Patent Publication Number: US-2019178046-A1

Title: Anti-settling Apparatus and Method and Apparatus for Checking the Same, and Apparatus for Preventing Settlement of Well

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of and claims the benefit of Chinese application No. 201711327747.8 filed Dec. 13, 2017, the contents of which are hereby incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The present invention relates to the field of drilling, in particular to an anti-settling apparatus and a method and apparatus for checking the same, and an apparatus for preventing settlement of a well. 
     BACKGROUND OF THE INVENTION 
     Since the beginning of the 21st century, with the rising demand for oil and natural gas in various countries of the world and the depletion of oil and gas resources in the conventional areas, the development of oil and gas resources has gradually developed toward the deep sea and the polar tundra. In 2009, the United States Geological Survey released a report saying that undiscovered oil reserves and natural gas reserves in the Arctic account for 13% and 30% of oil and natural gas reserves in the conventional areas respectively. The polar region may become another core area for oil and gas development after the ocean. Oil companies in countries such as Russia, Norway, the United States, and Canada all have a keen interest in polar oil and gas resources, and invest a lot of money into the basic research of oil and gas exploration and development. 
     Although there are abundant oil and gas resources in the polar tundra, its remoteness and low temperature have caused many difficulties for drilling. At the same time, due to the presence of permafrost in the near-surface areas of the polar region, it is usually necessary to inject the drilling fluid into the drill string during drilling, and to allow the drilling fluid to circulate in the drill string and well. High temperature drilling fluid returning upwards from the deep stratum may melt the ice layer in the permafrost, lower the strength of the rock layer in the permafrost, cause the settlement of the stratum, and may thus cause the wellhead to collapse. Specifically, during the drilling process, the drilling fluid enters the bottom of the well through the drill string and returns through an annular gap between the drill string and the well wall. Due to the high temperature in the depth of the stratum, the drilling fluid gains energy from the stratum and its temperature is thus increased. When the drilling fluid returns upwards to the near-surface permafrost, it increases the temperature of the permafrost, and the ice thaws. After the ice melts into water, its volume decreases, the strength of the reservoir and the lateral resistance to the casing also decrease. The settlement of the stratum is caused by the gravity of the overlying rock layer and the load of the wellhead, which causes the wellhead to sink. In severe cases, it may even cause the wellhead to collapse. 
     Wellhead settlement is the most important issue faced during drilling in the polar tundra, and many scholars have conducted in-depth studies on it. Regarding the uneven settlement of the wellhead foundation in the permafrost, a scholar proposed to prevent the permafrost from thawing by a thermal insulation layer (CONSTRUCTION, volume 8, p62-63, 2011). However, some unstable external environmental factors also damage the permafrost and cause natural settlement of the permafrost under external loads. Patent CN202850926U related to an anti-scouring underwater wellhead anti-settling apparatus used in deep water drilling engineering proposes to reduce scouring of the current at the bottom of the sea through an anti-scouring net and reduce the stress through a circular bottom plate, so as to prevent the settlement of the wellhead. However, this proposal is mainly aimed at the tilt and instability of the wellhead caused by scouring and movement of the submarine sediment in the deep water drilling process. It has substantive differences from the settlement of the entire wellhead caused by thawing of permafrost during the drilling process. Moreover, the method for calculating key parameters of the apparatus is not provided. 
     SUMMARY OF THE INVENTION 
     An object of the embodiments of the present invention is to provide an anti-settling apparatus and a method and apparatus for checking the same, which can reduce the settlement of a well during drilling in permafrost. 
     Another object of embodiments of the present invention is to provide an apparatus for preventing the settlement of a well, in which the anti-settling apparatus and the method and apparatus for checking the same are applied, so that the pressure imposed by various drilling rigs on the wellhead and the permafrost beneath it can be reduced, whereby the settlement of the well can be reduced and stable drilling is made possible. 
     To this end, an embodiment of the present invention provides an anti-settling apparatus for well drilling in tundra disposed at wellhead region during drilling to prevent settlement of the well, the anti-settling apparatus comprising: an anti-settling base having an opening corresponding to the wellhead of the well and formed at the center of the anti-settling base, the anti-settling base being disposed in the wellhead region during drilling; and an interlocking member disposed at the opening to connect and fix a casing provided in the well with the anti-settling base, wherein the anti-settling base is a hollow multi-layer structure, and the anti-settling apparatus may further comprise a support structure disposed between the multiple layers of the hollow multi-layer structure for support. 
     The anti-settling apparatus may further comprise a truss provided along the circumference of the anti-settling base, one end of the truss being fixed on the anti-settling base at an edge of the anti-settling base, and the other end being fixed on the anti-settling base at the center of the anti-settling base. 
     According to another aspect of the present invention, there is also provided a method for checking the anti-settling apparatus, the method comprising: simulating a temperature field of an annular gap based on the flow rate of a drilling fluid and the temperature of the drilling fluid in a drill string; determining permafrost parameters of a thawing region of the permafrost where the anti-settling apparatus is on, based on the temperature field of the annular gap and a temperature field of the permafrost; determining the size of the anti-settling apparatus based on the permafrost parameters; determining the amount of settlement of the well based on the size; and adjusting the size of the anti-settling apparatus based on the amount of settlement and a settlement amount threshold. 
     Determining permafrost parameters of the thawing region of the permafrost based on the temperature field of the annular gap and a temperature field of the permafrost may comprise: determining a thawing boundary and a thawed amount of the thawing region based on the temperature field of the annular gap and the temperature field of the permafrost; and determining permafrost parameters of the thawing region based on the thawing boundary and thawed amount. 
     The permafrost parameters include a lateral resistance coefficient and a compressive strength of the permafrost, and determining the size of the anti-settling apparatus based on the permafrost parameters of the thawing region may comprise determining the size according to the following formula: 
     
       
         
           
             
               
                 
                   
                     
                       ( 
                       
                         1 
                         - 
                         f 
                       
                       ) 
                     
                      
                     G 
                   
                   + 
                   
                     
                       ρ 
                       e 
                     
                      
                     gSh 
                   
                 
                 S 
               
               ≤ 
               
                 σ 
                 p 
               
             
             , 
           
         
       
     
     wherein S indicates contact area of the anti-settling apparatus with the permafrost, G indicates the gravity of a wellhead apparatus and a casing installed in the well during drilling, f indicates the lateral resistance coefficient, h indicates the thickness of the permafrost, σ p  indicates the compressive strength, and ρ e  indicates the density of the permafrost. 
     The permafrost parameters further include the porosity of the permafrost in the thawing region, wherein determining the amount of settlement of the well based on the size of the anti-settling apparatus may comprise: determining a volume compression coefficient of the permafrost in the thawing region based on the porosity; and determining the actual amount of settlement corresponding to the size according to the following formula: 
     
       
      
       s=a 
       0 
       h+mνhΔp  
      
     
     wherein s indicates the amount of settlement, h indicates the thickness of the permafrost, a 0  indicates thaw-settlement coefficient of thawed permafrost, my indicates the volume compression coefficient of the permafrost, and Δp indicates the pressure of the anti-settling apparatus on the permafrost at the wellhead. In addition, adjusting the size of the anti-settling apparatus based on the amount of settlement and a settlement amount threshold may comprise: increasing the size by a predetermined value to re-determine the size of the anti-settling apparatus if the actual amount of settlement exceeds the settlement amount threshold. 
     According to another aspect of the present invention, there is provided an apparatus for checking the anti-settling apparatus, the apparatus comprising: a temperature field simulation module configured to simulate a temperature field of an annular gap based on the flow rate of an injected drilling fluid and the temperature of the drilling fluid in a drill string; a parameter determination module configured to determine permafrost parameters of a thawing region of the permafrost based on the temperature field of the annular gap and a temperature field of the permafrost; an anti-settling apparatus determination module configured to determine the size of the anti-settling apparatus based on the permafrost parameters; a settlement amount determination module configured to determine the amount of settlement of the well based on the size of the anti-settling apparatus; and an adjustment module configured to adjust the size of the anti-settling apparatus based on the amount of settlement and a settlement amount threshold. 
     The parameter determination module may be further configured to: determine a thawing boundary and a thawed amount of the thawing region based on the temperature field of the annular gap and the temperature field of the permafrost; and determine permafrost parameters of the thawing region based on the thawing boundary and thawed amount. 
     The permafrost parameters include a lateral resistance coefficient and a compressive strength of the permafrost, and the anti-settling apparatus determination module may be further configured to determine the size according to the following formula: 
     
       
         
           
             
               
                 
                   
                     ( 
                     
                       1 
                       - 
                       f 
                     
                     ) 
                   
                    
                   G 
                 
                 + 
                 
                   
                     ρ 
                     e 
                   
                    
                   gSh 
                 
               
               S 
             
             ≤ 
             
               σ 
               p 
             
           
         
       
     
     wherein S indicates contact area of the anti-settling apparatus with the permafrost, G indicates the gravity of a wellhead apparatus and a casing installed in the well, f indicates the lateral resistance coefficient, h indicates the thickness of the permafrost, σ p  indicates the compressive strength, and ρ e  indicates the density of the permafrost. 
     The permafrost parameters further include the porosity of the permafrost in the thawing region, wherein the settlement amount determination module may be further configured to: determine a volume compression coefficient of the permafrost in the thawing region based on the porosity; and determine the actual amount of settlement corresponding to the size according to the following formula: 
     
       
      
       s=a 
       0 
       h+m 
       v 
       hΔp  
      
     
     wherein s indicates the amount of settlement, h indicates the thickness of the permafrost, a 0  indicates thaw-settlement coefficient of thawed permafrost, m v  indicates the volume compression coefficient of the permafrost. In addition, the adjustment module may be further configured to: increase the size by a predetermined value to re-determine the size of the anti-settling apparatus if the actual amount of settlement exceeds the settlement amount threshold. 
     According to another aspect of the present invention, there is also provided an apparatus for preventing settlement of a well for drilling in permafrost, comprising the anti-settling apparatus and the checking apparatus. 
     The apparatus may further comprise: a thermal insulation casing disposed on wall of the well for preventing heat transfer from the well to the permafrost; and an interlocking member for connecting and fixing the thermal insulation casing with the anti-settling apparatus. 
     The thermal insulation casing comprises an outer thermal insulation casing, an inner thermal insulation casing, and a hollow portion formed between the outer and inner thermal insulation casings and filled with a gas. 
     The thermal insulation casing may further comprise: foam pellets disposed in the hollow portion and adhered to an inner wall of the hollow portion. 
     In another aspect, the present invention provides a machine-readable storage medium having instructions stored thereon for causing a machine to perform the method for checking the anti-settling apparatus. 
     According to the above technical solution, the present invention employees an anti-settling apparatus to mitigate the pressure imposed by various apparatuses on the wellhead and permafrost beneath it, so as to reduce the pressure experienced by the well and surrounding permafrost in the drilling process, and uses a checking apparatus to adjust the size of the anti-settling apparatus based on the temperature field of the well and the permafrost surrounding the well, whereby the amount of settlement of the well is minimized, preventing the well from tilting during drilling and thus achieving stable drilling. 
     Other features and advantages of the embodiments of the present invention will be described in detail in the following detailed description. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The drawings are provided to facilitate further understanding of the embodiments of the present invention, form a part of the specification and used to explain the present invention along with the embodiments described hereinafter, but in no way, limit the scope of the present invention. In these drawings: 
         FIG. 1  is a cross-sectional view of an anti-settling apparatus and a well when the anti-settling apparatus is installed in a wellhead region according to an embodiment of the present invention; 
         FIG. 2  is a top view illustrating a state where the anti-settling apparatus is disposed in the wellhead region according to an embodiment of the present invention; 
         FIG. 3  is a flowchart of a checking method according to an embodiment of the present invention; 
         FIG. 4  is a flowchart of a checking method according to another embodiment of the present invention; 
         FIG. 5  is a structural block diagram of a checking apparatus according to an embodiment of the present invention; and 
         FIG. 6  is a structural block diagram of an apparatus for preventing settlement of well according to an embodiment of the present invention. 
     
    
    
     LIST OF REFERENCE SIGNS 
     
         
         
           
               1 : wellhead apparatus;  2 : anti-settling base 
               3 : support structure;  4 : upper truss 
               5 : interlocking member;  6 : outer thermal insulation casing 
               7 : filling gas;  8 : foam pellets 
               9 : inner thermal insulation casing;  10 : annular gap 
               11 : drill string;  12 : open well wall 
               13 : cement sheath;  14 : permafrost 
               15 : common stratum;  16 : drill bit 
               100 : temperature field simulation module;  200 : parameter determination module 
               300 : anti-settling apparatus determination module;  400 : settlement amount determination module 
               500 : temperature field simulation module;  600 : parameter determination module 
           
         
       
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following describes the embodiments of the present invention with reference to the drawings. It would be appreciated that the embodiments described here are intended to illustrate and explain, rather than limit the present invention. 
       FIG. 1  is a cross-sectional view of an anti-settling apparatus and a well when the anti-settling apparatus is installed in a wellhead region according to an embodiment of the present invention.  FIG. 2  is a top view illustrating a state where the anti-settling apparatus is disposed in the wellhead region according to an embodiment of the present invention. 
     As shown in  FIG. 1 , the anti-settling apparatus of the present invention is installed in the wellhead region of the well during drilling to prevent the well from settlement. The anti-settling apparatus generally comprises: an anti-settling base  2  having an opening corresponding to the wellhead formed at the center thereof, the anti-settling base  2  being disposed in the wellhead region during drilling; and an interlocking member  5  disposed at the opening to connect and fix a casing provided in the well with the anti-settling base  2 . The casing is a device that is usually set in a well during drilling, in which a drill string  11  and a drill bit  16  may be arranged. During drilling, a drilling fluid is injected into the drill string  11  and further injected into an annular gap  10  between the drill string and the casing through the drill bit  16 , thereby forming a circulation of the drilling fluid. The arrow in  FIG. 1  indicates the flow direction of the drilling fluid. Lower part of  FIG. 1  provides a schematic diagram of a well in a common stratum  15  below the permafrost  14  while drilling, and reference sign  12  indicates the open well wall without casing. A cement ring  13  is also formed between the permafrost  14  and the well for fixing the well wall and connecting and fixing the casing. 
     The anti-settling base can be tiled in the wellhead region. Due to the large contact area between the anti-settling base and the permafrost at the wellhead region, gravity of wellhead apparatus  1 , casing and so on can be distributed, so that the pressure on the wellhead and surrounding permafrost is relieved, and the possibility of settlement is reduced. The wellhead apparatus  1 , which is commonly used for drilling, mainly comprises a casing head, a blowout preventer and the like. Its main function is to suspend the casing and ensure the safety of the drilling. In drilling, the wellhead apparatus is generally directly fixed on the ground. 
     As shown in  FIG. 2 , the anti-settling apparatus according to the present invention may preferably comprise a truss  4  provided along the circumference of the anti-settling base, one end of the truss  4  being fixed on the anti-settling base at an edge thereof, and the other end being fixed on the anti-settling base at the center thereof. The truss  4  is preferably formed along the circumference of the anti-settling base  2  at predetermined intervals. As shown in  FIG. 1 , the truss  4  may be formed to include a plurality of triangular braces, thereby making it possible to reduce its weight while making the entire anti-settling apparatus more stable. As shown in  FIG. 1 , the truss may be formed to be supported by a hollow triangular support from the edge side to the center side of the anti-settling apparatus. 
     In a preferred embodiment, the anti-settling base may be formed as a hollow multi-layer structure as shown in  FIG. 1 , and a support structure  2  is provided between the multiple layers for support. The support structure is preferably a wavy structure as shown in  FIG. 1 , thereby further reducing the weight of the anti-settling apparatus. 
     The anti-settling base and the truss may be formed into a foldable or retractable structure so as to adjust the size of the anti-settling base according to actual needs. 
       FIG. 3  is a flowchart of a checking method according to an embodiment of the present invention. As shown in  FIG. 3 , the method comprises the following steps. 
     At  310 , a temperature field of an annular gap is simulated based on the flow rate of a drilling fluid and the temperature of the drilling fluid in a drill string. The annular gap is the gap formed between the drill string and the well wall. During the drilling process, the drilling fluid circulates between the drill string, the drill bit and the annular gap, and the drilling fluid returning upwards from the annular gap exchanges heat with the casing. The temperature field of the annular gap may be simulated according to the following formula: 
     
       
         
           
             
               
                 
                   1 
                   
                     v 
                     a 
                   
                 
                  
                 
                   
                     ∂ 
                     
                       T 
                       a 
                     
                   
                   
                     ∂ 
                     t 
                   
                 
               
               - 
               
                 
                   ∂ 
                   
                     T 
                     a 
                   
                 
                 
                   ∂ 
                   z 
                 
               
             
             = 
             
               
                 
                   1 
                   A 
                 
                  
                 
                   ( 
                   
                     
                       T 
                       
                         e 
                         , 
                         0 
                       
                     
                     - 
                     
                       T 
                       a 
                     
                   
                   ) 
                 
               
               - 
               
                 
                   1 
                   B 
                 
                  
                 
                   ( 
                   
                     
                       T 
                       a 
                     
                     - 
                     
                       T 
                       p 
                     
                   
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 wherein 
                  
                 
                     
                 
                  
                 A 
               
               = 
               
                 
                   
                     c 
                     f 
                   
                    
                   w 
                 
                 
                   2 
                    
                   π 
                    
                   
                       
                   
                    
                   
                     r 
                     ci 
                   
                    
                   
                     U 
                     a 
                   
                 
               
             
             , 
             
               B 
               = 
               
                 
                   
                     
                       c 
                       f 
                     
                      
                     w 
                   
                   
                     2 
                      
                     π 
                      
                     
                         
                     
                      
                     
                       r 
                       pi 
                     
                      
                     
                       U 
                       p 
                     
                   
                 
                 . 
               
             
           
         
       
     
     In the above formula, v a  indicates the flow rate of drilling fluid in the annular gap, T a  and T p  indicates the temperature of drilling fluid in the annular gap and the drill string respectively, t indicates the time, z indicates the depth of well, c f  indicates the specific heat capacity of the drilling fluid, r ci  and r pi  indicate the inner diameter of the casing and the drill string respectively, U a  and U p  indicate the coefficient of total heat transfer from the annular gap to the stratum and from the drill string to the annular gap respectively, w indicates the flow rate of the drilling fluid, and T e,0  indicates the temperature of the permafrost adjacent to the casing, namely, the temperature at the interface between the casing and the permafrost (e indicates the permafrost, and 0 indicates that it is adjacent to the casing). In the above formula, the first item on the left side is the heat of the drilling fluid flowing into the drill string, the second item on the left side is the heat of the drilling fluid flowing out of the annular gap, the first item on the right side is the heat transferred from the stratum to the annular gap and the second item on the right side is the heat transferred from the annular gap to the drill string. 
     At S 320 , permafrost parameters of a thawing region of the permafrost is determined based on the temperature field of the annular gap and a temperature field of the permafrost. 
     With the heat exchange between the drilling fluid and the casing, heat transfer between the heated casing and the permafrost causes the temperature of the permafrost to rise, which in turn causes the permafrost to thaw. The temperature change in the permafrost depends on the relative magnitude of the heat absorbed from the surroundings, the energy consumed by the ice to thaw, and the energy absorbed by the melt water. Based on the above principle and principle of energy conservation, the following formula may be used to simulate the temperature field of the permafrost: 
     
       
         
           
             
               
                 
                   
                     ρ 
                     e 
                   
                    
                   
                     c 
                     e 
                   
                 
                 
                   k 
                   e 
                 
               
                
               
                 
                   ∂ 
                   T 
                 
                 
                   ∂ 
                   t 
                 
               
             
             = 
             
               
                 
                   
                     ∂ 
                     
                       φ 
                       s 
                     
                   
                   
                     ∂ 
                     t 
                   
                 
                  
                 
                   
                     L 
                      
                     
                         
                     
                      
                     
                       ρ 
                       s 
                     
                   
                   
                     k 
                     e 
                   
                 
               
               + 
               
                 
                   
                     ∂ 
                     
                       ( 
                       
                         T 
                          
                         
                             
                         
                          
                         
                           φ 
                           l 
                         
                       
                       ) 
                     
                   
                   
                     ∂ 
                     t 
                   
                 
                  
                 
                   
                     
                       c 
                       l 
                     
                      
                     
                       ρ 
                       l 
                     
                   
                   
                     k 
                     e 
                   
                 
               
               + 
               
                 
                   ∂ 
                   
                     T 
                     2 
                   
                 
                 
                   ∂ 
                   
                     x 
                     2 
                   
                 
               
               + 
               
                 
                   1 
                   x 
                 
                  
                 
                   
                     ∂ 
                     T 
                   
                   
                     ∂ 
                     x 
                   
                 
               
             
           
         
       
     
     wherein ρ e , ρ s  and ρ i  indicate the density of permafrost, ice and water respectively, T indicates the temperature of permafrost, φ s  and φ l  indicate the saturation of ice and water respectively, k e  indicates the thermal conductivity coefficient of permafrost, and x indicates the distance between permafrost and wells. 
     At S 330 , the size of the anti-settling apparatus is determined based on the permafrost parameters of the thawing region. The temperature of interface between the permafrost and the casing can be regarded as the consistent, so it can be used as a boundary condition to perform differential coupling to solve the temperature field equation for the wellbore, drill string, annular gap and permafrost, and melting rate equation for the permafrost permafrost, in order to get the relevant permafrost parameters. 
     At S 340 , the amount of settlement of the well is determined based on the size. 
     At S 350 , the size of the anti-settling apparatus is adjusted based on the amount of settlement and a settlement amount threshold. 
     In order to enable the size of the anti-settling apparatus to be adjustable based on the actual amount of settlement, the base and the truss of the anti-settling apparatus may be formed as a structure capable of being folded and contracted. 
       FIG. 4  is a flowchart of a checking method according to another embodiment of the present invention. 
     As shown in  FIG. 4 , the step of determining permafrost parameters of a thawing region of the permafrost based on the temperature field of the annular gap and a temperature field of the permafrost may preferably comprise the following steps: 
     At S 420 , a thawing boundary and a thawed amount of the thawing region is determined based on the temperature field of the annular gap and the temperature field of the permafrost. 
     At S 430 , permafrost parameters of the thawing region are determined based on the thawing boundary and thawed amount. 
     The permafrost parameters include a lateral resistance coefficient and a compressive strength of the permafrost. In this case, the size of the anti-settling apparatus may be determined according to the following formula: 
     
       
         
           
             
               
                 
                   
                     
                       ( 
                       
                         1 
                         - 
                         f 
                       
                       ) 
                     
                      
                     G 
                   
                   + 
                   
                     
                       ρ 
                       e 
                     
                      
                     gSh 
                   
                 
                 S 
               
               ≤ 
               
                 σ 
                 p 
               
             
             , 
           
         
       
     
     wherein S indicates contact area of the anti-settling apparatus with the permafrost, G indicates the gravity of a wellhead apparatus and a casing installed in the well during drilling, f indicates the lateral resistance coefficient, h indicates the thickness of the permafrost, σ p  indicates the compressive strength, and ρ e  indicates the density of the permafrost. The principle of this formula is such that the pressure imposed by the anti-settling apparatus, the wellhead apparatus and the casing to the permafrost is less than the compressive strength of the permafrost. For example, in case that the anti-settling base is formed to be circular, S can be ¼ πD 2 . 
     In determining the size of the anti-settling apparatus, a certain margin may be reserved as the actually determined size after the theoretical size of the anti-settling apparatus is determined based on the critical value in the above formula. For example, in case that the anti-settling base is formed to be circular, a 15% size margin can be reserved, that is, the size calculated based on the critical value in the above formula is multiplied by 1.15, so the size of the anti-settling apparatus may be calculated as: 
     
       
         
           
             D 
             = 
             
               2.3 
                
               
                 
                   
                     
                       ( 
                       
                         1 
                         - 
                         f 
                       
                       ) 
                     
                      
                     G 
                   
                   
                     π 
                      
                     
                       ( 
                       
                         
                           σ 
                           p 
                         
                         - 
                         
                           
                             ρ 
                             e 
                           
                            
                           gH 
                         
                       
                       ) 
                     
                   
                 
               
             
           
         
       
     
     In the above formula, D indicates the size of the anti-settling base, which may be, for example, the diameter of the anti-settling base when the anti-settling base is circular, G indicates the total gravity of the casing and the wellhead apparatus, f indicates the lateral resistance coefficient of the permafrost, h indicates the thickness of the permafrost, σ p  indicates the compressive strength of the permafrost, and ρ e  indicates the density of the permafrost. The anti-settling base is preferably circular in shape, but it may also be formed in a rectangular shape. In the case that it is in a rectangle form, the size of the anti-settling base may be its side length. 
     Reference is now made to  FIG. 4 , the permafrost parameters may further include the porosity of the permafrost in the thawing region, and the checking method may preferably comprise the following steps: 
     At S 450 , a volume compression coefficient of the permafrost in the thawing region is determined based on the porosity. The volume compression coefficient of the permafrost is determined by the nature of the permafrost itself. When the size of the anti-settling base changes, the volume compression coefficient of the permafrost covered by the anti-settling base will also change due to the change of the region covered by the anti-settling base, as well as the difference in heat transferred to the permafrost portions having a different distance from the casing in the well. The volume compression coefficient may be determined by a known method, and the present invention is not limited thereto. 
     At S 460 , the actual amount of settlement corresponding to the size is determined according to the following formula: 
     
       
      
       s=a 
       0 
       h+m 
       v 
       hΔp  
      
     
     wherein s indicates the amount of settlement, h indicates the thickness of the permafrost, a 0  indicates thaw-settlement coefficient of thawed permafrost, m v  indicates the volume compression coefficient of the permafrost, and Δp indicates the pressure of the anti-settling apparatus on the permafrost at the wellhead. 
     At S 470 , it is determined whether the actual amount of settlement exceeds the settlement amount threshold. 
     At S 480 , the size is increased by a predetermined value to re-determine the size of the anti-settling apparatus if the actual amount of settlement exceeds the settlement amount threshold. The predetermined value may also be a value determined according to the above formulae used to determine the anti-settling base. For example, a compressive strength value may be selected within a range smaller than the maximum compressive strength, and the size of the anti-settling base may be increased to the newly determined value after the size of the anti-settling base is recalculated. 
     With the above method, an appropriate anti-settling apparatus can be determined to relieve the pressure of various apparatuses on the permafrost, thereby reducing the amount of settlement of the well. 
       FIG. 5  is a structural block diagram of a checking apparatus according to an embodiment of the present invention. The checking apparatus provided by the present invention is used to carry out the above described method. As shown in  FIG. 5 , the checking apparatus comprises: a temperature field simulation module  100  configured to simulate a temperature field of an annular gap based on the flow rate of an injected drilling fluid and the temperature of the drilling fluid in a drill string; a parameter determination module  200  configured to determine permafrost parameters of a thawing region of the permafrost based on the temperature field of the annular gap and a temperature field of the permafrost; an anti-settling apparatus determination module  300  configured to determine the size of the anti-settling apparatus based on the permafrost parameters; a settlement amount determination module  400  configured to determine the amount of settlement of the well based on the size of the anti-settling apparatus; and an adjustment module  500  configured to adjust the size of the anti-settling apparatus based on the amount of settlement and a settlement amount threshold. The temperature field of the annular gap and the permafrost may be simulated according to above equations. 
     The parameter determination module  200  may be further configured to: determine a thawing boundary and a thawed amount of the thawing region based on the temperature field of the annular gap and the temperature field of the permafrost; and determine permafrost parameters of the thawing region based on the thawing boundary and thawed amount. 
     The permafrost parameters include a lateral resistance coefficient and a compressive strength of the permafrost, and the anti-settling apparatus determination module  300  may be further configured to determine the size according to the formula used for determining the size of the anti-settling apparatus as described above. 
     The permafrost parameters further include the porosity of the permafrost in the thawing region, the settlement amount determination module  400  may be further configured to: determine a volume compression coefficient of the permafrost in the thawing region based on the porosity; and determine the actual amount of settlement corresponding to the size of the anti-settling apparatus, wherein the actual amount of settlement may be determined according to the equation described in relation to the checking method. 
       FIG. 6  is a structural block diagram of an apparatus for preventing settlement of well according to an embodiment of the present invention. The apparatus for preventing settlement of well comprises the above described anti-settling apparatus  600  and the checking apparatus  700 . 
     In a preferred embodiment, in order to reduce heat transfer from the casing to the permafrost, the apparatus may further comprise: a thermal insulation casing disposed on wall of the well for preventing heat transfer from the well to the permafrost. In the case that the thermal insulation casing is provided, it may replace the common casing normally used for drilling. 
     As shown in  FIG. 1 , the thermal insulation casing may further comprise an outer thermal insulation casing  6 , an inner thermal insulation casing  9 , and a hollow portion formed between the outer and inner thermal insulation casings and filled with a gas  7 . The thermal insulation casings in the present invention are formed in a dual-layer structure, and the hollow portion between the two layers is in a sealed state. When the temperature in the annular gap between the casing wall and the drill string is increased, the hollow portion of the dual-layer thermal insulation casing may experience gas expansion and pressure increase due to temperature increase. In order to prevent gas expansion and pressure increase that may cause damage to the dual-layer thermal insulation casing, foam pellets  8  are filled in the hollow portion of the dual-layer thermal insulation casing. These foam pellets  8  adhere to the inner wall of the hollow portion and can absorb and release the gas when the gas in the hollow portion expands and shrinks, which ensures stability of the pressure in the hollow portion and thus avoids damaging the dual-layer thermal insulation casing. 
     In addition to the dual-layer thermal insulation casing, it is also possible to use other thermal insulation casings having thermal insulation effects. For example, a casing having a thermal insulation coating or the like can be used. In the case where the thermal insulation casing is provided, the temperature of the drilling fluid in the well can be effectively prevented from being transmitted to the permafrost so as to prevent the permafrost from thawing and reduce the amount of settlement of the well. 
     The foregoing has described in detail the optional implementations of the embodiments of the present invention with reference to the accompanying drawings. However, the embodiments of the present invention are not limited to the specific details of the foregoing implementations. Within the technical concept of the embodiments of the present invention, various simple variations may be made to the technical solutions of the embodiments of the present invention, and these simple variations all fall into the protection scope of the embodiments of the present invention. 
     In addition, it should be appreciated that the technical features described in the above embodiments can be combined in any appropriate manner, provided that there is no conflict among the technical features in the combination. To avoid unnecessary iteration, such possible combinations are not described here in the present invention. 
     Those skilled in the art can understand that all or part of the steps for implementing the method of the above embodiments can be accomplished by a program instructing relative hardware, which is stored in a storage medium with several instructions to make a single chip microcomputer, a chip or a processor to perform all or part of the steps of the method described in the various embodiments of the present application. The foregoing storage medium may include a U disk, a removable hard disk, a read-only memory (ROM), a random access memory (RAM), a magnetic disk, an optical disk, or any other medium that can store program codes. 
     Moreover, different embodiments of the present invention can be combined freely as required, as long as the combinations do not deviate from the ideal and concept of the present invention. However, such combinations shall also be deemed as falling into the scope disclosed in the present invention.