A review of thermal models of Atlante project regions starting from well data logs: the example of Sicily. Gianluca Gola CNR-IGG of Pisa (Italy)

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1 A review of thermal models of Atlante project regions starting from well data logs: the example of Sicily Gianluca Gola CNR-IGG of Pisa (Italy)

2 Where are geothermal resources found? Where are geothermal resources in Southern Italy strong enough to be economical for geothermal applications? Enel, Eni-Agip, CNR, ENEA (1988) National Geothermal Resources Inventory More than 2700 data points were available for the whole Italian territory at that time. The existing temperature maps (1000 m m m. depth from ground level) provided an initial overview of the Geothermal resources. ViDEPI Project Geothopica Project Additional industrial drilling temperature data, provided by UNMIG, were used to update the National Geothermal Database (BDNG).

3 Sicily and offshore areas wells locations Hydrocarbon exploratory wells Geothermal wells

4 1.1 Temperature Database More than 1500 temperature measurements - coming from about 280 wells - provided valuable information on the temperature distribution in the range 0-6 km. The database consists of: Hight precision depth-temperature logs coming from geothermal exploration wells Variable quality point measurements (bottom hole temperatures BHT, drill stem test temperatures DST) coming from hydrocarbon exploratory wells These data have been used to predict the temperatures beyond the depth of observation.

5 1.1 Temperature Database Due to the different conditions under which the temperatures are measured and the purposes for which they are used, these data constitute an heterogeneous database with differents degree of accuracy and spatial distribution. Temperature logs BHTs DST temperatures Continuous log Accuracy of ±0.05 C Static conditions of borehole Few logged wells Shallow depths Punctual measurements Accuracy of ±0.5 C Dynamic conditions of borehole Many wells Hight depths Referred to the productive interval Accuracy of ±0.5 C Flowing fluid temperature Sensitive data Hight depths

6 1.2 BHTs Correction to Static Conditions Why? BHTs reflect the thermal conditions of the drilling mud, not those of the undisturbed rock. The mud is cooler than the drilled formations, so it tends to cool the rocks near the bottom of the borehole. How? Most techniques treat temperature as a transient function, i.e., they involve progressive measurements of temperature (BHT) with time after cessation of circulation, to extrapolate the temperature at static conditions (SBHT)

7 1.2 BHTs Correction to Static Conditions Line source method The model assumes that the mud circulation acts as a heat sink An instantaneous line source/sink induces the following temperature perturbation (Carslaw & Jaeger 1959): BHT t c, t SBHT 2 2 Q r r E 1 E1 4 r 4 r tc t 4 rt Under the assumption: 2 r 4 t r 1 BHT Q 4 c t, t SBHT log c 1 Horner-type extrapolation r t t

8 Temperature ( C) 1.2 BHTs Correction to Static Conditions Horner plot Giaurone 001 well BHT = -249,7H + 116,6 R² = 0,9993 z = 3459 m BHT = -19,2H + 90,1 R² = 0,9465 z = 3221 m. 85 BHT = -43,9H + 83,0 R² = 0,9905 z = 2810 m ,00 0,05 0,10 0,15 LN(1+t c / t) BHT = -50,4H + 76,5 R² = 0,9993 z = 2134 m.

9 Temperature ( C) 1.2 BHTs Correction to Static Conditions Horner plot Ursitto 001 well BHT = -235,8H + 107,4 R² = 0,9977 z = 3813 m. 95 BHT = -90,7H + 93,2 R² = 0,9968 z = 3205 m BHT = -102,7H + 79,6 R² = 1 z = 2478 m. 65 0,00 0,02 0,04 0,06 0,08 0,10 0,12 LN(1+t c / t)

10 Depth from ground level (m) 1.3 Time-temperature series Temperature ( C) t (hours)

11 Depth from ground level (m) 1.3 Time-temperature series Temperature ( C) The data set account for about 160 groups of timetemperature series (BHT and t), a part of theme also having circulation time t c. When t c is not available, an empirical estimation as function of depth is applied As DST temperatures are supposed to be undisturbed temperatures, SBHT data are compared with them in wells where both data are available

12 Depth from ground level (m) 1.4 Drill Stem Test Temperatures Temperature ( C) Narciso field Lippone/Mazara fields Gagliano field Bronte/S.Nicola fields M.Pellegrino/Feudo Grande fields Catania field Gela field Cisina field 5000 Ponte Dirillo field 6000 Prezioso field Ragusa field Feudo Grande Gagliano Monte Pellegrino Lippone/Mazara Gela Pontedirillo Catania Bronte/S.Nicola Ragusa Cisina Prezioso Narciso

13 Depth from ground level (m) 1.5 Horner correction vs. DST temperatures Temperature ( C) Temperature ( C) Temperature ( C) Gagliano 101 Gela 86X Ragusa 56-dir SBHT SBHT DST DST SBHT DST

14 Depth from ground level (m) 1.5 Horner correction vs. DST temperatures Too few data exist to perform quantitative statistics, and comparison is difficult as temperatures are usually not at same depths, therefore only qualitative remarks can be made. Temperature ( C) Temperature ( C) SBHT are in agreement with the DST geotherm within ±5-10 C, which is the accuracy expected by a number of authors SBHT Prezioso 001 DST Prezioso Est 001 DST Prezioso SBHT Narciso 002 SBHT Narciso 001 dir SBHT Narciso 003 DST

15 T ( C) 1.6 Correction method for single BHTs Horner slope BHT SBHT ( C) BHT BHT Q 4 c t, t SBHT log c 1 r t t 2 c t, t SBHT az bzlog c 1 t t t (hours) A depth-time correction method, based on correlation between the Horner slope and depth, is here proposed and tested within the area of study

16 2.1 Hydrocarbon exploratory well logs

17 2.2 Geological model

18 3.1 Geothermal gradients well by well Mozia 001 In the cap rock the high temperature gradients imply a predominance of conductive heat transfer. Opposite the temperatures in the carbonate reservoir reveal very low geothermal gradients where the component of convective heat transfer is not negligible. Gazzera 001 Contrada Triglia Temperature ( C) Temperature ( C) Temperature ( C) G_caprock = 42,7 Ckm G_caprock = 65,8 Ckm G_caprock = 35,7 Ckm -1 The geothermal gradient of the cap rock reflects the trend of the underlying Mesozoic carbonate formations Contrada Triglia 001 Gazzera 001 Mozia G_reservoir = 11,3 Ckm -1 G_reservoir = 17,5 Ckm

19 3.2 Heat flux on Mt. Etna T Q k A 2k 2 z T z z o r r k = 1.7 W m -1 K -1 A = 1.0 µw m -3 As the magmatic chamber is so deep-seated (20 km), the backgrounf heat flux of the area is estimated in 0.12 W m -2. k = 2.2 W m -1 K -1 A = 0.5 µw m -3 k = 2.8 W m -1 K -1 A = 1.0 µw m -3 During normal periods of quiescence the heat flux from the top of the central conduit is 1.5 W m -2 and the maximum heat flux during eruptive phases is 7.5 W m -2. T = 1150 C D. Scandura et al. (2009). Thermo-mechanical modelling of ground deformation in volcanic areas. Proceeding in PHYSCON 2009, Catania, Italy, 1-4 September. A. Bonaccorso et al. (2005). Analytical and 3D numerical modeling of Mt. Etna (Italy) volcano inflation. Geophys. J. Int. 163, S.T. Minett & S.C. Scott (1985). Theoretical consideration of heat flux on Mount Etna, Sicily. Journal of Vulcanology and Geothermal Research, 25,

20 3.3 Geothermal gradient map: cap rock

21 3.4 Geothermal gradient map: reservoir

22 4.1 Thermal model: data input Topography Top of reservoir Cap rock geothermal gradient Reservoir geothermal gradient

23 +0 m. below sea level 4.2 Thermal model: results

24 +1000 m. below sea level 4.2 Thermal model: results

25 +2000 m. below sea level 4.2 Thermal model: results

26 +3000 m. below sea level 4.2 Thermal model: results

27 +4000 m. below sea level 4.2 Thermal model: results

28 +5000 m. below sea level 4.2 Thermal model: results

29 Conclusions In the cap rock the temperature gradients range from 18 to 170 C km - 1 with a mean value of 36 C km - 1. The high temperature gradients imply a predominance of conductive heat transfer. Opposite the temperatures in the carbonate reservoir reveal geothermal gradients ranging from 1 to 35 C km - 1 with a mean value of 12 C km - 1. At regional scale the geothermal gradient of the cap rock varies laterally and reflects the trend of the underlying Mesozoic carbonate formations, with higher values where the fluid convection inside the carbonates is more efficient. Regions of near-isothermal temperatures imply significant convective heat transfer and high permeability within the geothermal reservoir.