The  dynamics  of  porosity  and  reactive  surface  area  changes    controlling  water-­‐rock  interaction  

Supervisors:  Dr  Fiona  Whitaker1,  Prof  Tom  Scott2,  Prof  Chris  Perry3,  Prof  Maurice  Tucker1  

1.   School  of  Earth  Sciences,  University  of  Bristol  2.   Interface  Analysis  Centre,  School  of  Physics,  University  of  Bristol  3.   School  of  Geography,  University  of  Exeter  

The  challenge:  Understanding  reactive  surface  area  is  essential  for  quantitatively  modeling  water-­‐rock   interactions   including   chemical   weathering   and   the   fate   of   solutes.   Porosity   changes  associated  with  mineral  dissolution  or  precipitation  are  important  in  a  range  of  geological  settings  and   industrial  applications,   including  diagenesis,  karst  and  ore   formation,  well   stimulation  and  sealing,  and  CCS.  Associated  changes  in  pore  structure  and  volume  affect  both  flow  and  transport.  Porosity  change  can  be  highly  localized,  leading  to  pore  coalescence  and  formation  of  conductive  flow  channels.  Understanding  interactions  between  reactions,  pore  geometry,  mass  and  solute  transport   is   a  prerequisite   to  accurate  prediction  of   sources  and   fate  of   solutes   in  all   reactive  systems.  

The  problem:  Numerical  models  coupling  reaction  kinetics  and  flow  (RTM  –  reactive  transport  models)  may   fail   to   reproduce  laboratory   or   field   observations   in  heterogeneous  porous  media,   especially  where   chemical   and   flow   gradients   are  steep.   This   is   in   part   due   to   the   use   of  equivalent   rock   properties   averaged   at  the   scale   of   an   REV,   ignoring   the   pore  network   geometry   and   nature   and  disposition  of  minerals.  Reactive   surface  area  (RSA)  is  a  key  parameter  because  the  mineral-­‐fluid   interface   area   controls  reaction   rate   in   many   systems.   RSA   is  often   approximated   using   geometric  relationships   based   on   idealized  grain/crystal  shapes  which  can  bear  little  resemblance   to   reality   (Fig.  1).  However  direct   measurement   of   RSA   may  overestimate   reactivity   as   it   ignores   the  complex   relationship   between   the  mineral   surface   morphology,   diffusive  boundary   layer   thickness   and   reaction  rate.   To   model   reactive   transport   we  need  to  characterize  RSA  changes  during  water-­‐rock  interaction  and  to  incorporate  their  effects  into  RTMs.    

The  project:  will  identify  the  behaviour  of  RSA   in  natural  systems  and  evaluate  the  

impact  on  reactive  transport.  Using  flow-­‐through  experiments  you  will  simulate  dissolution  and  precipitation   in  materials  with  contrasting  RSA.  During   the  course  of   the  experiments  you  will  monitor   bulk   reactions   via   fluid   composition   and   changes   in   permeability   as   they   impact  macroscopic   flow   behavior,   and   use   XRT   to   directly   image   changes   in   pore   geometry.   Lab  experiments   could   be   complemented   with   field   experiments   in   a   natural   system   which   has  already   been   well   characterized.   You   will   directly   examine   changes   in   the   surfaces   and   pore  structure  of  experimental  substrates  using  3D   imaging  of  minerals  and  pores  via  micro-­‐CT  and  measure   RSA   by   gas   adsorption.   From   this   you   will   develop   pore   scale   models   of   flow   and  reactions  using  a  Lattice  Boltzmann  approach.  Finally,  you  will  use  the  data  from  these  studies  to  develop   novel   approaches   to   upscale   our   improved   understanding   of   feedbacks   between  porosity,   permeability   and   RSA   to   a   macroscopic   scale,   which   can   be   incorporated   into   next  generation  RTMs.  

Training:   This   project   offers   a   chance   to   develop   an   unusual   breadth   of   expertise,   spanning  laboratory  and  field  experiments,  physical  and  chemical  characterization  of  porous  media,  and  modeling   of   water-­‐rock   interaction   from   pore   to   macroscopic   scale.     This   work   is   of   direct  relevance  to  the  Oil  and  Gas  industry  as  it  is  central  to  understanding  diagenesis,  an  important  modified  of  reservoir  quality  in  many  carbonate  reservoirs,  and  may  be  run  as  an  Industrial  CASE  studentship.    

Background  Reading:  Gabellone  T.  &  Whitaker  F.,  Secular  variations  in  seawater  chemistry  controlling  dolomitisation  in  shallow  

reflux  systems:  insights  from  reactive  transport  modelling.  Sedimentol.  63,  1233-­‐1259  (2016).  Lai  P.  et  al.,  Pore-­‐scale  heterogeneity  in  the  mineral  distribution  of  reactive  surface  area  of  porous  rocks.  

Chem.  Geol.  411,  260-­‐273  (2015).  Norriel   C.   et   al.,   Changes   in   reactive   surface   area   during   limestone   dissolution:   and   experimental   and  

modelling  study.  Chem.  Geol.  265,  160-­‐170  (2009).  Steefel  C.  et  al.,  Micro-­‐continuum  approaches  for  modelling  pore-­‐scale  geochemical  processes.  Rev.  Min.  

&  Geochem.  80,  217-­‐246  (2015)