Force analysis and calculation of workover string in the slanted and horizontal well are the basis of designing and checking string strength, selecting tools and determining operation parameters, which determine the operation safety and success of engineering accidence treatment. In this paper, by comprehensive consideration of wellbore structure, string assembly, string load and workover operation conditions, the workover string mechanical model has been built under three kinds of working states of lifting, lowering and rotating. The downhole string mechanics has been analyzed and calculated. By field verification, the string assembly, tool selection and operation parameter optimization can be achieved, which can improve the safety and success rates of workover engineering accident treatment.
The casing in slanted and horizontal well is curved, so by the limits of the casing workover string is curved. It makes workover string not only by the force of its own gravity, but also by the friction between the casing and workover string as well as the torque caused by friction [
As a result, workover string mechanical model is established according to the characteristics of slanted and horizontal well. Stress analysis and calculation can guide the field construction, which has an important meaning to improve the safety and success rates of workover engineering accident treatment.
In the workover string force analysis of the slanted and horizontal wells, the tubing string is the analysis object, the string is taken as flexible pole to analyze its stress, and the mechanical models of workover string under lifting, lowering and rotating states are established.
1) The pulling force of the string (hook hoisting load) F1;
2) The weight of the string above the deflection point G1;
3) The weight of the string with the friction tape of casing AB is G2 and the friction force between the string and casing f1;
4) The weight of the string below the point C is G3 and the friction force between the string and the casing is f2;
5) The anchorage force of the string with the friction tape of casing AB as well as below is F2 and F3;
6) The pulling force F to the bottom of the string by the fish, for 0 when the fish is not retrieved.
The relationships of the forces are as follows:
At this time,
L1―the length of the vertical string, m;
L2―the length of the kickoff string, m;
L3―the length of the slanted string, m;
1) The pulling force of the string (hook hoisting load) F1;
2) The weight of the string above the deflection point G1;
3) The weight of the string with the friction tape of casing AB is G2 and the friction force between the string and casing f1;
4) The weight of the string below the point C is G3 and the friction force between the string and the casing is f2;
5) The anchorage force of the string with the friction tape of casing AB as well as below is F2 and F3.
The relationships of the forces are as follows:
1) The pulling force of the string (hook hoisting load) F1;
2) The weight of the string above the deflection point G1;
3) The weight of the string with the friction tape of casing AB is G2 and the friction force between the string and casing f1 as well as the moment T1 caused by f1;
4) The weight of the string below the point C is G3 and the friction force between the string and the casing is f2 as well as the moment T2 caused by f2;
5) The anchorage force of the string with the friction tape of casing AB as well as below is F2 and F3;
6) The pulling force F to the bottom of the string by the fish.
The relationships of the forces are as follows:
Well ST32131 is a new fractured well constructed by some job team. The string falls to well in the process of sand washing after fracture, fish top is 73 mm pipe couplings, depth is 2682.31 m, the length of fish is 710 m . Now the force of the well in the salvage construction and the torque of the string in rotating state are analyzed and calculated [
The string salvaged in well is the string assembly of 89 mm and 73 mm (outer thickening) of N80, the length is 1182 m and 1500 m. Known that the weight of 89 mm tubing in well is 117.6 N/m, and 73 mm outer thickening tubing is 83.5 N/m (density of well liquid is 1.0 g /cm3), and the fish in well is of 73 mm tubing of J55, the weight is 81.5 N/m.
1) Base data (
2) The base data about the string assembly of 89 mm and 73 mm (outer thickening) of N80 (
According to the above conditions the following can be worked out [
1) Before fish is retrieved F = 0, and
2) When fish is retrieved and jam is released 600 kN,
Casing external diameter | 139.7 mm | Casing thickness | 9.17 mm | Casing depth | 3410.0 m |
---|---|---|---|---|---|
Drilled depth | 3416.0 m | Cement return height | Wellhead | Cement quality | Qualified |
Artificial botoum hole | 3394.4 m | Fish top depth | 2682.31 m | Distance between tubing and bushing | 4.72 m |
Kickoff point | 1417.2 m | Biggest inclined point | 2201.2 m | Maximum angle | 37.49˚ |
Tubing | External diameter/(mm) | thickness/ (mm) | Inner diameter/ (mm) | Grade | weight/ (N/m) | volume/ (L/m) | tensile strength/ (kN) | Coupling OD/(mm) | Buckled torque/(N∙m) | ||
---|---|---|---|---|---|---|---|---|---|---|---|
minimum | best | maximum | |||||||||
89 | 88.9 | 6.5 | 76.0 | N80 | 134 | 1.67 | 708 | 107 | 2150 | 2850 | 3600 |
73UP | 73.02 | 5.5 | 62.0 | N80 | 95 | 1.17 | 645 | 93 | 2400 | 3200 | 4000 |
73 | 73.02 | 5.5 | 62.0 | J55 | 93 | 1.17 | 323 | 89 | 1100 | 1450 | 1800 |
1) Naturally rotating the string before salvaging fish, at this time
2) When salvaging tubing and picking up 300 kN to back off,
3) If back off from the bottom of fish (fall fish is buried by sand)
1) The load of uniformly lifting the string is 287.5 kN, while the load of lowering it is 224.5 kN, so the difference of the two loads is 63 kN; the solution force got by fish top is 231.2 kN when jam is released to 600 kN, the load loss is 81.3 kN and it accounts for a quarter of the rise load.
2) The torque that can make string rotate uniformly is calculated to be 1104.5 N∙m, with the carry load increasing, the friction torque of string also increases, that has effects on string’s back-off and leads to a wrong position [
3) In the actual construction, the load of string is verified. The load of lifting the string uniformly is 290 kN, while the load of lowering the string uniformly is 220 kN, and the difference is 70 kN. Oil pipe wrench torque table shows 940 N∙m when turning a string uniformly. It is found that the loads in lifting and lowering process are basically identical with calculated results [
4) In the construction using drill pipe, due to the large diameter of drill pipe couplings and large area of cross-sectional, the friction between the casing and drill pipe will further increase, that can have large effects on treatment success rate of all kinds of the conventional technology in the construction of slanted and horizontal wells [
1) Force analysis and calculation of workover string in the slanted and horizontal well are the basis of designing and checking string strength, selecting tools and determining operation parameters, which determine the operation safety and success of engineering accidence treatment.
2) Force analysis of workover string in the slanted and horizontal well must consider the factors of well structure, string combination, string load as well as the operation condition. The calculation of string load and the friction in every operation condition must consider the buckle deformation of the string and the constrain of the well structure and the fish.
3) Through the calculation methods and simplified formula of workover string load, deformation and stress in slanted and horizontal wells, the mechanics calculation about the downhole strings is made and the results are verified. Through the safety factor, the string assembly, tool selection and operation parameter optimization can be achieved, which can improve the safety and success rates of workover engineering accident treatment.