Patent Publication Number: US-8122951-B2

Title: Systems and methods of downhole thermal property measurement

Description:
RELATED APPLICATION DATA 
     The present application claims priority under 35 U.S.C. §119 to U.S. Provisional Application Ser. No. 60/657,207, naming Masafumi Fukuhara and Kasumi Fujii as inventors, and filed 28 Feb. 2005, which is incorporated herein by reference in its entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to methods and systems of deriving thermal properties of subsurface structures by downhole dynamic thermal measurements. In particular, the present invention relates to utilizing an in situ active cooling and/or heating device to disturb local downhole temperature in formations, such as oil/gas bearing structures, water saturated formations, gas hydrate bearing rocks and sediments, and measuring thermal response data thereof for purposes of characterizing the formation. 
     BACKGROUND OF THE INVENTION 
     Some naturally occurring resources, such as gas hydrates, dissociate or form, or are otherwise affected, when temperature and/or pressure conditions cross the equilibrium border. An understanding of the parameters for such behavior is important for efficient exploration and development of the resources, for example, gas hydrates and heavy oils as energy resources. In this, thermal measurements are one of the key components in characterizing subsurface structures, not only statically but also dynamically. 
     Conventional methods to estimate thermal properties, such as thermal conductivity, capacity and diffusivity, of subsurface formations include monitoring temperatures passively at several underground depth locations and interpreting the collected data with assumptions such as steady heat flow or relaxation from thermal disturbance by drilling and/or mud circulation, etc. In conventional systems, temperature changes caused by a production test or drilling/circulation operations are measured. Because the thermal properties are estimated based on several assumptions, the passive measurement methods described above leave large uncertainties in the estimated thermal properties of the subsurface structures. 
     On the other hand, active thermal property measurements may be undertaken in a laboratory and equipment is commercially available for these purposes. However, applications of the laboratory based active measurement methods to in situ subsurface formation measurements have many technical and logistical difficulties. 
     SUMMARY OF THE INVENTION 
     In consequence of the background discussed above, and other factors that are known in the field of thermal characterization of subsurface formations, applicants discovered methods and systems for active subsurface thermal property measurements based on the principle that the thermal response time of subsurface structures would vary in zones having different thermal properties. In this, the present invention contemplates utilizing active heating and/or cooling, i.e., known temperature disturbances or stimulations caused by sending a known signal into subsurface formations, as opposed to passive monitoring for purposes of characterizing the subsurface formations. 
     Applicants recognized that dynamic measurement methodology disclosed herein would have particular applicability to in situ active measurements of subsurface structures, such as gas hydrate and/or heavy oil bearing formations. 
     Applicants further recognized that in situ characterization of thermal properties is one of the key components for characterization of subsurface formations. 
     Applicants also recognized that acquiring thermal properties of downhole structures, such as hydrocarbon bearing formations and sediments, would be highly beneficial in designing efficient systems and methods for treating the subsurface structures for the development and production of natural reserves. 
     The present invention contemplates dynamic measurements of temperature time variance, i.e., the thermal response is not static in time, to derive static thermal properties for formation analysis of, for example, rocks, sediments and such other subsurface formations with deposits, such as oil, gas, methane hydrates, water, among others contemplated by the present invention. In this, characterizing a formation may comprise providing one or more answer products based on one or more thermal properties of the formation, for example, answer products relating to one or more of characteristics of hydrocarbons in the formation for heat treatment of the hydrocarbons; one or more physical parameters of the formation for delineating the formation; permanent monitoring of an operating well traversing the formation; among others that are realizable based upon the teachings of the present invention. 
     Applicants discovered that an algorithmic relationship may be utilized advantageously to derive thermal properties, such as thermal conductivity, based on configuring a heater and/or cooler and one or more sensors in localized and/or distributed arrangements to acquire time variance of local temperature data for subsurface formations under investigation. 
     In one embodiment of the invention, a distributed temperature sensing (DTS) downhole system having a fiber sensor may be utilized as a temperature sensor with a localized cooling device, such as a Peltier device, and/or a heating device, such as a resistance heater that is configured by, for example, applying current to a metal and/or plastic or similar tube around the fiber sensor. In other embodiments of the invention, the configuration may include a localized precision thermometer, such as a resistor temperature detector (RTD) or a Fiber Bragg Grating sensor, and a suitably configured power supply to provide temperature control of the cooling and heating device. 
     In accordance with one aspect of the instant invention, one method of deriving one or more thermal properties of a subsurface formation for characterizing the formation comprises creating thermal disturbance downhole, acquiring thermal response data for the formation based on time variation in temperature, and deriving a thermal property of the formation based on the thermal response data of the formation. According to other aspects of the present invention, a system for deriving thermal properties of a subsurface formation comprises at least one of a heating and cooling device configured for creating thermal disturbance downhole in a predetermined area of interest and a sensing system configured for acquiring thermal response data and deriving a thermal property of the formation based on time variation in downhole temperature. 
     Additional advantages and novel features of the invention will be set forth in the description which follows or may be learned by those skilled in the art through reading the materials herein or practicing the invention. The advantages of the invention may be achieved through the means recited in the attached claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate preferred embodiments of the present invention and are a part of the specification. Together with the following description, the drawings demonstrate and explain principles of the present invention. 
         FIG. 1  is a schematic view of one exemplary context in which the present invention may be used to advantage and depicts graphically expected temperature behavior, i.e., thermal response, of layers having different thermal properties. 
         FIGS. 2A and 2B  are schematic representations of a top view and a cross-sectional view, respectively, of a sensor cable-heater configuration used in an experimental system according to the present invention. 
         FIG. 3  is a schematic depiction of a model for the experimental configuration of  FIGS. 2A and 2B  in which layer  1  represents a sensor cable with plastic sheath configured as a heater; layer  2  represents sand; layer  3  represents air; and L represents cable length. 
         FIG. 4  is a graph of calculated thermal resistance versus radial distance for the model of  FIG. 3 . 
         FIG. 5  is a graphical representation of temperature increase in sand while heating. 
         FIG. 6  graphically represents preliminary results of estimation of apparent thermal conductivity vs. time. Broken gray line indicates the estimated apparent thermal conductivity in sand. 
         FIG. 7  is a computer simulation of heat transfer for a cross-section of a well. The sensor cable is located outside a casing that is a support pipe. 
         FIG. 8  is a graph representing influence of formation thermal conductivity (λf) on temperature for a well diameter of 30 cm. 
         FIG. 9  is a graph representing influence of formation thermal conductivity (λf) on temperature for a well diameter of 10 cm. 
         FIG. 10  is a graph showing influence of formation thermal conductivity (λf) on temperature for a well with metal casing. 
         FIG. 11  is a graph showing influence of formation thermal conductivity (λf) on temperature for a well with plastic casing. 
         FIGS. 12A to 12D  are schematic representations of exemplary systems for deriving thermal properties of subsurface structures according to the present invention. 
     
    
    
     Throughout the drawings, identical reference numbers indicate similar, but not necessarily identical elements. While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Illustrative embodiments and aspects of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in the specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, that will vary from one implementation to another. Moreover, it will be appreciated that such development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having benefit of the disclosure herein. 
     Stability of formation fluids, including hydrocarbons such as gas hydrates and heavy oils, is sensitive to variations in pressure (P) and temperature (T). In this, gas hydrates dissociate or form when pressure and/or temperature conditions cross the equilibrium border. As discussed above, conventional methods to acquire thermal properties of subsurface formations, such as thermal conductivity, diffusitivity, capacity, include core analysis in a laboratory and passive temperature measurements at several underground locations. For core analysis of hydrate bearing zones, for example, it is often difficult to keep the same conditions in a laboratory as the conditions that are found downhole. Data obtained are dependent upon coring conditions due to the dynamic dissociation/formation process of hydrates, compaction factor differences under different pressures, and sometimes upon missing core samples from certain depth intervals. For passive measurements at several underground locations, the acquired data are interpreted under certain assumptions that increase uncertainties, such as steady heat flow and relaxation from thermal disturbances by drilling and/or mud circulation. Furthermore, detailed thermal measurements in hydrate bearing zones have not been conducted. 
     The present invention contemplates utilizing thermal property measurement methods for in situ evaluation of formation thermal properties by actively creating a known thermal disturbance with a thermal source, such as a cooler and/or heater device, and monitoring the thermal response, i.e., time variant temperature data, of the target zone downhole. In this, the present invention contemplates measuring temperature of the formation area or areas in which the thermal disturbance is caused for a predetermined period of time so that the thermal response of the formation may be determined. The predetermined period of time for temperature measurements depends on surrounding conditions, such as formation properties, borehole fluids, configurations of the systems that are used, among other factors that determine a desirable or necessary period of time for the temperature measurements. Moreover, the predetermined period of time for temperature measurements may include the time period of active heating or cooling of the formation and a suitable period of time after the active heating or cooling so that sufficient thermal response data of the formation are acquired for purposes of the instant invention. 
     Thermal disturbance may be local heating or cooling, i.e., in a localized area of interest, so as to keep the thermal disturbance in an area or areas of the formation under investigation. The present invention contemplates actively heating or cooling the formation so as to cause dynamic thermal disturbance therein for a predetermined period of time so that the thermal response of the formation may be determined. The predetermined period or periods of time for active heating or cooling of the area/areas of interest depend on surrounding conditions, such as formation properties, borehole fluids, configurations of the systems, among other factors that determine a sufficient period of time for the heating or cooling. 
     Although as described herein, one embodiment of the present measurement methodology is based on a hot wire method for measuring thermal properties, other methods are within the scope of the present invention. In this, the present invention additionally contemplates systems and methods of measuring one or more thermal properties of hydrocarbon-bearing formations that do not utilize symmetrical and/or infinite length assumptions of the hot wire method. 
     From temperature relaxation with an infinite line heater and a temperature sensor on the heater in an infinite medium, thermal properties are derived for the surrounding medium. The relationship of thermal conductivity and temperature is shown in Equation 1. For example, from the slope of the logarithmic time (ln(t)) and temperature increase (T−T0), thermal conductivity λ is derived. 
                     T   -     T   0       ≅       q     4   ⁢           ⁢   π   ⁢           ⁢   λ       ⁡     [       ln   ⁢           ⁢     (   t   )       -   γ   -     ln   ⁡     (         r   2     /   4     ⁢           ⁢   α     )         ]               (   1   )               
wherein
     q=input power per length   γ=Euler constant   r=distance   α=thermal diffusivity   

     The above methodology is applicable to in situ downhole measurement conditions. As depicted in  FIG. 1 , a dynamic system  10  according to one embodiment of the present invention includes a long and slender heater  12 , such as a metal and/or plastic or other similar tube  20 , and an array of temperature sensors  14 , for example, a fiber optics sensor, located in a well  18  of a subsurface formation  16 . The tube  20 , such as a metal and/or plastic tube, may be deployed around the sensors  14  for protecting the sensors and for other purposes, such as distributed heating, as discussed in more detail below. Expected differences in temperature relaxation behaviors, i.e., thermal response, during heating and cooling, where the thermal properties of the formation differ, are shown graphically in  FIG. 1 . Herein, thermal conductivity measurements are used for purposes of describing the present invention. However, the present invention also contemplates other thermal property measurements, such as thermal diffusivity and thermal capacity. 
     Typically, borehole fluids would be directly surrounding the medium of the heater  20  and sensors  14  in a borehole. However, the present invention contemplates minimizing fluid convection effects as the first order of approximation. 
     In one embodiment of a system according to the present invention, an optical fiber sensor cable may be used as a distributed temperature sensing (DTS) system for the configuration depicted in  FIG. 1 . DTS systems are known in oilfield applications and will not be described in detail herein. Such DTS configurations provide consecutive temperature measurements along a well (note  FIG. 1 ). Advantageously, the fiber sensor cable may be configured to provide temperature information and the metal and/or plastic or other similar tube of the cable may be configured as a heater by applying current to the tube. 
     Applicants conducted experiments to evaluate the dynamic measurement methods and systems of the present invention.  FIGS. 2A and 2B  show top and cross-sectional views, respectively, of a sensor cable layout that was used in a modeling of the present invention. A cylinder shaped container  22  was filled with sand  24  with a sensor cable  12  positioned in the container  22  at the center and buried with the sand  24 . Thermal conductivity of the sand was experimentally derived. The sensor cable  12  included a fiber sensor  14  and a steel tube  20  with a plastic sheath outside the tube. Input power for heating the tube  20  was applied at both ends of the tube  20 , as depicted in  FIG. 2A . 
     Thermal conductivity of the sand was measured in advance with commercially available equipment and was found to be 0.22 watts/meter/Kelvin [W/m/K]. 
       FIG. 3  shows the experimental layout of  FIGS. 2A and 2B  in which homogeneous layers of cable with plastic sheath (layer  1 ), sand (layer  2 ), and air (layer  3 ) in the radial direction are assumed.  FIG. 4  shows the calculated thermal resistance, expressed in Equation 2 below, plotted relative to the radial distance. It is assumed that the thermal resistance and the slope in Equation 1 (˜1/λ) have a qualitatively similar tendency during a heat transfer process in time and radial distance. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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       FIG. 5  shows the measured temperature data during heating with 4.7 watts/meter [W/m]. The linear curve, as indicated by a broken black line, may be seen. This result versus time shows very similar response to the modeled thermal resistance curve versus radial distance in  FIG. 4 . The data were smoothed, i.e., polynomially fitted with moving windows, in time, then the derivative of dT/d(ln(t)) (the slope 1/4/pi/λ in Equation 1) was computed. The apparent thermal conductivity λ was obtained and plotted in  FIG. 6 . From the value of the almost flat part, as shown by the broken gray line in  FIG. 6 , the thermal conductivity was estimated as 0.22 W/m/K. The modeled result shows good correspondence with the pre-evaluated thermal conductivity in sand of 0.22 W/m/K. 
     Accordingly, through an experimental layout and modeling, applicants derived thermal conductivity in sand as a preliminary result, using methods and systems of the present invention. 
     Applicants further evaluated through numerical modeling applicability of a method of the present invention to in situ downhole conditions. A practical situation was assumed with a sensor cable supported by a pipe/casing put in a well (note  FIG. 7 ). A plastic sheath covers the sensor cable. The measurement duration for necessary temperature increase, i.e., the temperature increase required for evaluating formation thermal properties, with a practical input power was evaluated. As discussed above, the predetermined period of time for active heating depends on several factors. 
     Table 1 shows the parameters used in the numerical modeling.  FIG. 7  shows a computer simulation of a temperature field for a cross section of the well with a heater sometime after heating.  FIGS. 8 and 9  show graphically the temperature increase (ΔT) at the heater versus time during heating for well diameters of 30 cm and 10 cm, respectively. The pink lines in  FIGS. 8 and 9  indicate when the formation thermal conductivity (λf) is 3 W/m/K, and the blue lines indicate conductivity of 2 W/m/K. The red lines show a relative increase of {ΔT(=3)−ΔT(λf=2)}/ΔT(λf=3). The result suggests that smaller well size may be preferred to differentiate temperature behavior between different thermal properties, since heat energy consumption relates to the volume of the fluid. However, even with a hole having a diameter of 10 cm, more than 2 days are required to achieve a 10% relative increase in temperature. To reduce the necessary measurement duration,  FIGS. 10 and 11  show modeling focused on an effect of the material for the pipe.  FIG. 10  is for a pipe made of metal and  FIG. 11  is for a plastic pipe. In both cases, well diameter is 18 cm. A 10% relative increase in temperature is achieved in a few hours with the plastic pipe whereas the metal pipe required 20 hours. The plastic pipe works as a thermal isolator in the well to effectively transfer heat into the formation. By using a plastic pipe, the techniques of the present invention are enhanced for purposes of measuring temperature changes, and therefore thermal conductivity, in downhole environments. 
     The numerical modeling confirmed suitability of the present methodology to downhole applications, with some assumed conditions. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Parameters for numerical modeling 
               
            
           
           
               
               
               
               
            
               
                   
                 Thermal 
                 Density 
                 Specific heat 
               
               
                   
                 conductivity 
                 [kg/m3] 
                 [J/kg*K] 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Formation 
                 3 or 2 
                 3000 or 2000 
                 1000 
               
               
                   
                 Fluid 
                 0.6 
                 1000 
                 4200 
               
               
                   
                 Heater/sensor 
                 50 
                 7800 
                 500 
               
               
                   
                 Pipe 
                 50 
                 7800 
                 500 
               
               
                   
                 Plastic 
                 0.16 
                 1300 
                 1500 
               
               
                   
                 Air 
                 0.024 
                 1.25 
                 1000 
               
            
           
           
               
               
               
            
               
                   
                 Heater power 
                 30 W/m 
               
               
                   
                   
               
            
           
         
       
     
     Applicants&#39; experimental and modeling results confirmed applicability of in situ downhole thermal conductivity measurements according to the present invention. While evaluating factors such as fluid effect and appropriate cable and heater designs, preliminary experiments showed applicability of the present invention to measurements of thermal properties in surrounding media within an acceptable error range of around 10%. 
     In one embodiment of the present invention, a heating device may include a heater that utilizes an electrically insulated metal tube surrounding, for example, a fiber optics sensor, for generating heat in a subsurface structure by injecting current into the metal tube. In this, the metal tube may be sheathed with a suitable electrically insulating material, such as plastic and glass. For example, in situations where local heating of target depths in subsurface formations is preferred or desirable, current may be injected into an electrically insulated metal tube having higher resistivity at the target depths (note  FIG. 12A ). In this, a tube having appropriate resistivity as required by the localized heating may be utilized. Different resistivity in a tube may be achieved by using different materials for the tube. Alternatively, different heater density, for example, different numbers of winding around a fiber cable, may be utilized in combination with, or independently from, a heater tube having varying resistivity. 
       FIGS. 12A to 12D  are schematic depictions of some embodiments for dynamic measurements of thermal properties of subsurface formations according to the present invention.  FIG. 12A  depicts a system  10  having a distributed temperature sensing (DTS) system  12  with a fiber optics sensor  14  and a metal tube  20  deployed in a subsurface formation  16  having, for example, borehole fluid in a borehole  18 . Since the fiber optics sensor  14  is in the thin metal tube  20 , the metal tube  20  may be used as a heat generating or cooling device by, for example, sending a pulse-type or AC/DC current from the surface through the metal tube  20 . Appropriate electronics and processing and control capability may be provided as schematically depicted in  FIGS. 12A to 12D . In this, the configuration of  FIG. 12A  provides distributed temperature changes along a predetermined section of the well  18  and the formation structure  16 . 
     The configuration of  FIG. 12A  has various benefits such as utilizing existing oilfield equipment that may be set up at a well site without need for additional equipment. The depicted configuration provides homogeneous and distributed heat generation and distributed temperature outputs in predetermined area or areas of interest for purposes of determining formation thermal properties thereof. However, in some situations, based on particular needs, the embodiment of  FIG. 12A  may cause heat leak or provide unnecessary heating in non-targeted zones of the subsurface formation. Moreover, acquired data may be an averaged response for a layered zone due to limitations in depth resolution or spatial sensitivity that are inherent in DTS hardware. 
       FIG. 12B  provides one embodiment with a system  10  having a DTS system  12  with localized heaters  26  to heat selected, predetermined segments of the formation  16  having a well  18  therein. The embodiment of  FIG. 12B  having the DTS  12  with a localized heat generation device may be used in circumstances where the distributed heat generation configuration of  FIG. 12A  does not create adequate temperature disturbances in a well. Electrical current may be sent from the surface, or from a power device in the well  18 . As in the embodiment of  FIG. 12A , temperature along the well  18  may be monitored by the sensors  14 . In the embodiment of  FIG. 12B , the localized heaters  26  work efficiently in one or more target zones providing heat generation in the formation  16  as necessary or desirable. In this, more selective and targeted temperature variances may be generated.  FIG. 12B  depicts additional configuration that is required for the depicted embodiment, including a longer heater length that is sufficient to cover DTS spatial resolution. 
       FIG. 12C  depicts yet another embodiment of one system  10  having a DTS system  12  with localized heaters  26  and, in this embodiment, localized high precision temperature sensors  28 . In the embodiment of  FIG. 12C , the localized temperature sensors  28  may be used with a sensor cable  14  and localized heat generation devices  26 , as discussed in other embodiments above. In this, in an instance where the DTS system  12  does not give a sufficiently high resolution in temperature and/or depth, the system  10  of  FIG. 12C  may be used with one or more localized heaters  26  in combination with one or more localized temperature sensors  28 , such as Fiber Bragg Grating (FBG) sensors and resistor temperature detector (RTD) sensors, to provide suitable spatial and/or temperature resolution, as desirable or necessary. In the present embodiment, temperature variance may be monitored with the localized sensors  28  as well as the sensor cable  14 . 
     Localized heaters, such as depicted in  FIG. 12C , may be used to provide efficient, targeted heating in one or more predetermined zones of interest. In this, it is possible to acquire higher depth resolution by utilizing localized heaters and sensors, as depicted in the embodiment of  FIG. 12C , although configuration in addition to the one depicted in  FIG. 12A  may be required to set up the system. 
       FIG. 12D  depicts yet another embodiment of a system  10  for dynamic measurements of temperature having distributed metal and/or plastic or other similar tube heat generators  20 , for example, with varying resistivity in one or more tubes, and localized high precision temperature sensors  28  deployed in a well  18  of a formation  16 . In the embodiment of  FIG. 12D , a distributed heater, such as an electrically insulated metal tube heat generator  20 , may be used in configuration with localized high precision temperature sensors  28  in situations where a DTS system may not provide sufficiently high resolution in temperature and/or depth. In this, a heater  20  over the depth of the well  18  may be provided with localized temperature sensors  28 , such as FBG and/or RTD sensors. Sensors (not shown), such as cable sensors in a DTS system, may be combined with the system  10  in  FIG. 12D , if desirable or necessary. 
     In some embodiments of the present invention, dynamic temperature measurements may be acquired using an active heat generation device in a well, such as a hydrocarbon, carbon-di-oxide and/or water bearing well. The present invention contemplates applicability in methane hydrates and heavy oil bearing formations. As used herein, “heavy oil” refers to viscous oil deposits, such as heavy oil, tar sand, bitumen, oil sand, for which knowledge of thermal properties is desirable and/or necessary for purposes of development and extraction. For purposes of explanation, some embodiments of the instant invention are described herein with thermal disturbance/stimulation by a heat generation device; however, the present invention also contemplates utilizing a cooler, for example, a Peltier-type device, to disturb the temperature of a subsurface formation for purposes of thermal characterization of the formation. In this, the present invention contemplates wide applicability of the methods and systems disclosed herein to a range of endeavors that involve in situ characterization of thermal properties of subsurface structures for purposes of exploration and/or development of the structures. 
     The preceding description has been presented only to illustrate and describe the invention and some examples of its implementation. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. 
     The preferred aspects were chosen and described in order to best explain principles of the invention and its practical applications. The preceding description is intended to enable others skilled in the art to best utilize the invention in various embodiments and aspects and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims.