John P. Crawshaw

Spontaneous Imbibition in Nanopores of Different Roughness and Wettability

The spontaneous imbibition of liquid in nanopores of different roughness is investigated using coarse grain molecular dynamics (MD) simulation. The numerical model is presented and the simplifying assumptions are discussed in detail. The molecular-kinetic theory introduced by Blake is used to describe the effect of dynamic contact angle on fluid imbibition. The capillary roughness is modeled using a random distribution of coarse grained particles forming the wall. The Lucas-Washburn equation is used as a reference for analyzing the imbibition curves obtained by simulation. Due to the statistical nature of MD processing, a comprehensive approach was made to average and smooth the data to accurately define a contact angle. The results are discussed in terms of effective hydrodynamic and static capillary radii and their difference as a function of roughness and wettability.

Preventing buoyancy-driven flows of two Bingham fluids in a closed pipe - Fluid rheology design for oilfield plug cementing

The mechanically unstable situation of a heavy Bingham fluid resting on top of a light Bingham fluid in an inclined closed-ended pipe can be stabilised if the fluids have sufficiently large yield stresses. This paper focuses on determining the yield stresses that are sufficient to keep the fluids statically stable for a given fluid density difference, pipe diameter and pipe inclination. The results are applicable to a broad class of practically observable flows. This situation provides an idealised model for the oilfield process of plug cementing.

Three-dimensional imaging of porous media using confocal laser scanning microscopy

In the last decade, imaging techniques capable of reconstructing three‐dimensional (3‐D) pore‐scale model have played a pivotal role in the study of fluid flow through complex porous media. In this study, we present advances in the application of confocal laser scanning microscopy (CLSM) to image, reconstruct and characterize complex porous geological materials with hydrocarbon reservoir and CO2 storage potential. CLSM has a unique capability of producing 3‐D thin optical sections of a material, with a wide field of view and submicron resolution in the lateral and axial planes. However, CLSM is limited in the depth (z‐dimension) that can be imaged in porous materials. In this study, we introduce a ‘grind and slice’ technique to overcome this limitation. We discuss the practical and technical aspects of the confocal imaging technique with application to complex rock samples including Mt. Gambier and Ketton carbonates. We then describe the complete workflow of image processing to filtering and segmenting the raw 3‐D confocal volumetric data into pores and grains. Finally, we use the resulting 3‐D pore‐scale binarized confocal data obtained to quantitatively determine petrophysical pore‐scale properties such as total porosity, macro‐ and microporosity and single‐phase permeability using lattice Boltzmann (LB) simulations, validated by experiments.

Advanced Reservoir Characterization for CO2 Storage

Abstract Injection of CO2 into the subsurface is of interest for CO2 storage and enhanced oil recovery (EOR). There are, however, major unresolved questions around the multiphase flow physics and reactive processes that will take place after CO2 is injected, particularly in carbonate rock reservoirs. For example, the wetting properties of CO2-brine-rock systems will impact the efficiency of EOR operations and CO2 storage but reported contact angles range widely from strongly water-wet to intermediate wet. Similar uncertainties exist for properties including the relative permeability and the impact of chemical reaction on flow. In this presentation we present initial results from laboratory studies investigating the physics of multiphase flow and reactive transport for CO2-brine systems. We use traditional and novel core flooding techniques and x-ray imaging to resolve uncertainties around the CO2-brine contact angle, relative permeability, residual trapping, and feedbacks between chemical reaction and flow in carbonate rocks.

Preparation of microporous rock samples for confocal laser scanning microscopy

In this paper we describe an improved sample-preparation technique for applying confocal laser scanning microscopy to image the void space of porous geological media, particularly various kinds of carbonate rocks with significant microporosity. We have improved the existing sample-preparation technique for confocal imaging by introducing a positive-pressure application step. This additional step helps to force the fluorescent-doped epoxy mixture inside the submicron pores (the microporosity) which make up a significant fraction of the total porosity of the carbonate rocks being characterized using confocal laser scanning microscopy. We also provide additional technical details and discuss practical aspects important to consider when imaging carbonate rock samples using this technique.

Influence of system size and solvent flow on the distribution of wormlike micelles in a contraction-expansion geometry

Abstract.Viscoelastic wormlike micelles are formed by surfactants assembling into elongated cylindrical structures. These structures respond to flow by aligning, breaking and reforming. Their response to the complex flow fields encountered in porous media is particularly rich. Here we use a realistic mesoscopic Brownian Dynamics model to investigate the flow of a viscoelastic surfactant (VES) fluid through individual pores idealized as a step expansion-contraction of size around one micron. In a previous study, we assumed the flow field to be Newtonian. Here we extend the work to include the non-Newtonian flow field previously obtained by experiment. The size of the simulations is also increased so that the pore is much larger than the radius of gyration of the micelles. For the non-Newtonian flow field at the higher flow rates in relatively large pores, the density of the micelles becomes markedly non-uniform. In this case, we find that the density in the large, slowly moving entry corner regions is substantially increased.

Gas Channelling and Heat Transfer in Moving Beds of Spherical Particles

Gas-to-solid heat transfer coefficients measured in moving beds of solid particles are an order of magnitude smaller than those predicted by conventional correlations which work well for fixed beds. The authors have elsewhere proposed a cause: gas flow in moving beds involves gross channelling which falsifies the plug-flow heat transfer analysis which yields the coefficients. In the present work, end effects have been removed from measured gas RTDs using a Fourier Transform technique applied to data from beds of different lengths. This refinement of the data analysis reveals gas RTDs for moving beds to be characteristically bimodal, conclusively demonstrating the predicted channelling. Radial profiles of axial gas velocities have also been measured, using the time-of-flight of tracer gas pulses injected and sampled at known radial positions in the packing. Two distinct channels are identified: an annular zone near the bed wall of thickness six to ten particle diameters, and a cylindrical zone in the bed centre where the gas velocity is higher, by a factor of some 1.14–1.25, than that in the annulus. A simple model of flow and heat transfer is used to show that the degree of channelling observed is of the correct order of magnitude to account for the erroneous heat transfer coefficients reported.

Micromodel observations of evaporative drying and salt deposition in porous media

Most evaporation experiments using artificial porous media have focused on single capillaries or sand packs. We have carried out, for the first time, evaporation studies on a 2.5D micromodel based on a thin section of a sucrosic dolomite rock. This allowed direct visual observation of pore-scale processes in a network of pores. NaCl solutions from 0 wt. % (de-ionized water) to 36 wt. % (saturated brine) were evaporated by passing dry air through a channel in front of the micromodel matrix. For de-ionized water, we observed the three classical periods of evaporation: the constant rate period (CRP) in which liquid remains connected to the matrix surface, the falling rate period, and the receding front period, in which the capillary connection is broken and water transport becomes dominated by vapour diffusion. However, when brine was dried in the micromodel, we observed that the length of the CRP decreased with increasing brine concentration and became almost non-existent for the saturated brine. In the exp...

Kinetics of carbonate mineral dissolution in CO2-acidified brines at storage reservoir conditions.

We report experimental measurements of the dissolution rate of several carbonate minerals in CO2-saturated water or brine at temperatures between 323 K and 373 K and at pressures up to 15 MPa. The dissolution kinetics of pure calcite were studied in CO2-saturated NaCl brines with molalities of up to 5 mol kg-1. The results of these experiments were found to depend only weakly on the brine molality and to conform reasonably well with a kinetic model involving two parallel first-order reactions: one involving reactions with protons and the other involving reaction with carbonic acid. The dissolution rates of dolomite and magnesite were studied in both aqueous HCl solution and in CO2-saturated water. For these minerals, the dissolution rates could be explained by a simpler kinetic model involving only direct reaction between protons and the mineral surface. Finally, the rates of dissolution of two carbonate-reservoir analogue minerals (Ketton limestone and North-Sea chalk) in CO2-saturated water were found to follow the same kinetics as found for pure calcite. Vertical scanning interferometry was used to study the surface morphology of unreacted and reacted samples. The results of the present study may find application in reactive-flow simulations of CO2-injection into carbonate-mineral saline aquifers.

Ultrafiltration and nanofiltration membrane fouling by natural organic matter: Mechanisms and mitigation by pre-ozonation and pH

The fouling of ultrafiltration (UF) and nanofiltration (NF) membranes during the treatment of surface waters continues to be of concern and the particular role of natural organic matter (NOM) requires further investigation. In this study the effect of pH and surface charge on membrane fouling during the treatment of samples of a representative surface water (Hyde Park recreational lake) were evaluated, together with the impact of pre-ozonation. While biopolymers in the surface water could be removed by the UF membrane, smaller molecular weight (MW) fractions of NOM were poorly removed, confirming the importance of membrane pore size. For NF membranes the removal of smaller MW fractions (800 Da-10 kDa) was less than expected from their pore size; however, nearly all of the hydrophobic, humic-type substances could be removed by the hydrophilic NF membranes for all MW distributions (greater than 90%). The results indicated the importance of the charge and hydrophilic nature of the NOM. Thus, the hydrophilic NF membrane could remove the hydrophobic organic matter, but not the hydrophilic substances. Increasing charge effects (more negative zeta potentials) with increasing solution pH were found to enhance organics removal and reduce fouling (flux decline), most likely through greater membrane surface repulsion. Pre-ozonation of the surface water increased the hydrophilic fraction and anionic charge of NOM and altered their size distributions. This resulted in a decreased fouling (less flux decline) for the UF and smaller pore NF, but a slight increase in fouling for the larger pore NF. The differences in the NF behavior are believed to relate to the relative sizes of ozonated organic fractions and the NF pores; a similar size of ozonated organic fractions and the NF pores causes significant membrane fouling.