Geothermal well logging : temperature and pressure logs

Abstract

Temperature and pressure logs are the most important well logs in geothermal exploration and development. They are used extensively throughout the life time of wells. Electronic tools with surface are used in low temperature wells (T< 150°C), but for wells with temperatures, in the range of 150-380°C, the geothermal industry have used mechanical Kuster/Amerada temperature gauges for decades. During the last ten years electronic high temperature tools have become available and the T&P tool that is most widely used in geothermal today is a K10G from Kuster Company, which is a memory tool with the electronics inside a Dewar flask to shield the electronics from the high well temperatures and maintain internal tool temperatures below 175°C for hours even at 350°C well temperatures. The ultimate goal of temperature and pressure logging in geothermal investigation is to determine formation temperature and reservoir pressures, but even during drilling when the well temperatures are highly disturbed by drilling fluid circulation and cold water injection in to the well the temperature profiles provide valuable information on the location of aquifers (feed zones) and their relative size (permeability). Internal flow often exists in very permeable wells with multiple feed zones. This flow is clearly seen in temperature logs and sometimes the internal flow rate can be estimated based on temperature transients. Bottom-hole formation temperature is sometimes estimated by extrapolation of a short term heating up temperature survey at bottom using Horner plot or other extrapolation algorithms.

Pressure in wells is also influenced by fluid circulation, injection and production during drilling. Pressure transient test do, however, give information on well injectivity and productivity as well as other hydrological parameters. The temperature and pressure disturbances in a well during drilling will fade away gradually when the drilling stops. The wells will heat-up and reach thermal equilibrium with the surroundings in matter of several months and the well pressures will also recover after drilling and reach equilibrium with the permeable feed zones of the well. Temperature and pressure logs during the heating/recovery period after drilling are the most important data to estimate formation temperatures and reservoir pressure. T&P logs at later stages can improve the estimates. Monitoring of temperature and pressure becomes an essential tool for the management of the reservoir, when utilization commences