European Geothermal Drilling Experience-Problem Areas and Case Studies

Geothermal drilling has long been restricted in Western Europe to the sole dry steam field of Larderello in Italy. In the last few years, a wider experience is building up as a consequence of intensified exploration and development programs carried out for evaluation and production of both low- and high-enthalpy geothermal resources. A sample of some 40 boreholes indicates the problem areas which are given

Effects of autologous bone marrow stem cell transplantation on beta-adrenoceptor density and electrical activation pattern in a rabbit model of non-ischemic heart failure

BACKGROUND: Since only little is known on stem cell therapy in non-ischemic heart failure we wanted to know whether a long-term improvement of cardiac function in non-ischemic heart failure can be achieved by stem cell transplantation. METHODS: White male New Zealand rabbits were treated with doxorubicine (3 mg/kg/week; 6 weeks) to induce dilative non-ischemic cardiomyopathy. Thereafter, we obtained autologous bone marrow stem cells (BMSC) and injected 1.5–2.0 Mio cells in 1 ml medium by infiltrating the myocardium via a left anterolateral thoracotomy in comparison to sham-operated rabbits. 4 weeks later intracardiac contractility was determined in-vivo using a Millar catheter. Thereafter, the heart was excised and processed for radioligand binding assays to detect β(1)- and β(2)-adrenoceptor density. In addition, catecholamine plasma levels were determined via HPLC. In a subgroup we investigated cardiac electrophysiology by use of 256 channel mapping. RESULTS: In doxorubicine-treated animals β-adrenoceptor density was significantly down-regulated in left ventricle and septum, but not in right ventricle, thereby indicating a typical left ventricular heart failure. Sham-operated rabbits exhibited the same down-regulation. In contrast, BMSC transplantation led to significantly less β-adrenoceptor down-regulation in septum and left ventricle. Cardiac contractility was significantly decreased in heart failure and sham-operated rabbits, but was significantly higher in BMSC-transplanted hearts. Norepinephrine and epinephrine plasma levels were enhanced in heart failure and sham-operated animals, while these were not different from normal in BMSC-transplanted animals. Electrophysiological mapping revealed unaltered electrophysiology and did not show signs of arrhythmogeneity. CONCLUSION: BMSC transplantation improves sympathoadrenal dysregualtion in non-ischemic heart failure

Of Black Swans and Tossed Coins: Is the Description-Experience Gap in Risky Choice Limited to Rare Events?

When faced with risky decisions, people tend to be risk averse for gains and risk seeking for losses (the reflection effect). Studies examining this risk-sensitive decision making, however, typically ask people directly what they would do in hypothetical choice scenarios. A recent flurry of studies has shown that when these risky decisions include rare outcomes, people make different choices for explicitly described probabilities than for experienced probabilistic outcomes. Specifically, rare outcomes are overweighted when described and underweighted when experienced. In two experiments, we examined risk-sensitive decision making when the risky option had two equally probable (50%) outcomes. For experience-based decisions, there was a reversal of the reflection effect with greater risk seeking for gains than for losses, as compared to description-based decisions. This fundamental difference in experienced and described choices cannot be explained by the weighting of rare events and suggests a separate subjective utility curve for experience

Evaluation of the mutagenic effects of SV40 in mouse, hamster, and mouse-human hybrid cells

We have examined the ability of SV40 to induce changes in drug or temperature resistance in mouse, hamster, and mouse-human hybrid cells. SV40 induced a substantial increase of cells resistant to 5-bromodeoxyuridine + trifluorothymidine in Balb/c 3T3 cells and induced an increase of hybrid cells resistant to 6-thioguanine. SV40 was found to be nonmutagenic or weakly mutagenic in other test systems. The 3T3 cells were T-antigen positive, exhibited a marked reduction in TK activity, were heterogeneous for [ 3 H]BrdU incorporation by autoradiography, and exhibited instability of the drug-resistance phenotype, suggesting that SV40 may be inducing resistance by an epigenetic process. SV40-induced 6-thioguanine resistance in the hybrids appears to occur predominantly by chromosome loss.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45539/1/11188_2005_Article_BF01233058.pd

EUROPEAN GEOTHERMAL DRILLING EXPERIENCE- PROBLEM AREAS AND CASE STUDIES

ABSTRACT Geothermal d r i l l i n g h a s long been restricted i n Western Europe t o t h e sole d r y s t e a m f i e l d of L a r d e r e l l o i n I t a l y . I n t h e l a s t f e w y e a r s , a wider e x p e r i e n c e i s b u i l d i n g up a s a consequence of i n t e n s i f i e d explor a t i o n and development programs c a r r i e d o u t for e v a l u a t i o n and product i o n of both l o w -and high-enthalpy geothermal resources. A sample Of some 40 boreholes i n d i c a t e s the following problem areas. 1. Low-Enthalpy D r i l l i n 9 Due t o s i m i l a r s e t t i n g s --h o t water system flowing i n sedimentary u n i t s a t t e m p e r a t u r e s and d e p t h s ranging f r o m 40" t o 140°C (104" t o 284°F) and from 1,000 t o 3,500 metres (3,281 t o 11,484 f e e t ) , r e s p e c t i v e l y --the technology h e r e i s s t r o n g l y dependent on o i l and g a s d r i l l i n g p r a c t i c e . S t i l l , s p e c i f i c problems remain i n t h e areas o f multipler e s e r v o i r reconnaissance and w e l l completion a t production and r e i nj e c t i o n l e v e l s , p a r t i c u l a r l y i n p o o r l y c o n s o l i d a t e d f l u v i o -d e l t a i c sequences l e a d i n g t o sand c o n t r o l and s w e l l i n g c l a y problems. Expert i s e needs t o be developed t o minimize costs, s e c u r e h i g h production c a p a c i t i e s , long lifetimes, and minimum maintenance compatible w i t h t h e economics and t h e l a c k of s u i t a b l e workover f a c i l i t i e s . . High-Enthalpy D r i l l i n g Exploratory d r i l l i n g i s c u r r e n t l y combining w i l d c a t t i n g and deeper i n v e s t i g a t i o n s of known f i e l d s . Lost c i r c u l a t i o n , d r i l l s t r i n g corros i o n , t u b u l a r s , mud, cementing, and d e v i a t i o n c o n t r o l are t h e m o s t f r e q u e n t l y encountered d i f f i c u l t i e s w h i l e d r i l l i n g i n h o s t i l e waterdominated environments. Formation temperatures i n excess o f 300°C (572°F) are o f t e n t h e r u l e , and r e c e n t d r i l l i n g conducted i n v o l c a n i c areas have h i t f l u i d s approaching s u p e r c r i t i c a l s t a t e . Whenever these problems do n o t remain under c o n t r o l , t h e y r e s u l t i n r i g standby and e x t r a costs which e v e r e l y p e n a l i z e an i n d u s t r y which needs s h a r p improvements t o be Geothermal w e l l s t i m u l a t i o n i s t h e r e f o r e a f i e l d of growing i n t e r e s t , b u t it lacks adequate procedures. Current geothermal d r i l l i n g p r a c t i c e i n Europe i s i l l u s t r a t e d by three t y p i c a l case s t u d i e s . l l y r e l i a b l e and cost e f f e c t i v e . 5-1 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. Ib!TRODUCTION ci P r i o r t o 1974 geothermal energy (GE) was more o r l e s s regarded b y European (EC) S t a t e s an e x o t i c c u r i o s i t y o f marginal impact although, n o t mentioning Roman ages, a t l e a s t two c o u n t r i e s had-long been i n v o l v e d i n geothermal m a t t e r s -I t a l y i n power generation from d r y steam sources (Larderello, 19041 and France i n d i r e c t use of low enthalpy a q u i f e r s (Melun 1'Almont d i s t r i c t h e a t i n g doublet, 1969). I n t h e 1960's ENEL, t h e i t a l i a n power agency, who had b u i l t up i t s d r i l l i n g and p r o d u c t i o n experience on t h e L a r d e r e l l o fieldflextended e x p l o r a t i o n t o o t h e r areas o f Tuscany and Northern Latium. T h i s l e d t o t h e d i s c o v e r y o f t h e T r a v a l e and Monte Amiata d r y steam and t h e Cesano p r e s s u r i z e d water systems, Since then, and as a consequence o f t h e energy p r i c e c r i s i s , EC c o u n t r i e s r e v i s i t e d GE and a pronounced involvement c o u l d be noticed. I n p a r t i c u l a r a R & D programme i n GE, common t o n i n e member Statesa, was launched i n 1975 by t h e Commission o f t h e European Communities (CEC) adressing a l l aspects o f geothermal research, from e x p l o r a t i o n t o production, i n which w i l d c a t t i n g t o o k a n important p a r t ( 8 ) . I n s p i t e of a very recent a c t i v i t y i n d r i l l i n g f o r low grade heat and i n a c t i v e v o l c a n i c areas, t h e european geothermal w e l l record i s n o t n e g l i g i b l e as . shown i n Table 1. I t p r o v i d e s a l r e a d y a s u i t a b l e data base f o r r e v i e w i n g major problem areas, associated w i t h e x p l o r a t i o n and p r o d u c t i o n d r i l l i n g , w i t h respect t o (i) d r i l l i n g and completion technology, (ii) environmental c o n s t r a i n t s and (iii) economics. For t h e sake o f s i m p l i f i c a t i o n , d r i l l i n g i s d i v i d e d according t o t h e enthalpy o f t h e geothermal f l u i d , as d r i l l i n g o f h i g h and low enthalpy w e l l s adresses i n f a c t c o n t r a s t e d g e o l o g i c a l and thermal environments and d i f f e r e n t usage and economics. Three case studies, s e l e c t e d among deep e x p l o r a t i o n p r o j e c t s supported b y t h e CEC, w i l l i l l u s t r a t e t h e p r a c t i c a l problems encountered i n geothermal d r i l l i n a . GEOTHERMAL EP!VIRONMENTS The g e o l o g i c a l s t r u c t u r e o f Europe and consequently i t s geothermal environments a r e commanded by t h e geodynamic e v o l u t i o n o f t h e Eurasian Plate. I t d i s p l a y s a v a r i e t y o f geodynamic settings, most p l a t e t e c t o n i c a t t r i b u t e s b e i n g present except t h e oceanic crust. Western Europe a t l a r g e i s an area o f o l d and r i g i d c o n t i n e n t a t c r u s t c h a r a c t e r i z e d b y c r y s t a l l j n e massifs, i n t r a c r a t o n i c and foredeep b a s i n s and c o n t i n e n t a l r i f t i n g . A younger c r u s t s t r e t c h e s over t h e Mediterranean area, where t h e A f r i c a n and Eurasian P l a t e s c o l l i d e , e x h i b i t i n g t y p i c a l f e a t u r e s -subduction, m a r g i n a l basins, i s l a n d a r c s and e x t e n s i o n a l h o r s t and graben systems. c of Germany, France, Ireland, I t a l y , Luxembourg, Greece w i l l j o i n t h e EC i n 1981. 5-2 These environments a r e i r r e g u i a r l y d i s t r i b u t e d though. An important consequence i s t h a t t h e m a j o r i t y of E€ States a r e faced w i t h t h e s o l e low enthalpy outlook, h i g h enthalpy sources being l i m i t e d t o Central and Southern I t a l y and t o i a s t e r n Greece. -h i g h e n t h a l y r e s e r v o i r s placed along t h e mediterranean b e l t , c h i e f l y t h e Tuscanian d i s t e n s i v e t e c t o n i c system, t h e Northern Latium and Campanian p l i oquaternary volcanism and t h e subduction magmatism o f t h e E o l i a n Arc. DRILLING DATA EASE Over 800 r e l l s have been d r i l l e d i n Western Europe f o r geothermal purposes, since t h e e a r l y 1900's, w i t h a success r a t i o o f 56 % c l o s e t o t h e w o r l d average. Note,in work over LOW ENTHALPY DRILLING Most o f t h e o i l and gas d r i l l i n g background i s t r a n s f e r a b l e t o h o t water d r i l l i n g which r e q u i r e s a s i m i l a r technology owing t o a s i m i l a r environmenta q u i f e r s f l o w i n g i n sedimentary rocks a t depths and temperatures v a r y i n g from 40 t o 1 4 O O C and 1000 t o 3500 m r e s p e c t i v e l y . S t i l l , t h e r e a r e a few s t r i k i n g d i f f erences i n t h e areas o f w e l l completion, reconnaisance, d r i l l i n g f l u i d s , environmental c o n s t r a i n t s and,of course,economics. Basically,low grade heat b e i n g o f low market v a l u e i s n o t y e t t r a n s p o r t a b l e economically over d i s t a n c e s g r e a t e r t h a n 5 km. D r i l l i n g costs,elsewhere,are h i g h (see bounds develgpment t o d i s t r i c t h e a t i n g o f l a r g e c i t i e s . L Well Completion ---------------A major d i f f i c u l t y i n geothermal energy i s t h e adequate cementing o f columns i n c l u d i n g l i n e r hangers, a problem w i t h which one has t o l i v e . Most carbonate geothermal r e s e r v o i r s a r e produced (and i n j e c t e d ) i n open h o l e -Dogger i n t h e P a r i s Basin, carboniferous k a r s t i f i e d limestone i n Belgium and createceous d o l o m i t e i n A q u i t a i n e -and exhibit,due t o t h e i r f r a c t u r e dominated porosities, h i g h y i e l d s . T r i a s s i c c l a s t i c d e p o s i t s (sandstone and interbedded c l a y ) a r e l e s s dependable r e s e r v o i r s because o f random p o r o s i t y t r e n d s (compaction, diagenesis) and r e q u i r e s u i t a b l e w e l l completion. The rule,below 1500 m,i s t o a v o i d t h e placement o f a g r a v e l pack which i s considered a t o o r i s k y operation. A good substitute,already experienced i n underground gas storage,is t o set down h o l e w i r e wrapped screens o f e i t h e r o i l and gas o r superweld t y p e ( 1. A c l a s s i c a l design would be a 0 6" 518 s t a i n l e s s 316L superweld s c r e e $ w i t h a 0.8 mm slot,on a w e l l p r o d u c i n g from a sand and sandstone m a t r i x . No sand c o n t r o l problems have y e t appeared. I n t h e c e n t e r o f t h e P a r i s B a s i n c o e x i s t t h e p o s s i b i l i t y o f producing e i t h e r t h e Dogger o r t h e T r i a s r e s e r v o i r s . The Dogger i s a r e l i a b l e open h o l e producer but i s c o o l e r whereas t h e T r i a s i s h o t t e r but random as t o p o r o s i t i e s and moreover i t r e q u i r e s a completion. Only d i r e c t assessment by t e s t i n g can o p t i m i z e t h i s balance. There a r e t h r e e p o s s i b l e s t r a t e g i e s : i. Target r e s e r v o i r : T r i a s 5-4 -P r o d u c t i o n o f t h e Dogger by r e l e a s i n g t h e "Ken Flow" through p e r f o r a t i o n s or -The 0 9" 5 / 8 becomes t h e r e i n j e c t i o n column. -D r i l l i n g o f t h e p r o d u c t i o n borehole, o b j e c t i v e Dogger. t h e b 8" 112 open h o l e a f t e r p u l l i n g out t h e 0 7" l i n e r . O i l Based Muds _-------------I n general, r e l u c t a n c e i s f r e q u e n t l y n o t i c e d among p r o f e s s i o n a l s towards using o i l base muds d e s p i t e t h e i r many advantages,principally because o f environmental (disposal and f i r e ) and economical considerations. I n one case use o f these muds i s reported s a t i s f a c t o r y ( 9 ) d u r i n g most o f t h e d r i l l i n g sequence. The s i t e c o n s t r a i n t s were such -urban area, l i t t l e a v a i l a b l e space, t a r g e t f o r m a t i o n d i s t a n t from t h e d r i l l i n g p l a t f o r m -t h a t i t was necessary t o d r i l l i n a 0 8" 1 / 2 diameter a t a 400 d r i f t angle w i t h l i g h t muds ( d e n s i t y o f 1.04 -1.051, no reserve p i t and no p o l l u t i o n o f t h e environment. O i l based mud were found t o best meet these o b j e c t i v e s by t h e i r a b i l i t y t o (i) secure f a s t d r i l l i n g r a t e s by minimizing f r i c t i o n and s t i c k i n g r i s k s associated w i t h s l a n t d r i l l i n g , (ii 5-5 I t enabled t o remove 54 % of t h e s o l i d s and t o recover 89 % o f t h e emulsion o f t h e d e s i l t e r heavy e f f l u e n t s . D r i l l i n g could be completed i n 20 days from 200 t o 1330 m ( w i t h l e s s than 5 % r e d r i l l i n g time, and below t h e p r o j e c t e d budget. Density was always kept a t 1.05, Noise -----Conventional, non e l e c t r i c , petroleum r i g s used i n geothermal d r i l l i n g can generate n o i s e up t o €5 dbA whereas l e g a l environmental r e g u l a t i o n s set t h e l i m i t s i n urban areas, w i t h heavy t r a f f i c , t o 45 ?. 15 = 60 dbA and 45 + 5 = 50 dbA d u r i n g day and n i g h t t i m e . Excess n o i s e margins are of 5 and 3 dbA r e s p e c t i v e l y . Waiting f o r e l e c t r i c i t y d r i v e n rigs, temporary measures can achieve t h e o b j e c t i v e o f d r i l l i n g a t t o l e r a b l e n o i s e levels. They consist, among others,of : -suppressing generator sets,replaced by connections t o t h e grid, -o r i e n t i n g t h e d r i l l i n g p l a t f o r m and n o i s y engines according t o t h e d i r e c t i o n -s e t t i n g m u f f l e r s and hoods on engines, -b u i l d i n g a n t i -n o i s e walls, b a t t e r s or wooden screens, -avoiding n o i s y operations a t n i g h t : stem manoeuvers, cementing, logging. They r e s u l t i n a d d i t i o n a l costs estimated a t 100,000 US S.. o f dominant winds, O i l and gas d r i l l i n g p r a c t i c e versus geothermal r e s e r v o i r e v a l u a t i o n The f o l l o w i n g case study (6) may w e l l i l l u s t r a t e t h e problems l i k e l y t o be encountered when a p p l y i n g s t r a i g h t forward petroleum r o u t i n e t o geothermal explorat i o n . 5-6 Major events : incomplete cementing o f t h e 13" 3/8 -9" 5/8 l i n e r hanger. A t 2000 m a mini e r u p t i o n o f o i l and d i s s o l v e d gas i s c o n t r o l l e d b y t h e crew. I t causes a strenghtening o The f i r s t caused denser d r i l l i n g f l u i d s and exaggerated h o l e control, t h e second r e s u l t e d i n f o r m a t i o n damage, t h e t h i r d i n a funnel shaped columr),the cheapest mud formula and t o o f a s t d r i l l i n g r a t e s a t r e s e r v o i r l e v e l . Consequences : s i d e t r a c k from r e s e r v o i r top, w i t h s a l t w a t e r as a d r i l l i n g f l u i d and a m o d i f i e d h o l e geometry (7" casing t i e d back t o t h e former 7" -9" 5/23 l i n e r hanger). Subsequent e x t r a costs a r e estimated a t 1.2 m i l l i o n US 8 . HIGH ENTHALPY DRILLING Genera 1 -------For many years h i g h enthalpy d r i l l i n g -w a s bound t o Tuscan d r y steam f i e l d s and i t i s o n l y r e c e n t l y t h a t more d i v e r s i f i e d s e t t i n g s have entered t h e geothermal d r i l l i n g and p r o d u c t i o n p i c t u r e . O f t h e 814 w e l l s d r i l l e d a t temperatures above 15OOC about 30, d r i l l e d i n t h e past decade, d e a l t w i t h water dominated systems. Relevant environments can e x h i b i t formation temperatures i n excess of 4OOOC and a bottom h o l e t e m p e r a t u r e o f 300OC i s a q u i t e common f i g u r e . CO c o n t e n t s a s h i g h as 80 % a r e sometimes n o t i c e d i n t h e e a r l y producing stage (Torre ALfina). Reservoir rocks include : q u a r t z i t e , p h y l l i t e and a n h y d r i t e (Larderello, Travale, Monte Amiata, P i a n Castagnaio) -Carbonate (limestone and d o l o m i t e ) rocks (Larderello, Travale, Torre A l f i n a , Cesano, L a t e r a l -a l k a l i n e tuffaceous products (Mofete, San V i t o ) -micaschists, c r y s t a l l i n e and metamorphic rocks (Milos). But i n general i t i s l e s s t h e n a t u r e o f t h e rock than i t s f r a c t u r e d porosity, t h e dominant f e a t u r e o f geothermal reservoirs, which i s a problem. Hard and a b r a s i v e rocks a r e d r i l l e d a t e s w i t h conventional t o o l s . Lengths f 75, 100 and even 125 m d r i l l e d w i t h t h e same t o o l a r e n o t uncommon i n geotherma b i t records. I t i s more a m a t t e r o f t o o l t o f o r m a t i o n adequacy than o f adequate t o o l a v a i l a b i l i t y . 2 Fractures, e s p e c i a l l y when massive as i n t h e L a r d e r e l l o f i e l d , associated w i t h h i g h temperatures cause t h e major problems as t o l o s t c i r c u l a t i o n control, cementing jobsand s e t t i n g o f m u l t i p l e casingsThese d i f f i c u l t i e s a r e t h e major reasons t o t h e t w o f o l d increase o f geothermal d r i l l i n g costs as compared t o e q u i v a l e n t o i l and gas operations. S t a t i s t i c s , c a r r i e d out on a sample o f 20 w e l l s d r i l l e d a t depths o f 1000 t o 1500 m w i t h s i m i l a r technologies show (Munier Jolain, personal communication, 19801, f o r hydrocarbon versus geothermal : t o t a l d u r a t i o n (days) 30/50, e f f e c t i v e d r i l l i n g t i m e (%I 35/20, t o o l l i f e t i m e i d e n t i c a l , c o n t r o l (losses, cementing. ..) and i n s t r u m e n t a t i o n (%I 13/30. L Although l o s t c i r c u l a t i o n s a r e n o t t o be overlooked (see example f u r t h e r ) t h e y a r e u s u a l l y kept under c o n t r o l by cement p l u g s -up t o 17 a r e r e p o r t e d o n a j o b a t 120 m depth. So cementing i s t h e major p u z z l e e s p e c i a l l y when c o n s i d e r i n g t h a t squeeze and stage cementing a r e r u l e d o u t i n geothermal wells. A r u l e f r e q u e n t l y adopted c o n s i s t s o f p l a c i n g t h r e e successive columns (17" 112 -13" 3 / 8 -9" 518 -7") continuously, cemented b u t n o t f i e c e s s a r i l y over t h e e n t i r e length, w i t h expansion s p o o l s a t w e l l head. S l o t t e d l i n e r s a r e more a makeshift t h a n a need. A standard cement u t i l i z e d i n t h e EC i s t h e I t a l c e m e n t i (ieotherm Class G cement, based o n 40 % S i l i c a f l o w e r which copes M i t h temperatures o f 3OOOC u s i n g adequate a d d i t i v e s and r e t a r d e r s ( 4 ) . A t y p i c a l h i g h temperature w e l l i s shown i n l i g n i t e s and non-ionic surfactants; b e n t o n i t i c muds w i t h FCL/CL s t a b i l i z e d b y Synergetic Polymer Blend; b e n t o n i t i c muds w i t h h i g h ferro-chromo-lignosulfonate/ chromo-Lignite contents, p r o t e c t e d and s t a b i l i z e d by a s p h a l t i c o i l d i s p e r s e d products. I t can be seen i n I n general h i g h temperature do n o t cause major problems p r o v i d e d t h e r e i s adequate cooling. Some l o g g i n g companies c l a i m temperature t o l erance up t o 26OOC f o r main t o o l s but these l i m i t s were n o t checked i n h o l e i n EC wells. As t o bottom h o l e measurements Ameradas and p r e f e r a b l y K u s t e r mechanical devices m o d i f i e d f o r h i g h temperature service ,are found t h e most r e l i a b l e and have been used up t o 370OC o v e r s e v e r a l hours. S i n g l e shot i n s t r u m e n t s mounted o n t h e rmal s h i e l d s have proved e f f e c t i v e a t 4OOOC and 1.5 hours, ( 4 ) . Open h o l e packers have f a i l e d a f t e r 12 hours o p e r a t i n g i n a 2OOOC environment i n t h e Phlegreaen f i e l d s . Down h o l e h i g h temperature hardware i s being developed w i t h t h e support o f t h e EC i n t h e f o l l o w i n g areas : pressure and temperature gauge (3OO0C, 500 bars), flowmeter-caliper (240OC, 250 bars), f l u i d sampler (24OoC, 250 bars). Research i s a l s o conducted f o r t h e design o f an i n h o l e o p t i c a l f i b e r p r i n c i p l e f o r power and s i g n a l t r a n s m i s s i o n A l o g g i n g f a c i l i t y w i t h a 4000 m T e f l o n i n s u l a t e d c a b l e i s c u r r e n t l y o p e r a t i n g i n I t a l y . I n Europe a h i g h enthalpy w e l l i s considered commercial f o r a n e t p r o d u c t i v e c a p a c i t y o f 1.5 MW . Costs o f geothermal w e l l s sunk a t depths o f 2 000 and 2 500 m average 1.9 and 2.8 m i l l i o n US 8 r e s p e c t i v e l y . S l i m h o l e ( p r o j e c t e d n o t a c t u a l ) costs f o r s i m i l a r depths amount t o 500,000 and 650,000 US 8 . Environmental const r a i n t s may add 30 % t o t h e b i l l (Campi F l e g r e i ) . According t o j o i n t AGIP and 5-8 S l i m h o l e w i l d c a t t i n g w i l d c a t s p r o g r e s s i v e l y replace t h e so c a l l e d shallow g r a d i e n t holes u t i l i s e d i n t h e past f o r heat f l o w mapping purposes. These s l i m holes, d r i l l e d e s s e n t i a l l y on geop h y s i c a l i n d i c e s down t o depths of 2000 m w i t h a 0 6" t e r m i n a l open h o l e diameter, a r e shown t o be t h r e e times cheaper than an o r d i n a r y e x p l o r a t o r y borehole. They a r e n o t designed as p r o d u c t i o n w e l l s and t h e i r use i s t h e r e f o r e r e s t r i c t e d t o g e o l o g i c a l reconnajssance, pressure -temperature measurements and r e s e r v o i r evaluation. As a result, s l i m h o l e w i l d c a t t i n g should enable,(i) t o speed up exploration, (ii) t o cut down e x p l o r a t i o n costs, (iii) t o c a l i b r a t e geophysics and d e r i v e r e l e v a n t conceptual models and, the

European Geothermal Drilling Experience. Problem Areas and Case Studies

Geothermal drilling has long been restricted in Western Europe solely to the dry steam field of Larderello in Italy. In the last few years wider experience has been gained as the consequence of intensified exploration and development programs carried out for evaluation and production of both low- and high-enthalpy geothermal resources. A sample of some 40 boreholes indicates the following problem areas. 1) Low-enthalpy drilling Due to similar settings -hot water system f lowing in sedi-mentary units at temperatures and depths ranging from 40° to 140°C (104° to 284°F) and from 1,000 to 3,500 metres (3,281 to 11,484 feet), respectively - the technology here is strongly dependent on oil and gas drilling practice. Still, specific problems remain in the areas of multiple-reservoir reconnaissance and well completion at production and reinjection levels, particularly in poorly consolidated fluvio-deltaic sequences leading to sand control and clay swelling problems. Expertise needs to be developed to minimize costs, secure high production capacities, long lifetimes, and minimum maintenance compatible with the economics and the lack of suitable workover facilities. 2) High-enthalpy drilling Exploratory drilling is currently combining wildcatting and deeper investigations of known fields. Lost circulation, drill string corrosion, tubulars, mud, cementing, and deviation control are the most frequently encountered difficulties while drilling in hostile water-dominated environments. Formation temperatures in excess of 300°C (572°F) are often the rule, and recent drilling conducted in voleanic areas have hit fluids approaching the supercritical state. Whenever these problems do not remain under control, they result in rig standby and extra costs which severely penalize an industry which needs sharp improve-ments to be fully reliable and cost effective. Geothermal well stimulation is therefore a field of growing interest, but it lacks adequate procedures. Current geothermal drilling practice in Europe is illustrated by three typical case studies