TEMPERATURE-DEPTH CURVE TUTORIAL

Maria Richards

SMU Geothermal Lab Coordinator

Southern Methodist University, Department of Geological Sciences

PO Box 75-0395, Dallas, TX 75275-0395

214-768-1975

mrichard@smu.edu

http://smu.edu/research/profiles-blackwell.asp

https://smu.edu/geothermal/temperat/frames/mainpage_temp_ex.htm

The following temperature-depth curves are from data collected by the SMU Geothermal Laboratory. A normal temperature curve is a consistent increase in temperature with depth. What is more commonly found are wells with increases and decreases in temperature because of the plethora of effects on wells. The temperature-depth curves shown in this tutorial will assist you in interpreting what is actually encountered in the field. If you are interested in more information or have questions about this material, please contact Maria Richards with detailed needs at mrichard@mail.smu.edu.

INTRODUCTION

Measuring the temperature in a well is useful for determining the Earth's gradient at that

location. Reviewing the gradient (change in temperature over the change in depth) gives

clues as to what is happening underground. When comparing multiple wells in a close

proximity, the subtle movements of water, gas, air and the transfer heat are noticeable.

Certain patterns are identifiable once there is an understanding of how temperature

reacts to different mediums and situations. The following sections give examples of how

temperature -depth curves can differ and what these differences mean.

Conductive Heat Transfer

This is the basic example of the Earth's temperature increasing consistently with

depth. The ability to transfer heat from one substance to another is referred to as its

thermal conductance. When heat transfers conductively in a homogenous or horizontally

layered media the temperature-depth curve is a straight line with little to no change in

gradient for then entire length of the curve.

Microclimate Effects

Variations in the nature of the ground cover and slope angle and azimuth within 50-200

meters of a borehole (well) collar can cause variations in gradient in the upper part of the

hole. Some examples that cause surface temperature changes are the Earth's average air

temperature warming, woods being clear-cut or growing, and pavement laid around a

well. Most often the effect is to change the gradients no more than 20- 30°C/km at 20-50

meters depth, and only less than 5°C/km at 100-150 meters depth. The effects are rarely

larger than this. The extrapolated surface temperatures from deeper than 150 meters is

usually within 2°C of actual surface temperatures.

Water Table Effects

In general there does not appear to be a systematic change in gradient (higher or lower)

associated with the water table. Even some porous rocks (Chalk Butte siltstones, f = 30%)

show no saturation effect. Coarser grained lithologic types (sand and gravel) may, but

water flow effects frequently obscure any systematic behavior. The speed of the flow and

the length of time the water has been flowing in the region will influence the amount of

effect the water table has on the surrounding rocks, and therefore, on the temperature-

depth curve.

Intra-Borehole Flow

When drilling a well, the new borehole may connect two aquifers (fracture zones,

lithologic horizons) with different heads (pressures, piezometric levels). This causes the

water to flow from the area of higher pressure to lower pressure through the open

borehole, or if not grouted properly, along the outside of the casing. The water may move

up or down the borehole. If the water comes up and exits the hole, then it is

artesian When drilling creates a pathway for two aquifers to interact it should be

removed because of potential contamination of the two aquifers. This is done with

proper casing and grouting of the well hole.

Regional Aquifers Flow - Slow Moving

Slow water flow rates in large scale aquifer systems generate a characteristic set of

temperature-depth curves. The recharge regions have temperature curves concave

toward the temperature axis, while the upflow regions of the system have temperature

curves convex toward the temperature axis. The extent of curvature depends on the

depth and size of the aquifer circulation.

Lateral Flow - Fast Groundwater Movement

In many shallow volcanic aquifers groundwater flow is so fast that the temperatures

through the aquifer have little to no change in temperature from the top of the aquifer to

the bottom. In this scenario the temperature curve through the aquifer is a vertical line.

The rocks below the aquifer seem to treat it as if it were the ground surface. The water

temperature tends to be very cold in these regions, allowing the denser water to sink and

not thus temperatures do not warm with depth.

Just because there is fast moving groundwater does not necessarily mean that the bottom

of the aquifer will be similar in temperature to the top, the last few examples are places

where the lithology ignores the aquifer.

Lithology Effect - Vesicular Basalt

Vesicular basalt is a very porous rock type. Above the water table the pore space is filled

with air allowing the surface temperatures to effect the rock temperatures deeper than

normal. Because cold air is more dense and sinks, it flows through the vesicular basalt

causing the temperature to decrease with depth, rather increase in normal scenarios. In a

temperature curve the air effect appears as an exaggerated annual wave effect (in

amplitude and depth). Typically these wells exhale and inhale air depending on

atmospheric conditions. Temperatures below the water table are frequently disturbed by

large amounts of water flow in the porous rocks. One of the most likely places to find

vesicular basalt is in the region of the Snake River.

Lateral Flow - Transient Activity

Transient lateral flow is very common in geothermal systems. Areas where there have

been recent faulting are the most likely to find the transient flow. The term transient is

used here on a geologic time-table. The length of time the flow will occur is dependent on

the source of fluids and their chemistry. Faults will seal over time from crystallization of

the minerals in the water precipitating out. The unusually hot water flows convectively up

the fault until it reaches an aquifer or the water table. Where the fault and aquifer

intercept is where the flow will become lateral. The heated fluid flowing laterally can be

very deceiving when determining the size of a geothermal area. Many a driller has

thought that they hit the geothermal jackpot only to keep drilling and watch the

temperatures decrease with depth once through the lateral flow. These temperature-

depth curves do contain information on the duration of flow, velocity, distance to

recharge point and temperature of recharge.

Geothermal Systems Examples

The areas shown are related to data available in the Western Geothermal

Database. When reviewing an entire geothermal system, a person may find many

different types of temperature curves. It is normal to find curves that are related to the

background temperature and consistently increase with depth, curves that represent

transient lateral flow, isothermal temperatures, and the desirable elevated temperatures.

Roosevelt Utah and COSO California

Roosevelt Utah and COSO California

NEWBERRY CRATER