New Methods Analyze Asphaltene Deposition and Fouling in Reservoirs

Fig. 1—(a) Deposition profile from micromodel experiment. (b) Deposition profile from simulation.

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Asphaltene precipitation and deposition in production tubing and surface facilities are well-documented issues, and different methods are available to manage them. However, the problems that asphaltenes may cause in the reservoir, especially in the near-wellbore region, are much less understood. This project aimed to develop experimental procedures and modeling methods to establish whether impairment caused by asphaltene deposition in reservoirs is a real problem and to develop an understanding of the mechanisms by which asphaltene precipitates, alters wettability, and obstructs flow by potentially depositing in the formation.

Introduction

Asphaltenes are a polydisperse mixture of the heaviest and most polarizable fractions of crude oil. Understanding the asphaltene-deposition problem and the factors affecting it is of great importance to the oil industry because of the costs associated with production loss and remediation activities, such as solvent wash and removal of deposited asphaltenes.

Several modeling approaches in the literature are used to model asphaltene deposition in porous media. Different mechanisms, such as adsorption, surface deposition, entrainment of deposits, pore-throat plugging, and pore-throat opening, have been considered in these approaches. However, which mechanisms play the most significant role and which mechanisms can be neglected in modeling asphaltene deposition inside porous reservoir rocks remains unclear.

In this work, a series of tools has been developed and used to deepen the understanding of the mechanisms by which asphaltenes can precipitate and deposit in the reservoir. A new technique based on an automated chromatographic method was implemented to separate and quantify the content of saturates, aromatics, resins, and asphaltenes (SARA) in two crude-oil samples. Microfluidic devices were used to visualize the formation of asphaltene deposits in the porous media under various experimental conditions. Also, a quick and inexpensive novel technique to cast core holders using a special epoxy resin was developed to accelerate the execution of coreflood experiments. Finally, a simulation tool based on the lattice Boltzmann method (LBM) was developed for modeling asphaltene deposition in porous media.

Crude-Oil Characterization

Because of the complex nature of crude oils, performing a detailed compositional analysis for every substance in the oil is not readily achievable. Instead, a ­hydrocarbon-group analysis is preferred. SARA separation is one of the approaches for the hydrocarbon-group analysis. It separates the crude-oil components, on the basis of their solubility and polarity, into four parts: saturates, aromatics, resins, and asphaltenes.

In this work, a new method of SARA analysis is proposed to overcome some of the limitations of previous techniques. After removing the asphaltene fraction of the oil sample by the conventional method, the remaining mixture (maltene) is analyzed using the proposed method, which is called the automated chromatographic-columns system for maltenes analysis.

The new maltene-analysis method uses two chromatographic columns, one filled with clay and the other with silica gel. Columns are connected in series, and the effluent is recirculated using a high-­performance liquid-­chromatography pump. On the basis of polarity and polarizability, the resins are selectively adsorbed onto the clay in the first column, while the aromatics are adsorbed onto the silica gel in the second column. The effluents of the two columns contain the saturate fraction. The efficiency of the process is verified by ultraviolet spectroscopy. The next step after the adsorption process is desorption. Desorption of resins and aromatics is performed simultaneously in two separate processes. The columns are washed separately with a relatively polar solvent (e.g., dichloromethane or a mixture of dichloromethane and hexane). After that, the solvents are recovered in a distillation column and the residues are collected. After removing the excess solvent, resins and aromatics fractions are independently dried and weighed.

Coreflooding Experiments

In this work, a new technology has been developed for preparing the core holder. The decision to develop this technology was made because of several limitations of conventional metallic core holders. First, metallic core holders are costly to make, especially those with internal pressure taps, which could cost more than USD 20,000. Having the internal pressure taps is necessary to study the flow behavior in porous media, and it is even more important in the case of studying asphaltene deposition. Making a core holder with internal pressure taps using the new technology developed in this work, however, is easy, quick, and inexpensive. Each core holder costs less than USD 200; therefore, several core holders can be prepared and several experiments can be performed at the same time.

The main idea of this new technology is to mix the proper resin epoxy with its hardener and pour the mixture around the core plug except at the two faces on the sides, which will be used as the inlet and outlet ports for the injection fluid. The epoxy will form a powerful bond with the core plug and will withstand high pressures and high temperatures.

Coreflooding-Test Results

Two crude-oil samples, A and C; two rocks of different permeability; two different flow rates; and two asphaltene precipitants were chosen for the coreflood tests in this work. Two asphaltene precipitants, n-pentane and n-heptane, were used for the coreflood experiments to understand the tendency of different asphaltene fractions to deposit in porous media and to cause formation damage.

For all but two of the experiments, no significant pressure drop was seen across the core plug during the asphaltene-deposition test.

The two experiments that showed significant pressure drop were conducted using Crude Oil A mixed with n-heptane and injected into a low-permeability core plug. The conclusion of this observation is that the asphaltenes precipitated by n-heptane (n-C7+ asphaltenes), which are more rigid and solid-like compared with the asphaltenes precipitated by n‑pentane (n-C5+ asphaltenes), are probably more prone to deposit and plug the pore space in the porous media, particularly for low-permeability cores. Once n-C7+ asphaltenes deposit, they cannot be removed easily by shear forces. Unlike the n-C7+ asphaltenes, n-C5+ asphaltenes contain a large amount of ­liquid-like asphaltenes and, therefore, are lighter and less rigid compared with the n-C7+ asphaltenes. As a result, n-C5+ asphaltenes are less prone to deposit in the porous media and those that deposit in the core plug might be removed easily by the fluid flow.

Microfluidic Experiments

In this work, a solvent-resistance microfluidic device that mimics the flow through porous media has been developed and tested successfully to study asphaltene deposition at dynamic conditions. This microfluidic setup provides a unique way to study the mechanism of asphaltene deposition and the effects of different variables, such as precipitation driving force and shear rate, on the deposition process. With the in-house microfluidic setup, the process of asphaltene deposition is readily visualized and analyzed at microscale. This will contribute to the prediction of risk factor and correct assessment of rate of formation damage caused by asphaltene deposition in porous media in the near-wellbore region. Efficient treatment methods and frequency of treatments will then be suggested to project development and execution as needed.

Observations of asphaltene deposition in porous media under dynamic conditions are reported from data obtained in the microfluidic experiments. The obtained results indicate that deposition of asphaltene in porous media varies mainly with respect to changes in the aggregation process and in the affinity of asphaltenes to the surface and to themselves. The strong self-affinity between asphaltenes tends to make a major contribution to the deposition. The small and large asphaltene aggregates result in different mechanisms of deposition and formation damage in porous media.

Computational-Fluid-Dynamics Simulation

In this work, the Navier-Stokes equations, a continuity equation, a mass-transfer equation, and an ordinary-­differential equation for the amount of deposited asphaltene were combined to simulate the process of asphaltene deposition. Two mechanisms of surface deposition and particle entrainment were considered in the asphaltene-deposition model. The pore-throat-plugging mechanism was not included because it does not seem to have a significant effect on asphaltene deposition, according to the results obtained from the microfluidic and coreflood experiments.

The model proposed in this work was validated against the experimental data obtained from the microfluidic experiments.

Fig. 1a (above) is a picture of the asphaltene deposition in the micromodel. The fluid flows from left to right. White circles are the obstacles in the microchannel, and the dark areas represent the deposited asphaltenes. Asphaltene deposition is in the form of conical shapes upstream of the different obstacles. The LBM result for the shape of deposited asphaltenes is shown in Fig. 1b. The color scale represents the fraction of deposited asphaltene, which ranges from red for the highest amount to blue where no deposition occurs.

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 27933, “Asphaltene Deposition and Fouling in Reservoirs,” by Mohammad Tavakkoli, Peng He, Pei-Hsuan Lin, Sara Rezaee, Maura Puerto, Rocio Doherty, Jefferson Creek, SPE, Walter Chapman, Sibani Lisa Biswal, and Francisco M. Vargas, Rice University; Jianxin Wang, SPE, and Greg Kusinski, SPE, Chevron; and Joseph Gomes, Deepstar, prepared for the 2017 Offshore Technology Conference, Houston, 1–4 May. The paper has not been peer reviewed. Copyright 2017 Offshore Technology Conference. Reproduced by permission.

New Methods Analyze Asphaltene Deposition and Fouling in Reservoirs

01 September 2017

Volume: 69 | Issue: 9

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