Undisturbed Formation Temperatures Research Articles

Uncertainty Assessment of Corrected Bottom-Hole Temperatures Based on Monte Carlo Techniques

Most temperature predictions for deep geothermal applications rely on correcting bottom-hole temperatures (BHTs) to undisturbed or static formation temperatures (SFTs). The data used for BHT correction are usually of low quality due to a lack of information and poor documentation, and the uncertainty of the corrected SFT is therefore unknown. It is supposed that the error within the input data exceeds the error due to the uncertainty of the different correction schemes. To verify this, we combined a global sensitivity study with Sobol indices of six easy-to-use conventional correction schemes of the BHT data set of the Bavarian Molasse Basin with an uncertainty study and developed a workflow that aims at presenting a valid error range of the corrected SFTs depending on the quality of their input data. The results give an indication of which of the investigated correction methods should be used depending on the input data, as well as show that the unknown error in the input parameters exceeds the error of the individual BHT correction methods as such. The developed a priori uncertainty-based BHT correction helps to provide a real estimate of the subsurface temperatures needed for geothermal prospecting and probabilistic risk assessment.

A novel cyclical wellbore-fluid circulation strategy for extracting geothermal energy

Both analytical and numerical modeling methods have helped explore power generation by producing steam with the binary organic Rankine cycle or ORC. A steady-state fluid circulation rate becomes the norm in a closed-loop system in most studies, assuming a stable geothermal gradient. In this study, we explored the suitability of those assumptions, given that the geothermal gradient is a time-variant entity at a well's proximity. Furthermore, the near-wellbore formation cooling necessitates a variable-circulation rate for the cold surface fluid to ensure a stable, wellhead-fluid temperature.This article presents a new transient analytical modeling approach for geothermal energy extraction by invoking the cyclical fluid circulation rate to ensure a stable surface fluid temperature. This analytical model entails the reverse circulation of water down the annulus and up the tubing to the surface. Specifically, we introduce a cycle (a stepwise increasing circulation rate sequence, followed by the stepwise decreasing rate sequence) to retain a stable, wellhead-fluid temperature. Of course, this cycle gets repeated to manage transient heat transfer from cold fluid circulation and the wellbore formation.Reproduction of the undisturbed formation temperature from the temperature line-source solution at each step of the cycle provided the necessary proof of the proposed approach. Model validation entailed comparing the suggested analytical model results to demonstrate that multiple transient-rate steps yielded the same initial temperature, thereby reaffirming the stepwise increasing and decreasing rate sequence strategy. We also explored the model's application in diverse settings in a probabilistic frame. As expected, the geothermal gradient, vertical-well depth, and insulation strength became the most influential on the dependent variable, wellhead fluid temperature, among the various independent variables studied.

Estimating the initial-formation temperature and flowing-temperature gradient with transient-temperature analysis: Applications in gas reservoirs

The formation temperature constitutes one of the essential factors that affect the wellbore fluid temperature behavior in production mode. The amount and direction of the radial heat transfer between the surrounding formation and the wellbore depend on the temperature differential and thermal properties of the media.Traditional studies probing geothermal gradients depended principally on extrapolating the transient bottomhole temperature to infinite shut-in time for obtaining the undisturbed formation temperature. After that, a straight line from the surface to the bottomhole condition constituted the geothermal gradient. Of course, the implicit assumption in this approach is that a linear geothermal gradient exists in light of unavailability of depth-wise temperature measurements.This study presents a new transient-temperature analysis approach to determine the undisturbed formation temperature. Specifically, we show the application of the line-source solution that has roots in the temperature-diffusivity equation. Secondly, using the distributed temperature measurements associated with transient-pressure testing, we proved that the bottomhole or static temperature is time-dependent, leading to alteration of the geothermal gradient in a well's proximity. Finally, the lowering of the temperature-derivative plateau with increasing depth during well shut-in periods indicated an increase in thermal conductivity, resulting in a nonlinear geothermal gradient.

Exploring alteration of near-wellbore geothermal gradient during fluid circulation and production

Abstract The constant undisturbed formation temperature profile controlling heat transfer into the wellbore appears counterintuitive in light of transient cooling of the formation that occurs upon fluid circulation in drilling and also heating during fluid production. This study presents a mathematical model that shows that the heat transfer occurs from the wellbore/formation interface, not from some distance away from the wellbore wherein the initial formation temperature profile remains undisturbed. This new model allows investigation of heat transfer behavior from the formation into the wellbore during drilling or fluid circulation, and from the wellbore into the reservoir in the production mode. Application of the line-source solution for the temperature diffusivity equation for a steady-state system provides the necessary ingredients for computing the temperature behaviors at different times and radii. This line-source solution can be used throughout the wellbore to determine the undisturbed formation temperature, which may be used to obtain the geothermal gradient dependent on the radius and fluid circulation time. Therefore, the initial geothermal gradient works as a time-dependent variable, and the resultant second-order polynomial relationships can describe the undisturbed formation temperature. This study provides the required tools to assess the wellbore heat-transfer behavior and their effect on the wellbore temperature profiles. Also, the new mathematical model illuminates the impact of heat transfer by comparing its performance with the original formulations. Besides, this paper presents a complete derivation of the line-source solution of the temperature diffusivity equation to justify the proposed approach.

Thermal solar energy storage in Jurassic aquifers in Northeastern Germany: A simulation study

This contribution studies the usability of aquifer thermal energy storage (ATES) for seasonal solar heat storage by means of thermo-hydraulic modeling. The geological setting refers to the North East German Basin (NEGB), specifically a site approx. 50 km west of Berlin, Germany. The considered storage formation is located in Jurassic sandstones at about 270 m depth below surface, showing an in-situ (undisturbed) formation temperature of around 17 °C and appropriate hydraulic storage properties. The paper considers idealised doublet systems in faulted as well as unfaulted reservoir domains and studies the energy- and mass transport of simulated ATES systems. Five perennial loading/unloading series of solar thermal energy are investigated, assumed to be harvested by a hectare-sized flat plate collector field which is modeled employing climate data of the considered region. The simulation results exemplarily show how the storage system develops temperature-conserving recovery fractions of up to 80% heat recovery during the first years of operation.

Geothermal and rheological regime in the Po plain sector of Adria (Northern Italy)

We present a thermo-rheological study of the Adriatic crust in the central Po plain (Northern Italy), which records the convergence of the Alpine and Apennine external fronts. The present thermal regime of the crust is inferred from geological and geophysical data from oil exploration. A set of temperature data from 37 deep boreholes (bottom hole, drill stem, and production test temperatures) is used to estimate undisturbed formation temperatures down to 6-7 km of depth. Best quality data are used to estimate upper crustal geotherms, which are subdivided into four groups taking into considerations the following factors: i) presence/absence of relevant fluid circulation; ii) conductive thermal refraction; iii) recent sedimentation, and iv) other disturbing tectonothermal processes (e.g., recent tectonics). Assuming average corrections for temperature data affected by recent sedimentation effects, geotherms fall within the same range and provide a homogeneous thermal signal. Temperature measurements in the upper ~7 km are used to constrain a 1-D thermal model at the crustal scale. The best fitting geotherm gives a thermal gradient of ~23 ± 5 mK m-1 (over 7 km of depth), a surface heat flow of ~46 ± 9 mW m-2, and an estimated Moho temperature of 620 ± 80 °C.Using the estimated crustal structure and composition and the best-fitting geotherm, a representative strength envelope is obtained.For hydrostatic and supra-hydrostatic pore pressures, the calculated total crustal strength of this sector of the Adria microplate (not including the upper mantle contribution) is in the 1.8-2.5 TN m-1 range. Considering also the lithospheric mantle contribution, the total lithospheric strength is estimated to be between 23 (wet peridotite)and 39 (dry peridotite) TN m-1.

Results from Sleipner gravity monitoring: Updated density and temperature distribution of the CO2 plume

Abstract To help monitor the evolution of stored CO 2 , we have made precision seafloor gravity measurements at 30 seafloor stations above the Sleipner CO 2 plume in the years 2002, 2005 and 2009. Each epoch of gravity data has an intra-survey repeatability of about 3 μGal (standard deviation), obtained using state-of-the-art instrumentation on top of pre-deployed seafloor benchmarks, with typically three visits on each location during a survey. We used three relative quartz-spring Scintrex CG-5 gravimeters in a unique offshore instrument package. Ocean tidal fluctuations and benchmark depths were determined using both pressure gauges on the gravity survey tool and stationary reference pressure gauges on the seafloor. We analyzed and accounted for multiple sources of changes in gravity to obtain an estimate of in situ CO 2 density. First, the injected CO 2 , 5.88 million tonnes during this time period, displaces denser formation water, causing a negative gravity change above the plume. This is the signal of interest for this study. At the same time, hydrocarbon gas production and water influx into the deep, nearby gas reservoir cause an increase in gravity of higher amplitude and longer wavelength. Finally, by observing vertical depth changes of the seafloor benchmarks between surveys to mm precision, we quantified vertical benchmark movements caused by sediment scouring. Some of the benchmarks have experienced more than 10 cm vertical movement over the 7 year duration of the experiment, and erosional topography can be seen in a >10 m broad area around some of the benchmarks. The shifting sediment can also cause a change in the observed vertical gravity gradient. We inverted the gravity changes for simultaneous contributions from: (i) injected CO 2 in the Utsira Formation, (ii) water flow into the Sleipner gas reservoir, and (iii) vertical benchmark movements. We estimate the part of the change in gravity caused by CO 2 injection to be up to 12 μGal. If we assume a geometry of the plume as seen in 4D seismic data, the best match to the 30 stations requires an average CO 2 density of 720±80 kg/ m 3 , neglecting dissolution of CO 2 into the formation water. While the CO 2 in the Utsira Fm. at Sleipner is supercritical, it is fairly close to the critical point; therefore only a slight increase in temperature could lower the density significantly. Density is also sensitive to impurities, which make up 1–2% of the injected material at Sleipner and reduce the density slightly. In the absence of down hole gauges in the injection well, we estimate the well-bottom CO 2 temperature to be 48 °C and pressure to be hydrostatic (∼105 bar). These conditions give a calculated density of 485±10 kg/ m 3 at the perforation. Density is expected to increase away from the well as CO 2 cools down from contact with the cooler formation, up to a maximum of about 710 kg/ m 3 . The distribution of temperature and density within the plume is difficult to model exactly, but most of the CO 2 is expected to cool down to initial reservoir temperature (∼35.5 °C at the perforation) except for a central high-temperature region where CO 2 is still near the injection temperature. Because the undisturbed formation temperatures and the injection temperature are fairly well known, the 2002–2009 gravity change can be used to constrain the rate of dissolution of CO 2 into the formation water. Dissolved CO 2 is invisible in seismic data. The contribution from gravimetric data could therefore be highly valuable for monitoring this process, which is important for long-term predictions of the CO 2 stored in the Utsira Fm. We give an upper bound on the dissolution rate of 1.8% per year.

Determination of formation temperatures from temperature logs in deep boreholes: comparison of three methods

The most important data on the thermal regime of the Earth's interior come from temperature measurements in deep boreholes. The drilling process greatly alters the temperature field of formations surrounding the wellbore. The temperature change is affected by many factors and the exact determination of formation temperature at any depth requires a certain length of shut-in time. In theory this shut-in time is infinitely long. There is, however, a practical limit to the time required for the difference in the temperature between the well wall and surrounding reservoir to become a negligible quantity. In this paper, we conducted a comparison of three methods of predicting the undisturbed formation temperatures from shut-in temperature logs in deep wells. Long-term temperature surveys in five wells were selected to estimate the formation temperatures and to compare measured and predicted transient formation temperatures. It was found that the best agreement is observed with values calculated by the ‘three-point method’ and measured during shut-in period transient temperatures.

Application of a spherical-radial heat transfer model to calculate geothermal gradients from measurements in deep boreholes

This paper presents estimates of the undisturbed formation temperatures in a geothermal exploration well drilled in the Ceboruco area in the western part of the Mexican Volcanic Belt. The method used assumes that during the warming-up period following fluid circulation, heat transfer from the surrounding formation to the wellbore is spherical-radial in the bottomhole region of the well. According to this model, the transient temperatures display a linear T versus 1 / t behavior, with an intercept equal to the undisturbed formation temperature.

Prediction of formation temperatures in permafrost regions from temperature logs in deep wells—field cases

AbstractImportant data on the thermal regime of the Earth's interior come from temperature measurements in deep boreholes. Drilling greatly alters the temperature field of earth materials surrounding the wellbore. In permafrost regions, due to thawing of adjacent strata during drilling, representative data can be obtained only by repeated observations over a long period of time. In this paper we predict undisturbed formation temperatures (and geothermal gradients) from shut‐in temperature logs in deep wells. The main features of the method are: (1) in the permafrost section of the well, the starting point in the well thermal recovery is moved from the end of well completion to the moment of time when the refreezing of enclosing strata was completed; it takes into account the refreezing of thawed material in a temperature interval; and (2) below the permafrost base, the starting point in the well thermal recovery is moved from the end of well completion to the moment of time when the first shut‐in temperature log was taken. A generalized formula to process field data (for the well sections below and above the permafrost base) is presented. Temperature logs conducted in five wells verify the method. Copyright © 2003 John Wiley & Sons, Ltd.

Heat transfer by free convection from an isothermal vertical round plate in unlimited space

Three high resolution temperature logs recorded after shut-in of the main borehole KTB-Oberpfalz HB of the German Continental Deep Drilling Program are used to determine a first estimate of the undisturbed formation temperature, using the line source model of Bullard [Bullard, E.C., 1947. The time necessary for a borehole to attain temperature equilibrium. Monthly Notices Royal Astronomical Society, Geophysical Supplement 5, 127–130.]. The resulting formation temperature profile and the corresponding temperature gradient, deviate from previous estimates [Wilhelm, H., Baumann, C., Zoth, G., 1995. Some results of temperature measurements in the KTB main borehole, Germany. Geothermics 24, 101–113; Clauser, C., Giese, P., Huenges, E., Kohl, T., Lehmann, H., Rybach, L., Safanda, J., Wilhelm, H., Windloff, K., Zoth, G., 1997. The thermal regime of the crystalline continental crust: implications from the KTB. Journal Geophysical Research 102, 18,417–18,441; Wilhelm, H., 1998. Comparison of predicted and logged temperatures in the KTB main hole. In: Popov, Yu, Pimenov, V. (Eds.), Proc. Int. Conf. The Earth’s Thermal Field and Related Research Methods. Moscow State Geological Prospecting Academy, Moskow, pp. 280–281], which were derived from bottomhole measurements and from multiple temperature logs recorded during drilling pauses at 4500, 6000 and 7200 m depth. Although the database is not large enough to allow a formal error estimate, the noise in the extrapolated formation temperature indicates a mean error of about ±0.15 K between 2500 and 5200 m depth and a mean error of ±0.5 K below 5200 m depth.

Undisturbed temperature in the main drillhole of the German Continental Deep Drilling Program predicted from temperature logs recorded after shut-in

Three high resolution temperature logs recorded after shut-in of the main borehole KTB-Oberpfalz HB of the German Continental Deep Drilling Program are used to determine a first estimate of the undisturbed formation temperature, using the line source model of Bullard [Bullard, E.C., 1947. The time necessary for a borehole to attain temperature equilibrium. Monthly Notices Royal Astronomical Society, Geophysical Supplement 5, 127–130.]. The resulting formation temperature profile and the corresponding temperature gradient, deviate from previous estimates [Wilhelm, H., Baumann, C., Zoth, G., 1995. Some results of temperature measurements in the KTB main borehole, Germany. Geothermics 24, 101–113; Clauser, C., Giese, P., Huenges, E., Kohl, T., Lehmann, H., Rybach, L., Safanda, J., Wilhelm, H., Windloff, K., Zoth, G., 1997. The thermal regime of the crystalline continental crust: implications from the KTB. Journal Geophysical Research 102, 18,417–18,441; Wilhelm, H., 1998. Comparison of predicted and logged temperatures in the KTB main hole. In: Popov, Yu, Pimenov, V. (Eds.), Proc. Int. Conf. The Earth’s Thermal Field and Related Research Methods. Moscow State Geological Prospecting Academy, Moskow, pp. 280–281], which were derived from bottomhole measurements and from multiple temperature logs recorded during drilling pauses at 4500, 6000 and 7200 m depth. Although the database is not large enough to allow a formal error estimate, the noise in the extrapolated formation temperature indicates a mean error of about ±0.15 K between 2500 and 5200 m depth and a mean error of ±0.5 K below 5200 m depth.

TEMLOPI: a thermal simulator for estimation of drilling mud and formation temperatures during drilling of geothermal wells

This paper describes the development and application of the numerical code TEMLOPI v1.0, a useful tool for estimating the temperature distribution of the fluids employed for drilling geothermal wells. The simulator also allows estimation of the thermal disturbance of the surrounding rock caused by fluid circulation and well shut-in. TEMLOPI v1.0 is based on a mathematical model which considers the main heat transfer mechanisms and the heat exchange between the circulating fluid and the surrounding rock formation that occur during drilling of geothermal wells. The simulator was written in Fortran 77 using modular (block) programming. It runs on most IBM compatible personal computers and can be used in-situ. Input data includes the well geometry, the fluid and flow characteristics and the initial (undisturbed) formation temperature. Output files contain the transient temperature distribution (temperature vs depth) in the fluid flowing down the drill pipe and the annulus, the well inner face and the radial distribution in the surrounding rock. The software code model, architecture, input and output files and the solution algorithm are described in detail. Results obtained were validated by comparison with data published in the specialized literature and with data from well Az-29 from the Los Azufres Mexican geothermal field.

Temperature field distribution from cooling of a magma chamber in La Primavera Caldera, Jalisco, Mexico

The temperature field distribution in La Primavera geothermal area, Jalisco, located in the western part of the Mexican Volcanic Belt (MVB), has been simulated from cooling of a shallow magma chamber (assumed as the primary heat source) during the entire volcanic history of the caldera. Similar to the other two geothermal fields of the MVB (Los Humeros and Los Azufres), it is considered that the evolution of the magma chamber is controlled by the processes of fractional crystallization as well as magma recharge. Besides these processes, heat contribution is also taken into account from decay of natural radioactive elements, U, Th, and K, present in all geological materials. In some models presented in this work, convection in the geothermal reservoir is simulated by assigning higher values of thermal conductivities (up to 20 times the rock conductivities) to respective geologic units. The heat transfer equation has been solved by a finite element implicit method. The results of temperature simulations from the magma chamber are compared with undisturbed formation temperatures in three drill wells. The subsurface depth of the top of the magma chamber is varied from 5 to 7 km. Similarly, the horizontal dimensions of the chamber are varied from 12 km (which is approximately the diameter of the La Primavera caldera) to 10 km. The thermal effects of this change in depth and horizontal dimensions of the magma chamber are readily seen in the predicted temperature distribution for this rather young caldera.

Some results of temperature measurements in the KTB main borehole, Germany

High-resolution temperature measurements in the main borehole “KTB-Oberpfalz HB” of the German Continental Deep Drilling Program are used to determine the undisturbed formation temperature (UFT) by extrapolating the logged values in time using three models: the line source model, the cylindrical source model, and the cylindrical source model with thermal contact resistance. In order to stabilize the inversion algorithm, a priori information about the model parameters must be introduced. The line source model and the simple cylindrical source model yield almost identical results, which, in the lower part of the borehole, lie above the temperature values extrapolated to depth from the almost undisturbed temperature measured in the 4000-m-deep pilot-hole. The simulated UFT obtained from the cylindrical model with thermal contact resistance shows a reduction with respect to the UFT from the other two models in the interval 4000–6000 m for the data from the 6000-m log runs. This difference becomes very small when the measurements from the 7200-m runs are used.

Estimation of undisturbed formation temperatures under spherical-radial heat flow conditions

A new method to estimate undisturbed formation temperatures from measurements taken during the return to equilibrium is presented. The proposed conceptual model considers, as an approximation, the return to equilibrium under spherical-radial heat flow with unit initial normalized temperature in the perturbed region and zero outside. Analysis shows that temperature transients travel linearly as 1/√t at large times and the undisturbed formation temperature can be obtained directly from the intercept of a T vs 1/√t plot. Circulation time is not explicitly required in the approach presented in this paper, which represents an advantage since, in practice, its estimation is uncertain and difficult to evaluate with sufficient accuracy using conventional methods. Data from examples previously reported in the literature are analyzed to illustrate the method.

Evaluation of Local Geothermal Gradients on North Slope of Alaska: ABSTRACT

The U.S. Geological Survey is conducting a detailed assessment of worldwide natural-gas hydrate occurrences. Thermodynamic conditions controlling hydrate occurrences of northern Alaska have been examined. Pressure and temperature conditions on the North Slope indicate that hydrates would be potentially stable both above and below the permafrost base. Geothermal gradients needed to predict the thickness of the hydrate stability zone are not easily obtained. A survey of preliminary data suggested wide variations in averaged regional geothermal gradients across the North Slope. To evaluate regional variations of geothermal gradients, 2 techniques were employed to calculate local gradients. The first method used bottom-hole temperatures recorded during successive wireline logging runs and corrected by Horner crossplots to determine undisturbed formation temperatures. The Horner crossplot method requires a series of recorded bottom-hole temperatures. However, in most of the North Slope production wells, only 2-3 log runs are conducted per well, thus limiting the number of bottom-hole temperatures. To overcome this limiting factor, a second method has been developed to evaluate local geothermal gradients. This new technique uses permafrost depths delineated from well-log data to project geothermal gradients. Gradients within the permafrost zone have been projec ed from the base of permafrost, which is in equilibrium at -1°C. A series of mean ground temperatures has been used to project the upper extent of each gradient. Geothermal gradients change abruptly at the base of the permafrost. In order to calculate the gradient below the permafrost base, a constant generated from subsurface temperature data was used to correct for this change in geothermal gradient. Data from 398 wells were examined by each method to develop a series of geothermal gradient maps. The gradient maps generated by the 2 methods compare favorably; trend-surface comparisons indicate a high degree of similarity. End_of_Article - Last_Page 245------------

The establishment of the thermal equilibrium in a deep bore hole in the northern part of the German Democratic Republic

In deep bore holes (> 5000 m) in the G.D.R., temperature measurements were performed for different time intervals after the mud circulation had been stopped. From an analysis of the depth and time dependence of the process of establishing the temperature equilibrium it follows that three depth ranges have to be distinguished. In the upper and lower parts of the bore hole the temperatures measured immediately after the mud circulation had been stopped deviate strongly from the undisturbed formation temperatures. Temperature correction values can be derived for both these depth ranges. Between them there is a transition zone in which the time variations of the temperature can be neglected. Therefore, the temperature gradient within this transition zone should be used to determine the heat flow density.

Computing Downhole Temperatures in Circulation, Injection, and Production Wells

A computer model is presented for predicting downhole wellbore temperatures in flowing or shut-in fluid streams, in casing and cement, and in formations. Flowing options include injection/production, forward/reverse circulation, and drilling. Model predictions agree with field temperature data. The influences of temperature, flow rate, and depth on downhole temperatures are presented. Introduction Temperatures in a well are important for many aspects of drilling, completion, production, and injection. A few applications that require an understanding of the downhole temperature history in a well include (1) cement composition, placement, and setting time, (2) drilling mud and annulus fluid formulation, (3) packer design and selection, (4) logging tool design and log interpretation, (5) wax deposition in production tubing, (6) corrosion in tubing and casings, (7) thermal stresses in casings and tubing, (8) permafrost thaw and refreezing, (9) wellhead and production equipment design, (10) drill bit design, and (11) elastomer and seal selection. Of course, many other applications may exist. Two interesting possibilities are computing undisturbed formation temperatures from flowing stream temperature measurements and anticipating abnormal pressure zones from fluid temperature changes while drilling.Hostile environments present even more challenging needs for downhole temperatures, while making it more difficult to determine what temperatures to expect. Any unusual temperature conditions can make previous temperature experience and intuition unreliable. Abnormal temperature gradients can cause a rapid rise in flowing fluid temperature, and unusually deep wells expose fluids to hotter temperatures for longer times. In arctic environments, cool permafrost zones can cause unexpectedly low temperatures throughout the length of a well, both in and below the permafrost. Conversely, geothermal wells exist in unusually hot regions, resulting in abnormally high temperatures in a well. An understanding of downhole temperatures is needed in hostile environments for the same applications as more conventional wells and perhaps for additional needs, but determining those temperatures is more difficult.Determination of downhole wellbore and earth temperatures is a complex task. Many variables influence temperatures, which are continuously changing with time. Temperature recording devices have been developed, but these provide only isolated data points for a transient quantity and, furthermore, cannot provide sufficient information to establish the relative importance of variables influencing temperatures. Therefore, a means of computing downhole temperatures is needed to determine important design criteria, such as maximum temperature and time for exposure to high temperatures. Many simplified analytical techniques and some correlations of experimental data have been constructed in the past, each with limited success in predicting downhole temperatures. Experience has demonstrated that a computer model is needed to account for complexities of heat transfer in a well. JPT P. 1509＾

Analysis of Cerro Prieto well logs: Some results and problems

The evaluation of well logs from the Cerro Prieto geothermal field is made difficult by the wellbore conditions existing during the logging runs and by the complex minerology of the hydrothermal reservoir. In order to obtain well logs at the Cerro Prieto geothermal field, the wellbore must be cooled to at least the maximum operational temperature of available well-logging equipment, normally 120°C. Since undisturbed formation temperatures are usually in excess of 250°C at Cerro Prieto, the cooling of the wellbore creates an acute thermal gradient between the wellbore and the undisturbed formations. An understanding of the influence of this gradient on logging responses is necessary to establish confidence in well log evaluations. Similarly, since the mineralogic characteristics of the formations affect the well-log responses, quantitative mineralogic studies of wellbore cuttings and core make an important contribution to log evaluation. This influence is pronounced in a hydrothermal reservoir where complex mineralogies occur. An appreciation of the effects of wellbore cooling and complex mineralogies on log responses should increase confidence in the evaluation of Cerro Prieto geothermal well logs.