or gases such as CO2 or light hydrocarbons. Prior to any field exploitation or gas injection project it is therefore highly desirable to anticipate the event of asphaltene precipitation by conducting representative laboratory tests.2 In cases where asphaltene deposition has already occurred in a given application, it may also be necessary to assess the efficiency of remediation means3,4 in the laboratory. These laboratory tests have been used to determine the asphaltene deposition envelope in a range of thermodynamic conditions (i.e., temperature, pressure and fluid compositions, e.g., oil+solvent) close to those of the field application. Most of these tests consist of detecting flocculation, e.g. by spot test measurements,5 light transmission or diffusion,6 conductimetry7 or viscosimetry.8,9 Because these methods are sensitive to bulk properties, they may not work for oils with a low asphaltene content. The oil from the Hassi-Messaoud field, for instance, contains as low as 0.1 wt.% asphaltene but nevertheless forms deposits in the pipes.10 In addition, the utility of these tests for predicting formation damage may be questioned. Permeability reduction of porous media by asphaltene-contaning crudes is in fact a complex process caused not only by the formation of large, pore-blocking aggregates but also by mono- and multi-layer asphaltene adsorption (or deposition) on pore walls.11,12 In this paper we present a capillary flow method for determining the asphaltene deposition envelope which appears to be more representative of the formation damage process than the bulk tests mentioned above; yet it is much less expensive and time-consuming to perform than permeability damage experiments.