Abstract

The aggregation and deposition of asphaltenic material in reservoir rock are significant problems in the oil industry and can adversely affect the producibility of a given reservoir. To obtain a fundamental understanding of this phenomenon, we have studied the deposition and aggregation of colloidal asphaltene in capillary flow by experiment and simulation. For the simulation, we have used the stochastic rotation dynamics (SRD) method, in which the solvent hydrodynamics emerge from the collisions between the solvent particles, while the Brownian motion emerges naturally from the interactions between the colloidal asphaltene particles and the solvent.

We compare our simulation results with flow experiments in glass capillaries where we use extracted asphaltenes only in toluene, re-precipitated with n-Heptane and also asphaltenes precipitated from the whole oil. In the experiments, the asphaltene precipitation and deposition dynamics were monitored in a slot capillary using optical microscopy under flow conditions similar to those used in the simulation.

Maintaining a constant flow rate of 5µL/min, we found that the pressure drop across the capillary first increased slowly, followed by a sharp increase, corresponding to a complete local blockage of the capillary. Subsequently the pressure fell sharply as asphaltenes were re-entrained. This condition was confirmed by the visual observations that showed the slow build-up of asphaltenes deposit followed by the sudden erosion of a channel through the deposit at the time when the pressure suddenly decreased.

We calculate the change in the dimensionless permeability as a function of time for both experiment and simulation. By matching the experimental and simulation results, we obtain information about 1) the interaction potential well depth for the particular asphaltenes used in the experiments, and 2) the flow conditions associated with the asphaltene deposition process. The data obtained will also be used as input parameters for a deep-bed filtration model.

Introduction

The deposition of asphaltenes may cause problems in flowlines, production facilities, and oil reservoirs near wellbores. It is the latter issue that we will be dealing with in this paper. Recently, a number of capillary flow experiments have been carried out to investigate asphaltene deposition. Assuming a uniform thickness of the layer deposited Broseta et al (2000) calculated an effective hydrodynamic thickness of a deposited asphaltene layer in flow experiments in a metal capillary. Wang et al. (2004) studied the deposition of asphaltene on metallic surfaces using the homogeneous deposition hypothesis.

In this paper, we develop a predictive model of asphaltene deposition under flowing conditions based on colloid dynamic simulations that are compared to microscopic experiments. We study asphaltene deposition in a glass capillary, which allows for direct visual observation of the deposition as a function of distance from the capillary entrance, in addition to the measurement of pressure drop as a function of time. We compare our experimental results with computer simulations using a novel modelling method called stochastic rotation dynamics (SRD). Experiment and simulation are compared directly by calculating the dimensionless conductivity of the capillary. The aim of our work is to contribute to a physically based model using colloidal interaction potentials to predict permeability damage in reservoir rock due to asphaltene deposition.

Experimental Procedure and Materials

The experimental setup comprised of a rectangular glass capillary, a camera-microscope, two constant flow-rate syringe pumps, and a data acquisition system as shown in Fig. 1.

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