EFFECTS OF NATURAL RECHARGE ON GAS COMPOSITION IN THE LAXDERELLO - CASTELNUOVO AREA. Franco D'Amore*, Romano Celati",Claudia Galore*, Rino Bertrami**

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1 Procdigs Nith Workshop Gothrmal Rsrvoir Egirig Staford Uivrsity, Staford, Califoria, Dcmbr 983 SCP-TR-7 EFFECTS OF NATURAL RECHARGE ON GAS COMPOSITION IN THE LAXDERELLO - CASTELNUOVO AREA Fraco D'Amor*, Romao Clati",Claudia Galor*, Rio Brtrami** "CNR-Istituto Itrazioal pr l Ricrch Gotrmich PISA,Italy **ENEL-Uits Nazioal Gotrmica PISA,Italy ABSTRACT A study of th tmporal bhaviour of gas compositio i th Lardrllo-Castluovo ara shows that this bhaviour varis, dpdig o whthr th wlls ar affctd or ot by atural rcharg. Whr o atural rcharg xists, gas compositio sms to b govrd by chmical quilibria. Th rcharg watr, ad th stam it producs, mix with th fluid alrady xistig i th rsrvoir. At Lardrllo (far from absorptio aras) th gas compositio rsultig from this mixig dos ot udrgo furthr chags by chmical ractios. This is du ithr to a lack of ractivity whr thr is o liquid phas or to a iffctiv fluid-rock itractio. At Castluovo (clos to absorptio aras) th HS ad CH cotts i th gas hav gradually dcrasd with th icras i rcharg ffcts. Th dcras i HS ca b attributd to dissolutio i liquid watr ad oxidatio. Various hypothss hav b forwardd for th mtha. Th corrlatio xistig btw CH ad Np coctratios, v wh th mtha dcrass i th aras affctd by rcharg, suggsts that CH may b govrd by mor tha just th Fishr-Tropsch ractio. INTRODUCTION I a arlir papr (Calor t a.,98) th spatial ad tmporal variatios i gas/stam ratio ad th isotopic compositio of th stam wr studid i th Lardrllo-Castluovo ara. This study showd that fluid compositio i ths zos of th fild is maily cotrolld by a mixig btw th fluid origially prst i th rsrvoir ad rct mtoric watrs. Th arias affctd by atural rcharg wr also gradually xpadig.this papr dals with th variatio i ucodsabl gas compositio i th ara affctd by atural rcharg.a compariso is also mad with fluid compositio i aras uaffctd by this phomo. FLUID COMPOSITION IN THE LARDERELLO-CASTELNUOVO AREA - H ad HS coctratios i th ucodsabl gas ar plottd i Figs. ad vrsus gas/ stam ratio. Th volutio with tim of gas/ stam ratio is diffrt i th wlls uaffctd by rcharg (D'Amor & Trusdll, 979; D'Amor t a.,98; D'Amor t a.,98) compard to wlls affctd by this phomo (Calor t a.,98). I th lattr cas th typical trd xhibits strog dcrass i gas/ stam ratio withi a rlativly short tim, accompaid by a shift i 6 I80 towards mor gativ valus ad, i may cass, by th apparac or icras of th tritium cott i th stam. This bhaviour has b attributd to a mixig btw th stam origially prst i th rsrvoir ad th stam formd by th vaporizatio of th rcharg watr cotaiig tritium, with vry gativ 6 l8o ad almost o gas. I th zos uaffctd by rcharg of rct mtoric watrs, th H ad HS cotts i th stam (Figs. ad ) vary ivrsly with th gashtam ratio (D'Amor t a.,98). Th bads show i ths Figurs iclud th Lardrllo wlls with stimatd rsrvoir tmpraturs i th Oo-6O0C rag. Th trd is th sam for diffrt wlls ad for ach sigl wll i diffrt priods, whthr th gaslstam ratio icrass or dcrass. I th wlls affctd by rcharg, th prctag of H i th ucodsabl gas rmais mor or lss costat with th dcras i gaslstam ratio, both at Lardrllo ad Castluovo. This mas that th H/H0 ratio varis proportioally with th gas/h0 ratio, as if th origial stam was simply dilutd by a gas-fr vapour. Th H S cott i th ucodsabl gas xhib- its a diffrt trd at Lardrllo from that at Castluovo. I th formr ara it rmais --

2 N =3 x o \ N = 3 x gas/stam( litrs STP/Kg) C gadstam ( litrs STP/ Kg ) Figur. H (molar prct i th ucodsabl gas) vrsus gas/stam ratio. A)Lardrllo ara; B) Castluovo ara. )wlls uaffctd by rcharg; )wlls affctd by rcharg. Th bad i Fig. A is show i Fig.lB for compariso. practically costat, whras i th lattr it shows a strog dcras with th dcras i gaslstam ratio. Figur 3 shows th zo of th fild for which thr is o vidc of rcharg ffcts ad th zos, affctd by rcharg, whr th HS cott rmais costat (Lardrllo) or dcrass (Castluovo). Th prctag of CH i th ucodsabl gas shows o corrlatio with th gaslstam ratio i th aras uaffctd by rcharg (Tabl ); o th othr had, its aral distributio i that ara is wll dfid, ad similar tmporal trds ar show by groups of ighbourig wlls. Figur illustrats th CH trd i th Lardrllo ara bfor rcharg had ay ffct o th zo. Th Castluovo ara is xcludd from this map as thr wr ot ough data availabl to dfi ay tmporal bhaviour bfor th ffcts of rcharg. Two zos ca b distiguishd i Fig. i which th CH molar prctag rmaid costat with tim, at valus of ad - rspctivly. I th itrmdiat zo th %CH icrasd cosidrably (as much as 00%) bfor rcharg ffcts bcam vidt. I th Castluovo ara th mtha cott i th gas showd a wll-dfid aral distributio durig th priod It icrasd rgularly from about 0. o th southrmost margi to -.5 o th orthr margi. Rcharg producd a gral strog dcras i th CHq/gas ratio at Castluovo (Tabl ).At Lardrllo th most cospicuous dcrass causd by rcharg occurrd i th wlls that iitially had th highst prctags of CH, whras i th wlls with th lowst iitial valus th lattr rmai costat or dcras vry littl. It is difficult to distiguish ay trd i th cas of th lowst valus, howvr, as th aalytical rrors ar of th sam ordr of magitud as th variatios. A gral corrlatio was otd throughout th whol Lardrllo fild btw CH ad N.Thi.5 corrlatio is show by D'Amor t a.(983). Figur 5 shows that this corrlatio holds v wh %CH dcrass i th wlls affctd by rcharg. --

3 \oo t\ % = 3 x "I gas/stam (litrs STP/Kg) C *W" \ L I I I I I gas/stam (litrs STP/Kg ) Figur. HS (molar prct i th ucodsabl gas) vrsus gas/stam ratio. S Fig. for symbols. DISCUSSION Fluid compositio i zos uaffctd by rcharg Accordig to th cocptual modl of D'Amor d Trusdll (979). diffrt parts of th rsrvoir with diffrt fluid compositios ad liquid saturatios ca b distiguishd i a vapour-domiatd systm. Tmporal variatios i th gaslstam ratio hav b xplaid by th fact that xpasio of th draiag volum is accompaid by a tmporal variatio i th cotributio to productio of various zos of th rsrvoir. D'Amor t a.(98) hav show that H ad H S cotts i th fluid ar govrd by th followig two quilibria i adquat rdox coditios (oxyg ad sulphur fugacity): H 0 = H 0 () H S = H IS () Equatios () ad (7) i D'Amor t a.(98) hav b modifid, for oxyg fugacity, accordig to D'Amor & Gialli (98): log f 0= /T-.075 x 06/T (3a) ad, for sulphur fugacity, accordig to th followig mpirical corrlatio (valid for th Lardrllo fild): log fs /T (3b) Ths quatios ca b rwritt as: H H? fps Wy) = log(y) ), ( g o l H O WH gas WH H0 WH 6 = /T.038 x 0 /T -Y % (Y - () log(7) HS = logh-) HS log(- gas ) H0 WH gas WH H0 WH = /T log T.038 x 6 -Y 0 /T log( y - ) BHS (5) -3-

4 ~ ~~ I ~ 7 t 3 ) 0 3) j 9 Lardrllo 5 m.a,,? J. &7 "i 't I.. : '3,. *. 8 0 km Figur 3. Locatio of wlls with diffrt volutio of %HS i th ucodsabl gas. )Wlls uaffctd by rcharg; )wlls affctd by rcharg, with costat %HS; 3) wlls affctd by rcharg, with dcrasig %HS. lol k; Lo9 0" I km. Castluovo VC I) ) 0 3 Figur. Aral distributio ad tmporal volutio of CH(molar prct i th ucodsabl gas) for th Lardrllo wlls bfor rcharg ffcts bcam vidt. l)%ch costat i th rag ; )%CH costat i th rag -; 3)%CH icrasig with tim towards valus of % Nz Figur 5. Corrlatio btw ZCH ad %N i th ucodsabl gas from i wlls affctd by atural rcharg. whr is th umbr of mols, T is tmpratur i OK, Bi is th distributio cofficit of th spcis i btw stam ad liquid watr, ad y is th molar fractio of stam i th stam-watr mixtur i th rsrvoir. Ths quatios ar valid i th followig coditios (D'Amor & Clati,983) : th fluid producd coms from o two-phas sourc i chmical ad phas quilibrium; - th two-phas fluid prst i th rsrvoir is trasfrrd to th wll with o chmical ractios ad o gai or loss of mass, although it may udrgo phas chags (liquid boilig). From th abov quatios it follows that th H ad HS coctratios i th gas vary ivrsly with th gas/stam ratio (for costat T ad y), ad icras as a fuctio of T ad y. Th fild data show i Figs. ad ar clustrd withi rlativly arrow bads for two rasos: a) th slctd wlls hav rathr uiform rsrvoir tmpraturs; b) all othr coditios big qual (tmpratur, fluid compositio i th stam phas, tc.), th gas/stam ratio icrass with y. --

5 Tabl givs fluid compositio for som wlls at Lardrllo durig a priod i which thir gaslstam ratio rmaid mor or lss costat. Also show ar stimatd rsrvoir tmpraturs basd o productio data ad i-hol logs. I ordr to vrify whthr qs.() ad (5), at ths tmpraturs, ar cosistt with fluid aalyss, w calculatd y for ach wll from qs.() ad (5) (y ad y ). Th y H S valus obtaid ar plausibl, ad thr is usually a satisfactory agrmt btw th y valus from ths two quatios. As rgards CH, assumig that th Fishr- Tropsch ractio C0 H = CH H 0 is i quilibrium i a two-phas systm, ad that th hypothss mad for qs.() ad (5) ar valid, th: log( ) c0 0g(- H gas WH H0 WH /T log (Y -Y 5 Equatio (7) dos ot satisfactorily xplai th variatios i CH /gas ratio obsrvd i th ara. Accordig to th y valus calculatd from q. (7) (y i Tabl ) th low CH /gas FT ratios should b tid to high y ~alus.~th y valus obtaid from q.(7) i this cas ar, howvr, systmatically highr tha thos obtaid from qs.() ad (5). Not that qs.(), (5) ad (7) ar valid oly for o two-phas sourc of fluid. Followig th modl of D'Amor & Trusdll (979), howvr,t:i fluid may com from various zos of th rsrvoir with possibly diffrt fluid compositios ad physical charactristics. Th y valu obtaid wh assumig a sigl sourc is still maigful i th cas of various sourcs, if th lattr hav th sam tmpratur ad stam phas compositio. I this cas th calculatd trm y -- rprsts th wightd av- B rag of ths trms of th various sourcs. Th corrlatio btw CH ad N at Lardr- llo caot b xplaid by cosidrig ractio (6) as th oly possibl quilibrium cotrollig mtha coctratio. If at last som of th mtha drivs from th products of dcompositio of orgaic itrogous compouds (CYN,H), o th othr had, th CH ad N could b likd togthr i th sam chmical ractio. D'Amor t a.(983) hav cosidrd th ractio: C in 7 H = CH NH 3 I th sam hypothss o chmical quilibria ad th boilig procss formulatd for ractios (),() ad (6), w obtai: log (7 "3 ) log( 7 CH 7 H - - log(- H0 h?h H0 WH H0 WH - -log( -% = IT H0 WH Th y valus calculatd from q.(9> for th fild data (ycn i Tabl ) ar grally i good agrmt with thos calculatd from th H ad H S coctratios (qs.() ad (5)). Th tmporal icrass i CH /gas ratio obsrv- d at Lardrllo occurrd i a zo lyig btw th ctral zo of Lardrllo, with a rlativly high CH/gas ratio, ad th southwstr zo, with a low CH/gas ratio. Sic svral sourcs at varyig dpths cotribut to fluid productio, w may assum that th cotributio from th dpst sourcs gradually icrasd with tim i th zo with a icrasig CH/gas ratio. Fluid compositio thus gradually cam closr to that i th adjact ctral zo, whr th cotributio from dpth was alrady quit larg (D'Amor & Trusdll, 979). Fluid compositio i th zos affctd by rcharg At Lardrllo th costat valus of H ad HS (Figs.lA, A) ar idicativ of a mixig i th stam phas with o furthr chags du to chmical ractios. Th rcharg watr durig its flow towards th ctr of th fild is assumd to b totally vaporizd aftr a rathr short distac. Th stam producd i this way has probably a low gas cott. Durig its flow withi th fild this 'rcharg stam' mixs with th stam ad gas risig from th dpr boilig zos, ad th rsultig fluid is, i part, xtractd -5-

6 through th productiv wlls alog this flowpath. Th gradits obsrvd i th gas/stam ratio ad i isotopic compositio (Calor t a.,98) ca b ascribd to this procss. This gradual mixig procss dos ot chag th H ad HS coctratios i th gas,sic ths coctratios ar almost uiform i th dp stam all alog th flow-path, as idicatd by th H ad HS coctratios obsrvd bfor rcharg ffcts bcam importat i th ara. This is ot th cas for mtha, whos coctratio was ot uiform i th ara. Th lowst valus occur i th southr zo, i.. i th first part of th flowpath of th rcharg fluid. Thus th mixig producd a dcras i CH coctratio i th gas of th wlls furthst from th rcharg aras, which had a high CH cott bfor mixig. \% a xcss of watr is prst as stam i th fluid rsultig from th mixig, th quilibria cosidrd i th systm uaffctd by rcharg grally o logr hold for th mixtur. A w quilibrium is appartly ot attaid for H. This may b th rsult of th lack of a liquid phas, or of th limitd rock volums capabl of itractig with th fluid: if th fluid flows prvaltly through a small umbr of larg fracturs, th th cotact surfac will b small ad th buffr capacity of th rock xt to th walls of th fractur may b usd up or covrd. Not that i th lattr cas quilibrium () may b attaid with o dtctabl chag i %H, but with a icras of f. Th mixig producs 0 a dcras i H/H0 ratio with rspct to th origial fluid, ad fo may icras to a w quilibrium valu by dissociatio of a miimum amout of H0, big fo << fh. This hypothsis could b vrifid zy idpdt dtrmiatio of fo (.g. by CO/CO ratio ma- surmts). With rgard to ractio (, th dilutio phomo dos ot affct th H,;!S ratio i th gas, so that fs should rmai costat. Ractio (6) i th stam phas, o th othr had, could produc importat variatios i gas compositio, as th fluid cotais apprciabl quatitis of all th spcis ivolvd i this ractio. A dcras i K / H~G ratio might lad to th dcras i%hq!c0 ratio obsrvd i svral wlls. Howvr, ractio (6) dos ot xplai th simultaous dcras i N/gas ratio. At Castluovo, which is th zo arst th rcharg ara, fluid compositio is probably affctd by th prsc of a larg amout of liquid watr i th rsrvoir. HS is probably th gas compot most affctd by th prsc of liquid, as it has a high solubility ad ca b quickly oxidizd to HSO-, Vry youg ifiltratio watrs ca b xpctd to rtai a crtai oxidizig capacity. Not that HS could quilibrat with HSO- i th prsc of CaC03 ad Cas0 (with 0.7%H S i th gas ad a gaslstam ratio of 5 litrs STP/ kg as typical valus of th ara), with log fo I = at 00'C.This is still a vry low valu (th valu xpctd i a watr-rock quilibrium at th sam tmpratur is log fo = -). Thr is o idicatio of ay trd to rstor th prvious H/H0 ratio i th fluid wh th gaslstam ratio dcrass, as th majority of th Castluovo data idicat a approximatly costat coctratio of H i th gas. This dos ot xclud that q.(l) r-quilibrats with a highr oxyg fugacity. Calculatios mad at th sam fo cosidrd for HS oxidatio rproduc th oesrvd HIH0 ratio with plausibl low valus of y. Mor procsss ca b cosidrd possibly rsposibl for th strog dcras i %CH i th gas. ) Eaitial ~ =~u~lib~a~i~-a~t~r-m~xi~. At Castluovo th mixig of 'origial stam' ad rcharg fluid occurs i th prsc of a liquid phas which may facilitat chmical ractios. If ractio (6) taks plac, th dcrasd H/H0 ratio tails a strog dcras i %CH i th gas. Howvr, oc agai ractio (6) caot xplai th simultaous dcras i CH ad N Equatio (9), for fixd ' tmpratur ad y ca b r-writt as: CH H CH ) gas WH gas WH N wh log () = 7og ( - m(y) I th study ara / ad cp ar H gas mor or lss costat, so that / should CH gas dcras with a icras i NH3 / gas ad a dcras i oas ~ /H?O. A strog icras i - / ad a dcras i $as/stam ratio ""3 gas hav actually b obsrvd at Castluovo. -6-

7 ... A complt r-quilibratio appars ulikly i th uppr parts of th rsrvoir, cosidrig th fast circulatio of rcharg watr, th imprfct mixig ad th short tim availabl for chmical ractios. ) IaLi_atLozs-iE Zlzw-pfitLp. Th fluid variatios obsrvd at Castluovo durig xploitatio suggst latral fluid flow from th outr towards th ctral ara. Icrasd cotributio from th southwst zo to productio i th ctral ara of Castluovo must lad to a dcras i CH i th gas,with mior chags to H ad HS. $ fact, th southwst zo is charactrizd by vry low coctratios of CH ad N, ad H ad H S coctratios similar to te ctral zo. This procss could also xplai th simultaous variatio i CH ad N. REFERENCES Calor, C., Clati,R., D'Amor, F., Noto, P. (98),"Gochmical Evidc of Natural Rcharg i Lardrllo ad Castluovo Aras", Proc.8th Workshop Gothrmal Rsrvoir Egirig, Staford, Dc.-6,98, D'Amor, F., Calor, C., Clati, R. (98), "Tmporal Evolutio of th Compositio of th Fluid from Srrazzao Zo (Lardrllo)", Proc. 7th Workshop Gothrmal Rsrvoir Egirig, Staford, Dc.5-7, 98, D'Amor, F., Clati, R. (983),"Mthodology for Calculatig Stam Quality i Gothrmal Rsrvoirs", Gothrmics, vo., o.3, 9-0. D'Amor, F., Clati, R., Calor, C. (98) "Fluid Gochmistry Applicatios i Rsrvoir Egirig (Vapor-Domiatd Systms)", Proc, 8th Workshop Gothrmal Rsrvoir Egirig Staford, Dc.-6,98, D'Amor, F., Clati, R., Calor, C. (983) "Hypothss o Possibl Equilibria btw NH ad Othr Gass Prst i Gothrmal 3 Fluids", this Workshop. D'Amor, F., Gialli, G. (98), "Miral Assmblags ad Oxyg ad Sulphur Fugacitis i Natural Watr-Rock Itractio Procsss", Gochim. t Cosmochim. Acta, i prss. D'Amor, F., Trusdll, A.H. (979)," Modls for Stam Chmistry at Lardrllo ad Th Gysrs", Proc. 5th Workshop Gothrmal Rsrvoir Egirig, Staford, Dc.-, 979, 83-97, -7-

8 Tabl. Avrags of gaslstam ratios ad major gas aalyss from Stam sampls from Lardrllo wlls bfor ay rcharg ffcts. Compositios ar i mol prct of total dry gas. "3 is i ppm i th stam codsat. yh, ys, yft ad ycn ar th molar fractios of stam i th stam-watr mixtur i th rsrvoir, computd from qs.(),(5),(7) ad (9) rspctivly. W Yars Trs Gaslstam OC litrs STP/kg %H %H S %CH %N ZCO "3 yh PPm ys 'FT 'CN I N N C3 L? a l i A f * t f f f f.. f. 9.8 f. 5. f.5 3. i f.0 8. f f f. 3. f * f f f i f f.0. f i f..96 * io..53 f f ko..77 f f0.0.9 f f f f h f f 0.0. f f f i f f f i f f f i f f 0..0 f 0.5. f f i i i i * i i i 0..3 i f i i f f i i f i t f f f f f i * f i f f * i * f i i f t f f i i i f i f i * 0..6 * 0..8 i A f t i f f f i i f f * i & I i f f f f * f A t i t A a i 0. 9 f f 0. 7 i A f t 0. 3 f i 0.39 A i f f i i f f f i f f i f * * f f t f i f f f * f f 0.0 f _ i * 0. 7 i f 0. 8 i i & f 0. io8 f f i f i f i

9 Tabl. Variatios i fluid compositio with tim for som wlls affctd by rcharg Wll Yar Gas f s tam %H XH S XCH %N %CO NE 3 litrs STP/kg PPm O O O