What laboratory-induced dissolution trends tell us about naturaldiagenetic trends of carbonate rocks

TIZIANA VANORIO1*, YAEL EBERT1 & DENYS GROMBACHER2

1Stanford Rock Physics Laboratory, Mitchell Earth Sciences Building,

397 Panama Mall, Stanford University, CA 94305-2115, USA2Environmental Geophysics Laboratory, Mitchell Earth Sciences Building,

397 Panama Mall, Stanford University, CA 94305-2115, USA

*Corresponding author (e-mail: tvanorio@stanford.edu)

Abstract: The pivotal idea of this study is to unravel the processes that control heterogeneity in theattributes of the pore space in carbonate rocks (i.e. stiffness, connectivity and tortuosity), and, inturn, in the transport and elastic properties. We use starting rocks of variable fabric (i.e. a deposi-tional-dependent microstructure) to induce a specific process (e.g. chemical dissolution understress) and then observe the development of the microstructure, permeability, porosity and velocitydue to the induced chemomechanical processes.

We find that the changes in the two end members of the analysed rock types (mudstones andpackstones) can lead to two different evolutionary trends of permeability and velocity, dependingon the effectiveness of dissolution with respect to compaction. The balance between the twodepends on: (a) the fraction of the carbonate phases characterized by large surface area; and (b)the pore stiffness of the rock. Packstones are characterized by low pore stiffness and compact sig-nificantly upon dissolution. This behaviour leads to a decrease in velocity because of a reduction inthe stiffness at the grain contacts and a slight increase in permeability. The latter is curbed by theongoing compaction. Mudstones are characterized by higher pore stiffness, experiencing minimalor negligible compaction. This behaviour leads to a slight change in porosity and velocity.However, large permeability changes are observed, related to enhanced connectivity or decreasedtortuosity of the pathways.

Carbonate rocks are well known to be prone todiagenetic alterations as a result of chemical andphysical changes from pressure, temperature andthe presence of subsurface fluids. Compared tofluids generated by early burial processes (i.e. com-pacting shales and/or evaporite dehydration), meso-diagenetic fluids (Schmidt & McDonalds 1980)appear likely to cause local dissolution and pre-cipitation in carbonates. Organic–inorganic inter-actions in petroleum-producing sedimentary basinshave a direct involvement in rock post-diagenesisbecause many organic alteration products partici-pate in processes that create or destroy sedimentporosity and permeability (Seewald 2003). Spe-cifically, organic maturation and thermochemicalmineral transformation during mesodiagenesis cre-ate aggressive fluids charged with organic acid,carbon dioxide and hydrogen sulphide (Mazzullo& Harris 1992), which may either reduce porositythrough compaction and cementation, or enhancepre-existing pores or create new ones throughcement dissolution and pore pressure build-up(Mazzullo & Chilingarian 1992). The change ofsmall primary crystals into micrite (2 to 15 micronin diameter) implies some modification of themicrofabric, resulting in intercrystalline pores that

are 2–15 micron wide and porosities exceeding20% (Moshier 1989). There is also agreement inthe literature that leaching and/or dissolution ofcarbonate cement and minerals forming the rockframework is the most important factor in creat-ing secondary porosity in carbonates (Mazzullo& Harris 1991; Mazzullo 2004). The concentra-tion of these acids, along with mineral catalysis,seems to be sufficient (Goldstein 1983) to overridethe buffering from carbonate-rich systems andthe reverse solubility of calcite with temperature(Lippmann 1973).

Several studies in the past literature focusedon the decrease in permeability due to pore clog-ging in unconsolidated sand as a result of chang-ing mineral paragenesis and fluid chemistry duringhydrothermal flow-through experiments (Tenthoreyet al. 1998; Gouze & Coudrain-Ribstein 2000;Yasuhara et al. 2006). Other studies addressedpressure solution in carbonates resulting frommass transfer due to stress-induced solubility gra-dients (Sprunt & Nur 1977; Robin 1978; Bakeret al. 1980; Renard et al. 2004). Nevertheless, ourunderstanding of the development of secondaryporosity in carbonates is incomplete. Despite thesimpler mineralogy of carbonate rocks compared

From: Agar, S. M. & Geiger, S. (eds) 2015. Fundamental Controls on Fluid Flow in Carbonates: CurrentWorkflows to Emerging Technologies. Geological Society, London, Special Publications, 406, 311–329.First published online May 22, 2014, http://dx.doi.org/10.1144/SP406.4# The Geological Society of London 2015. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics

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to siliciclastic rocks, the proclivity of carbonates todissolution, compaction and decarbonation exposesthese rocks to continuous changes during post-depositional diagenesis, eventually creating sig-nificant heterogeneity in both texture and fabric.The continuous superimposition of processes onone another complicates first-order relationshipsbetween geophysical observables, such as seismicvelocity and electrical resistivity, and measurablerock properties, such as porosity and permeability.The scatter in the observed correlations makes itdifficult to identify data trends, hampering themodelling of seismic and well data. Therefore, westill have much to learn about how dissolution andcompaction modify the pore space of carbonates,and whether different depositional microstruc-tures predispose carbonates to particular types ofmodification.

The fundamental argument driving this study isthat the spatial distribution of carbonate reservoirproperties is a product of interactions between: (1)the specific diagenetic processes acting upon therock: (2) the parameters controlling fluid type andcirculation (e.g. pH and flow rate); and (3) the orig-inal depositional facies defining the intrinsic mech-anical response of the rock to chemical changes.Specifically, we want to investigate whether dif-ferent styles of chemically induced pore-volumemodification are imposed by the intrinsic pore-space compressibility, 1/KF, of the original rock.In a porous medium, pore-space compressibilityis defined as the ratio of the fractional change inpore volume, vp, to an increment of applied stress,s (Mavko et al. 1998). The way that pore spacedeforms under stress controls rock compaction,stress redistributions and pore-fluid effects oneffective moduli. Pore compressibility is thus amodel-independent parameter based on pore geo-metry and, under constant burial conditions, itmay change significantly when small amounts ofmatrix or contact cement are added or removed.Sprunt & Nur (1977) experimentally showed thatspherical holes drilled centrally within slabs ofdifferent materials increased in size because of dis-solution under stress; the holes also flattened in thedirection of stress.

Thus, the key question is this: when we introducea chemically reactive fluid into a carbonate rockunder stress, how does the pore space change inresponse to stress redistribution and microstructureadjustment as the fluid–rock system attains a newchemomechanical equilibrium? Because unravel-ling continuous sequences of diagenetic eventsdue to dissolution and compaction is difficult, weuse a forward-experimental approach: we directlymonitor the evolution of porosity, permeability,compressional and shear velocities (PPV trends) ofrock types from different depositional carbonate

facies. The different rock types are characterizedby different textures and, in turn, variable poretype and pore compressibility. To study the evol-ution of the pore-space character, we induce chemi-cal dissolution by flooding the rock samples with acarbonic acid solution (pH ¼ 3.5) while monitoringthe mechanical deformation under stress, as well asthe induced changes in transport and elastic proper-ties. In our experiments, we monitor the evolution ofboth transport and elastic parameters to: (1) compre-hensively look at what attribute of the pore space(i.e. stiffness, connectivity and tortuosity) is morelikely to be responsible for the changes, given theoriginal texture of the rock; and (2) learn whetherthe induced modifications to the microstructure ledto systematic changes and/or trends in the rockproperties that are characteristic of specific carbon-ate facies.

Fig. 1. (a) Klinkenberg-corrected permeability of thecarbonate samples as functions of helium porosity. Dataare colour-coded as a function of the specific rock type:(1) (orange) mudstones, MSA (Monte Sant’Angelo)Formation; (2) (dark red) mudstones–grainstones, MA(Monte Acuto) Formation; (3) (green) mudstones–packstones, FP (Peschici) Formation; and (4) (light red)grainstones, GR (Gravina) Formation. (b) P-wavevelocity of the carbonate samples as functions of heliumporosity. Data are colour-coded as in Figure 1a.

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Sample description and preflooding

property characterization

Figure 1a, b report the Klinkenberg-corrected per-meability and the P-wave velocity of carbonatesamples as functions of helium porosity (Scotellaroet al. 2008; Vanorio et al. 2008). Samples span a

variety of rock types from different depositionalcarbonate facies, porosities (from 1 to 52%) andpore types. The samples were collected both fromthe Gargano Promontory (Scotellaro et al. 2008)and the Murge region, both belonging to the ApuliaCarbonate platform, the stable and relatively unde-formed Southern Apennine foreland (Borgomano

Fig. 2. (a) Backscatter SEM images showing the microstructure of a bioclastic (shell fragments) mudstone from theMSA formation. (b) Backscatter SEM image showing a magnified area of (a). A mud-supported framework with aninterlocked mosaic of micro-rhombic crystals of micrite and tight spar cement predominantly characterizes themicrostructure of these rock plugs. The arrows show examples of locations where changes occurred in the post-injectionsamples (see Fig. 13).

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2000; Pomar & Tropeano 2001; Morsili et al. 2004).The samples from the Monte S. Angelo Formation(MSA) come from the Monte S. Angelo Sequence(Late Albian–Santonian), consisting mainly ofslope and basinal, fully pelagic sediments (Boselliniet al. 1999; Morsili et al. 2004). The depositionalfabric of the MSA samples corresponds to tight bio-clastic mudstones characterized by an interlockedmosaic of micritic matrix (Fig. 2b). The samplesfrom the Monte Acuto Formation (MA–MonteS. Angelo Sequence) come from a depositionalsequence that is typical of a transgressive systemstract (TST). It consists of stacked, coarsening-upwards cycles of basal pelagic mudstones passingupwards into coarse calciturbidites (Bosellini et al.1993, 1999). The depositional fabric of the MA For-mation varies from chalky, whitish grainstonesto lime mudstones (Fig. 3) with variable micritecontent (Vanorio & Mavko 2010). The samplesfrom the Peschici Formation (FP) come from the

Monte Saraceno Sequence (Middle Eocene–Lute-tian), which is represented almost entirely by slopeand base-of-slope deposits. This formation consistsof a succession of calciturbidites alternating withpelagic mudstones (Fig. 4). The mudstones arecharacterized by a tight microstructure consistingof an interlocked mosaic of micritic matrix. Ourdataset of the FP samples comprises both deposi-tional fabrics, characterized by two distinct rangesof porosity, permeability and velocity (Fig. 1a, b,green diamonds). The collected samples of theGravina Calcarenite (Bradanic Trough; middle Plio-cene–lower Pleistocene) come from a typical shore-line facies. The depositional fabric consists of apoorly consolidated grainstone lacking in micritematrix (Fig. 5) (Pomar & Tropeano 2001). For thisrock type, only pre-injection measurements arereported. Samples fell apart during injection,which prevented us from measuring any time-lapsephysical property.

Fig. 3. Different magnifications of backscatter SEM images showing the microstructure of samples from theMA Formation. Samples vary from chalky, whitish grainstones to lime mudstones with variable micrite content.(a) Grain-dominated packstone rock fabric with mud-size crystals controlling pore-size distribution. The arrows showexamples of locations where changes occurred in the post-injection samples (see Fig. 15). (b) A magnified patch ofround micrite crystals (1–4 mm). (c) Grain-dominated rock fabric showing the presence of macropores. (d) Magnifiedbackscatter SEM image of (c) highlighting the presence of interlocked mosaic of micro-rhombic crystals of micrite andspar cement.

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The scanning electron images of Figures 2–5clearly highlight the depositional fabric of thedifferent rock types over a range of scales (from50 mm to 1 mm). This heterogeneity translates intoa wide spectrum of effective pore stiffnesses of the

rock samples (Fig. 6) that varies as a function ofporosity. We computed pore stiffness by knowingporosity (F), the bulk modulus of the mineralphase (Ks ¼ 76.8 GPa) and the bulk modulus ofthe dry carbonate frame (Kdry) (Mavko et al. 1998).

Fig. 4. Different magnifications of backscatter SEM images showing the microstructure of samples from theFP Formation. The collected samples range from (a) higher porosity packstone characterized by the presence ofmacropores to (c–e) lower porosity pelagic mudstones. (b) Higher magnification of the backscatter SEM imageof (a) showing macropores characterized by an average size of 100–200 mm. (d) Higher magnification of thebackscatter SEM image of (c) showing the tighter microstructure of the pelagic mudstones, which consists of aninterlocked mosaic of micritic matrix. Arrows show examples of locations where changes occurred in the post-injectionsamples (see Fig. 14).

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Kdry is computed from the measured velocities andbulk density:

1

Kdry= 1

Ks+ F

KF.

In Figure 6, both pore stiffness (KF) and the bulkmodulus of the dry carbonate frame (Kdry) are

normalized with respect to the bulk modulus ofthe mineral phase (Ks). While mudstones (MSAand low-porosity FP samples) are characterized bya wider spectrum of larger magnitude pore stiff-nesses (from 0.1 to 0.5), grainstones and packstones(MA, FP higher porosity and GR) span a narrowerrange of much smaller pore stiffnesses, rangingfrom 0.1 to 0.01 (Fig. 6). Although a direct

Fig. 5. Two different magnifications of backscatter SEM images (a) and (b) showing the microstructure of samplesfrom the Gravina Formation. This rock type shows characteristics of a shoreline facies with a depositional fabricconsisting of a poorly consolidated grain-supported bioclastic calcarenite lacking in micrite matrix. Owing to the poorconsolidation, samples are characterized by high porosity and permeability, as well as low pore stiffness (Fig. 6). Theseproperties contribute to the weak framework of the samples from this formation, which led the plugs to fall apart duringinjection.

Fig. 6. Variation of pore stiffness as a function of helium porosity for all carbonate rock samples. Data are colour-coded as a function of the specific formation: mudstones from the MSA Formation (orange); mudstones–grainstonesfrom the MA Formation (dark red); packstones and pelagic mudstones from the FP Formation (green); andgrainstones from the GR Formation (light red). Contours of constant, dry rock bulk modulus are also reported. Bothpore stiffness (Kf) and bulk modulus of the dry carbonate frame (Kdry) are normalized with respect to the bulk modulusof the mineral phase (Ks).

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relationship is difficult to establish, the comparisonbetween Figures 1a and 6 highlights an interestingsimilarity: tight mudstones span a wide range ofpermeability and pore stiffnesses compared to moregrain-dominated fabrics.

Method

We conducted several laboratory experiments toinvestigate and monitor the evolution of the porenetwork attributes responsible for variations inporosity, permeability, and P- and S-wave velocitiesof carbonate rocks (Table 1). The experiments con-sisted of flow-through injections of an aqueous CO2

solution into samples (1 inch diameter × c. 1 inchlength) subjected to an applied confining pressureand ambient temperature. Figure 7 describes the

experimental protocol used in this study. First, thesamples were loaded up stepwise to an effectivepressure of 15 MPa to simulate a range of pressuresthat are appropriate for carbonate reservoirs. Dur-ing this stage, samples were dry, and P- andS-wave velocities were recorded along the loadingpressure path (pre-injection measurements, openred circles, PV ¼ 0; Fig. 7, left panel). Sampleswere then saturated under pore pressure rangingbetween 10 and 13 MPa, with an increasing porevolume (i.e. volume of injected fluid normalizedby the sample pore volume) of a pre-mixed,aqueous CO2 solution. During injection, we main-tained a nearly constant downstream flow rate of5 ml min21, and an effective pressure (Pe ¼ Pc–Pf) that ranged between 2 and 5 MPa, dependingon the trade-off between sample permeability andflow rate. Several cumulative injections were

Table 1. List of the sample core plugs used for the experiments, summarizing their rock type, depositionalenvironment, and the range of porosity, permeabilityy, and pore stiffness

Plugname

Formation Rock type Depositionalenvironment

Porosity(%)

Permeability(mD)

Pore stiffness(GPa)

MSA Monte S.Angelo

Mudstone Basin to slope 0.2–10 0.8–1000 0.08–0.6

FP Peschici Packstoneandmudstone/wackestone

Base-of-slopeto slope

0.3–1520–40

0.8–20040–1100

0.02–0.1

MA MonteAcuto

Mudstone tograinstone

Basin to slope 20–30 70–120 0.01–0.1

GR Gravina Grainstone Slope crest 40–55 .900 0.01–0.05

Fig. 7. Outline of steps in the experimental procedure describing the injection of the aqueous CO2 solution. The samplewas first loaded stepwise to a confining pressure simulating the range of pressures that are appropriate for carbonatereservoirs (solid red line). At this stage the sample was dry (i.e. pore fluid pressure is equal to zero), so differentialpressure was equal to the confining pressure. Both P- and S- wave velocities were recorded along the loading pressurepath (open red circles, left panel). Samples were then first saturated under constant pore fluid pressure and then injectedwith a fixed pore volume of a pre-mixed, aqueous CO2 solution (dash blue line). The differential pressure was then heldconstant (dash red line) during injection. After injection, confining pressure was decreased and helium gas was injectedthrough the sample to ensure drying within the vessel (open red circles). This step allowed us to monitor the change ofproperties of the rock frame alone. Velocity measurements are taken in both undersaturated (cyan circle) and dry (opencircle) conditions. After drying, the sample was reinjected with an increasing pore volume of CO2 solution to monitorthe change in the elastic and transport properties of the rock frame as injection proceeds.

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performed during each experimental run. The mainobjective was to expose the samples to increasingpore volumes of fluids and to understand the roleof the continued exposure on the trends of themeasured properties. During the experiment, theplugs were jacketed with rubber tubing to isolatethem from the confining pressure medium. Aftereach injection, pore and confining pressures werereleased, and helium gas flowed at 150 psi throughthe sample for 8 h to ensure drying within the ves-sel. This allows us to measure and monitor the vari-ation of the properties of the frame alone whileminimizing dispersion effects on velocity due tohigh frequencies (Mavko & Jizba 1991). Then,Klinkenberg-corrected nitrogen permeability wasmeasured by using the constant-head technique.After each flooding–drying cycle, we also charac-terized the samples’ porosity and ultrasonic P- andS-wave velocity under pressure conditions. PZTcrystals mounted on steel endplates of the coreholder were used to generate P- and S-waves(1 MHz for P-waves and 0.7 MHz for S-waves).

The porosity change throughout loading andinjection has two contributions: the loss in porositydue to compaction under stress; and the enhance-ment of porosity from the dissolution of the frameand/or mechanical removal of particles occurring

during injection (Vanorio et al. 2011). The formercan be approximated by measuring the samplestrain, while the latter is determined by measuringthe Ca and Mg cation concentration in the collectedaqueous solution. For strain, three linear potenti-ometers were used to measure length changes ofthe samples as a function of stress. The lengthchanges were related to changes in porosity byassuming that pore contraction was the main causeof strain. The concentration of cations enabledus to calculate the change in porosity due to dissol-ution:

DFc(ti) =

!n

1

Dmn

Vbulk rg

=Vinj f (ti )

!n

1Cn

Cation Mw min

Vbulk rg

where DFc(ti) represents the change in porosity cal-culated from the measured concentration of the dis-solved cations over the period [ti21; ti] between twofluid samplings, Vbulk is the volume of the sample, rg

is the grain density and Dmn is the change in massdue to the nth dissolved mineral over the period[ti21; ti]. The mass change was calculated fromthe mean concentration C Cation

n of the dissolvedcations from the nth mineral, the molecular weight(Mw min) of the dissolving mineral species and

Fig. 8. Variation of (a) the bulk- and (b) shear-modulus as functions of injected pore volumes. Data refer to a micrite-rich grainstone from the MA Formation and are colour-coded as a function of confining pressure. (c) Variation in P-wave velocity as a function of confining pressure. Data are colour-coded as a function of the injected pore volumes.(d) Variation in the concentration of the calcium ion in the collected fluid as a function of the injected pore volumes.

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the volume of the injected fluid, Vinj f (ti )(note: the

laboratory measurements of water hardness arein mg l21 and are equivalent to the product,C Cation

n Mw min). The total net change in porositywas thus the resulting porosity from each com-ponent. We estimate the accuracy in velocity,porosity and permeability measurements to beapproximately +1, +1 and +2%, respectively.Finally, time-lapse SEM imaging of the sampleswas performed after each run to relate changesin the physical properties to those in the rockmicrostructure.

Results

Pressure-dependence of velocity and

compaction on dissolution

The experimental results show that the bulk andshear moduli of the dry frame of the injected car-bonate samples change upon injecting increasingvolumes of the aqueous CO2 solution. Nevertheless,the magnitude and the trends of the measured prop-erties strongly depend on the rock type. As the firstpore volumes were injected (14.3 pore volume (Pv))

in the micritic grainstone/chalk (MA Formation;F ¼ 25.2%), both velocities (and, by proxy, theelastic moduli) decreased in magnitude (Fig. 8a,b), implying either an increase in porosity, areduction in the stiffness at the grain contacts orboth. The decrease in elastic moduli levelled offover time despite continued injection of the acidicsolution (Fig. 8a, b). Similarly, the amount ofCa2+ measured from the effluent fluid at theoutlet also levelled off, implying a reduced rate ofcalcite dissolution (Fig. 8d). In addition, thedecrease in velocity was pressure-dependent. Simi-larly to the elastic moduli, post-injection P- and S-wave velocity also decreased with respect to theirpre-injection values as the confining pressurereduced from 15 to 5 MPa (Fig. 8c). As injectionproceeded, velocity data showed an increased sensi-tivity to pressure, implying a change in pore stiff-ness. Concurrently, the porosity of the rock samplealso evolved (Fig. 9) as a result of two differentmechanisms. At first, the sample experienced mech-anical compaction due solely to the effect of confin-ing pressure (injected pore volume ¼ 0) noted bythe porosity decrease from approximately 25.2%(black circle) to about 23.2% (blue diamondsat 15 MPa) as pressure increased (Fig. 9a). As

Fig. 9. Variation of the porosity (a) of the MA micrite-rich grainstone as a function of the injected pore volumes.The sample experienced dissolution-driven compaction under the effect of the applied stress, eventually leading toporosity change. (b) The change in porosity throughout loading and injection had two contributions: the loss in porositydue to compaction under stress and the enhancement of porosity from frame dissolution and/or mechanical removal ofparticles occurring during injection (orange circles) computed by titration of the collected fluid.

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injection proceeded, the sample experienced twocompeting mechanisms. On the one hand, dissol-ution of calcite led porosity to increase (orangecircles, Fig. 9b). On the other hand, the sampleexperienced a concurrent, pressure-dependent com-paction driven by dissolution (Fig. 9a). Compactionwas probably due to internal creeping under con-stant stress, resulting from the decreased rockstrength. Thus, the total net evolution in porosityof the sample upon dissolution under stress(Fig. 9b) was the sum of both processes and wascalculated by considering the change in porositydue to each component. Figure 9b also shows thatporosity enhancement due to dissolution over-whelmed the loss in porosity due to compac-tion, resulting in a net porosity increase afterflooding.

However, the evolution of the elastic propertiesof tight bioclastic mudstones upon injection seemsto follow different trends. Figure 10 shows the vari-ations in elastic moduli, velocity and Ca2+ concen-tration upon injection through a tight mudstone(MSA Formation; F ¼ 7.73%). Before injection,the mudstone exhibited a much lower velocity–pressure sensitivity than did the micritic grain-stone (Figs 8c & 10c, blue diamonds), implying astiffer pore network. Micrite-rich grainstones and

mudstones are, in fact, characterized by averagerelative pore stiffnesses of 0.05 and 0.3, respec-tively (Fig. 6). The different pressure-sensitivitytranslates to a much smaller porosity loss in themudstones due to pressure-dependent compaction(Figs 9a & 11a, data points at Pv equal to zero).After injection, there were three main differencesbetween the tight mudstones and the micritic grain-stone. First, the magnitude of the decrease in elasticmoduli (and by proxy velocity) of the tight mud-stones was smaller than that of the micrite-richgrainstone (Figs 8 & 10). Also, the decrease did notlevel off as much as observed in the micrite-richgrainstones. Second, the tight mudstone experi-enced less dissolution, as highlighted by the smallerconcentration of Ca2+ measured within the outletfluid (Figs 8d & 10d). This behaviour led eventuallyto a smaller porosity enhancement from dissolu-tion (Figs 9b & 11b). Third, as the injected porevolumes increased, the tight mudstone experiencedless dissolution-induced compaction than did themicrite-rich grainstone (Figs 9a &11a). As a result,porosity enhancement due to dissolution did notstrongly overwhelm the already minimal porosityloss due to compaction. This behaviour resulted ina minimal porosity increase overall (Fig. 11b). Justas the grainstones showed an increased sensitivity

Fig. 10. Variation of the (a) the bulk- and (b) shear-modulus as functions of injected pore volumes. Data refer toa tight limestone from the MSA Formation and are colour-coded as a function of confining pressure. (c) Variationin P-wave velocity as a function of confining pressure. Data are colour-coded as a function of the injected pore volumes.(d) Variation in the concentration of the calcium ion in the collected fluid as a function of the injected pore volumes.

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of velocity to pressure with flooding (Fig. 10c), sodid the limestones. This trend was particularlyvisible at low pressure ranges.

Dependence of the original depositional

facies on dissolution

We compare the evolution of velocity andpermeability of the treated samples to the naturaldiagenetic trends of untreated (no injected fluid) car-bonate samples (grey symbols, Fig. 12). Both per-meability (Fig. 12a) and velocity (Fig. 12b) dataare plotted against porosity. The observed trendsreflect how the modifications in the pore attributesand microstructure due to dissolution under stressaffect velocity and permeability. Although thenumber of injected pore volumes was nearly thesame for each sample, the trends of the evolvingproperties vary across the specific rock type, withsome similarities among samples belonging to thesame facies/formation. In chalky, micritic pack-stones (MA and high-porosity FP formations),both porosity and permeability increased uponinjection, while velocities strongly decreased. Inmudstones (tight MSA and low-porosity FP for-mations), porosity and velocity exhibited negligible

change upon injection, while permeability greatlyincreased after reaching a critical threshold.Another noteworthy and surprising result is thatthe evolution of both velocity and permeability ofthe injected samples followed the natural diagene-tic trends. Dissolution seemed to primarily affectthe permeability of tight mudstones while leavingporosity and velocity essentially unaltered. Con-versely, dissolution was a major influence on thedecrease in velocity of micrite-rich carbonates, therate of which increases going from grainstones tolime mudstones. Both permeability and porosityslightly increased as dissolution proceeded, withtheir evolution strongly curbed by compaction.

The set of SEM images referring to samplesfrom the MSA, FP and MA formations showsthat the induced changes to the rock depended onthe microstructure (Figs 13–15). Mud-dominatedrock types (MSA (Figs 2 & 13) and low-porosityFP formations (Figs 4 & 14)) exhibited a minimalincrease in porosity, negligible compaction andgrain sutures along crystal faces (arrows in Figs 4& 14b). Conversely, grain-dominated packstonesexperienced an increase in porosity that developedat the expense of the micrite phase, as well as com-paction due to pore collapse and grain rearrange-ment (arrows in Figs 3 & 15).

Fig. 11. Variation of the porosity (a) of the MSA tight mudstone as a function of the injected pore volumes. The sampleexperiences a dissolution-driven compaction under the effect of the applied stress, eventually leading to porositychange. (b) The change in porosity throughout loading and injection has two contributions: the loss in porosity due tocompaction under stress and the enhancement of porosity from frame dissolution and/or mechanical removal ofparticles occurring during injection (orange circles), computed by titration of the collected fluid.

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Discussion

Transport and elastic properties depend on differentattributes of the pore space: stiffness at the graincontacts, porosity and pore compliance control vel-ocity (Anselmetti & Eberli 1993; Mavko et al. 1998;Weger et al. 2009); while pore connectivity, tortuos-ity and pore size control permeability (Carman1961). Any change to these parameters during

diagenesis defines the evolution of permeability,porosity and velocity. Several papers in the litera-ture highlight the importance of chemical processesin lowering the grain–grain contact stiffness of car-bonates (Vanorio et al. 2010; Vanorio et al. 2011;Croize et al. 2013). This paper shows that to under-stand the evolution of the transport and elasticproperties of carbonates, it is vital to look at theproblem comprehensively, and to understand how

Fig. 12. Evolution of (a) permeability and (b) P-wave velocity of the injected samples as functions of porosity. Data arecompared to the natural diagenetic trends of untreated carbonate samples (grey circles). Data refer to dry samplesmeasured after being injected, and the different colours describe different injected samples (MA, micrite-richgrainstones; FP, packstones and mudstones; and MSA, tight mudstones); the black-circled symbol refers to the valuemeasured for each sample before injection.

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dissolution, stress and the original pore stiffness ofthe rock are coupled.

As diagenetic processes alter the original deposi-tional fabric of the different carbonate rock types,the number of possible deviations from first-orderrelationships between geophysical observablesand rock properties (e.g. porosity and permeabil-ity) increases enormously. Previous studies that

involved uncoupling the chemical and mechani-cal processes acting on a deposition-dependentmicrostructure have not provided sufficient under-standing of how the pore-space parameters controlthe scatter in the velocity and permeability trendsof carbonates. The result of this lack of knowledgeis that rock-physics modelling is unconstrainedby physics- and chemistry-driven principles.

Fig. 13. Two different magnifications of backscatter SEM images showing the changes in the microstructure of abioclastic mudstone from the MSA Formation due to the injection of the CO2 aqueous solution. (b) The magnified imageshows changes at the same locations highlighted in Figure 2b. As a result of dissolution under stress, changes involved aminimal increase in porosity, negligible compaction, cementation and microcrystal welding along crystal faces.

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A comparison of data from micrite-rich grain-stones and tight mudstones shows that the evolutionof permeability and velocity in carbonates, due toleaching and/or more advanced dissolution, ismicrostructure-dependent. Under constant stress,

carbonates experience dissolution-induced compac-tion (Fig. 9 & 11) as a result of: (1) mass removalaffecting the strength of the rock; and (2) themicrostructure-dependent pore stiffness. The weak-ening results exclusively from the removal (either

Fig. 14. (a) Backscatter SEM images showing the changes in the microstructure of packstone from the FP Formationdue to the injection of the CO2 aqueous solution. The images show the same rock location as shown in Figure 4a, b (top,sample FP10c) and highlight the main changes experienced by this rock type as a result of dissolution under stress.Changes involved collapse of larger pores and minor compaction. (b) Backscatter SEM images showing the changes inthe microstructure of pelagic mudstones from the FP Formation due to the injection of the CO2 aqueous solution. Theimages show the same rock location as shown in Figure 4c, d (bottom, sample FP14c) and highlight the main changesexperienced by this rock type as a result of dissolution under stress. Changes involved negligible compaction,cementation and microcrystal welding along crystal faces.

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due to chemical dissolution, mechanical removalor both) of carbonate phases with high surfacearea (i.e. micrite and cement). Images show largergrains experiencing only minor etching. Neverthe-less, the processes of dissolution and compac-tion compete and feed back upon each other. Theresults from injection into different rock typesshow that the final outcome of these competing pro-cesses depends on: (1) the specific pore-stiffness ofthe rock, which controls compaction; (2) the amountof micrite and/or cement, which is responsible

for selective dissolution; and (3) the tightnessof the formation, which, by controlling permeabil-ity, influences the effective fluid circulation andthe balance between dissolution and precipitation.Altogether, these factors determine how the velo-city and permeability of carbonates evolve understress.

In micrite-rich packstones, several factors favourdissolution and dissolution-induced compaction:higher porosity and permeability, the presence ofpores that are more compliant than those present

Fig. 14. Continued.

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in tighter, mud-dominated facies, and the presenceof high-surface-area phases (i.e. microcrystallinecalcite). Dissolution of cement and micrite accom-modates compaction, as well as slippage and/orlocal rearrangement of grains (Fig. 15), ultimatelyreducing the stiffness at the grain contacts. If con-tacts are not promptly recemented, the velocity ofthe rock framework is permanently reduced asfluid circulation proceeds. In addition, by control-ling the reactive surface area, compaction curbsthe chemical rock–fluid interaction and reducesthe subsequent decrease in the elastic moduli.Vanorio et al. (2011) showed that injecting increas-ing volumes of fluid into a compacting sample doesnot lead to further decrease in the elastic moduli. Asa consequence, elastic moduli level off as injectionproceeds. The change in the velocity of the rock fra-mework is particularly important when monitor-ing the injection of reactive fluids into micrite-richpackstones by time-lapse seismic. In addition,

compaction continuously compensates for theenhancement in porosity and the enlargement ofpore throats resulting from dissolution, curbing theincrease of the permeability in these rocks.

In tight mudstones, the finely interlockedmosaic of micrite rhombohedra (Fig. 2) makes thedeposition-inherited microstructure of these rocksvery stiff (i.e. they show high pore stiffness). Suchmicrostructure makes this rock type slightly sensi-tive to compaction, favouring the enlargement ofpore throats and microporosity due to dissolution.In addition, the low permeability favours a poordewatering of the Ca2+ ion-rich water and supersa-turation, which leads calcite to precipitate and weldsmicrograins together under the effect of pressure(Figs 13 & 14b). The direct consequence of theseprocesses is that the enhancement of pore throatsdoes not contribute much to the total porosity orto the decrease in velocity. However, it does affectpore connectivity (and probably tortuosity),

Fig. 15. Backscatter SEM images illustrating the changes in the microstructure of plugs from the MA Formation due tothe injection of the CO2 aqueous solution. Changes are reported for different magnification scales. The images show thesame rock location as shown in Figure 3 and highlight the main changes experienced by this rock type as a result ofdissolution under stress. Changes involved an increase in porosity that was predominantly created at the expense ofmicrite (a & b) and non-negligible compaction (c). High-magnification images (d) show that microcrystal welding andsutures also formed as a result of dissolution-induce compaction under stress.

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leading the permeability of these tight microstruc-tures to greatly increase. Time-lapse nuclear mag-netic resonance (NMR) measurements reported byGrombacher et al. (2012) showed that mudstonesfrom the same rock type (FP10) exhibit significantchanges in their pre- and post-injection distributionof the transverse relaxation time (T2). After injec-tion, the most notable changes were in the appear-ance of merged T2 signal peaks and a signal atshorter relaxation times. The observed variabilityin FP10c’s T2 distribution (Grombacher et al.2012) (Fig. 6d) was attributed to an increased con-nectivity, caused by pore coupling, and/or to theopening of very small pore throats, which were satu-rated and detected post-injection.

It is often discussed in the literature that largenumbers of injected pore volumes coupled withhigh flow rates (i.e. higher Peclet–Damkohlernumbers) lead to unbalanced geochemical reactionsand localized dissolution patterns (i.e. hetero-geneous dissolution), especially near the injectionarea. Although these parameters are largely usedin chemical engineering to relate the timescale ofchemical reactions to processes occurring in aporous system, it is worth noting that they: (a)apply to homogeneous porous media supportingno stress; and (b) are very often independent of thechemistry of the fluid (Fredd & Fogler 1998). High-resolution imaging showed that the heterogeneousdissolution observed in our carbonate samples ispredominately due to the natural distribution ofgrain sizes occurring in carbonates. Different grainsizes imply a different surface area, which is thenultimately responsible for dissolution, weakeningand compaction that is heterogeneous through thesample. Also, injection led neither to the formationof wormholes nor to the formation of larger macro-pores on the surface of the samples. These featureshave been observed in previous experiments(Bemer & Lombard 2010), as well as predicted bydynamic models (Fredd 1999) attempting to formand develop wormholes during matrix acidizingtreatments using HCl. The reactive fluid used inthis study is an aqueous CO2 solution. This fluid isnotoriously a weak acid and thus is characterizedby a much lower ionic strength compared to HCl(with pH being equal). Therefore, the CO2 solutioncauses much less dissolution when reacting withcarbonates. Most important, we observe that as thefluid–rock system attains a new chemomechanicalequilibrium, the overall microstructure of the rockreadjusts under stress through grain slippage, poredeformation and/or closure. Dissolution-drivencompaction prevents the mere subtraction of thethree-dimensional (3D) computerized tomography(CT)-scanned imaged rock volumes (Vanorio et al.2011) due to deformation and shrinkage. Mostdynamic models simulating pore-scale, reactive

fluid flow based on uncoupled chemomechanicalprocesses neglect these factors.

One of the most important results of this study isthat the evolution of both velocity and permeabilityof the injected samples follows the natural diage-netic trends of the rock type. The similaritiesbetween the induced evolution of properties andthose depicted by the natural diagenetic trendsimply that attributes of the pore network (such asconnectivity, stiffness and tortuosity) change, andthe rock microstructure readjusts through commonrules that are characteristic of each carbonatefacies. Thus, the original depositional texturedefines the intrinsic mechanical response of therock to chemical changes, playing a major role inthe way that pore space modifies under chemome-chanical processes. The changes in the pore spaceseem to create systematic patterns in rock proper-ties, whose evolution is controlled mainly by com-paction, and, in turn, by surface area and porestiffness. Depending on the effectiveness of com-paction, changes in the rock lead either to areduction in contact stiffness (in rocks experiencinghigh compaction) or to connectivity enhancement(in rocks experiencing low compaction). Thesefactors are key to future modelling strategies and/or pore-scale computation of property changes incarbonate reservoirs.

Conclusion

We monitored the evolution of porosity, per-meability and velocity of different carbonatemicrostructures upon injection of an aqueous CO2

solution under stress. The experimental resultsprovide a better understanding of how chemome-chanical processes alter the pore-space attributes(i.e. pore stiffness, connectivity and tortuosity) ofcarbonate rocks under dissolution-induced compac-tion. Since the experimental results mirror diage-netic trends, they also provide new insights topredict the evolution of rock microstructure duringreservoir diagenesis. The agreement between theevolution of velocity and permeability found inthe laboratory, and that expressed by the naturaldiagenetic trends of the rock types simply showsthat there are basic rules controlling the way poreattributes modify during diagenesis. These rulesare dictated by the original microstructure thatformed during deposition, which closely controlsgrain/matrix ratio and pore stiffness of the rock.In turn, these two parameters, respectively, controlthe bulk surface area of the rock and its suscepti-bility to volumetric compaction during diagenesis.Depending on the balancing of pore stiffnessto reactive surface area, fluid–rock chemical inter-actions in carbonate reservoirs may lead either to

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reduction in contact stiffness (high compaction) orto connectivity enhancement (low compaction).Eventually, these two possibilities are likely tolead to two evolutionary patterns of velocity andpermeability in carbonate rocks when they areexposed to circulating aggressive fluids in thesubsurface.

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