SOLUTION MINING RESEARCH INSTITUTE 105 Apple Valley Circle Clarks Summit, PA 18411, USA Technical Conference Paper Telephone: +1 570-585-8092 Fax: +1 570-585-8091 www.solutionmining.org Permanent Blanket-Brine Interface monitoring by temperature monitoring in Salt Caverns Dr. Stephan Grosswig Bernhard Vogel GESO GmbH Jena, D-07747 Jena, Germany SMRI Spring 2008 Technical Conference 28-29 April 2008 Porto, Portugal
Solution Mining Research Institute Spring 2008 Technical Conference Porto, Portugal, 28-29 April 2008 Permanent Blanket-Brine Interface monitoring by temperature monitoring in Salt Caverns Dr. Stephan Grosswig and Bernhard Vogel GESO GmbH Jena, Germany Abstract The solution mining process for salt production in caverns must be controlled to reach an optimal yield, and to get a high mechanical stability of the produced cavern. In order to prevent uncontrolled washing in the top of the cavern during continuation of the process, a so-called blanket will be filled into the cavern. Mostly a special mineral oil is used for it, because it swims on the brine. During the solution mining process the level of the blanket-brine interface can changes; therefore it is very important, to permanently know whose position (depth), in order to be able to intervene. A monitoring system using the distributed fiber-optic temperature sensing technology is developed by GESO GmbH Jena in collaboration with experts of the esco - european salt company GmbH & Co. KG, Werk Bernburg, Germany. A special fiber-optic sensing cable will be installed into the annulus of the well during the construction period or during a work over period. Using the difference of the thermal conductivity between brine and mineral oil the position (depth) of the interface can be pinpointed. The value of the interface depth will be evaluate quasi continuously. The blanket-brine interface monitoring system can work over the whole period of the cavern leaching process while the sensing cable is installed in the borehole. It s possible to monitor up to 8 caverns simultaneously with one central processing unit. Key words: Cavern Development, Caverns for Brine Extraction, Instrumentation and Monitoring Introduction The solution mining of salt is a very effective technology to product raw material for many products in the chemistry. Otherwise caverns in salt rock formations are good to use for the storage of fluids and gases. For it the size of the cavern must be optimized to guarantee the stability over a long period of time. Therefore the solution process must be monitored to have any possibility to control it to reach an optimal size. A monitoring system for it using the distributed fiber-optic temperature sensing technology is developed by GESO GmbH Jena, and will be presented. - 2 -
Basic Principle Using the distributed fiber optical temperature sensing technology an monitoring system for the blanket-brine interface were designed. The measuring principle is based on the RAMAN effect and the so-called Optical Time Domain Reflectometry (OTDR) method. Laser pulses are coupled into the fibers where the photons interact with the molecules of the fiber material. Some photons are scattered backwards and they carry information on the thermal motion of the molecules they were scattered by. Consequently, the spectrum of the backscattered light carries information on temperature of the fibers. Thus can be used to measure temperature along the optical fiber. The spectral analysis is combined with measuring the propagation time of the laser pulses along the fiber (radar principle) as the speed of light in the fiber is known. Scanning the entire length of the fiber by short intervals (e. g. 0.5 m) the temperature profile along the fiber is determined. The technical equipment of the distributed fiber optical temperature sensing technology consists of two main components: fiber optical sensor cable and the measuring device. The optical fibers which are integrated into robust cables, are the temperature sensitive elements and allow for measuring temperature profiles at arbitrary times, quasi-continuously with a high spatial resolution along the cable. This is a requirement for the investigation of thermal processes. Technical Solution The designed technical solution should be described with the following picture: picture 1: the fiber optical sensing cable installed into the annulus - 3 -
To realize this measuring task a so called fiber optical hybrid sensing cable with two electrical conductors inside is required: picture 2: cross section of the fiber optical sensing cable How to see on the picture 1 the sensing cable directly goes through the blanket-brine interface. The both electrical conductors are connected at the end of the cable (bottom), the outcome of this is an electrical circuit. Supplying with an electrical current by the electrical power supply unit the cable will be heated up over the whole length at a few degrees. The induced heat energy mostly goes into the surrounding fluids of the cable. Therefore the temperature in the cable, measured by the fiber-optical measuring device, strongly depends on the thermal conductivity properties of these fluids. That means that the measured temperature in the cable is higher at these locations, where the thermal conductivity of the surrounding fluid is lower. The designed method uses the fact that the thermal conductivity of the blanket (fuel) is lower than the thermal conductivity of the brine. The following picture shows the temperature in the cable for two different surrounding fluids: Temperature [ C] heating period 0:00:00 0:15:00 0:30:00 0:45:00 1:00:00 1:15:00 time [h:mm:ss] cable section in fuel cable section in brine picture 3: temperature difference in the cable for different surrounding fluids To find the position of the blanket-brine interface (depth) the temperature profiles measured first before the electrical supply will turned on, and second at the end of the heating period must be compared. The following picture shows the temperature difference between both profiles by depth: - 4 -
Additionally to the difference curve the trend line is added. The so called inflection point of this curve stands for the blanket-brine interface depth. In this case the created software tool found this level at a depth of 604.5 m. picture 4: temperature difference and trend line of it The GESO GmbH Jena has created a mathematical algorithm, and developed a software to control the measuring process and to calculate the blanket-brine interface depth. The created system including the software were tested at a production site of the esco - european salt company GmbH & Co. KG, Werk Bernburg, Germany. The above shown results we achieved at this site in collaboration with experts of the esco company. Adventures In comparison with the state of the technology the designed GESO system has the following adventures: Permanent monitoring of the blanket-brine interface level, High accuracy of the level value, Possibility to save costs, because work over activities during the solution process can be saved, The solution process of the cavern will be finished in a shorter time; so the cavern can be used for other tasks (e.g. gas storage) in a shorter time. Outlook The available measuring device has up to 4 measuring channels, and it is able to measure up to distances of 8 km, up to 8 caverns can be monitored by one central system. The GESO GmbH Jena will continue to develop this system for the practice of blanket-brine interface monitoring at brine production sites. Further test results will be presented at the SMRI meeting in Porto. - 5 -