Asphaltene precipitation from live crude oils that occurs due to pressure reduction can foul and clog oil production equipment, at the well surface, in the borehole, and even in the subsurface formation, thus is of considerable interest to oil operating companies. We employ near-infrared (NIR) spectroscopy to characterize this asphaltene precipitation process; in particular, the independent measurements on asphaltene flocculation of wavelength dependence of optical scattering and of sedimentation rates are performed. Here, it is established that different asphaltene flocs form during depressurization of crude oil. Furthermore, the initial precipitate is probably not problematic in the production of crude oil, relaxing constraints imposed by asphaltene considerations. Additionally, the asphaltene precipitation process is shown to be largely reversible in the minutes time frame, but subtle irreversibilities are suggested. Compressibility is measured using NIR techniques to validate our methods. Optical spectroscopy on optically thin samples is found to be a powerful and indispensable tool to characterize asphaltene precipitation.

Asphaltenes are the n-pentane or n-heptane insoluble fractions of crude oil that remain in solution under reservoir temperature and pressure conditions. They are destabilized and start to precipitate when the pressure, temperature, and/or composition changes occur during primary production. The precipitated asphaltene particles will then grow in size, and may start to deposit onto the production string and/or flowlines, causing operational problems. In this paper, our emphasis is to identify the first pressure and/or temperature conditions at which the asphaltene will start to precipitate for two reservoir oils. Four different laboratory techniques were independently used to define the onset of the asphaltene precipitation envelope. These methods are:gravimetric;acoustic resonance;light scattering; andfiltration. The gravimetric method was found to be precise, and within the accuracy of the analytical methods. However, the method was time consuming. The acoustic resonance technique (ART) was fast and less subjective, but it did not define the lower asphaltene boundary. The interpretation of the onset pressure from the near-infrared (NIR) light-scattering technique (LST) was subjective to a degree. However, the NIR response defined the upper and lower boundaries of the asphaltene envelope and the bubblepoint pressure, as did the gravimetric technique. In a manner similar to those of the gravimetric technique and LST, the filtration technique can also define the upper and lower asphaltene phase boundaries, in addition to the bubblepoint pressure. The filtration technique is fast compared to the gravimetric technique, but takes more time than the ART and LST methods. Introduction Asphaltenes remain in solution under reservoir temperature and pressure conditions. They start to precipitate when the stability of the colloidal dispersion is disturbed. This disturbance can be caused by changes in pressure, temperature, and/or composition of the oil. Precipitation and deposition of asphaltenes have reportedly caused operational problems, ranging from plugging of tubulars and flowlines(1–3) to clogging of production separators(4). Leontaritis and Mansoori present a comprehensive description of field problems caused by asphaltene deposition(5). Figure 1 schematically presents the asphaltene-related problems that may occur in the field. Asphaltene precipitation problems can be categorized as follows:Precipitation can be caused by the changes in temperature and/or pressure during primary depletion.Precipitation can be caused by blending or commingling of two noncompatible reservoir fluid streams (i.e., subsea completions), acid stimulation and/or enhanced recovery injection gases (CO2, H2S, or rich gas). The correct operating procedure to minimize the asphaltene problem is not well understood. We believe a better understanding of the fundamental processes leading to solids precipitation is a prerequisite to management and prevention of production problems. Primarily, two theoretical approaches have been presented in the literature to compute phase separations of asphaltene during primary production. These approaches include association modeling(6, 7) and calculation of asphaltene solubility parameters with the Flory Huggins polymer phase separation technique.(8, 9) Asphaltene destabilization caused by solvent injection and consequent alteration of the rock surface wettability have also been reported in the literature, but are not discussed here.(10).

Asphaltenes are the n-pentane or n-heptane insoluble fractions of crude oil that remain in solution under reservoir temperature and pressure conditions. They are destabilized and start to precipitate when the pressure, temperature and/or composition changes occur during primary production. The precipitated asphaltene particles will then grow in size and may start to deposit onto the production string and/or flowlines, causing operational problems. In this paper, our emphasis is to identify the first pressure and/or temperature conditions at which the asphaltene will start to precipitate for two reservoir oils. Four different laboratory techniques were independently used to define the onset of the asphaltene precipitation envelope. These methods aregravimetric,acoustic resonance,light scattering, andfiltration. The gravimetric method was found to be precise and within the accuracy of the analytical methods. However, the method was time consuming. The acoustic resonance technique (ART) was fast and less subjective, but it did not define the lower asphaltene boundary. The interpretation of the onset pressure from the near-infrared (NIR) light-scattering technique (LST) was subjective to a degree. However, the NIR response defined the upper and lower boundaries of the asphaltene envelope and the bubblepoint pressure, as did the gravimetric technique. In a way similar to those of the gravimetric technique and LST, the filtration technique can also define the upper and lower asphaltene phase boundaries in addition to the bubblepoint pressure. The filtration technique is fast compared to gravimetric technique, but takes more time than the ART and LST methods. Introduction Asphaltenes remain in solution under reservoir temperature and pressure conditions. They start to precipitate when the stability of the colloidal dispersion is disturbed. This disturbance can be caused by changes in pressure, temperature, and/or composition of the oil. Precipitation and deposition of asphaltenes have reportedly caused operational problems ranging from plugging of tubulars and flowlines1–3 to clogging of production separators.4 Leontaritis and Mansoori present a comprehensive description of field problems caused by asphaltene deposition.5Fig. 1 schematically presents the asphaltene-related problems that may occur in the field. Asphaltene precipitation problems can be categorized as follows:Precipitation can be caused by the changes in temperature and/or pressure during primary depletion.Precipitation can be caused by blending or commingling of two noncompatible reservoir fluid streams (i.e., subsea completions), acid stimulation and/or enhanced recovery injection gases (CO2, H2S or rich gas). The correct operating procedure to minimize the asphaltene problem is not well understood. We believe a better understanding of the fundamental processes leading to solids precipitation is a prerequisite to management and prevention of production problems.

The near-infrared (NIR) light scattering technique, commercially known assolids detection system (SDS) is widely used to determine the asphalteneprecipitation pressure in reservoir fluids. In this study the SDS is used tocompare samples collected from the same zone in the same well at the same timebut in different sample chambers. The SDS technique is further compared withhigh pressure filtration for the same fluid studied by SDS In addition, asphaltene precipitation as a function of pressure was determined. The results of this study indicate that the SDS test provides a quick toolto determine the asphatene precipitation pressure but has limited sensitivity.Furthermore, the bulk of asphaltenes tended to precipitate within a narrowpressure range near the precipitation onset pressure for the fluid sampletested in this study. Introduction Asphaltene precipitation and deposition from reservoir fluids due topressure depletion can be a serious flow assurance problem. This hassignificant impact on the development of deep-water reservoirs due to enormouscost associated with inhibition and remediation of asphaltenes. Remediationcost increases with water depth for offshore fields (MMS/DeepStar Workshop onProduced Fluids, Offshore Technology Conference, Houston, TX; May 4, 1995):Deepwater projects may spend $5–10MM for asphalteneremediation/control.Each well intervention incident may cost $0.5–1MM neglecting lostproduction, and each flow line mechanical remediation may cost about$25MM. Therefore, the understanding of asphaltene precipitation and depositionpotential from reservoir fluids is an important consideration in the designphase of the development of remote / deep-water reservoirs. Deposition of asphaltenes can occur in the tubing, flow lines and surfacefacilities leading to operational problems and loss in production. For the Gulfof Mexico (GoM) fluids the asphaltene instability increases with decreasingtemperature. Thus, the asphaltene deposition (precipitation) in the long subseatiebacks, tubing and surface facilities will increase with a reduction in fluidtemperature. Though there is considerable discussion of the reversibility ofashphaltene precipitation, the asphaltenes precipitated from GoM reservoirfluids are proven to be reversible (at least partially). A detailed review ofthe literature on asphaltene related production problems has been published byKokal et al.