Pressure Buildup Monitoring of the Krafla Geothermal Field, Iceland

A break in electrical power generation from the Krafla geothermal plant was planned from beginning of May to early September 1984. Early in June most of the production wells were shutin and their pressure recovery monitored. A regular monitoring of the pressure buildup was carried out on a well to well basis until mid-August, when the wells were put back into production except for wells 12 and 16. They were used to monitor the pressure drawdown due to the start of production. This was abruptly brought to an end by a nearby volcanic eruption in early September. The pressure buildup in the two-phase geothermal reservoir at Krafla is described and the first results presented. The results are compared with parameters determined on the completion of the wells and with predictions from numerical simulations of the reservoir. Finally the status of the Krafla geothermal system is discussed with regard to the comparison

World geothermal assessment

ABSTRACT The terrestrial energy current flowing from the mantle to the surface of the Earth is more intense at plate boundaries than within the tectonic plates. At the surface of the Earth, the most obvious manifestations of this energy current are active volcanoes and high temperature geothermal fields. Geothermal assessments have only been carried out for a limited number of countries or regions, while the distribution of active volcanoes in the world is fairly well known. As the volcanoes and the high temperature geothermal fields are manifestations of the same energy current, the distribution of active volcanoes should reflect the geothermal potential of the world. An empirical relation between the number of active volcanoes and the technical potential of high temperature geothermal fields in 8 regions of the world has been established in the paper. This relation is consequently used to estimate the technical potential of high temperature geothermal fields in the world as a whole. The result is that the most likely value for the technical potential of geothermal resources suitable for electricity generation is 240 GW e . Theoretical considerations based on the conditions in Iceland and USA reveal that the magnitude of hidden resources is expected to be 5-10 times larger than the estimate of identified resources. If this is the case for other parts of the world, the upper limit for electricity generation from geothermal resources is in the range 1 -2 TW e . Furthermore, the frequency distribution of the temperature of geothermal resources in Iceland and USA indicates that the magnitude of low-temperature geothermal resources in the world is about 140 EJ/year of heat. For comparison, the world energy consumption is now about 420 EJ/year. Comparing estimates of the generating capacity for individual geothermal fields obtained on one hand by simulation models and on the other hand by the volumetric method indicates that the volumetric method gives on the average 4 -5 times higher values than simulation models. Taking the results of the simulation models as an estimate of the lower limit of the geothermal potential, the lower limit of the world geothermal potential for electricity generation is estimated to be about 50 GW e and the corresponding value for direct use to be 1 TW th

PRESSURE BUILDUP MONITORING OF THE KRAFLA GEOTHERMAL FIELD, ICELAND

Below there is a two-phase reservoir with temperatures and pressures correspondlng t o the boiling curve with depth. This reservoir directly underlies a confining layer at 1100-1300 II depth and extends to depths greater than 2200 m. This division into upper and deeper reservoirs does not extend across the Hveragil gully and in the Sudurhlidar-field only the two-phase liquid-dominated reservoir seems to be present. The reservoirs in the Leirbotnar-Sudurhlidar fields seem to be connected near the Hveragil gully. In accordance with sales contracts, a break in electrical power generation from the Krafla geothermal power plant was planned from the beginning of May to early September 1984. This break was used to monitor the pressure recovery in the production fields in Leirbotnar and Sudurhlidar. Preparation for the work started in late May with the condltfon o f the wells being checked. A drillout operation was planned for wells 3 and 9 in July, but had to be put toward to early June. Due to this well 9 was shutin on June lst, or before the regular monitoring project started. Therefore, pressure buildup started in the upper reservoir in the Leirbotnar-field before the other wells were shutin. -'17 7- The monitoring project started on June 4th with the shutin of well 16 in the Sudurhlidarfield. Other wells there were shutin two days later. The same procedure was used for the Leirbotnar-field and started on June 7th with the shutin of well 12. Well 7 was kept in production to keep the pipelines hot. Wells 9 and 3 were drilled out during the period June 13-20th. Soon after shutin a high wellhead pressure 0 7 0 bar) had built up in well 14. T o ,eliminate the risk of damaging the wellhead equipment, the .well was opened up again only two dayis after shutin. Similarly, well 13 had to be put on restricted flow five days after shutin. The p l a n was t o m o n i t o r u n t i l mid-September t h e i r p r e s s u r e drawdown due t o t h e s t a r t o f p r o d u c t i o n . However, t h i s was a b r u p t l y b r o u g h t t o an end by a nearby v o l c a n i c e r u p t i o n on September 4 t h , 1984. The v o l c a n i c a c t i v i t y has p r e v i o u s l y caused in t h e l i q u i d p a r t of t h e system a l a r g e p r e s s u r e i n c r e a s e , w h i c h is an o r d e r o f magnltude l a r g e r t h a n t h e e f f e c t caused by p r o d u c t i o n in t h e f i e l d ( S t e f a n s s o n , Pressure was monitored regularly in wells 1981; S i g u r d s s o n and Tiab, 1983). ANALYSIS OF DATA The wells where w a t e r l e v e l was measured r e g u l a r l y had n o t been p r o d u c i n g f o r y e a r s b e f o r e t h e s h u t i n o f t h e f i e l d s , e x c e p t f o r well 3. I n t h e L e i r b o t n a r -f i e l d t h e s e wells The e a r l y b u i l d u p is due t o t h e p r e m a t u r e s h u t i n o f well 9, b u t t h e n t h e r e is a change in t h e s l o p e a f t e r about 320 hr, w h i c h is due t o t h e s h u t i n o f well 3 . U s i n g t h e t i m e o f i n t e r c e p t l o n and o t h e r a v a l la b l e d a t a f o r t h e upper r e s e r v o i t ( P r u e s s e t . a l . , 1984). t h e d i s t a n c e between w e l l s 10 and 3 is c a l c u l a t e d as 535 m, b u t t h e measured d i s t a n c e between t h e i r w e l l h e a d s on t h e s u r f a c e is about 540 m. p o r o s i t y a n a l y t i c a l model. R e s u l t s f r o m t h e match a r e p r e s e n t e d i n t a b l e 1, and i n d i c a t e 'a n e g a t i v e s k i n f o r t h e well, h i g h w e l l b o r e s t o r a g e and f r a c t u r e s w i t h r e s t r i c t e d f l o w c a p a c i t i e s . Uell 13 in L e i r b o t n a r is d i r e c t i o n a l l y d r i l l e d t o e a s t and c u t s a NNE-SSU d i r e c t e d n e a r v e r t i c a l f r a c t u r e a l o n g t h e H v e r a g i l g u l l y . The well had n o t y e t s t a b i l i z e d , so t h e d a t a is f i t t e d w i t h an i n f i n i t e a c t i n g svstem. I t i n d i c a t e s a h i o h w e l l b o r e s t o r a g e w i t h t h e and a p o s i t i v e s k i n . T h i s -agrees f a c t t h a t s c a l i n g is o c c u r r i n g in Due t o t h e h i g h w e l l b o r e s t o r a g e and p o i n t s a match w i t h a v e r t i c a l f r a c was n o t o b t a i n e d . t h e w e l l . few d a t a u r e model I n F i g u r e 5 t h e d a t a p o l n t s and match f o r well 15 i n L e i r b o t n a r is shown. Some d i f f l c u l t i e s were in m a t c h i n g t h e d a t a because of i t s b e h a v i o r f o r t h e f i r s t 30 h o u r s , w h i c h may b e caused by t h e h i g h gas c o n t e n t o f t h i s well's f l u i d . The f i g u r e shows a match w i t h a d o u b l e p o r o s i t y model. However, t h e model does n o t f n d i c a t e any major c o n n e c t i o n t o a f r a c t u r e . The d a t a f r o m well 16 in S u d u r h l i d a r a r e p r es e n t e d in F i g u r e 6 and show a b e t t e r match w i t h a d o u b l e p o r o s i t y model t h a n t h e a l t e r n a t i v e v e r t i c a l f r a c t u r e model. The match i n d i c a t e s t h a t t h e well i n t e r c e p t s a f r a c t u r e in a r o c k mass o f r a t h e r low p e r m e a b i l l t y . There is a s a a l l r e s t r i c t i o n i n t h e f r a c t u r e . T h i s well was m o n i t o r e d f o r t h r e e weeks a f t e r o t h e r w e l l s i n S u d u r h l i d a r had been p u t i n t o p r o d u ct i o n a g a i n . D u r i n g t h a t p e r i o d no p r e s s u r e d e c l i n e was observed in t h e well. However, a p r e s s u r e p u l s e o f 3.7 b a r caused by t h e v o l c a n i c a c t i v i t y was observed. T h i s i n d i c a t e s t h a t a t l e a s t in t h e e a s t e r n p a r t o f t h e S u d u r h l i d a r -r e s e r v o i r t h e f l u i d 1s s t i l l m o s t l y s i n g l e -p h a s e l i q u i d . R e s u l t s f o r w e l l 17 in S u d u r h l i d a r a r e shown in F i g u r e 7. The d a t a is matched w i t h an i n f l n i t e a c t i n g system. The match r e s u l t s in a l a r g e n e g a t i v e s k i n , w h i c h is caused by a t h i n and h i g h l y permeable near h o r i z o n t a l l a y e r o r f r a c t u r e , i n t e r s e c t e d by t h e well. Well 20 in S u d u r h l i d a r is d i r e c t i o n a l l y d r i l l e d t o t h e n o r t h and i n t e r s e c t s two n e a r l y v e r t i c a l f r a c t u r e s or f a u l t s . A match w i t h a v e r t i c a l f r a c t u r e model was n o t as good, because t h e a n a l y t i c a l model was n o t a b l e t o h a n d l e a s t r o n g e a r l y w e l l b o r e s t o r a g e b e h a v i o r , p o s s i b l y enhanced by t h e r m a l e f f e c t s in t h e well. T h i s may cause t h e d o u b l e p o r o s i t y model t o g i v e t o o low e s t i m a t e s f o r t h e t r a n s m i s s i v i t y ( U i l l e r , 1980). On t h e o t h e r hand t h e d o u b l e p o r o s i t y model i n d i c a t e s , t h a t t h e well i n t e r s e c t s a r e l a t i v e l y l a r g e volume f r a c t u r e . .. DISCUSSION OF RESULTS I n t a b l e 2 a comparison is made between t h e t r a n s m i s s i v i t y v a l u e s o b t a i n e d in an i n j e c t i o n t e s t a t t h e end o f d r i l l i n g and t h o s e p r e s e n t l y e s t i m a t e d . The g e n e r a l t r e n d l e a d s t o a s l i g h t l y lower e s t i m a t e now, t h a n a t t h e end o f d r i l l i n g . T h i s comparison a l s o i n d i c a t e s t h a t t h e s h o r t n o n -i s o t h e r m a l i n j e c t i o n t e s t s , p e rformed a t t h e c o m p l e t i o n o f t h e wells, g i v e f a i r l y r e l i a b l e e s t i m a t e s o f r e s e r v o i r t r a n sm i s s i v i t y . The p r e s e n t e s t i m a t e f o r well 13 is 3 . t i m e s g r e a t e r now t h a n a f t e r d r i l l i n g . T h i s may r e f l e c t t h e t r a n s m i s s i v i t y o f t h e f r a c t u r e i t s e l f t h r o u g h t h e H v e r a g i l g u l l y , b u t n o t t h e combined f r a c t u r e r o c k t r a n s m i s s i v i t y , because t h e d a t a f r o m well 13 have a s h o r t t i m e span. -178- The estimated transmiSslvity for well 20 is most probably about 50% too low due t o the analytical model used to match the data. As mentioned earlier this could be related to a thermally enhanced wellbore storage effect. The initial pressure of the main feed points extrapolated to the depth of measurement are presented for each vell in table 3 followed by the average reservoir pressure as presently estimated. A comparison of these values reveals no pressure drawdown in the eastern part of the Sudurhlidar-field, but possibly a small pressure decline in the northen part. However, this difference is not significant. 2. . 4. An interference i

Nonisothermal Injectivity Index can Infer Well Productivity and Reservoir Transmissivity

In geothermal wells injection tests are commonly used to obtain well and reservoir data. These tests are typically conducted in a series of step rates followed or preceded by a complete shutin. Usually the temperature of the injected fluid is different from that of the reservoir fluid. Because of the strong temperature dependence of fluid viscosity and to a lesser extent, fluid density, nonisothermally related pressure responses must be considered. The nonisothermal isjectivity index obtained from these tests depends on the mobility ratio of the cold region to the hot reservoir and the extent of the cold spot. This paper proposes a method which accounts for these effects and relates the nonisothermal injectivity index to the isothermal injectivity index

A summary of modeling studies of the Krafla Geothermal Field, Iceland

A comprehensive modeling study of the Krafla geothermal field in Iceland has been carried out. The study consists of four tasks: the analysis of well test data, modeling of the natural state of summary of the the field, the determination of the generating capability of the field, and modeling of well performance. The results of all four tasks are consistent with field observation