Single-and-two-phase-fluid-flow-properties-of-cataclastic-fault-rocks-in-porous-sandstone_2012

Single-and two-phase fluid flow properties of cataclastic fault rocks in porous sandstoneChristian Tueckmantel a ,*,Quentin J.Fisher a ,Carlos A.Grattoni b ,Andrew C.Aplin caCentre for Integrated Petroleum Engineering and Geoscience,School of Earth and Environment,University of Leeds,Leeds LS29JT,United Kingdom bRock Deformation Research Limited,School of Earth and Environment,University of Leeds,Leeds LS29JT,United Kingdom cSchool of Civil Engineering and Geosciences,Drummond Building,Newcastle University,Newcastle upon Tyne,NE17RU,United Kingdoma r t i c l e i n f oArticle history:Received 24February 2011Received in revised form 30May 2011Accepted 20July 2011Available online 27July 2011Keywords:Fault seal PermeabilityCapillary pressure Two-phase flow Cataclasite90-Fathom faulta b s t r a c tUnderstanding the impact of faults on fluid flow in the subsurface is important for the extraction of oil,gas and groundwater as well as the geological storage of waste products.We address two problems present in current industry-standard work flows for fault seal analysis that may lead to fault rocks not being represented adequately in computational fluid flow models.Firstly,fluid flow properties of fault rocks are often measured only for small-scale faults with throws not exceeding a few rge seismic-scale faults (throws >20m)are likely to act as baf fles or conduits to flow but they are seldom recovered from subsurface cores and consequently fault rock data for them is sparse.Secondly,experi-mental two-phase fluid flow data is lacking for fault rocks and,consequently,uncertainties exist when modelling flow across faults in the presence of two or more immiscible phases.We present a data set encompassing both single-and two-phase fluid flow properties of fault and host rocks from the 90-Fathom fault and its damage zone at Cullercoats Bay,NE England.Measurements were made on low-throw single and zones of deformation bands as well as on slip-surface cataclasites present along the w 120m throw main fault.Samples were analysed using SEM and X-ray tomography prior to petro-physical measurements.We show that single deformation bands,deformation band zones and slip-surface cataclasites exhibit dissimilar single-and two-phase fluid flow properties.This is due to grain-size reduction being more pronounced in slip-surface cataclasites and changes in microstructure being fault-parallel for deformation bands but mostly fault-perpendicular for slip-surface cataclasites.A trend of fault rocks with low absolute permeabilities exhibiting lower relative permeabilities than more permeable rocks at the same capillary pressure is evident.Ó2011Elsevier Ltd.All rights reserved.1.IntroductionAn important objective in disciplines concerned with the extraction of oil and gas,groundwater resource management or the geological storage of waste products,such as CO 2,is to understand and predict the movement of fluids in the subsurface.This knowledge is key in determining the ideal placement and speci fi-cations of production and injection wells.The planning process is usually aided by computational fluid flow models,which are capable of simulating the behaviour of subsurface reservoirs (e.g.Dake,2001).Faults can have a major impact on fluid flow so it is important to correctly incorporate their flow properties in production simulation models (Fisher and Jolley,2007;Jolley et al.,2007;Zijlstra et al.,2007).Three main mechanisms have been identi fied by which faults may impact fluid flow in subsurface reservoirs (Manzocchi et al.,2010).Firstly,a fault may juxtapose lithologies with different fluid flow properties against each other.Secondly,fault rocks may have different fluid flow properties than the undeformed host rock.Thirdly,faults may provide open,fault-parallel pathways which may function as fluid conduits.In this study we focus on the second mechanism and,in particular,on the impact of cataclastic fault rocks on cross-fault flow.Assessing and predicting cross-fault flow is usually referred to as fault seal anal-ysis in the oil and gas industry.We address two key problems present in current industry-standard work flows for fault seal analysis that may lead to fault rocks not being represented adequately in computational fluid flow models.Firstly,fluid flow properties of fault rocks are often measured only for small-scale faults with throws not exceeding a few centimetres (e.g.Fisher and Knipe,1998,2001).Large*Corresponding author.Present address:Shell Global Solutions International B.V.,Kessler Park 1,2288GS,Rijswijk,The Netherlands.Tel.:þ31704475215.E-mail address:c.tueckmantel@ (C.Tueckmantel).Contents lists available at ScienceDirectMarine and Petroleum Geologyjournal h omepage:ww w.elsevi /locate/marpetgeo0264-8172/$e see front matter Ó2011Elsevier Ltd.All rights reserved.doi:10.1016/j.marpetgeo.2011.07.009Marine and Petroleum Geology 29(2012)129e 142seismic-scale faults with throws>20m are likely to act as baffles or conduits tofluidflow but they are seldom recovered from subsurface cores and,consequently,fault rock data for them is sparse.It is still unclear whether it is possible to estimate prop-erties of fault rocks along large-scale faults based on the analysis of small faults in their damage zones(Antonellini and Aydin,1994; Fisher and Knipe,2001;Tueckmantel et al.,2010).A second problem when modellingflow across faults is the lack of experimental data on the two-phasefluidflow properties of fault rocks.Fault rock permeability is usually determined by flowing either gas or water through a respectively dry or water saturated sample,which gives the single-phase or absolute permeability value.However,often two or morefluid phases are present in subsurface reservoirs(e.g.brine and gas or brine and oil).In the presence of more than one immisciblefluid,the effective permeabilities of each phase are reduced relative to the single-phaseflow case as a function of the phase saturation,which in turn depends on the capillary pressure.The parameter relating absolute to effective permeabilities is called the relative perme-ability.To our knowledge,Al-Hinai(2007)and Al-Hinai et al. (2008)report the only published relative permeability data for fault rocks.Al-Hinai et al.(2008)compare their results with capillary pressure data reported for gas reservoirs and conclude that failure to take two-phaseflow into account could lead to an overestimation of the cross-fault transmissibility by several orders of magnitude.However,two-phasefluidflow data needs to be extended to encompass a variety of fault rock types and faulting regimes before predictions can be made with confidence.Here we present experimental data for different types of cata-clastic fault rock collected from outcrop to help tackle the two problems identified above.Our aim is to investigate whether small-scale faults can be used to predict the single-and/or two-phase fluidflow properties of large-scale faults.To this end we used a combination of microstructural analysis,X-ray tomography and laboratory measurements of single-and two-phasefluidflow properties to characterise fault and host rock samples,which we collected from the90-Fathom fault at Cullercoats Bay in NE England (Jones,1967;Collier,1989).The analysed fault rocks encompass single deformation bands and deformation band zones exposed in the damage zone as well as slip-surface cataclasites present along the main fault.The term slip-surface cataclasite is used here to describe cataclasites that delineate the slip surface of a fault on one or both sides without interspersed host rock(Tueckmantel et al., 2010).All the sampled fault rocks developed within porous sand-stone of the Permian Yellow Sands in the hanging wall of the90-Fathom fault.Fault throw ranges from a few millimetres for single deformation bands to w120m for slip-surface cataclasites along the main slip surface of the90-Fathom fault.The data reported presents thefirst published comparison of two-phase fluidflow properties of different fault rock types.In the discus-sion we integrate our results with fault rock data from published studies.Furthermore,we review two-phasefluidflow data measured for tight gas sandstones to assess whether these may function as an analogue for cataclastic fault rocks.2.The90-fathom faultThe90-Fathom fault is part of the E e W to ENE e WSW trending Stublick-90-Fathom normal fault system(Kimbell et al.,1989).The fault system down-throws to the north and constitutes the southern border of the Carboniferous Northumberland-Solway Basin,which covers around6500km2of northern England and southern Scotland(Chadwick et al.,1993).In NE England,the Stu-blick-90-Fathom fault system represents the border between the early Carboniferous Northumberland basin to the north and the Alston block,a granite-centred structural high,to the south(Collier, 1989;Kimbell et al.,1989;Fig.1).The fault system initiated as an E e W trending normal fault due to Early Carboniferous N e S extension that ended during the Namurian,followed by thermal subsidence during the Westphalian(Kimbell et al.,1989).According to Kimbell et al.(1989),the Lower Carboniferous Dinantian is more than4.2km thick in the fault hanging wall but only a few hundred metres thick in the footwall,indicating syndepositional faulting. The fault showed renewed activity in Permian to Mesozoic times, documented by displaced Upper Carboniferous and Permian strata. Fault reactivation was extensional or transtensional and probably associated with the early stages of rifting in the North Sea basin (Collier,1989).According to De Paola et al.(2005),the90-Fathom fault was reactivated as a dextral transtensional fault during NE-SW regional stretching in post Carboniferous times.We studied the90-Fathom fault in a coastal section at Cullercoats Bay between Tynemouth and Whitley Bay,NE of Newcastle(Fig.1). At this locality the fault trends E e W and juxtaposes Permian Yellow Sands in its hanging wall against Upper Carboniferous Coal Measure shales and mudstones associated with coal seams in its footwall (Collier,1989).According to Jones(1967),fault throw at Cullercoats Bay is w120m.Where the main fault is exposed it constitutes an approximately planar,polished slip surface,which dips with40 to the NNE.The sandstone adjacent to the slip surface consists of cat-aclasites of at least15cm thickness without interspersed host rock, referred to as slip-surface cataclasites.The well exposed Yellow Sands in the hanging wall consist offine-to medium-grained aeolian sandstone that shows a complex pattern of deformation bands. Regionally the Yellow Sands dip towards the south but close to the 90-Fathom fault at Cullercoats Bay they dip with w20 to the west. Deformation bands in the Yellow Sands are either isolated with displacements of up to a few centimetres or organised in up to w0.5m thick zones with cumulative displacements of up to w1m. The majority(>90%)of deformation bands trend E e W,parallel to the main fault(Collier,1989;Knott et al.,1996).The E e W trending deformation bands are organised in conjugate fault sets,syn-and antithetic to the main fault with angles of20e30 between them (Harris et al.,2003).Collier(1989)reports a subordinate fault set trending roughly N e S.De Paola et al.(2005)describe ESE e WNWFigure1.The Stublick-90-Fathom normal fault system in NE England(modified from Kimbell et al.,1989).The90-Fathom fault was studied in a coastal section at Cullercoats Bay(arrow).C.Tueckmantel et al./Marine and Petroleum Geology29(2012)129e142 130trending dextral strike-slip faults in the hanging wall,mutually cross-cutting the E e W striking deformation band set and therefore broadly contemporaneous.Knott et al.(1996)report that the hanging-wall damage zone is300m wide with fault frequency diminishing gradually away from the main fault.The background fault density defining the boundary of the damage zone is one fault per10m(Knott et al.,1996).3.Experimental methodologyHost and fault rock blocks with dimensions of up to 0.5mÂ0.3mÂ0.3m were collected from the90-Fathom fault and its hanging-wall damage zone.The blocks were cut and cored in the laboratory to produce cylindrical plugs with a diameter of w3.6cm and length varying from1.8to5.5cm for CT scanning and petro-physical measurements.Small blocks with dimensions of up to 2cmÂ2cmÂ1cm were cut as well for scanning electron microscopy(SEM)and mercury injection porosimetry.All samples were cleaned in a Soxhlet extractor using an azeotropic methanol-dichloromethane mixture followed by methanol to extract any salt and organic matter present.The samples were oven dried prior to measurements.3.1.X-ray tomographyPlugs drilled from host and fault rock blocks were analysed using a Picker PQ2000dual energy computed tomography(CT) scanner prior to petrophysical measurements.Differences in the degree of X-ray attenuation,which is material and energy dependent,are visible in the recorded CT images.In particular,X-ray tomography can visualise density differences and provides a non-destructive way to image the internal structure of a rock sample.It is therefore particularly useful for the current study because the cataclastic fault rock has a higher density than the undeformed sandstone.Furthermore,open fractures and heavy mineral cement can easily be recognised due to their respectively very low and very high density.All samples were scanned along two perpendicular planes containing the longitudinal axis of the cylindrical plugs.3.2.Microstructural analysisResin impregnated polished blocks of host and fault rocks were examined by back-scattered electron(BSE)and cathode lumines-cence(CL)imagery as well as by an energy dispersive X-ray spectrometer(EDS)using a Camscan CS44SEM.Images were saved in8-bit(256grey-levels)digital form and imported into the image analysis software ImageJ.As the BSE signal is proportional to the mean atomic number of the analysed material,different mineral phases appear in different shades of grey.Heavy minerals usually appear as very bright and the pore space as black areas.Different phases were identified by EDS,which,in conjunction with thresh-olding the images using ImageJ,allowed the estimation of porosity and mineral abundances.3.3.Absolute and effective gas permeabilityAbsolute and effective gas permeabilities were measured using a pulse-decay technique(Brace et al.,1968)for permeabilities<0.1 mD and a steady-state technique(API,1998)for more permeable samples.Measured steady-state gas permeability was corrected for the Klinkenberg gas slippage effect(Klinkenberg,1941).Both types of measurements were made byflowing helium across dry and partially water saturated samples.Flow was always parallel to the axis of the cylindrical plugs and perpendicular to deformation bands and the main fault in order to assess cross-fault permeability. Pulse-decay measurements were made under a confining pressure of2500psi and a pore pressure of1000psi.Measurements con-ducted with the steady-state method employ a confining pressure of1600psi and a pore pressure that was varied between5and100 psi to apply the Klinkenberg correction.Assuming an effective stress coefficient of unity,this experimental setup resulted in an effective stress of w1500psi for both the steady-state and pulse-decay measurements.Plugs containing deformation bands also contain a large proportion of host rock(Fig.2).The measured permeability therefore represents an average permeability across the whole sample.The true fault rock permeability,k f,was deconvolved from the average permeability,k av,using a method based on work by Cardwell and Parsons(1945)and outlined,for example,in Tueckmantel et al.(2010).3.4.Mercury threshold pressureMercury injection curves were measured using a Micromeritics Autopore II9220mercury injection porosimeter using a technique first employed by Purcell(1949).Tests were performed on rectan-gular blocks cut from plugs for which permeability was measured previously.Fault rock samples were sealed on all but one side with epoxy resin.The unsealed face was oriented parallel to either the deformation band or the slip surface.This procedureguaranteesFigure2.CT images of typical host rock(sample90F2H2B),single deformation band(sample90F2-1)and deformation band zone(sample90F5)plugs used for petrophysical measurements.The shade of grey is proportional to the density;i.e.the darker the image the more dense the material is.C.Tueckmantel et al./Marine and Petroleum Geology29(2012)129e142131that mercury has to invade the fault rock to form an inter-connected pathway throughout the sample.Host rock samples, on the other hand,were tested unsealed so that mercury was able to enter from all sides.As a comparison,three slip-surface cata-clasite samples were also tested unsealed.The mercury injection curves were used to determine the mercury threshold pressure,which is defined as the pressure at which mercuryfirst forms an inter-connected pathway throughout the sample(Katz and Thompson,1986,1987).The threshold pres-sure was picked at the inflection point of the mercury injection curves as described by Katz and Thompson(1987).Single defor-mation band samples contain a proportion of host rock the mercury can invade before reaching the fault.This configuration results in mercury injection curves with two distinct inflection points,the one at lower pressure representing the threshold pressure of the host and the one at higher pressure representing the fault-rock threshold pressure.3.5.Two-phasefluidflow propertiesTwo-phasefluidflow properties,including gas relative perme-ability,capillary pressure and water saturation,were measured for fault and host rock samples.Both drainage and imbibition tech-niques were used to change the air e water capillary pressure and consequently the water saturation of the samples.A combination of experimental methods was used to determine relative permeabil-ities over a large range of capillary pressures and saturations as each method yields only a limited range of capillary pressures.The effective gas permeability of the samples was measured using either a pulse-decay or steady-state method once equilibrium between capillary pressure and water saturation had been estab-lished.For a comparison of various methods for measuring capillary pressures,including humidity chamber and porous plate,the reader is referred to Newsham et al.(2004).Drainage experiments are based on a non-wetting phase being forced into a brine saturated sample.To saturate the samples,they werefirst placed in a saturator under vacuum for at least12h.The saturator was thenflushed with CO2,left for an hour and evacuated to remove the CO2.The chamber was thenfilled with1%NaCl brine and the pressure increased to2000psi.Samples were left to saturate at2000psi for48h.The brine saturated samples were then placed in a porous plate extractor or a humidity chamber to force brine out at a specified capillary pressure.Imbibition exper-iments were carried out by placing samples into humidity cham-bers within which pure water acts as the wetting phase invading initially dry samples.In this study the term water saturation will be used to refer to both water and brine saturation to simplify the presentation of the results and the discussion.3.5.1.Humidity chambersThe humidity chamber or vapour desorption method is based on the vapour pressure above a liquid being dependant on the curvature of the liquid’s surface as reportedfirst by Thomson (1871).Five different salts in a supersaturated brine solution were used to control the vapour pressure and hence the air e water capillary pressure.The salt solutions employed in this study yielded capillary pressures ranging from606to5728psi.Initially dry sample plugs were placed in a series of humidity chambers for imbibition experiments.The water saturation of the samples was monitored by weight.It was assumed that capillary equilibrium between the vapour phase and the pore water was reached when the sample weight was constant for more than one week.Samples were then removed from the chamber and their effective gas permeability measured.Measurements were repeated forfive different salt solutions corresponding to progressively lower capillary pressures and therefore higher water saturations. For each humidity chamber,it took up tofive weeks until equilib-rium between the capillary pressure and the water saturation was achieved.Humidity chambers were also used to conduct drainage exper-iments.In contrast to the imbibition experiments described above, a fully brine saturated sample was placed in a humidity chamber and the weight loss was monitored as the brine saturation of the sample began to equilibrate with the respective capillary pressure. When the weight loss slowed down to less than10mg per week samples were removed and their gas permeability was measured. This technique was useful to achieve high brine saturations in highly porous and permeable samples.The disadvantage is that the capillary pressure is unknown as equilibrium was not achieved between the brine saturation and the capillary pressure in the chamber.Due to equilibrium not being reached the desaturation process may also have led to inhomogeneous distribution of the brine within the sample.However,as samples were measured only when weight loss due to drainage was low,it is likely that the brine distribution was approximately uniform.3.5.2.Porous plateThe porous plate method was used for drainage experiments (Longeron et al.,1989;Wilson et al.,2001).Fully brine saturated samples were placed on top of a brine saturated porous plate within a Soilmoisture pressure plate extractor.The porous plate used has a maximum pore size of0.16m m.The highest capillary pressure achievable depends on the size of the pores in the plate and the pressure plate extractor used.The setup employed in this study can be operated at capillary pressures of up to220psi.The pressure plate extractor wasfilled with nitrogen gas and the gas pressure set to a specified value which was kept constant.Brine was forced out of the sample through the porous plate as a result of the applied gas-brine capillary pressure.The production of brine was monitored until the system reached equilibrium and no more brine was produced.The samples were then removed and the effective gas permeability measured.After the measurements the samples were placed on top of the porous plate again and the gas-brine capillary pressure increased to the next pressure step.This procedure was carried out for up tofive pressure steps(2,20,50,100and200psi).4.Sample descriptionThe host sandstone and three types of fault rock(slip-surface cataclasites along the main fault,a deformation band zone and single deformation bands)were sampled in the hanging wall of the 90-Fathom fault.A slip-surface cataclasite block was sampled directly adjacent to the main fault.A w20cm wide deformation band zone which accommodates a cumulative throw of45cm was sampled160m north of the main fault.The deformation band zone strikes parallel to the main fault and dips with80 to the NNE. Several host rock blocks with and without single deformation bands were collected130m north of the main fault.The sampled deformation bands strike approximately parallel to the main fault and are typically w1mm thick.Measured offset varies from5to 13mm for the analysed single deformation bands.Table1lists all the rock samples,their weight and dimensions as well as their absolute gas permeability and mercury threshold pressure. Dimensions given are for cylindrical plugs on which absolute and relative permeability measurements were conducted.4.1.X-ray tomographyNone of the host and fault rock plugs exhibits open fractures visible in CT images and significant amounts of heavy mineralC.Tueckmantel et al./Marine and Petroleum Geology29(2012)129e142 132cement(>1%)were found only in slip-surface cataclasite plugs. Typical plugs of host rock,a single deformation band and a defor-mation band zone are shown in Figure2.The density of the fault rock in the deformation bands is markedly higher than the host density.Only small pockets of undeformed low-density host are visible within the high-density fault rock in the deformation band zone plug.X-ray tomography of the slip-surface cataclasite plugs revealed an uneven distribution of heavy mineral cements(black patches in Fig.3).SEM analysis showed that the precipitated heavy minerals are barite and iron oxide.Thefirst2to4cm of fault rock directly next to the main slip surface typically show little or no heavy mineral cement.Cement begins to occur more frequently a few centimetres away from the slip surface and is unevenly distributed.Heavy mineral cement was observed directly adjacent to the main fault in only one sample along a cemented fracture oriented approximately perpendicular to the slip surface.This sample was not used for petrophysical measurements.For the zone affected by cementation only a minimum thickness of approximately15cm can be reported due to the limited length of the drilled plugs.Plugs used for petrophysical measurements were trimmed to include little or no heavy mineral cement based on the CT images taken as shown in Figure3.This procedure ensured that we measured the properties of the cataclasites directly next to the90-Fathom fault without heavy mineral cement affecting the results.4.2.Microstructure4.2.1.Host rockThe undeformed Permian Yellow Sands are a high porosity sandstone(Fig.4),with well rounded,fine-to medium-sand sized grains(w60e500m m diameter).Image analysis suggests the following composition:58e70%detrital quartz,7e9%K-feldspar, 5e16%kaolin,0e1%calcite,<1%authigenic quartz,<1%iron oxide and18e20%porosity.A small number of the observed K-feldspar grains are partly dissolved.Several quartz grains exhibit very thin rims of authigenic quartz,observable on CL images.It is unclearTable1Spatial andfluidflow properties of host and fault rock samples from the90-Fathom fault.The deconvolved fault rock permeability,k f,is given for deformation band samples.The offset is indicated in brackets for single deformation bands.Sample name Weight(g)Length(cm)Diameter(cm)Absolute gaspermeability(mD)Hg-thresholdpressure(psi)a)Host rock90F2H2A93.44 4.25 3.6115515 90F2H2B93.04 4.26 3.6117113 90F2BH283.91 4.19 3.5630410 90F2BH381.20 4.05 3.574118 90F2BHP186.03 4.36 3.553415 Arithmetic mean27710b)Single deformation bands90F2-1(8mm)104.92 4.85 3.610.29560 90F2-4(5mm)100.71 4.70 3.60 2.7315 90F2B2(8mm)93.29 4.59 3.6031020 90F2B5(9mm)87.25 4.50 3.55 6.3540 90F2B6(11mm)94.48 4.73 3.5610.360 90F2C2(13mm)92.81 4.40 3.56 2.5540 Arithmetic mean55.339c)Deformation band zone90F1A91.63 4.12 3.62 1.13125 90F1B47.41 2.16 3.63 2.48140 90F2121.38 5.46 3.63 2.28250 90F3111.29 4.90 3.64 1.20300 90F4109.87 5.06 3.62 2.62250 90F5102.07 4.45 3.630.955200 90F677.55 3.48 3.62 1.37e Arithmetic mean 1.72211d)Slip-surface cataclasites90FM168.56 2.72 3.630.002561000 90FM350.14 1.96 3.640.001504000/1000a 90FM464.28 2.55 3.640.06293000/650a 90FM5109.49 4.28 3.640.01283250 90FM684.32 3.31 3.650.0080513000 90FM959.19 2.30 3.640.005355000 90FM1045.96 1.84 3.640.0180400/200a Arithmetic mean0.01594236/617aa Sealed/unsealedsamples.Figure3.CT images of two typical untrimmed slip-surface cataclasite plugs.Thedarker the image the more dense the material is(heavy mineral cement appearsblack).The main slip-surface constitutes the upper boundary of the plugs.The verticalbars indicate the extent of the trimmed plugs used for petrophysical measurements.Note the absence of heavy mineral cement in the upper part of the plugs adjacent tothe slipsurface.Figure4.BSE image of typical Yellow Sands host sandstone from the hanging wall ofthe90-Fathom fault.Dark grey grains are quartz and light grey ones are K-feldspar.C.Tueckmantel et al./Marine and Petroleum Geology29(2012)129e142133。