On the Physical and Chemical Stability of Shales


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Journal of Petroleum Science and Engineering 38 (2003) 213 – 235 www.elsevier.com/locate/jpetscieng On the physical and chemical stability of shales Eric van Oort Shell E&P Company, New Orleans, LA, USA Abstract The stability of clay-rich shales is profoundly affected by their complex physical and chemical interactions with drilling fluids. In this paper, an attempt is made to clarify the intricate links between transport processes (e.g. hydraulic flow, osmosis, diffusion of ions and pressure)
  On the physical and chemical stability of shales Eric van Oort  Shell E&P Company, New Orleans, LA, USA Abstract The stability of clay-rich shales is profoundly affected by their complex physical and chemical interactions with drillingfluids. In this paper, an attempt is made to clarify the intricate links between transport processes (e.g. hydraulic flow, osmosis,diffusion of ions and pressure), physical change (e.g. loss of hydraulic overbalance due to mud pressure penetration) andchemical change (e.g. ion exchange, alteration of shale water content, changes in swelling pressure) that govern shale stability.It is shown that shale–fluid interactions can be manipulated to enhance cuttings and wellbore stabilization as well as improvinghole-making ability in shale formations. The mode of shale-stabilizing action of a wide variety of water-based fluid additives isdiscussed and the merits of various mud systems are ranked. It is shown that shale stabilization normally achieved using oil- based/synthetic-based muds is now becoming achievable with economical and environmentally friendly water-based drillingfluids. D 2003 Elsevier Science B.V. All rights reserved.  Keywords: Physical stability; Chemical stability; Borehole stability; Shales; Water-based mud (WBM); Oil-based mud (OBM); Synthetic-basedmud (SBM) 1. Introduction The problem of wellbore stability in shales hasfrustrated oil-field engineers from the start of oil andgas well drilling. Wellbore instability is in fact themost significant technical problem area in drilling andone of the largest sources of lost time and trouble cost (van Oort et al., 1996a).A typical example of prob-lems encountered in the field is given inFig. 1.The 8 1/2 in. section of this well, drilled with a water-basedmud, was enlarged up to 25 in. despite the presence of additives used especially for shale-stabilization pur- poses. Operational problems that derive from suchinstabilities may range from high solids loading of themud requiring dilution, to hole cleaning problems dueto reduced annular velocities in enlarged hole sec-tions, to full-scale stuck pipe as a result of well cavingand collapse.Wellbore stability is almost a trivial issue with oil- based and synthetics-based muds. Once mud weight and invert emulsion salinity are properly established,stability can virtually be guaranteed (except for a fewcases such as fractured shale formations, which may be rapidly destabilized by such muds when they penetrate the fracture network, lubricate fracture sur-faces, and equilibrate pore pressure with wellbore pressure). Moreover, oil and synthetic based muds ingeneral drill wells much faster than water-based mudsas they are much less prone to cause bit balling.Much more problematic and enigmatic have beenthe adverse interactions of shales with water-basedfluids. Such muds are potentially attractive alterna-tives for oil and synthetic muds from an environ- 0920-4105/03/$ - see front matter  D 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0920-4105(03)00034-2  E-mail address: eric.vanoort@shell.com (E. van Oort).www.elsevier.com/locate/jpetsciengJournal of Petroleum Science and Engineering 38 (2003) 213–235  mental point-of-view, but they are still outmatched bythe latter in overall drilling performance (exclusivefocus in this paper is on shale stability—note that additional factors, such as fluid loss control, lubricity,mud rheology, etc., need to be considered also whencomparing differences indrilling performance betweenmud types).The central issue explored in this paper is: ‘‘whichmeans can be exploited to achieve shale stabilizationand improve operational drilling performance withwater-based drilling fluids?’’ The fundamentals of the shale instability problem must be understood first in order to answer this question. This requires appre-ciation of: (1) transport processes in shales, (2) physio-chemical changes caused by this transport,and (3) implications of these changes for mechanicaland chemical shale stability. 2. Fundamentals of shale behavior 2.1. A balance of forces Fig. 2gives a simplistic but practical model for theforces acting on a shale system containing clays andother minerals (primarily quartz) at silt size. They can be subdivided into mechanical and physio-chemicalforces. The former include:  the in-situ vertical (overburden) and horizontalstresses;  the pore pressure;  the stress acting at intergranular contact points, e.g.at cementation bonds.The latter, acting primarily in the clay fabric,include:  the van der Waals attraction;  the electrostatic Born repulsion;  short-range repulsive and attractive forces that arederived from hydration/solvation of clay surfaces Fig. 1. Typical caliper example of shale instability and wellbore problems. This 8 1/2 in. hole (dotted line) was enlarged up to 25 in.in the shale sections, whereas the sands are near-gauge to slightlyunder-gauge due to the presence of a poor quality filtercake.Fig. 2. A schematic representation of downhole forces acting on ashale system, simplified as a single set of clay platelets connected toa pore. The forces include the in-situ vertical and horizontal stresses,the pore pressure, the swelling pressure acting between the clay platelets, and tensile or compressive forces in the cementationdeveloping upon compressive or tensile loading of the shalematerial, respectively.  E. van Oort / Journal of Petroleum Science and Engineering 38 (2003) 213–235 214  and the ions that are present in interlayer spacings(adsorbed or free).The latter forces are usually lumped together toform the ‘‘hydration stress/pressure’’ or ‘‘swellingstress/pressure’’, since they are responsible for thecharacteristic swelling behavior of clays and shales.The term ‘‘swelling pressure’’, well-accepted in oil-field practice, will be used exclusively below. 2.2. The swelling pressure The van der Waals attraction and Born repulsionwere combined successfully in DLVO theory(vanOlphen, 1977), which has worked well in explainingthe behavior of clay colloidal suspensions. However,DLVO is a continuum theory that breaks down at small clay interplatelet distances (i.e. distances<20A˚) present in most well-consolidated shales encoun-tered in the field. At such distances, short-rangerepulsive forces that bear the mark of the discrete,quantizednature of matter become dominant.Fig. 3ashows the results of a molecular dynamics(MD) study to simulatethe swelling pressure insodium montmorillonite(Karaborni et al., 1996).The pressure profile displays oscillations that relateto the layering of water betweenthe clay platelets. Thedensity distributions inFig. 3bshow that Na-mont-morillonite during swelling jumps from two water layers at a platelet spacing of 9.7 A˚, to three layersat 12.0 A˚, to five layers at 15.5 A˚, to seven layers at 18.3 A˚, etc. The states in-between, i.e. four, six andeight water layers, were all found to be stronglyrepulsive and therefore unstable. The simulationresults show good correlation with experimental deter-minations of the equilibrium st ates of Na-montmor-illonite(Karaborni et al., 1996).This example shows the complicated nature of the swelling pressure andexplains why attempts to explain clay–shale swelling behavior on the basis of simplistic models (such as theosmotic model of swelling) have met with littlesuccess.For decades, the standard oil-field solution to clay– shaleproblemshasbeen‘‘inhibition’’,atermsrcinallyderived from the ability of certain additives, most notably salts, to ‘‘inhibit’’ yielding of bentonite inwater (Darley and Gray, 1988).The term is confusing since the colloidal behavior of clays and swelling inwell-consolidated shales are two separate and, to alarge extent, unrelated issues. For instance, the effi- Fig. 3. (a) Swelling pressure in Na-montmorillonite as a function of interplatelet distance/basal spacing d100. Contribution of DLVO forces isnot included. Stable states are indicated by arrows. (b) Density distribution of oxygen atoms in water as a function of the distance Z from theoctahedral sheet. Results are shown for the stable states with spacings at 9.7, 12.0, 15.5, 18.3 and 20.7 A˚.  E. van Oort / Journal of Petroleum Science and Engineering 38 (2003) 213–235 215  ciency of clay flocculation governed by DLVO forcesdecreases with ionvalence (the well-known Schulze– Hardyrule,seee.g.vanOlphen,1977).Bycomparison,swelling pressure governed by non-DLVO forces suchas ion hydration follows quite the reverse trend,e.g. K  + is much more effective than Ca 2 + or Mg 2 + in reducingtheswellingpressureinmontmorillonite.Inthefollow-ing, the well-accepted oil-field terms ‘‘inhibition’’ and‘‘inhibitor’’ will apply strictly to additives that areaimed at reducing the swelling pressure. ‘‘Inhibition’’,however, is not necessarily a synonym for ‘‘shale-stabilization’’ as we shall see.The effectiveness of K  + ions in minimizing swel-ling pressures in montmorillonite is believed to berelated to the small degree of hydrationof these ionsin water, resulting in low ion repulsion(Karaborni et al., 1996). The effects of ion hydration, however, are non-trivial.Fig. 4shows the results of oedometer experiments, measuring the degree of swelling of a pre-loaded montmorillonite-rich shale sample that wasimmersed in concentrated solutions of KCl andKCOOH. Swelling was measured during an unloadingsequence and was quantified in terms of a swellingindex. At low salt concentrations, i.e.<20% w/w, areduction in swelling (showing as a reduced swellingindex) was seen with an increase in K  + content. At high salt levels, however, swelling was again seen to increase . Similar eff ects have been documented inopen literature (e.g.Christenson et al., 1987; Israel-achvili, 1991).These contra-intuitive results are explained byconsidering the increased ion repulsion that derivesfrom the introduction of an excess of hydrated ions inthe interplatelet clay spacings. At first, the introduc-tion of low concentrations of potassium salt is bene-ficial in lowering the swelling pressure due to K  + ionsreplacing ‘‘less-inhibitive’’, more hydrated ions at theclay surface. However, the swelling pressure willincrease when an excess of hydrated cations and  anions with increased mutual repulsion builds up inthe interplatelet clay spacings. Note that the above results were both obtained for ashale system with very high-salinity brine as the onlyfluid between the clay platelets. Such situations willhardly ever occur in actual field practice, where trans- port of solutes from the mud to the shale (e.g. diffusionof ions) dilutes the concentration of solutes. Theseresults should therefore not be used as an argument todiscard concentrated KCl or KCOOH brines as basefluids for shale muds. The results just serve to placeswelling pressure in a different light and to highlight the complexity of ion repulsion phenomena.A full discussion on other unique features of theswelling pressure, most of which are ill-understood incurrent oil-field practice, falls beyond the scopeof this paper. An excellent review can be found inIsrael-achvili (1991)for interested readers. Important to the present discussion are the following:1. The swelling pressure is always present in clay-richshales, acting as a tensile force on clay platelets: it does not suddenly develop when the shales arecontacted by water-based drilling fluids. However,chemical changes caused by shale-drilling fluidinteractions may change its magnitude (either  beneficially or adversely).2. Even the best inhibitors cannot bring the swelling pressure down to zero (seeFig. 4andIsraelachvili, 1991; Bol, 1986; Bol et al., 1992); there will always be residual repulsion between the platelets due tohydration of the clay surfaces and sterical interfer-ence between hydrated ions and water molecules,unless complete dehydration and platelet collapseoccurs. Studies claiming otherwise(Steiger, 1993)usually do not take into account the fact that  Fig. 4. Oedometer test result for a shale containing 68% total clay,of which 76% montmorillonite, immersed in solutions of KCl andKCOOH of increasing salinity. The test shows an initial decrease inswelling for increase in salinity (note that the swelling index doesnot go to zero, i.e. there always is a residual swelling pressure), after which swelling increases again with the increase in salt content.  E. van Oort / Journal of Petroleum Science and Engineering 38 (2003) 213–235 216
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