Mesoscale simulation of the flow of complex oilfield fluids:

wormlike micelles and asphaltene colloidal suspensions

Edo Boek ^(1,2) and Johan Padding ^(1,2,3)
(1) Schlumberger Cambridge Research, UK
⁽²⁾ University of Cambridge, UK
⁽³⁾ University of Twente, NL

We have developed mesoscopic simulation models to calculate the flow of complex oilfield fluids. These include 1) the rheology of wormlike micellar fluids and 2) the aggregation and deposition of asphaltene colloidal suspensions in capillary flow. First, we develop a multi-scale simulation model for the rheology of wormlike micelles, This model can be used to predict the shear rheology and normal stress differences, particularly at high shear rates [1]. Such a model gives valuable insight, particularly because the available rheological equations of state [2] are incomplete with respect to extensional rheology [3] and predict the wrong behaviour for normal stress differences. The aim of the model is to understand the link between mechanical properties of the wormlike micelle on the one hand, and the dynamics and rheology of a solution of entangled wormlike micelles on the other hand. A mesoscopic approach is necessary because it is impossible to reach large enough length and time scales using atomistic simulation methods. The simulation method is based on Brownian Dynamics (BD) of coarse-grained pieces of wormlike micelle. To guarantee the predictive power of the BD simulations, the mesoscopic interaction parameters are derived as much as possible from the results of Molecular Dynamics (MD) simulations. We calculate the mechanical properties of a small segment of a wormlike micelle, including the persistence length and elastic modulus, from MD simulations. Then we use these properties in the mesoscopic BD simulation, where the smallest unit is the persistence length. In the BD simulation, the ends of wormlike micelles can approach each other and, if a certain activation barrier is overcome, fuse together. The specific kinetics with which this happens can be investigated for different scission energies, activation barriers and environment parameters such as temperature, concentration and deformation rates. Entanglements are very important for the rheology of a solution of concentrated wormlike micelles. Entanglements emerge naturally whenever a wormlike micelle tries to cross another. By means of the TWENTANGLEMENT algorithm, originally developed for polymer melt simulations, these crossings are monitored and entanglement points are inserted whenever a crossing is imminent. Preliminary results are very encouraging. For example, the predicted shear thinning of EHAC surfactants is in good agreement with experimental results. Second, we study the aggregation and deposition of asphaltene colloids in capillary flow. The deposition of asphaltene solids in oil pipe lines and reservoir rock is poorly understood and hard to study experimentally. Therefore we have developed a mesoscopic simulation model, using the Stochastic Rotation Dynamics techniques. We study the aggregation and deposition of asphaltene colloids in capillary flow, as a function of flow rate and interaction potential between colloid and the walls. We find that the colloidal aggregates and deposits break up with increasing flow rate. Also we observe an decrease of the permeability with increasing depth of the interaction potential well.

[1] J.T. Padding, E.S. Boek and W.J. Briels, ``Rheology of wormlike micellar fluids from Brownian and Molecular Dynamics simulations'', J. Phys.: Condens. Matter 17, S3347S3353 (2005).

[2] O.Manero, F.Bautista, J.F.A.Soltero and J.E.Puig, J. Non Newton. Fluid Mech. 106,1-15(2002).[3] E.S. Boek, J.T. Padding, V. Anderson, P. Tardy, J. Crawshaw and J.R.A. Pearson,``Constitutive Equations for Extensional flow of wormlike micelles: Stability analysis of the Bautista-Manero model'', J. Non-Newtonian Fluid Mech. 126, 39-46 (2005).