59th Annual Report on Research 2014

Enhanced oil recovery techniques are becoming and increasingly important component of the petroleum production portfolio, particularly from declining reservoirs. Waterflooding has been practiced for almost a century, and now accounts for over one half of global oil production, yet much is unknown about the detailed mechanisms responsible for waterflood efficiency. This is particularly true for situation in which low salinity brines are used, with mixed and unpredictable success. Despite its vast potential, the underlying physical mechanisms of low salinity waterflooding remain unclear. To address this gap, we have developed microfluidic tools that more accurately simulate the morphology and chemistry of reservoir rock than traditional ÒmicromodelsÓ. We have introduced new microfluidic fabrication techniques to produce these tools, which have been utilized to investigate hypotheses regarding interfacial and transport phenomena of low salinity waterflooding. We have hypothesized that the ionic strength of the brine used in low salinity floods can mediate the interfacial elasticity of the oil, this exacerbating or suppressing the dispersion of continuous oil phases into emulsions. The goals of this project have therefore been twofold: 1) To develop novel fabrication techniques for the creation of micromodels with arbitrary cross sections. This capability would allow the production of micromodels that are more accurately representative of the confining geometry of reservoir rock. 2) Use these new platforms to isolate individual experimental phenomena, allowing investigation of single-variable effects in otherwise highly complex multiphase systems. These goals build upon one another to develop platforms that can be used to deconstruct the phenomena that contribute to highly unpredictable multi-scale behavior. The results of the first year of our project are discussed below.

Goal 1: Fabrication Technique Development

Varying the cross-sectional geometry of microfluidic channels can dramatically alter fluid flow behavior, but most fabrication techniques are planar and therefore incapable of easily generating variations in depth. We have therefore developed femtosecond pulsed laser ablation-based fabrication to create microchannels with highly controlled triangular cross sectional geometry. Triangular cross sections are of particular interest because they more accurately represent the pore-throat occurring in sandstones and other sedimentary reservoirs. We have demonstrate that femtosecond pulsed laser ablation is an effective tool to reliably produce channels with triangular cross sections, which can be subsequently replicated to produce elastomeric microfluidic devices. Alternately, we have also developed a novel apparatus for the rapid thermal bonding of glass substrates for the creation of all-glass microfluidic devices. Our apparatus was designed to bong glass autonomously, in a small, self-contained, affordable instrument. Figure 1 displays a variety of triangular channels that were machined in glass and subsequently replicated to form microfluidic networks

Figure 1. Images of the triangular cross sections (A) images of the soft lithography intermediates and (B) images of soft lithography replicas. Reported channel dimensions were measured from soft lithography intermediate via optical profilometry.

Goal 2: Low Salinity Waterflooding Mechanisms

The channels shown in Figure 1 were used as an experimental platform in which to observe two-phase flow and evaluate the capillary pressures required to initiate flow within triangular capillaries. Applying existing Mayer, Stowe and Princen (MS-P) theory, critical Òbreak throughÓ capillary pressures were predicted for varying cross sections and compared to experimental observations. Results indicate excellent correlation between the produced channels capillary pressures calculated by the MS-P method. The fabrication techniques and validation of predictive frameworks presented here provide a powerful approach to microfluidic experimental design for experimental multiphase flow.

We have subsequently designed microscale hydrodynamic focusing networks to evaluate the effects of brine salinity upon emulsion drop "snap off". The geometry of these networks are precisely defined and therefore the interfacial forces are controlled and predictable. As shown in Figure 2, oil being drawn through a constriction at the channel junction is subjected to shear forces, which are balanced by surface tension and interfacial elasticity. The interfacial elasticity can be isolated and compared for brines of different salinity. Figure 2 also clearly shows the stark difference in oil snap off behavior when aged in high and low salinity brines. Other than the ionic strength of the brine, the oil, device geometry and flow conditions are identical. This experiment highlights the unconventional nature of the device platform that we have developed: instead of using micromodels that recapitulate the geometric properties and therefore complexity of reservoirs, we have introduced controlled geometries that isolate individual phenomena, allowing the problem of multiphase flow to be simplified.

Our flow focusing platform will next be applied to study the influence of different brine types (beyond concentration) in order to illuminate the molecular mechanisms of interfacial elasticity evolution. We will also utilize our micromachining technology to create pore-throat networks that isolate the effects of geometry on snap off during pore filling and drainage. These networks will be of varying depth and channel cross section, allowing for a rigorous comparison of geometric parameters, instead of settling for a fabrication-limited approximation. Figure 2 A series of photographs illustrating the dynamic differences in the evolution of a snap-off event for a given crude oil after aging in low salinity brine (1%) and high salinity brine (100%). The final frame in the figure represents the Òcritical frameÓ or the frame immediately preceding the formation of an oil droplet. The apparent dynamic differences that result after aging the crude oil in low salinity brine are evidence of the formation of an interfacial film with dramatic elasticity differences. Crude oil aged in low salinity brine therefore develops the ability to suppress the snap-off event.