TEMPERATURE LOGGING IN PERFORATED WELLS

Transcription

1 PROCEEDINGS, Thirty-First Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 30-February 1, 2006 SGP-TR-179 TEMPERATURE LOGGING IN PERFORATED WELLS Per-Gunnar Alm and Leif Bjelm Engineering Geology, Lund University P.O.Box 118 S , Lund, Sweden ABSTRACT The deep geothermal well drilling in Lund during 2002 and 2003 reached 3700 m. The lower 1700 m was drilled in crystalline basement rocks. Casing was installed to 3310 m. In order to test and evaluate the well both open hole and perforated casing testing was carried out. The report brings to attention the usefulness of repeated temperature logging over the selected sections. The measurements were carried out under drawdown and built up conditions. For more than 300 million years there have been several highly active tectonic periods along the zone with of fairly calm conditions. Even today activities have been noted along this zone. All pump testing was performed as air lift operations. Retrieved data has proven to be useful when for example evaluating perforation success, thermal heterogeneity and temperature disturbances. DEEP GEOTHERMAL PROJECT The city of Lund in the southern part of Sweden has the only operational geothermal heat plant in Sweden. It is a low enthalpy plant operating with two heat pumps, which in two steps extract the energy. The delivery temperature to the district heating net is at the most 85 deg C. The plant has been in operation since 1984 and is described in various papers. In 2002 a new geothermal project was started in the vicinity of Lund, the Deep Geothermal Project. The aim of the project was to drill into a major deformation structure of the Tornquist zone. The southernmost part of Sweden is located within the border of the Baltic Shield and one of the major deformation zones of Europe, the Tornquist zone. Locally the width is about 100 km and several thousands kilometer long. It is a major zone traversing northern Europe. The extent is from the North See, in the northwest, down to the Black See in southeast. Its border zone, locally, passes just north of Lund. Figure 1. Scandinavia with Sweden Two deep boreholes were planned down into the basement. Studies of the geothermal gradient in the area suggested that it could be possible to find water in the fractured zone of > 100 deg C. The temperature would in such case be high enough for a direct heat exchange. GEOLOGY The basement in DGE#1 was encountered at a depth of 1950 m, only slightly higher than anticipated. The

2 upper 1950 m of the borehole consists of sedimentary sequences. Cretaceous, Jurassic and Triassic strata overlaying the Precambrian basement. All the sedimentary sequence and basement are strongly faulted divided into several fault blocks. (Erlström, Sivhed 2003). The selection of the potential production zones were based on studies of cuttings in combination with geophysical logging data from the borehole. The lithology description for the perforated given here is based on the well- site report. (Erlström, Sivhed 2003). The perforated section between 1895 and 1905 m consists of a well sorted fine to medium grained, silica cemented sandstone. The sandstone belongs to Early Cretaceous. The section is in between a claystone at the top and a limestone below. Between 1827 and 1853 m is a section with the same type of sandstone as in the previous section but in this case the sandstone is in-between layers of claystone. The section 1685 to 1717 m actually consists of two subsections. The upper part between 1685 and 1708 m, is an alternating sequence of claystones and poorly sorted, fine to medium grained, carbonate cemented sandstone. The lower part 1708 to 1717 m consists of fine to medium grained, carbonate cemented, sandstone with poor porosity interbedded with coal and claystone. Above and below the total section are layers of claystone. This sandstone also belongs to Early Cretaceous. The uppermost geological interval can be divided into four sub. The uppermost between 1427 and 1463 m consists of a sandy claystone and carbonate cemented sandstone/siltstone. Below, 1463 to 1477 m, is a fine to coarse-grained, poorly sorted, carbonate cemented sandstone. Further down between 1477 and 1490 m is similar to the top section between 1427 and 1463 m. The deepest section between 1490 and 1530 m consists of fine- to coarse-grained, carbonate cemented sandstone. The layers above and below the consist of claystone. The lithologies of all these sections belong to Late Cretaceous. WELL COMPLETION The borehole DGE #1 was drilled to a total depth of 3701 m, the second deepest borehole in Scandinavia. During the drilling operation several drilling methods were tested. (See more in paper by Bjelm, 2006) The crystalline basement was encountered at 1950 m. In order to be able to test potential production zones in the sedimentary part, down to about 1950 m, of the borehole, several perforation runs through a dual casing had to be run. The borehole has earlier been cased down into basement to 3310 m. Below that point the completion was an open hole. Before each casing installation a logging program, consisting of a normal set of open hole logs, was carried out. A production logging program was also performed between 1950 and 3310 m. After well completion the casing installations are as follows: 30 0 m to 155 m 20 0 m to 1004 m 13 5/8 930 m to 1975 m 13 5/8 0 m to 930 m 9 5/ m to 3310 m 9 5/8 900 m to 1880 m Perforations After production testing of the potential production zones in the basement parts of the dual casing, between 900 m and 1000 m, was perforated in order to test sandstone formations in the sedimentary section. This was done in steps with flow tests after each new perforation. In the sedimentary sections the following were perforated in the first run m to 1905 m 1827 m to 1840 m 1827 m to 1853 m 1685 m to 1717 m All these represent rather clean sandstones with high porosity. A second perforation was carried out of some to increase the shot density and to reduce the skin. (Rosberg, 2006). Later perforations were also carried out in the following m to 1525 m 1427 m to 1472 m 1427 m to 1531 m Part of this section has been perforated more than once in order to increase the productivity and to open up further sections. FLOW TESTS Two major flow test periods were carried out in DGE #1, one in the spring and the second in the end of Several flow tests, both production as well as injection of formation fluid, was performed during these test periods. (See more about this in paper by Rosberg, 2006). The injection tests have clearly affected the geothermal temperature distribution in

3 the borehole and its surroundings as will be shown later. The well was flow tested after each perforation. To dispose of the water when the mud pit was filled up the water was reinjected into the well. Before the autumn temperature measurements the following injection tests were carried out according to table 1 Table1. Some of the injections carried out during the second test period. Date Injection time 2003 Average injection rate (l/s) Injection point (m) Injection (m) h 40min h 10min h 10min logging tools used in these operations were Halliburton tools. A PLT survey in the crystalline part of the borehole was run by Baker. All logging operations after well completion were conducted by the department of Engineering Geology, Lund University. During drilling phase As temperature was one of the most important parameters in the project, every opportunity to record the temperature was taken. Temperature was recorded both as maximum recording (often bottom hole temperature) as well as conventional temperature logs. TEMPERATURE SURVEYS Equipment and logging operators Several different temperature devices were used throughout the project. Temperature recording was done with maximum temperature readers and memory gauge recorders. Temperature logs have been recorded both with a temperature sensor on a slick line as well as with a standard logging system operating with a four conductor logging cable. On the slick line primarily a Metrolog pressure and temperature memory gauge has been used. The sensor has a temperature range deg C and an accuracy of 0.3 deg C. The memory capacity of the gauge is recording points. The unit record values against time so therefore when using it on the slick line the depth devise from the standard logging unit was used to correlate the depth with time recordings. During recording the Metrolog the gauge was stopped every 25 m to allow for temperature stabilization and to correlate the depth with a time log. The logging system used during the temperature survey was a Mount Sopris system with a temperature sond that has an accuracy of 0.1 deg C. For recording of the bottom hole temperature (i.e. maximum temperature), Hg-thermometers and thermo strips were used. When used they were placed in the rod of the deviation measurement device or run on a slick line. Responsible for open hole logging and production logging in the open section in basement was Geofizyka Torun under contract of Halliburton. The Figure 2. Temperature recordings in the well DGE#1. Figure 2 shows the temperature recordings accomplished during all the drilling phases and figure 3 shows the same recordings for the lower part (crystalline basement) of the borehole. It should be noted that the specific temperature logs that were run during the temperature survey in the perforated sections in the sedimentary part of the borehole are not included in the figures. They will be presented later in this paper as a separate temperature study. Figure 2 also includes a temperature log (upper red line in figure) that was not recorded in borehole

4 DGE#1. The log was run in an adjacent borehole, Flackarp-1, which was drilled 1983 into the cretaceous sediments south of Lund., some 10 km west of DGE #1. The log was run in June 2003 and the well had not been used for injection or production for several years so it is safe to say that that log shows the geothermal temperature and gradient for the area around Lund. The total depth of Flackarp-1 is 823 m. In figure 2 and 3, a temperature gradient curve is also plotted. The line represents the estimated local geothermal gradient of 30 deg C per km. As can be seen the gradient follows the temperature curve recorded in Flackarp-1 quite well. Figure 3. Temperature recordings in DGE#1, lower part. There are three Bottom Hole Temperature (BHT) measurements shown in the figures 2 and 3, (black dots). The temperature curve from was recorded after the drilling with fluid. On several occasions there were maximum readings by use of Hg-thermometers and thermo strips. Theses are shown as red dots in the lower part of the figures. Before the 9 5/8 casings were installed a production test was done between 1880 and 3310 m and a series of logging runs were carried out with a production tool. Three of the temperature curves recorded during this session are shown in the figures. They are named PLT15, PLT17 and PLT18. In figure 2 they are in the lower part and in figure 3 they are show over the complete diagram. In several of the curves spikes with an increased temperature can be seen. Figure 3 show a zoom in of the lower part of the borehole. The temperature peaks (green and blue) coincide quite well with borehole enlargements shown by the inserted caliper log. Increased borehole diameter has been interpreted as fractured and jointed sections. The operator could not explain the peculiar behavior of the temperature sensors. Results from the temperature recordings during the drilling phase It can be seen from figures 2 and 3 that there is a wide spread of temperature readings. This can clearly be seen if the continuous recordings are compared to the maximum readings. The highest variations in the readings occurred in the HG and thermo-strips readings. There is a spread of up to 25 deg C between the lowest and the highest readings within a borehole length of less than 150 m. If the geothermal gradient is considered, the variation would have been no more than 5-6 deg C. Local influx of hot water from the formation doesn t seem likely why a malfunction of sensors is possible. The thermostrips and the Hg-thermometers is a contradiction but also quite unsafe methods. In figure 2 there is one curve that differs from the rest. It is the curve recorded , (yellow color). This curve has a higher starting temperature and a smaller gradient than the rest. The temperature was recorded directly after the drilling with fluid had ended. The borehole and its surrounding had over a long time been heated by circulating mud in the borehole. A new temperature log, red line, was run to approximately the same depth about half a year later, This was about two month after well completion. It can be seen that the formation in the upper part has cooled down, but the temperature at the bottom of the curves is the same. The temperature in the borehole is adjusting towards the local geothermal temperature gradient. During flow tests Before the flow- and temperature tests, autumn 2003, significant amount of drilling fluids from air drilling operations was injected to the borehole. For comparison a steady state temperature recording was done several months prior to the production and injection tests. This recording was done about 2 month after the drilling and more than 6 month before the start of the temperature survey. It is therefore quite certain that the recorded temperature and its gradient is fairly close to the local geothermal

5 temperature and gradient for the area. This recording is shown in figure 4, 5 and 6 as a dashed line. Measurements during production flow Over the test period several temperature surveys were run. They were run both in near static as well in dynamic condition. Only 2 of these runs are presented in figure 4. As can be seen the temperature increase from the lower part of perforation ( m and m) indicates warmer formation water is entering the well. The section 1685 to 1717 m doesn t seem to contribute with any inflow of water. In the uppermost perforated section between 1427 and 1528 m it is primarily the middle part that seems to contribute. Depth (mrkb) Lund DGE#1 - Static and Dynamic Temperature Surveys on 26/27/28 November 2003 Perforated Static Temp Survey at 23:00 hrs on 26/11/03 Dynamic Temp Survey at 17:30 hrs on 27/11/03 Measurements after shut in A series of temperature measurements were run after shut in and three of them are shown here. The first temperature log under pressure build-up, the green curve in figure 5, was recorded about half an hour after shut in. There is an increase in temperature at the lower part of section 1827 to 1853 m. For the rest of the borehole there is a decrease of temperature towards the temperature before the airlift. Due to the great amount of cool water that had been injected into the perforated sections it will take several months before the formations reach equilibrium with the geothermal temperature. Lund DGE#1 - Static and Dynamic Temperature Surveys on 26/27/28 November Static Temp Survey at 23:00 hrs on 26/11/ Perforated 1427 Temp Survey at 07:00 hrs on 28/11/ Temperature (Deg C) Figure 4. Temperature recording during production. Figure 4 shows the temperature just before any water had been produced from the borehole (blue curve). Depth (mrkb) The static temperature curve (blue) clearly shows that there had been a great amount of cool water, injected into the formation. It can be seen that the major part of the injected water had entered the upper part of the perforated interval between 1827 and 1853 m. In the upper perforation, 1427 to 1531 m, it can be seen that the inflow has been higher in the middle of the perforated section. After that recording, the temperature sond was lowered below the lowest perforation and left over night. The next day water was pumped from the well by air lifting for about 5.5 hrs, with an average flow of 65 l/s. During the airlifting several temperature logs were run both up wards as well as down wards. Figure 4 shows one of these recordings (red curve) Temp Survey at 19:30 hrs on 27/11/ Temperature (Deg C) Figure 5. After shut-in. In the beginning of the shut in period the cooling of the upper perforated section is more pronounced compared to pretest conditions (blue curve). On the red curve, recorded about 12 hours after shut in, the temperature has gone back to the initial temperature. This is however not the case for the 2 deepest

6 perforated sections. Here there is an ongoing temperature increase. Some results from the temperature recordings during the temperature surveys If we look in more detail at the lower part of figure 5 two zones with inflow of gas can be seen. They are shown in figure 6. The gas influx zones are located in the upper part of the perforated section 1827 to 1853 m and in the lower part of the perforated interval 1895 to 1905 m as indicated in figure 6. Lund DGE#1 - Static and Dynamic Temperature Surveys on 26/27/28 November 2003 other hand when there is production, convection will involve the porous zones and increase temperature faster. Taken this into consideration, figure 5 and 6, the 2 deepest perforated sections in relative terms seams to have the highest permeability since the temperature, after the production (curve red and green), stays higher in front of these compared to the uppermost. CONCLUSIONS As can be seen from the temperature recordings, during the drilling and flow testing, it is important to know under which circumstances the temperature was recorded in order to explain the behavior and variations of the temperature data Perforated Temp Survey at 19:30 hrs on 27/11/03 Recording and comparing temperature data in perforated wells under static and dynamic conditions can enhance the understanding of the hetogeneity of the formation and of different flow regions ACKNOWLEDGMENT We would like to mention that our Drilling Manager in charge was Virgil Welch, USA, possessing extensive Air drilling experience from for example geothermal operations in California. Depth (mrkb) Temp Survey at 07:00 hrs on 28/11/03 Gas Static Temp Survey at 23:00 hrs on 26/11/03 Further acknowledgment is directed to Derek Howard Orchard, England, reservoir engineer and invaluable in all matters related to drilling engineering Gas Temperature (Deg C) Figure 6. Zones with inflow of gas. During injection of cool water, the borehole and its surroundings will be affected. Except for injection time, flow and pressure there are two thermal conditions controlling the temperature impact convection and conduction. In a porous part of a formation both conduction and convection will take place. Thus, in a porous part the cool water will reach farther away from the borehole wall, than in a less porous. After injection when there is only limited movement of fluids most of the heat energy influencing the temperature is related to heat conduction. In such a case the nonporous part will reach geothermal equilibrium faster than the porous part since the cool water has reached deeper into the formation. On the REFERENCES Bjelm, Leif (2006) Under Balanced Drilling and Possible Well Bore Damage in Low Temperature Geothermal Environments. Proceedings, Thirty-First Workshop on Geothermal Reservoir Engineering. Stanford University, Stanford, California, January 30-February 1, 2006 SGP-TR-179 Erlström, M, Sivhed, U (2003) Well Site Report, DGE1/1b. Geological Descriptions and Composite Litho-Log Deep geothermal Energy project Skåne. Lund, Sweden Lunds Energi AB (2003) Lund DGE#1 Deep Geothermal Energy Project Well Evaluation Report, Lund, Sweden Rosberg, Jan-Erik (2006) Flow Test of a Perforated Deep Dual Cased Well. Proceedings, Thirty-First Workshop on Geothermal Reservoir Engineering. Stanford University, Stanford, California, January 30-February 1, 2006 SGP-TR-179