https://backend.orbit.dtu.dk/ws/files/176654859/MARAC_1_s2.0_S0920410519303869_main.pdf

Mechanisms of smart waterflooding in carbonate oil reservoirs - A review

We present rock mechanical test results and analytical calculations which demonstrate that a negative surface charge, resulting from sulfate adsorption from the pore water, impacts the rock mechanical behavior of high-porosity chalk. Na2SO4 brine flooded into chalk cores at 130 °C results in significantly reduced bulk modulus and yield point compared with that of NaCl brine at the same conditions. The experimental results have been interpreted using a surface complexation model combined with the Gouy-Chapman theory to describe the double layer. The calculated sulfate adsorption agrees well with the measured data. A sulfate adsorption of about 0.3 μmol/m2 and 0.7–1 μmol/m2 was measured at 50 and 130 °C, respectively. Relative to a total sites of 5 sites/nm2 these values correspond to an occupation of 4 % and 8–13 % which sufficiently explains the negative charging of the calcite surfaces. The interaction between charged surfaces specifically in the weak overlaps of electrical double layer gives rise to the total disjoining pressure in granular contacts. The net repulsive forces act as normal forces in the grains vicinity, counteracting the cohesive forces and enhance pore collapse failure during isotropic loading, which we argue to account for the reduced yield and bulk modulus of chalk cores. The effect of disjoining pressure is also assessed at different sulfate concentrations in aqueous solution, temperatures, as well as ionic strength of solution; all together remarkably reproduce similar trends as observed in the mechanical properties.

The Impact of Surface Charge on the Mechanical Behavior of High-Porosity Chalk

This volume highlights key challenges for fluid-flow prediction in carbonate reservoirs, the approaches currently employed to address these challenges and developments in fundamental science and technology. The papers span methods and case studies that highlight workflows and emerging technologies in the fields of geology, geophysics, petrophysics, reservoir modelling and computer science. Topics include: detailed pore-scale studies that explore fundamental processes and applications of imaging and flow modelling at the pore scale; case studies of diagenetic processes with complementary perspectives from reactive transport modelling; novel methods for rock typing; petrophysical studies that investigate the impact of diagenesis and fault-rock properties on acoustic signatures; mechanical modelling and seismic imaging of faults in carbonate rocks; modelling geological influences on seismic anisotropy; novel approaches to geological modelling; methods to represent key geological details in reservoir simulations and advances in computer visualization, analytics and interactions for geoscience and engineering.

Fundamental Controls on Fluid Flow in Carbonates: Current Workflows to Emerging Technologies

The primary aim of this study was to investigate the “dilution effect”, where dilution of the ionic concentration of the fluid injected into oil wells has been found to enhance oil recovery. We have measured crude oil/brine/carbonate surface (calcite) interactions using a variety of dynamic techniques including contact angles, surface forces apparatus, atomic force microscopy, interfacial tension, X-ray photoelectron spectroscopy, and other physical and chemical surface characterization techniques. The effects due to different brine (ionic electrolyte) solutions and temperatures, as well as the dynamics (time-dependence) of these effects, were investigated. Ionic strengths varied from pure water to 350 000 ppm, and temperatures varied from 20 to 75 °C. We found that upon exchanging solutions (as occurs for waterflooding using dilute solutions), three different dynamic processes occur that have very different time scales: (1) the initial, rapid (seconds to minutes) physical ion exchange with the surfaces t...

Effects of Salinity on Oil Recovery (the “Dilution Effect”): Experimental and Theoretical Studies of Crude Oil/Brine/Carbonate Surface Restructuring and Associated Physicochemical Interactions

A geochemical model for interpretation of chalk core flooding experiments

Effect of the aqueous chemistry on the mechanical strength of chalk has extensively been studied during the last decade. At high temperatures (~130°C), chalk exposed to seawater is significantly weaker compared to chalk exposed to distilled water when considering the hydrostatic yield strength and the following creep phase. The explanation of these experimental results must be of a chemical nature, as the density and viscosity of the aqueous phase vary little among these different brines. We present the results from simplified aqueous chemistry using MgCl2 brines, and compare these results with seawater. Previous studies show that different ions, e.g. Ca2+, Mg2+, $${{\rm SO}{_{4}}^{2-}}$$
 in the injected brine, as well as the chalk mineralogy have an impact on the stability of the rock. We performed mechanical tests on chalk cores from Liege and Stevns Klint; it was found that these two outcrop chalks exhibit an unexpected difference in their mechanical responses when comparing cores flooded with NaCl and MgCl2 at 130°C. The results of this study show that the effects of magnesium seem to be governed not only by the differences in mineralogy, but also a time dependency on chalk deformation is additionally observed. Independent of the chalk type tested, the chemical analyses performed show that when MgCl2 is flooded through the core, a significant loss of magnesium and a considerable additional amount of calcium are detected in the effluent. The experimental observations fit very well with the time-dependent chemical changes gained from the mathematical model of this study that accounts for transport effects (convection and molecular diffusion) as well as chemical processes such as precipitation/dissolution. Based on the calculations and chemical analyses, we argue that the loss of magnesium and the production of calcium cannot solely be a consequence of a substitution process. The calculations rather indicate that magnesium is precipitated forming new mineral phases and in this process not only calcite, but also silicates are dissolved. The amount of dissolved calcium and silicon from the rock matrix is significant and could thus cause an additional deformation to take place. Both the retention of magnesium in the chalk core and the formation of newly precipitated magnesium-bearing carbonates and/or magnesium-bearing clay-like minerals after flooding with MgCl2 brine were demonstrated using scanning electron microscopic methods. In addition, precipitation of anhydrite as a result of flooding with seawater-like brine was proven. The water-induced strain not only depends on the ion composition of the injected brine; moreover, the presence of non-carbonate minerals will most likely also have a significant influence on the mechanical behaviour of chalk.

Chemical Alterations Induced by Rock–Fluid Interactions When Injecting Brines in High Porosity Chalks

A laboratory test program, which simulated reservoir conditions of pressure and temperature, was conducted on outcrop and reservoir chalk samples of various porosities. All the samples experienced a stress path following uniaxial strain condition K 0 that led to compaction failure, i.e. pore collapse. The experiments were loaded by depletion of pore pressure conducted under load controlled conditions. This depletion phase was followed by a creep period, where time-dependent deformation was monitored. The intention of creating such reservoir condition in these laboratory experiments was to gain knowledge of the nature of chalk compaction. Chalk is an important reservoir rock for the oil and gas industry with unique storage capability with porosities up toward 50%. However, this rock is also very weak which has resulted in significant reservoir compaction and in turn severe seabed subsidence and casing failure. Mapping of the mechanical behavior of chalk in terms of deformation is thus decisive for a proper understanding of these reservoirs. The results of this study show that chalk is indeed a rate-dependent material under laboratory loading conditions as time effects were revealed as the loading rate was varied. However, the results raise uncertainty about the importance of rate dependency for chalk under completely drained conditions. Further, such high-porosity chalk suffers for substantial plastic strains and obvious strain hardening. Indeed, a relation between deformation/porosity and hardening is proposed by the introduction of real-time modulus values. Time-dependent deformation, also called creep was influenced by the depletion phase, as consolidation or transient creep influenced the deformation response for as much as 175 h after a change in load. This indicates that transient creep is dependent on the stress history. However, observations suggest the existence of a universal mechanism for steady state creep, governed by neither the initial porosity nor the stress history or chalk type, which thus seems to be an independent strain contributor. Finally, time dependence is found on the K 0 development for chalk tested at typically laboratory rates, which has been discussed as a reflection of the nature of the grain re-arrangement during failure and plastic deformation. Ultimately, such time dependence of the K 0 may contribute to the understanding of stress path data deduced from field data.

Deformation Behavior of Chalk Studied Close to In Situ Reservoir Conditions

Experimental Investigation of the Effective Stress Coefficient For Various High Porosity Outcrop Chalks

Induced Geometry in Chalk during Hydrochloric Acid Stimulation


 The drilling of extended-reach wells is an increasingly common practice for improving the recovery from mature fields, and for producing distant oil and gas resources using existing infrastructure. From a geomechanical point of view, the drilling and completion of extended-reach wells may become technically very challenging, as these wells often have long sections drilled at high inclinations that can be prone to borehole instability problems such as pack-offs or collapse of the wellbore wall.
 The case of an extended-reach well drilled in the Ekofisk field in the North Sea where borehole stability issues were observed and eventually resulted in the loss of the well is presented. A wellbore stability assessment is performed with well-specific stress and formation strength data that explores the possible failures that may have resulted in the loss of the well. In particular, a plane of weakness model is used to model possible shear failure along the bedding of the overburden shale formations. The uncertainty in the rock matrix strength is accounted for, as well as the cohesion, friction factor and orientation of the bedding plane, on the mud window using a Monte Carlo approach. This paper focuses in particular on how the properties of the bedding plane affect the minimum required mud weight, and compare to the actual mud weight used in operation.
 The generated mud window acknowledging failure along the weakness planes suggests that this type of failure was a relevant failure mechanism over the 13 1/2-in section of the well, as the mud weight employed was not high enough to avoid it. Accounting for uncertainty and the failure along the weakness planes in extended-reach wells to be drilled in the Ekofisk area may generate safer mud windows that in turn may reduce the occurrence of wellbore instability in the field.

E. Omdal

Papers

Chemical Alterations Induced by Rock–Fluid Interactions When Injecting Brines in High Porosity Chalks

Deformation Behavior of Chalk Studied Close to In Situ Reservoir Conditions

Experimental Investigation of the Effective Stress Coefficient For Various High Porosity Outcrop Chalks

Induced Geometry in Chalk during Hydrochloric Acid Stimulation

Rock Mechanics Analysis of Observed Borehole Instabilities in an Ekofisk Field Extended-Reach Well