G E OL. C RO A T. 49/2 295 - 298 l Tab. 2 Pis. ZAG REB 1996 SCiCllf{j ll' /l o l e Effectiveness of the Acidizing Treatment in Wells Based on the Petrographic Results of Thin-Section Acidizing Experiments Ncvcnka RAS KA.J . Edita GASL.JEVIC-MIL.JO VSKI and Mclika HOT-GRG IC PROCEEDINGS Key word s: Acid izing treatment in th in sections, Petro­ graphic analyses, Solubility of minera ls. Abstract The ac idi zing (reatment is a method usuall y used for Ihe cnhacc ­ menl of produc tivi ty i ll o il-gas fie lds . Treati ng thin -sec ti ons for microscopi c pe trog ra phi c anal yses wit h comm on acid s, establishes the so lubilit y and dispersion o f minerals, as wel l as secondary forma­ tio n damage. The best res ults were achieved by treating carbonale rocks and rocks wilh carbonate const ituents. Si liceolls minera ls, such as q uart? and micas arc nOI dissolved, but the sol ubili l}' o f fe ldspars illcreascs w ith the COl-component in plagioclase ,md thc dcgrce or al te ra ti on. Cl ay minera ls ,tre s ubj ect 10 mec hanica l d is integra tion w hic h causes fUrlher fo rmation damage. Anhyd ri te Hnd o paq ue min­ e ral s arc insolubl c. J. INTRODUCTION Th is method cont ri butes to the beller understand ing o f the effec tiveness or the aci dizi ng treatm ent in well form ati ons , by using lhe ac idi zed th in-section s [or mi croscopic pe trographi c analyses. No simila r method has been round in Ihe literature. Dependi ng on the m inera l composition, th in-sec ­ ti ons were ac id ized with hydrochlori c ac id and with a mixture o f hydrochloric and hydro fl uo ric ac ids which arc mostly useel for treat ment (BI. HUG HES Inc .• 1972; FARLEY e t a i. , 1970; GDANSK I & SHUCHART. 1996) . The main reason fo r thin-section ac idizing is the variabl e minera l com positi o n of secti o ns w ithin the same roek sample. With ac idizi ng treatment o r thin-sec­ tions the solubility o r minerals is di rectly demonstrated as well as the secondary effects caused by the acids. Al though this treat ment is mainly Llsed for enhanc­ ing permeabil ity in sedi menta ry rocks, metamorph ic and effusive rocks were a lso Ircaled . A few examples from the result s are represcntcd. 2. PROCEDURE T hin sections were prepared fro m the rocks o f dif­ fe rent wells (i.e. di fre rent types of sandstones, dol omite wi th anhydr ite, andes ite-basalt and garnet miea schi st), using convent ional meth ods. An acidproo [ resin was Kljucnc rijeci: kise li nska obracla mikroskopskih izbru­ saka, pe trogra[ske ana lize, topi vost mineral.:l. Saictak Kiselinska obrada st ijcna je metoda koja se kori sti za pobo ljsanje prod uktivnos ti naftno- plill ski h po lja . Trc ti ranje m izbru s.tka za m ikros kopskc pct rografske ;lnali zc odgovarajucim ki sc linama ulvrdcna je topivo sl mincra la kao i nak nadna o.~ t ccenj a ro nn acija . Naj bo lje rezultalc topivost i po kazuj u karbo na ti. S ilika lni m inera li poput kvarCil i linjaca su nelopiv i, dok se topivost fe ldspata poveCavil sa sadri ajern Ca u plag ioklasima i st ll pnjem aheracija. Topivosl g tina je upilna zbog nj ihove dis pe rzivnosti u dod iru S otapalom. Anhidrit i pi!" it Sli nc topivi. chosen [or preparation. T he samples were treated under laboratory condi tions, according to standard methods for so lubil ity (API-56) where the temperat ure, the time o f treatin g, type and concent ra ti on of ac id we re sa ti s­ fied , whi le the press ure and infl uence of pore fl ui ds were not. For thin-secti on ac idiz ing two acid so luti ons were lI sed which arc [requently practiccd in Laborato ry or INA Naft aplin : 15% II CI fo r carbonates anel 3% HF + 12% Il e l for silica tes. Each thin sect ion is separa te ly immersed in the appropriate heated so lution dependi ng on the mineral composit ion at a temperature o f 60°C for approx imately 30 minutes, Hnd then immediate ly micro­ scopi cally re-analyzcd and pho tographed , so Ih al the influ ence of ac ids on the minera ls can be de te rm ined . This procedure was app lied for Ihe first lime in the Lab­ oratory o f INA-Naftaplin. 3. RES ULTS Comparison of the petrographic analyses is given on Table I. It is obvious that metamorphic rocks predominantl y composed of quartz and micas are prac ticall y insol uble (Pl ate I, Fig. 1). Pene tratio n o f ac ids is onl y poss ib le a long the open or carbonate cemented fractures. This is con fi rmed in the Gr- IZ well , where the increased pro­ ducti vity aner acidizing is mainly connected wi th disso­ lu tion of the carbonate reservoir rocks. INA-Nartaptin, Laboratory Re ... earch Dept., Lovinciccva [ . HR- l0000 Zagreb, C roa ti.l. 296 Minenlls Quartz K-Fcldspar Plagioclase Micas Clays Sulfates (anhydrite) Pyrite Calcite, dolomi te SOlubility in 15% Hel no no high Solubility in 12 % He l + 3%HF no low to moclerllle low to high no low 10 moderate? no high Table I So lu bil ity of mi neral s accordi ng 10 restllts or pctfo,graphic analyses. Th e solubility of fe ld spars increases with the Ca­ component in plagioclase and the degree of alteration. Thi s is well docu me nted by microph otog raphs of tra­ chyancics itcs (PI. I, Fig. 2), from the oil - fi e ld C rnac, so the aciciizi llg treatme nt should probably improve reser­ voir productivit y. T o e nable the correct determ ination or c lay solubi li ­ ty, additional SEM anal yses arc recomme nd as the magnification or the petrographic mic roscope 's optical sys tcm is too low, anci further morc c lay min era ls arc subject to mec hani ca l disintegration and d ispers ion in contact with solut ion. This propcl1y of pclit ic sediments ve ry oft en cause formatio n pluggin g (13J. HUG H ES Inc ., 1972). Carbonate m incrals show an exce llent reac­ tion wi th the 15% I-ICI sol ution whethe r as prim ary o r secondary constituent s in rocks (Plat e II ). Anhydrite is not di ssolved in 15% He l, but associated wit h dolomi tc it dis integrated afte r the dolomite dissolved (Plate II ). Ac idi zing sand stones en larges the po re space and inc reases the produc ti vi ty of the reservoir, bu t a lso destroys the roek tex ture. Wi th HCI a ll carbona te com­ pone nts (ceme nt and dc tr itus) are di ssolved (Plate II , Fi g. I ) . Hyd rofluori c ac id partly di ssolved and di s­ persed both the c layey matrix and coating . " Matrix Geologia CrO:tlica 49/2 free" insoluble g rains may be re leased and collapsed, causing the I"onnation damage. Opaque minerals c.g. pyrite and il menite are mainly res istant (Plate J). 4. CONCLUSIONS AND RECOMMENDAT IONS The result s indi cate th at ac id izing treatm e nt wi th hydrochloric acid is complete ly sufficient for carbona te rock s, and in clastic sediments cemented by carbonates, as we ll as for clastic sedime nts wi th high co nte nt s o f carbonate de tritus (rock fragments, foss ils, e tc .), i. e . in li th ic a re ni tes, calc li th ites and calcare nites . In "'dirty" sand s tones (g raywac kcs) w ith a m at rix mainl y com ­ poscd of a mi c roc rys ta lline aggregate of quart z and seri c it e , c lays, sodic plagioclase e tc. ac idi z in g with mixture of hydrochloric and hydrofluoric acid seems to be rather ineffective than useful. Th e acicii z in g treat­ me nt is a lso recomme nded in s tron g ly tcc tonized nonsedimc ntary rocks with a hi gh fracture porosi ty o r with ves ic ul ar " porosity" as in e ffus ives. ror the compre he ns ive e ffec tiveness o f