Geothermal Short Course: Reservoir Characteristics Interpreting Temperature and Pressure Measurements from Wells

Rick Allis Utah Geological Survey Penrose Conference, Park City, UT, Oct 19-23, 2013

Themes

• • • • • • • Conductive thermal regimes (and heat flow) Convective thermal regimes (interpreting geothermal well data) Importance of water density and pressure data in hot wells Importance of feedzone control in high permeability wells Pressure drawdown effects due to production High permeability effect on deep basin pressure regime DSTs, and Injectivity Tests as indicators of permeability

Cascades

Yellowstone

Snake River Plain Great Basin Imperial Valley Colorado Rockies Rio Grande rift

Regional heat flow of the conterminous U.S.

(SMU geothermal lab; Blackwell et al., 2011)

Prospective reservoirs with attractive temperature and hi-permeability stratigraphic target

Gulf Coast

Heat Flow Measurements – A Critical Component of Geothermal Exploration

However, this map doesn’t have the resolution required for discovery of geothermal reservoirs (which require adequate temperature, and adequate permeability)

Pavant Butte- Black Rock Desert, Utah: a high heat-flow basin

Heat Flow (mW/m 2 ) = Thermal Conductivity (W/m°C) x Temperature Gradient (°C/km)

From Gwynn et al., 2013

Thermal Conductivity is important!

MIT, (2006) unconsolidated basin fill, mudstone, shale sandstone, many bedrock lithologies

Heat Flow (mW/m 2 ) = Thermal Conductivity (W/m°C) x Temperature Gradient (°C/km)

Hypothetical stratigraphy

H.F. = 100 mW/m 2

Exploration Oil-Well Data: often the main source of data on possible geothermal potential Correcting Bottom Hole Temperatures (BHTs) for the drilling disturbance can be tricky. A variety of methods, but basically, BHTs are a noisy source of data; However, they need to be corrected, and it can be a tedious process

The Geothermal Resource

•

Background HF 80-85 mW/m² = High Heat Flow Basin

•

High HF (>85 mW/m²) anomaly

•

350 km² > 150°C at 3 km

•

60 km² > 200°C at 3 km

•

Focused heat source superimposed on basin-scale thermal regime

•

Cooling intrusion?

•

Why is southern end of basin cooler (lower heat flow?)

Oil Exploration Old Gradient New Gradient

Heat flow from Wilson and Chapman (mW/m 2 )

The perils of outflow plumes!

(Roosevelt HS)

Groundwater chemistry compiled by D. Cole

• What we see in

downhole temperature and pressure logs in deep exploration wells may only represent the formation conditions at a few points!

• However we can still

infer significant characteristics with careful detective work!

Logging tool being prepared for “run”

Deep-Well Temperature and Pressure Logs

•

Probably the most important data to have

– Temperature and permeability information is essential for understanding the reservoir (is there a thermal

resource, and can you sweep the heat out!)

• Logs can be “static” (i.e. shut-in); or flowing – under injection or production; sometimes this status is not recorded… • The best permeability (i.e. best “feed zone”) in the well dominates the flow regime (shut-in or flowing) – gives indication of formation pressure • May see inter-zonal flow as isothermal sections on temperature logs (sometimes difficult to decide whether flow is up or down)

Hot water is less dense than cold water: pressure profiles in wells depends on the fluid state in the well bore, which in the case below is equilibrating with the reservoir at 500 m depth (and 35 bar).

Note the pressure pivot, and the difference in water “level” in the well.

Pressure (bar) 0 10 20 30 40 50 60 0 70 80 100 hot water (200 ºC)

steam

casing 200 300

cold water

400 500 600 700 800 900 1000 pressure control point at 500 m (35 bar)

These pressure measurements are really important for figuring out the hydrology of the system: is high permeability extensive and controlling a large reservoir? Or is the region of uniform pressure limited?

The Roosevelt reservoir seemed to be over-pressured with respect to the ground surface in its natural state; it may have been in equilibrium with regional cool groundwater at about 3 km depth (10,000 ft. ) The Cove Fort reservoir is under pressured with respect to the local groundwater, but seems to be connected to the Twin Peaks region 10 – 15 km to the north – there is a dilute warm spring in that location

2000

Conceptual Model – Awibenkok, Indonesia

(volcano-hosted) Salak volcano summit

AWIBENGKOK GEOTHERMAL FIELD 1500 1000 500

Cl Spring hydrostatic from field elevation

Temperature

0

Reservoir Pressure

-500

hydrostatic from adjacent valley floor

-1000 -1500 0 50 100 150 200

northeast wells

250 Pressure(bar g); Temperature ( o C)

Reservoir

300

west wells

Allis, 1999 350

• Major loss zone (and rapid heat up) at 1410 m depth • Minor loss zone at 1200 m depth, and possibly minor loss zone at well bottom • Pivot in pressure curves as liquid in wellbore heats up and becomes less dense – formation pressure at 1410 m depth is 121 bar gauge • Temperature here is at least 300°C (Possibly conductive gradient down to about 1000 m depth) • Note under injection the cool water level is at about 200 m depth (water was being poured in); however heated water column probably has +ve WHP – implying reservoir outflows as hot springs • Note also under injection, the pressure in the wellbore exceeds likely formation pressure below pivot 0 DownHole Combined Pressure/Temperature 25 50 Pressure [bara] 75 100 125 150 175 1,200 1,300 1,400 1,500 1,600 1,700 1,800 100 200 300 800 900 1,000 1,100 400 500 600 700 T

under injection

P

Heating 1 week

P T 0 50 100 150 200 Temperature [°C] KA41 PT 20/12/2005 04:50 Well Temperature KA41 PT 27/12/2005 12:57 Well Temperature 250 300 KA41 PT 20/12/2005 04:50 Well Pressure KA41 PT 27/12/2005 12:57 Well Pressure 350

What would the discharging profile look like for a geothermal well?

500 600 700 800 900 1000 1100 1200 0 0 100 200 300 400 10

Steam zone pressure control pressure control

20

Somewhere down here, liquid zone

30

steam

40 Pressure (bar) 50 60 70 80 well 28-3, Roosevelt Field, UT 90 static 5/10 casing

liquid

100

Example of a discharging geothermal well

500 600 700 800 900 1000 1100 1200 0 0 100 200 300 400 10 20

depth of first boiling (flash) about 690 m

30

steam

40 Pressure (bar) 50 60 70 80 well 28-3, Roosevelt Field, UT 90 static 5/10 flowing 8/09 casing

liquid Hypothetical pump

100

100 0 0

What happens when we draw down a cold water well?

Pressure (bar g) 30 10 20 40 50 60

undisturbed pumped Draw down = 5 bar (50 m)

200 casing 300 400

reservoir

500 What about a geothermal well?

Groundwater Analogy: Shape of cone of depression depends on the permeability of the aquifer (reservoir)

Roosevelt Geothermal Field: 25 years of production, recently at 35 MWe level.

About 75% of the produced mass in injected back into the reservoir.

There has been pressure decline of about 35 bar (500 psi), and an increase in steaming ground activity.

0 0 10 20 250 Natural State

Roosevelt system has developed a steam cap due to the production-induced pressure decline, and it now is an attractive target for increased power generation

500 750 1000

3-1 casing

30 Pressure (bar g) 40 50 60 70 80 90 100 0

3-1 1975 3-1 05/02/1988 3-1 05/31/2001 3-1 11/09/2010 58-3 06/30/2008

1000

steam zone pressure decline deep liquid pressure decline

2000 3000 4000 1250 1500 0 250 500 750 Pressure (psi g) 1000 1250

Roosevelt Hot Springs – Blundell Power Plant (Pacificorp Energy)

Initial Pressure Regime (hot hydrostatic)

2000 0 200

ground level at plant Roosevelt hot springs

1500 1000 400 600 Pressure (psi g) 800 1000 1200

72-16

Initial reservoir pressure (1976 - 1984) (slope = 0.82 bar/m ≈ 240 C water)

52-21 35-3 82-33 3-1 54-3 27-3

1400 500

9-1

0

82-33 13-10 14-2 52-21

-500

12-35 25-15

100 Pressure (bar g) 150 200 1600 5720 4720 3720 2720 1720 720 -280 -1280 -2280 250 -3280 -1000 0 Pressure (bar g) 2000 0 1500 50 100

Roosevelt hot springs and Opal Mound

54-3 3-1 52-21 9-1 150 200 Initial reservoir pressure (1976 - 1984) 1000 25-15 58-3 500 28-3 54-3 13-10 58-3 71-10* 0 -500 -1000 0 500 1000 1500 2000 Pressure (psi g)

* based runs in 2008 and 2010, both with problems

2500 3000 3500 250 5720 4720 3720 2720 1720 720 -280 -1280 -2280 4000 -3280 50

Final Pressure Regime (hot hydrostatic; but drawn down by 35 bars, or 500 psi)

Drill Stem Tests: Standard test in oil exploration drilling to detect permeability, pore fluid composition, static pressure… Example of a drill stem test from well Rocky Ridge 33-1 drilled by Python Ag LLC in 2010 (report accessed at http://oilgas.ogm.utah.gov/wellfiles/027/4302750001.pdf

). In this case the interval tested is between 6648 and 6818 feet depth, the formation pressure derived from shut-in pressures is 2810 psi (absolute), and the temperature at that depth is 195°F (91°C).

Pressure trend with depth showing normally pressured gradient (0.43 psi/foot), significant overpressures at depth, and comparison with a hot hydrostatic trend in a geothermal system with a temperature of 220°C (428°F) (modified from Nelson, 2003). Two “apparent pressure” points are shown, which can be excluded when considering the trend from the other data.

In both basins in The Rockies, high permeability and normal (hydrostatic) pressures were found in the Leadville carbonate at 5 – 7 km depth. Temperatures were in the range of 200 – 240°C at these depths.

1989

Piceance Basin, Colorado

Wilson et al., 1998

Wind River Basin, Wyoming 20 – 40 MMcf/day 68% C2H6, 20% CO2, 12% H2S 68% C2H6, 20% CO2, 12% H2S