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CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application is a divisional claiming priority to pending U.S. patent application Ser. No. 10/457,645, filed Jun. 9, 2003 and entitled “Determination of Thermal Properties of a Formation,” which is hereby incorporated by reference herein in its entirety.  
         [0002]     The present application is also related to a continuation application, U.S. patent application Ser. No. ______, filed concurrently herewith on ______ and entitled “Assembly for Determining Thermal Properties of a Formation While Drilling or Perforating,” which is hereby incorporated by reference herein in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0003]     Not applicable.  
       BACKGROUND  
       [0004]     1. Field of the Invention  
         [0005]     The present invention relates to the evaluation of downhole formations from the in situ determination of thermal properties. More particularly, the present invention relates to the in situ determination of thermal properties, such as specific heat, thermal conductivity, and thermal diffusivity from wellbore temperature measurements. Still more particularly, the present invention relates to the in situ determination of thermal properties performed while utilizing a heat source employed in wellbore stabilization, drilling, or perforating.  
         [0006]     2. Description of the Related Art  
         [0007]     Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes the characteristics of the earth formations traversed by the wellbore, and the location of subsurface reservoirs of oil and gas. Samples of the formation and reservoir may be retrieved to the surface for laboratory analysis. However, to enhance drilling and production operations, it is much more valuable to provide real-time access to the data regarding downhole conditions. Thus, it has become commonplace to make in situ measurements of many formation properties via wellbore logging tools, such as logging-while-drilling (LWD) and wireline tools, that may be operated by electromagnetic, acoustic, nuclear or electromechanical means, for example. These logging tools enable in situ determinations of such properties as the porosity, permeability, and lithology of the rock formations; reservoir pressure and temperature in the zones of interest; identification of the fluids present; and many other parameters.  
         [0008]     It has been suggested that thermal properties of the formation, such as thermal diffusivity, thermal conductivity, and specific heat (or specific heat capacity), are valuable for determining rock and fluid properties. Thermal diffusivity enables a determination of rock characteristics, such as whether the formation comprises sand, limestone, shale, or granite, for example. Specific heat enables a determination of fluid properties, such as whether the formation is saturated with hydrocarbons or water. In particular, the specific heat of water is approximately twice the specific heat of a hydrocarbon, regardless of whether the hydrocarbon is a liquid or gas. Thermal conductivity enables further differentiation between liquid and gas hydrocarbon-saturated formations. In particular, the thermal conductivity of a water-saturated formation is approximately twice the thermal conductivity of an oil-saturated formation, and the thermal conductivity of an oil-saturated formation is approximately twice the thermal conductivity of a gas-saturated formation.  
         [0009]     Thermal conductivity (K), the physical property of a material that determines how easily heat can pass through it, is defined by equation (1), which relates several thermal properties as follows: 
 
 K≡ρ·C·κ   (1) 
 
 where ρ is the material density, C is the specific heat capacity, and κ is the thermal diffusivity. Specific heat capacity (C) is defined as the quantity of heat required to raise the temperature of one unit of mass of material by one temperature degree. Thus, the units of specific heat capacity (C) may be calorie/gram° C., for example. Thermal diffusivity (κ) is defined as the rate at which heat is conducted during unsteady state heat transfer. 
 
         [0010]     To make thermal property determinations, the most common method is to take a sample of the formation downhole, retrieve it to the surface, and then evaluate the sample in a laboratory. The laboratory method does not enable real-time data acquisition, and can only be accurate with respect to the particular sample acquired. Since formation properties typically change with depth, it is likely that the laboratory analysis will not provide complete data for all zones of the formation. Other methods include inferring thermal properties from ambient borehole temperatures measured by conventional wellbore logging tools. This method has some limitations, including the inability to make accurate measurements through casing to determine formation properties behind the casing and the cement that surrounds it.  
         [0011]     Yet another known method for determining thermal properties is to provide downhole a constant output heat source and to measure the temperature relaxation over time, which enables accurate measurements in both cased and uncased boreholes. U.S. Pat. Nos. 3,807,227 (&#39;227 Patent) and 3,892,128 (&#39;128 Patent) to Smith, Jr. disclose such thermal well logging methods for determining specific heat and thermal conductivity, respectively. A single heat source and preferably three spacially separated thermal detectors are disposed on a well logging tool that is moved vertically through a borehole while the thermal responses are recorded. In particular, one thermal detector measures ambient temperature of the borehole at a particular depth before the heat source passes that depth. The other two thermal detectors measure the temperature of the borehole at the same depth following the heat source, each at a different time. In one embodiment, the heat source is disclosed to be a heat drilling tip that melts the earth formations to produce a borehole. Examples of modern heat drilling tips are disclosed in U.S. Pat. No. 5,735,355 to Bussod et al. comprising a rock melting tool with an annealing afterbody that cools the molten rock, and U.S. Pat. No. 5,771,884 to Potter et al. comprising a spallation head with rotating, circumferentially spaced jets that dispense flame jets, very hot water, and/or air to spall the formation rock, or fuse the formation rock if spallation is not feasible.  
         [0012]     The &#39;227 Patent discloses that in a borehole environment, the change in temperature (ΔT) is related to the radiated energy (Q) from the heat source, the mass of the heated earth formation (M) and its composite specific heat (C) as given by the relationship of equation (2): 
 
 Q=C×M×ΔT   (2) 
 
         [0013]     Similarly, the &#39;128 Patent discloses that thermal conductivity (K) is proportional to the time rate of heat transfer in the formation. The one dimensional relationship governing the energy transfer (ΔQ) during a short period of time (Δt) in a formation having a temperature differential (ΔT) over a length (ΔX) is given by Equation (3):  
                 Δ   ⁢           ⁢   O       Δ   ⁢           ⁢   t       =     K   ×   α   ×       Δ   ⁢           ⁢   T       Δ   ⁢           ⁢   X                 (   3   )             
 
 where α is a constant dependent on the geometry of the borehole, the formation, and the well logging tool; and K is thermal conductivity. Thus, according to the &#39;227 Patent and the &#39;128 Patent, the specific heat (C) and the thermal conductivity (K) of the formation can be inferred using equation (2) and equation (3), respectively, based on temperature measurements. Using these inferred values, qualitative evaluations of likely locations of water and hydrocarbon deposits can be made. 
 
         [0014]     U.S. Pat. No. 3,864,969 (&#39;969 Patent) to Smith, Jr. discloses two methods for determining thermal conductivity (K) of the formation by heating one spot within the formation. In the first method, the formation is heated for a predetermined length of time to elevate the temperature. Then the heat source is removed and the rate of temperature decay is measured over time until the formation returns to ambient temperature. In the second method, the formation is heated by a constant output heat source and the formation&#39;s rate of temperature increase is measured to derive an indication of thermal conductivity (K).  
         [0015]     Similarly, U.S. Pat. No. 4,343,181 (the &#39;181 Patent) to Poppendiek discloses a method for in situ determinations of the thermal conductivity and thermal capacity per unit volume of the earth. The &#39;181 Patent teaches a probe containing a heater and two temperature sensors spacially displaced from one another. The probe is positioned in the borehole at the level of interest and maintained at that position for a period sufficient for the probe to be in thermal equilibrium with its surroundings. The probe is displaced from the borehole wall by a thin fluid annulus, and it is not in contact with the borehole wall. The thermal gradient between the two temperature sensors is recorded without heat being applied. Then, the heater is turned on to apply heat at a constant rate, and the thermal gradient between the temperature sensors is recorded. The thermal conductivity and thermal capacity per unit volume of the surrounding earth is determined by relating the actual temperature curve to a calculated theoretical curve by best-fit mathematical methods. At short times, the thermal capacity is said to dominate the temperature response curve, and at long times, the thermal conductivity is said to dominate.  
         [0016]     Each of these prior in situ methods proposes utilizing a downhole heat source that is provided for the sole purpose of taking thermal measurements. Although this approach is technically sufficient, and valuable formation characteristics can be determined using this methodology, this approach has largely been ignored in practice. One possible explanation is that operators are not willing to incur additional capital and operating costs for a heat source that is provided solely for thermal property measurements. Thus, most commercial downhole systems do not include heat sources that enable in situ measurements of thermal properties. Accordingly, at the present time, thermal property measurements are almost exclusively restricted to analysis of samples in laboratories.  
         [0017]     Further, although the &#39;128 Patent and the &#39;227 Patent mention the concept of a heat drilling tip that may also be used as a heat source for enabling in situ determinations of thermal properties, such heat drilling tips have proven to be too slow for commercial success. In particular, the heat drilling tip is designed to spall or actually melt the rock of the formation as the method of forming a borehole. However, because rock is very slow to spall or melt utilizing such techniques, the heat drilling tip progresses at only 3-6 feet per hour. Therefore, the heat tip has not achieved commercial recognition or success as a viable drilling alternative.  
         [0018]     The present invention addresses the deficiencies of the prior art by providing a convenient in situ method of measuring formation thermal properties, such as specific heat, thermal conductivity, and thermal diffusivity. The method is suitable at multiple depths using a commercially viable heat source provided downhole for wellbore stabilization, well drilling or well perforating.  
       SUMMARY  
       [0019]     The present disclosure is directed to apparatus and methods for making in situ thermal property determinations utilizing a downhole heat source that may also be employed for wellbore stabilization, well drilling, or well perforating. Formation temperature measurements are made downehole, and thermal properties of the formation may be inferred from these measurements using conventional formulas.  
         [0020]     In one aspect, the present disclosure is directed to a method for determining the thermal properties of a downhole formation comprising heating the formation adjacent a selected depth level within a wellbore extending into the formation, forming a temporary liner in the wellbore, measuring a first temperature of the formation at the selected depth level at a first time after heating the formation, measuring a second temperature of the formation at the selected depth level at a second time after heating the formation, and combining the temperature measurements to derive an indication of the thermal properties of the formation according to known mathematical relationships. The method may further comprise measuring the ambient temperature of the formation at the selected depth level.  
         [0021]     In various embodiments, forming the temporary liner in the wellbore may comprise melting the formation adjacent the wellbore with a laser beam, or forming the temporary liner in the wellbore may comprise heating a material that solidifies while cooling. In an embodiment, the method further comprises extruding the material onto a wall of the wellbore. In various embodiments, the material may be conveyed into the wellbore through a conduit, or the material may be stored in an assembly disposed in the wellbore.  
         [0022]     In various embodiments, heating the formation may comprise disposing a heat source on a composite coiled tubing drillstring and powering the heat source via conductors disposed in the wall of the drillstring, or heating the formation may comprise disposing a heat source on a drillstring and powering the heat source via a wireline disposed through the drillstring. In an embodiment, measuring the temperatures comprises spacing at least two temperature sensors axially with respect to the wellbore and moving each of the temperature sensors in sequence to the selected depth level. In another embodiment, measuring the temperatures comprises positioning a temperature sensor at the selected depth level and maintaining the sensor at the selected depth level while measuring the first temperature and the second temperature. In an embodiment, the temperature sensors comprise a fiber optic element. The thermal properties of the formation may comprise the thermal conductivity, the thermal diffusivity, the specific heat, or a combination thereof. The method may further comprise drilling to extend the wellbore while forming the temporary liner in the wellbore.  
         [0023]     In another aspect, the present disclosure is directed to an apparatus for determining the thermal properties of a downhole formation and forming a temporary liner in a wellbore extending into the formation comprising a coiled tubing drillstring extending into the wellbore, an assembly supported by the drillstring, the assembly comprising an extruder that extrudes a liner material onto a wall of the wellbore, a heat source that heats the liner material and the formation, and at least one temperature sensor that measures a temperature of the formation. The assembly may further comprise a drill bit and a drill motor.  
         [0024]     In various embodiments, the apparatus may further comprise a wireline extending through a bore of the coiled tubing drillstring to supply power to the heat source, or the apparatus may further comprise conductors disposed in a wall of the drillstring to supply power to the heat source. In an embodiment, the apparatus further comprises a processor that receives temperature signals from the at least one temperature sensor and derives an indication of thermal properties of the formation. In various embodiments, the apparatus may further comprise a wireline extending through a bore of the coiled tubing drillstring to conduct temperature signals from the at least one temperature sensor to the processor, or the apparatus may further comprise conductors disposed in a wall of the drillstring to conduct temperature signals from the at least one temperature sensor to the processor.  
         [0025]     In an embodiment, the apparatus further comprises a conduit extending through a bore of the coiled tubing drillstring to deliver the liner material to the extruder. The drillstring may further comprise at least two flow bores. In an embodiment, the apparatus further comprises a reservoir containing the liner material, and the apparatus may further comprise a valve for selectively delivering the liner material from the reservoir to the extruder. In various embodiments, the heat source may comprise a laser or a heat tip. In an embodiment, the at least one temperature sensor comprises a fiber optic element. In another embodiment, the at least one temperature sensor comprises at least two temperature sensors spatially separated along the assembly.  
         [0026]     In still another aspect, the present disclosure is directed to an apparatus for determining the thermal properties of a downhole formation and forming a temporary liner in a wellbore extending into the formation comprising a coiled tubing drillstring extending into the wellbore, an assembly supported by the drillstring, the assembly comprising a drill bit and a drill motor, a laser that melts the formation to form the liner, and at least one temperature sensor that measures a temperature of the formation. In an embodiment, the at least one temperature sensor comprises a fiber optic element. The apparatus may further comprise a processor that receives temperature signals from the at least one temperature sensor and derives an indication of thermal properties of the formation. In various embodiments, the apparatus may further comprise a wireline extending through a bore of the coiled tubing drillstring to conduct temperature signals from the at least one temperature sensor to the processor, or the apparatus may further comprise conductors disposed in a wall of the drillstring to conduct temperature signals from the at least one temperature sensor to the processor.  
         [0027]     Thus, the present disclosure is directed to apparatus and methods comprising a combination of features and advantages that address various disadvantages of prior methods and apparatus. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]     For a more detailed description of the embodiments of the present invention, reference will now be made to the accompanying drawings, in which like elements have been given like numerals, wherein:  
         [0029]      FIG. 1  is a schematic of a representative conventional drilling system;  
         [0030]      FIG. 2  is a schematic of one embodiment of a wellbore stabilization operation;  
         [0031]      FIG. 2A  is a cross-sectional end view of a representative multi-bore coiled tubing;  
         [0032]      FIG. 3  is a schematic of another embodiment of a wellbore stabilization operation;  
         [0033]      FIG. 4  is a schematic of a chemical oxygen-iodine laser (COIL);  
         [0034]      FIG. 5  is a schematic of one embodiment of a laser drilling system; and  
         [0035]      FIG. 6  is a schematic of one embodiment of a laser perforating system. 
     
    
     DETAILED DESCRIPTION  
       [0036]     Laser technology has flourished in recent years, largely through the expansion of atomic physics, the invention of fiber optics, and advances in military defense capabilities. Through these efforts, tremendous advances have occurred, for example, in laser power generation, efficiency and transmission capabilities. Improvements in lasers and other thermal technologies have made it possible to perform wellbore operations, such as wellbore stabilization, drilling, and perforating, utilizing new techniques. Because thermal technologies offer significant advantages over conventional methods, they are gaining rapid acceptance in the petroleum industry.  
         [0037]     Lasers and other thermal technologies provide a heat source downhole to perform a primary function, such as wellbore stabilization, drilling, or perforating through casing. The heat source may also be utilized for the secondary but important purpose of determining thermal properties of the formation. Accordingly, measurements can be made of the temperature response of a borehole after it is heated by a heat source. The temperature measurements are then utilized to infer thermal properties, such as thermal conductivity, thermal diffusivity, and specific heat, according to conventional calculations.  
         [0038]     Some of the perceived advantages of using lasers or other thermal technologies stem from problems encountered with conventional drilling methods. Thus, referring initially to  FIG. 1 , there is depicted a conventional rotary drilling operation, which comprises mainly three steps: drilling, casing, and completion. To drill a well in the conventional manner, a drilling rig  100  on the earth&#39;s surface  10  conveys a drillstring  110  and a drilling assembly  120  with a drill bit  125  on its lower end into a wellbore  130 , forming an annular flow area  135  between the drillstring  110  and the wellbore  130 . The drillstring  110  is rotated from the surface  10  by the drilling rig  100  while a dense fluid, known as “drilling fluid”, is drawn from a reservoir  140  by a pump  150  on the surface  10 . The pump  150  discharges the drilling fluid through pump discharge line  155  into the drillstring  110 , downwardly into the drilling assembly  120  as depicted by the flow arrows to power the drill bit  125  and to remove the cuttings from the wellbore  130 . The drilling fluid exits the drill bit  125  and returns to the surface  10  through the wellbore annulus  135 . After reaching the surface  10 , the drilling fluid is cleaned and returned to the reservoir  140  via the pump return line  160 . Thus, the drilling fluid, which contains suspended drill cuttings, flows continuously through the drillstring  110  downwardly to the bottom of the wellbore  130  and upwardly through the wellbore annulus  135  back to the fluid reservoir  140  while removing cuttings (debris) from the wellbore  130 . During drilling, the density of the drilling fluid, commonly referred to as the “mud weight”, is optimized to counterbalance the earth&#39;s fluid pressure in the formation  175 , thereby preventing the uncontrolled flow of fluids from the formation  175  into the wellbore  130 , or preventing the drilling fluid from fracturing and flowing into the formation  175 . The earth&#39;s fluid pressure is caused by the presence of water, crude oil, and pressurized gas such as CO 2 , H 2 S, and natural gas, within the formation  175 .  
         [0039]     In more detail, as the wellbore  130  is drilled, the mud weight may be varied from one formation zone to another to counterbalance the pressure of the formation  175 . For example, if the lower zone  179  of the formation  175  is overpressured, the mud weight must be increased when drilling through zone  179  to balance the formation pressure. However, by using a heavier mud weight in the wellbore  130 , the upper zone  177  of the formation  175  could be fractured if that zone  177  is underpressured. In contrast, if the lower zone  179  is underpressured, the mud weight must be decreased when drilling through zone  179  to balance the formation pressure. However, by using a lighter mud weight in the wellbore  130 , a blow-out may occur in the upper zone  177  of the formation  175  if that zone  177  is overpressured. Therefore, before drilling into the lower zone  179  with a different mud weight, a steel tubular known as “casing”, such as casing  180 , is installed to line the wellbore  130  in the upper zone  177 , thereby isolating that section of the wellbore  130  from the surrounding formation  175 . The casing  180  is then cemented  185  against the formation  175  to protect fresh water supplies and other nonhydrocarbon fluids from contamination, and to provide zonal isolation between hydrocarbon and non-hydrocarbon bearing zones. Thus, the cement  185  provides a barrier to separate zone  177  from zone  179  behind the casing  180 . This procedure of casing installation and cementing is repeated for each section of the wellbore  130  corresponding to each formation zone  177 ,  179  as the well is drilled deeper. Each subsequent casing string, such as casing  190 , is set by lowering the casing  190  through the previously set casing  180 . Therefore, casing  190  will have a smaller diameter than the previously set casing  180 . Thus, when drilling a well, multiple casing strings  180 ,  190  are disposed along the wellbore  130  and cemented  185 ,  195  into place, thereby isolating each zone  177 ,  179  in the formation  175 . If the zones  177 ,  179  are not isolated by casing  180 ,  190  and cement  185 ,  195 , problems such as blowouts, lost circulation, and unwanted fracturing of the formation  175  could occur. The last section of the well to be drilled is the production borehole  132 , which is in contact with the fluid reservoir  170 .  
         [0040]     Once all the casings  180 ,  190 , have been installed and cemented  185 ,  195  into place, the drillstring  110  and drilling assembly  120  are removed and another assembly (not shown) is run into the wellbore  130  to make perforations through the casings  180 ,  190  so that hydrocarbons will flow from the formation  175  into the wellbore  130 . Perforations are made through the casings  180 ,  190  that line hydrocarbon-bearing zones  177 ,  179  of the formation  175 , and the cement  185 ,  195  prevents fluids, such as water, for example, from non-hydrocarbon bearing zones from flowing behind the casings  180 ,  190 , and through the perforations. A production tubular (not shown) is then inserted into the wellbore  130  to carry the hydrocarbons back to the surface. The process of perforating the casings  180 ,  190  and inserting the production tubing is known as “completion”.  
         [0041]     Some of the perceived advantages of using lasers or other thermal technologies stem from problems encountered with the above-described conventional methods of well drilling and completion. For example, many drilling problems occur before the casing  180 ,  190  is run and cemented  185 ,  195  into place because the wellbore  130  is unsealed and unprotected. An unsealed wellbore  130  adjacent an overpressured zone enables the influx of fluids from the formation  175 , which can result in pressure “kicks.” Further, an unsealed wellbore  130  adjacent an underpressured zone enables the loss of drilling fluids into the surrounding formation  175 , which may lead to lost circulation, formation damage, differential pressure sticking, borehole swelling, borehole fracture, and even borehole collapse. Accordingly, there is a need for a method to protect the wellbore  130 , even temporarily, after a new section has been drilled until a permanent casing  180 ,  190  can be installed and cemented  185 ,  195  into place. Further, because the casings  180 ,  190  telescope down in size from the top of the wellbore  130  to the bottom of the wellbore  130 , and the driller may not know in advance the number of pressure zones  177 ,  179  that will be encountered, it is difficult to predict with certainty the number of casings  180 ,  190  that will be required and the size of the production borehole  132 . In some cases, the production borehole  132  is so small that the hydrocarbons cannot be produced fast enough to make the well economically viable. Accordingly, there is a need for a method to protect and isolate the wellbore  130  without setting intermediate casing strings that reduce the diameter of the production borehole  132 .  
         [0042]     Referring now to  FIG. 2 , there is depicted a schematic of one embodiment of a wellbore stabilization operation that protects and isolates the wellbore  130 .  FIG. 2  depicts a coiled tubing drilling system  300  having a power supply  305 , a surface processor  310 , and a coiled tubing spool  315 . An injector head unit  320  feeds and directs the coiled tubing  325 , which may be either metal or composite coiled tubing, from the spool  315  downwardly into the wellbore  130 . The coiled tubing  325  supports a wellbore stabilization and drilling assembly  200  within the wellbore  130  that comprises a standard drill bit  125  driven by a drill motor  205 , an extruder  225 , a heat source  220 , and preferably a plurality of temperature sensing devices  230 ,  240 ,  250 , each of which is capable of measuring the temperature of the formation  175  in its vicinity. A reservoir  270  on the surface  10  connects to a pump  265  via an inlet conduit  275 , and the pump  265  connects to an exit conduit  255  that extends downwardly through the bore  322  of the coiled tubing  325  to the extruder  225 . A standard wireline  260 , such as a 7-conductor wireline, extends downwardly through the bore  322  of the coiled tubing  325  to conduct power from the surface power supply  305  to the wellbore stabilization and drilling assembly  200  for operating the heat source  220 . The wireline  260  further conducts signals from the temperature sensing devices  230 ,  240 ,  250  to the signal processing unit  310  on the surface. It will be appreciated by those skilled in the art that the wellbore stabilization and drilling assembly  200  also contains appropriate power supply circuitry and data transmission circuitry (not shown) for operating the heat source  220  and temperature sensing devices  230 ,  240 ,  250 , and for transmitting measurements made thereby to the surface for further processing.  
         [0043]     In operation, drilling fluid is pumped through the bore  322  of the coiled tubing  325  to power the drill motor  205 , which in turn powers the drill bit  125 . Simultaneously, the extruder  225  and the heat source  220  enable placement of a temporary liner  280  to line the wellbore  130  in sections where casings have not yet been installed, such as along the lower borehole section  134  of  FIG. 2 . In particular, pump  265  pumps a fusible liner material  210 , such as polyethylene or polypropylene, for example, from the reservoir  270  through the exit conduit  255  and downwardly to the extruder  225 , which extrudes the liner material  210  onto the wall  136  of the lower borehole section  134 . The fusible liner material  210  may be provided in the form of liquid or small solid pellets, for example, or any other suitable form. As the wellbore stabilization and drilling assembly  200  reaches the lower borehole section  134 , preferably the ambient temperature of the formation  175  is measured by sensor  230  at a predetermined depth D. Then, after the fusible liner material  210  is extruded onto the wall  136  of the lower borehole section  134 , the heat source  220  is activated to heat the fusible liner material  210 , thereby melting it. As the wellbore stabilization and drilling assembly  200  advances, the melted fusible liner material  210  is forced into the porosity of the formation  175 , and as the liner material  210  cools, it creates a temporary liner  280  along the lower borehole section  134  as depicted in  FIG. 2 . In another embodiment, instead of pumping the liner material  210  through a separate conduit  255  extending through the coiled tubing bore  322 , the coiled tubing  325  could have more than one bore. For example, as depicted in cross-section in  FIG. 2A , the coiled tubing  325  could comprise two bores, such as a central fluid bore  321  and an outer annular bore  323 . The drilling fluid could flow through the central bore  321  and the liner material  210  could flow through the annular bore  323 , or vice versa. There are a variety of other possible configurations for pumping the liner material  210  to the extruder  225 .  
         [0044]     Thus, the heat source  220  melts the fusible liner material  210  to produce the temporary liner  280 , while simultaneously heating the formation  175  in the vicinity of the heat source  220 . As heat is applied to the formation  175 , measurements of temperature changes in the formation  175  induced in the vicinity of the temperature sensing devices  230 ,  240 ,  250  enables the determination of thermal properties of the formation  175 , as in conventional methods. In particular, as the wellbore stabilization and drilling assembly  200  moves downwardly, the temperature sensing devices  230 ,  240 ,  250 , which are spacially separated, each pass depth D in the formation  175  at different times such that temperature measurements can be made at the same depth D over time to measure the time rate of decay of the temperature at depth D. Alternatively, once the formation  175  has been heated to form the temporary liner  280 , the wellbore stabilization assembly  200  may be held stationary, thereby keeping sensors  230 ,  240 ,  250  stationary within the wellbore  130 . Then a plurality of temperature measurements may be made at the depths d 1 , d 2 , d 3  of each sensor  230 ,  240 ,  250  respectively, over time, to measure the time rate of decay of the temperature at each depth d 1 , d 2 , d 3 . The temperature differentials and other measurements are then used to infer thermal properties according to conventional calculations.  
         [0045]     Referring now to  FIG. 3 , there is depicted a schematic of another embodiment of a wellbore stabilization operation.  FIG. 3  depicts a coiled tubing drilling system  300  having a power supply  305 , a surface processor  310 , and a coiled tubing spool  315 . An injector head unit  320  feeds and directs the coiled tubing  325 , which is composite coiled tubing, from the spool  315  downwardly into the wellbore  130  to support a wellbore stabilization and drilling assembly  330  on its lower end. The power supply  305  may be connected by electrical conduits  307 ,  309  to electrical conduits in the wall of the composite coiled tubing  325 . Alternatively, the power supply  305  may be connected to a wireline  260  that extends through the bore  322  of the coiled tubing  325 , as depicted in  FIG. 2 . Further, the surface processor  310  includes data transmission conduits  312 ,  314  that may be connected to data transmission conduits also housed in the wall of the composite coiled tubing  325 . Alternatively, the surface processor  310  may be connected to a wireline  260  that extends through the bore  322  of the coiled tubing  325 , as depicted in  FIG. 2 .  
         [0046]     The wellbore stabilization and drilling assembly  330  includes many of the same components as the wellbore stabilization and drilling assembly  200  of  FIG. 2 . However, the assembly  330  of  FIG. 3  includes a downhole reservoir  340  rather than the surface reservoir  270 , pump  265 , and conduit  255  extending through the coiled tubing  325 , as shown in  FIG. 2 .  
         [0047]     In operation, drilling fluid is pumped through the bore  322  of the coiled tubing  325  to power the drill motor  205 , which in turn powers the drill bit  125 . Simultaneously, the extruder  225  and the heat source  220  enable placement of a temporary liner  280  to line the wellbore  130  in sections where casings have not already been installed, such as along the lower borehole section  134  of  FIG. 3 . To create the temporary liner  280 , the fusible liner material  210  is selectively dispensed, such as via an actuatable valve  345 , for example, from the downhole reservoir  340  into the extruder  225 , which extrudes the liner material  210  onto the wall  136  of the lower borehole section  134 . Then, after the fusible liner material  210  is extruded, the heat source  220  melts the fusible liner material  210 . In an alternate embodiment, the downhole reservoir  340  may be positioned above another type of heat source (not shown), such that the fusible liner material  210  may be selectively dispensed into the heat source to be heated therewithin before being extruded. Thus, in the alternate embodiment, the extruder  225  extrudes melted liner material  210  onto the lower borehole wall  136 . Once the liner material  210  is extruded and melted, then as the wellbore stabilization and drilling assembly  330  advances, it forces the melted fusible liner material  210  into the porosity of the wellbore  130  to create a temporary liner  280 .  
         [0048]     Thus, the heat source  220  or the alternate heat source (not shown) melts the fusible lining material  210  to produce a temporary liner  280 , while simultaneously heating the formation  175  in the vicinity of the heat source  220 . As heat is applied to the formation  175 , measurements of temperature changes of the formation  175  induced in the vicinity of temperature sensing devices  230 ,  240 ,  250  enable the determination of thermal properties of the formation  175 , as in conventional methods. In particular, as the wellbore stabilization and drilling assembly  330  moves downwardly, the temperature sensing devices  230 ,  240 ,  250 , which are spacially separated, will each pass depth D in the formation  175  at different times such that temperature measurements can be made at the same depth D to measure the time rate of decay of the temperature at depth D. Alternatively, once the formation  175  to form the temporary liner  280 , the wellbore stabilization assembly  330  can be held stationary, thereby keeping sensors  230 ,  240 ,  250  stationary within the wellbore  130 . Then a plurality of temperature measurements may be made at the depths d 1 , d 2 , d 3  of each sensor  230 ,  240 ,  250  respectively, over time, to measure the time rate of decay of the temperature at each depth d 1 , d 2 , d 3 . The temperature differentials and other measurements are then used to infer thermal properties according to conventional calculations.  
         [0049]     The heat source  220  in the assemblies  200 ,  330  of  FIG. 2  and  FIG. 3 , respectively, may comprise any source of heat, such as for example, a heat tip that reaches sufficient temperatures to melt the fusible liner material  210 . In another embodiment, the heat source  220  comprises a laser as will be described in more detail hereinafter. A laser can reach temperatures capable of melting the formation  175  rock, and when the molten rock cools, a liner is formed such that no fusible liner material  210  is required.  
         [0050]     Beyond wellbore stabilization, lasers are now being utilized for drilling. Unlike conventional drilling, in which the drilling rate is determined by the weight-on-bit (WOB), mud circulation (cuttings removal) rate, rotary speed, hydraulic horsepower, bit design and wellbore size, the rate of penetration achieved with a laser may only depend on wellbore size and delivered power. Moreover, no out-of-balance or out-of-axis turning is expected to occur with a laser-drilled wellbore, and, because a laser head does not contact the rock, there is no need to stop drilling to replace a mechanical bit.  
         [0051]     A laser is basically a device that converts energy of some form (electrical, chemical, heat, etc.) into photons, which is electromagnetic radiation. The photons created through stimulated emission form a narrow beam of monochromatic coherent light energy that when focused into an intense beam can be used to fragment, melt or vaporize rock, depending upon the power delivered, and the operating parameters associated with pulsing the laser.  
         [0052]     Referring now to  FIG. 4 , there is shown a schematic of one embodiment of a laser that could be used for drilling a wellbore; namely a chemical oxygen-iodine laser (COIL)  400 . The overall COIL process is conceptually simple. Basic hydrogen peroxide (BHP)  410  when mixed with chlorine gas  420  in a gas generator  430  produces oxygen in an excited state (called the oxygen singlet delta  440 ). The byproduct of this reaction is heat  435  and brine  445 , which is common in the oilfield. The oxygen singlet delta  440  is combined with molecular iodine  450  in a supersonic mixing nozzle  460 , which causes both the dissociation of molecular iodine to atomic iodine and produces iodine in an excited state, creating the laser gain region  465 . In the laser gain region  465 , laser cavity mirrors  470  stimulate excited iodine to form atomic iodine, releasing photons, or “packets” of light energy that is the laser beam  475 . The exhaust gases are scrubbed in a scrubber  480  to remove any residual iodine and chlorine.  
         [0053]     While a COIL has been described as one type of laser that could be used for purposes of laser drilling, a number of alternate laser systems could also be used, including hydrogen fluoride (HF), deuterium fluoride (DF), carbon dioxide (CO 2 ), carbon monoxide (CO), free electron laser (FEL), neodymium:yttrium aluminum garnet (Nd:YAG), and krypton fluoride excimer (KrF (excimer)) lasers, for example. Lasers can operate in continuous-wave (CW), pulsed, and repetitively pulsed (RP) modes. The main energetic parameter for a CW laser is the output power, for a pulsed laser the output energy, and for a RP laser the average power and pulsed energy. Each of these lasers operates at a specific wavelength range, except for the FEL, which may be tuned to virtually any wavelength in continuous wave (CW) mode.  
         [0054]      FIG. 5  schematically depicts one embodiment of a laser drilling system  500  as it is drilling a wellbore  130  into a formation  175 . The laser drilling system  500  comprises a laser  510  on the surface, such as a FEL or COIL, connected to one or more fiber optic elements  515  comprising a bundle that extends downwardly through a coiled tubing drillstring  325  and connects to a series of lenses  520 ,  525 ,  530  on the lower end of a laser drilling assembly  550 . In an alternate embodiment, a smaller laser, such as a diode laser (not shown), is small enough to fit within the wellbore  130 , such as at the lower end of the laser drilling assembly  550 , thereby eliminating the need for fiber optic elements  515 .  
         [0055]     In  FIG. 5 , the laser drilling assembly  550  is suspended within the wellbore  130  by a coiled tubing system  300  having a power supply  305 , a surface processor  310 , and a coiled tubing spool  315 . An injector head unit  320  feeds and directs the coiled tubing  325 , which may be either metal or composite coiled tubing, from the spool  315  downwardly into the wellbore  130  to support the laser drilling assembly  550 . The power supply  305  is shown connected to a wireline  260  that extends through the bore  322  of the coiled tubing  325  and conducts power from the power supply  305  to the laser drilling assembly  550 . Alternatively, the power supply  305  may be connected to electrical conduits in the wall of the coiled tubing  325 . Further, the surface processor  310  connects to the wireline  260  to form a bi-directional telemetry system. In particular, the wireline  260  conducts data signals from several temperature sensors  552 ,  554  and  556  disposed on the laser drilling assembly  550  to the surface processor  310 , and the surface processor  310  may also generate command signals that are conducted downhole via the wireline  260  to the laser drilling assembly  550  to alter the laser drilling operations. Alternatively, instead of the wireline  260 , the surface processor  310  may be connected to data transmission conduits housed in the wall of the coiled tubing  325  to form the bi-directional telemetry system.  
         [0056]     In operation, the laser drilling system  500  transfers light energy from the laser  510  on the surface, down the one or more fiber optic elements  515  to the series of lenses  520 ,  525 ,  530 . The lenses  520 ,  525 ,  530  direct a laser beam to cut the rock and extend the wellbore  130 . As the laser  510  transfers light energy downhole, heat is produced that will enable time rate of decay temperature measurements to be taken by the temperature sensors  552 ,  554 ,  556 . In particular, as the laser drilling assembly  550  moves downwardly to extend the wellbore  130 , the temperature sensors  552 ,  554 ,  556 , which are spacially separated, will pass a depth D in the formation  175  at different times such that temperature measurements can be made at the same depth D over time. Thus, a temperature differential can be determined from which thermal properties can be inferred by conventional methods.  
         [0057]     One of the primary benefits of making these real-time thermal property determinations is to enhance laser drilling operations. Thermal properties, such as thermal diffusivity, are especially valuable for determining properties of the formation rock. For example, the laser  510  utilized for drilling will have different pulse rates and different preferred operating parameters, such as how long and how often to pulse the laser, for example, depending upon the type of formation rock. In some cases, the same amount of laser energy will melt one type of rock while it will vaporize another. Thus, real-time information about formation properties enables improvements to be made in the laser drilling process. In particular, as drilling progresses, temperature measurements are made via temperature sensors  552 ,  554 ,  556  disposed on the drilling assembly  550 . The wireline  260  conducts signals from the temperature sensors  552 ,  554 , and  556  to the surface processor  310  where the data is processed to determine thermal properties of the formation  175 . Then, various parameters of the laser  510 , such as the pulse rate, for example, may be adjusted depending upon the characteristics of the formation  175 .  
         [0058]      FIG. 6  schematically depicts the laser  510 , the one or more fiber optic elements  515 , and the lenses  520 ,  525 ,  530  being utilized to perforate casing  190  in the middle section  138  of the wellbore  130 . In this configuration, the laser  510  transfers light energy down the fiber optic elements  515  to the series of lenses  520 ,  525 ,  530 . The lenses  520 ,  525 ,  530  then direct a laser beam to perforate the casing  190 , cement  195 , and formation  175  to form a perforation  176 .  
         [0059]     In another embodiment, since light energy from a laser, such as laser  510 , can be conducted along a fiber optic element  515  for short distances, the lenses  520 ,  525 ,  530  can be eliminated. In this embodiment, the laser  510  transfers light energy down the one or more fiber optic elements  515 , and the ends of the fiber optic elements  515  are positioned to direct a laser beam to perforate the casing  190 , cement  195 , and formation  175 . It would also be possible to point the end of each one of the fiber optic elements  515  to various different positions along the length of the casing  190  to form multiple perforations  176  simultaneously. For example, assuming there are ten (10) fiber optic elements  515  bundled to a single laser source  510 , the end of each one of the fiber optic elements  515  may be pointed to a different location along the casing  190 . Then light energy from the laser  510  may be conducted to the ends of the individual fiber optic elements  515  in sequence to create ten perforations  176 . In particular, since each pulse of the laser  510  lasts only briefly when perforating so as to spall rather than melt the rock, the light energy can be conducted (pulsed) to each fiber element  515  in sequence at a rapid pace to generate the ten perforations  176 , with the sequence returning to the first perforation in time for the next pulse. This sequence would be repeated multiple times to create completed perforations along the casing  190  length. The laser  510  could be located on the surface as shown in  FIG. 6 , or alternatively, the laser  510  could be disposed within the wellbore  130 . It may also be possible to form and extend a perforation tunnel  177  using a fiber optic element  515  by projecting the element  515  out into the perforation  176  and pulsing the laser  510  to continue spalling the rock and form a progressively longer perforation tunnel  177 .  
         [0060]     A properly pulsed laser  510  may create fewer but higher quality perforations  176  as compared to conventional perforating methods. In particular, using a conventional perforator that dispenses shape charges, a plurality of perforations can be created in a short period of time. However, many of these perforations do not extend deep enough into the formation, or they may be blocked with casing and rock debris that is forced into the perforation and surrounding formation by the shape charge, thereby reducing the effective produceability of the formation. Therefore, although a large number of perforations can be created using a conventional perforator, few of the perforations will produce. For example, if 50 feet of casing length is perforated, approximately only 5 feet of perforations may actually produce. In contrast, the perforations  176  created with a laser  510  do not get blocked with debris and can be extended to form a tunnel  177  in the formation  175  as needed. In fact, as reported in “Temperature Induced by High Power Lasers: Effects on Reservoir Rock Strength and Mechanical Properties”, SPE/ISRM 78154, data indicates that in many cases the effective permeability of the rock around the tunnel  177  is actually increased relative to that of the virgin formation  175 . Therefore, it is possible for a majority of the laser-created perforations  176  to produce.  
         [0061]     Once again, heat is generated in the formation  175  as a result of the laser perforating operation performed by any of the above-described methods. Temperature probes can be installed in a perforation tunnel  177  to measure the temperature relaxation at one or more positions within the perforation tunnel  177  over time. In one embodiment, the temperature probes comprise a fiber optic element  515 , which, as known in the art, can make distributed measurements along the length of the fiber. The fiber optic element  515  may be part of the bundle connected to the laser  510 , or it may be a separate fiber optic element connected to another laser. Accordingly, since a fiber optic element  515  can be used to spall rock to form perforations  176  and tunnels  177 , and can also be used to take distributed temperature measurements, the same laser  510  and fiber optic elements  515  can be used for both purposes. In such a case, the laser  510  would be operated in one mode to form the perforations  176  and tunnels  177  and in a different mode to make temperature measurements.  
         [0062]     In more detail, to make distributed temperature measurements using a fiber optic element  515 , the laser  510  is operated to pulse light energy down the fiber optic element  515 . Temperature measurements can be made at each point along the length of the fiber  515 . Most distributed temperature sensing systems utilizing fiber optic elements  515  rely on Optical Time Domain Reflectometry (OTDR), which is known in the art, to determine the spatial position of an individual measurement. OTDR is a standard method of determining losses along the length of an optical fiber  515 . The time it takes for the reflective light to return to the source indicates the precise position along the fiber  515  where the measurement is being taken. The characteristics of the reflective light are analyzed using known techniques, such as Raman backscattering, to determine the temperature at that precise position. Thus, for each pulse of the laser  510 , the operator can obtain reflective light measurements at different times corresponding to different positions along the fiber optic element  515 . The operator can then pulse the laser  510  again and repeat the measurement sequence at each position along the fiber optic element  515 , and so on. This will provide a number of temperature measurements at each position such that temperature differentials can be determined from which thermal properties can be inferred by conventional methods. Although the length of the fiber optic element  515  is located at approximately the same depth in the formation  175  when disposed within a perforation tunnel  177 , distributed temperature measurements along the length of the fiber  515  are valuable for determining the properties of the formation  175  with greater accuracy, and for determining the required depth of the perforation tunnels  177  to engage formation zones containing the most hydrocarbons.  
         [0063]     One of the primary benefits of making real-time thermal property determinations is to enhance laser perforating operations based upon the characteristics of the rock comprising the formation  175 . For example, as previously described, temperature measurements may be made within a perforation tunnel  177  via a temperature probe (not shown) or a fiber optic element  515 , and the temperature signals may then be transmitted to the surface processor  310  to determine thermal properties of the formation  175 . Then, various parameters of the laser  510 , such as the pulse rate, intensity, and duration, can be adjusted based upon the real-time determination of thermal properties of the formation  175 , thereby improving the laser perforating operation.  
         [0064]     Thus, the apparatus and methods described herein take advantage of the heat from a thermal drilling, wellbore stabilization, or perforating process to perform thermal measurements. The thermal measurements may be performed simultaneously or near simultaneously with the drilling operations. The sensors thus provide in situ temperature measurements that permit the computation of thermal properties of the formation as described in the earlier &#39;128 Patent, the &#39;227 Patent and the &#39;969 Patent to Smith, Jr.  
         [0065]     While embodiments of the present invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit or teaching of this disclosure. For example, a fiber optic element and laser could be utilized to take distributed temperature measurements along the wellbore during thermal drilling or wellbore stabilization operations in addition to perforating. Thus, the embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the methods and apparatus are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims which follow, the scope of which shall include all equivalents of the subject matter of the claims.

Summary:
A method comprises heating a formation adjacent a selected depth level within a wellbore extending into the formation, forming a temporary liner in the wellbore, measuring a first temperature of the formation at the selected depth level at a first time after heating the formation, measuring a second temperature of the formation at the selected depth level at a second time after heating the formation, and combining the temperature measurements to derive an indication of thermal properties of the formation. An apparatus comprises a coiled tubing drillstring extending into a wellbore, an assembly supported by the drillstring, the assembly comprising an extruder that extrudes a liner material onto a wall of the wellbore, a heat source that heats the liner material and a formation, and at least one temperature sensor that measures a temperature of the formation.