Mechanical Resistance of a Superficially Treated Alloy Drill Pipe during Onshore Drilling

Abstract

A theoretical study was conducted to investigate the limit depth reached by aluminum alloy drill pipes combined with steel pipes for deep well drilling in the Algerian country. Therefore, the present study is based on various parameters that have an impact on the fatigue behavior of these tubes, focusing particularly on the damage caused by the gravity of the dogleg in the crooked path in addition to their mechanical behavior, and determines the extent to which the aluminum drill pipe can drill without failures using well-engineering software modeling which provides the expected loads in the drilling and provides results more or less close to reality. This analysis indicates that the aluminum drill pipe has good fatigue resistance, despite the cumulative presence of axial and bending stress concentrations in the dogleg zones.

1. Introduction

The increased depth of drilling over 4500 m has affected the complexity of drilling operation as well as drilling rigs and their practices. Due to this, modern drilling technologies are designed to increase the capacity of drilling equipment. Because the possibility of increasing the drive power is quite limited and economically impractical, the application of alternative materials such as aluminum alloys becomes the most cost-effective and relevant target [1]. For instance, drilling deep, ultra-deep, and especially horizontal wells is extremely important to ensure the high operational reliability of drill string, to reduce its stress-strain state, and to ensure trouble-free operation at extreme loads and high temperatures. Drill string assembly and its weight significantly affect the technical and economic parameters of the well drilling process, the behavior of resisting forces, and specify the level of load in the parts of a drilling rig.

Aluminum alloys possess several valuable physical and mechanical properties that favorably distinguish them from steel, which is a traditional material in the drill pipe. The following should be referred to as the basic properties of aluminum alloys:

• Low specific weight;

• Reduced modulus of longitudinal elasticity and shear;

• Workability in pipe production using extrusion process;

• Corrosion resistance in aggressive environment and, first of all, in Н₂S and СО₂.

Aluminum alloys are still of relatively limited use in drilling, since the temperature is of crucial importance here, since in deep wells, depending on the geological section, the temperature at 3500–7000 m can reach 42,315 K, and in some cases 52,315–82,315 K [2, 3] for example D16 and 1953 alloys. D16 alloy has high strength characteristics at room and elevated (up to 43,315 K) temperatures, medium ductility, but very low corrosion resistance in drilling fluids.

• Non-magnetic and vibration resistance properties;

• High rate of weight reduction in solutions of various density [1, 4].

These properties of aluminum alloys specify the basic efficiency in light alloy aluminum drill pipe (LAIDP) application in drill strings while constructing oil and gas wells. Therefore, the work process in this paper is to first determine the limit up to which the aluminum drill pipe can drill without failures by means of the Decision Space Well Engineering software and second, point out the expected loads during drilling [5].

2. Superficially treated 2024 aluminum alloy technology

The method of oxidization by micro arcs is an electrochemical surface treatment; it is similar to the anodization, where we use a very high voltage in order to produce electric discharges. It is a chemical conversion of the substrate by its oxide, therefore an excellent adhesion. The oxide layer is formed by applying an electric potential (at least 200 V), whereas the piece is immersed in an acid electrolyte. Discharges are produced, agglomerate, and make the oxide dense and partially converted to amorphous alumina in crystal form such as the cordon. The structure of coatings applied by the micro-arc oxidation method to DI6 aluminum alloy) was investigated with the use of scanning electron microscopy and chemical and X-ray diffraction analyses. It was found that the coating is inhomogeneous. Its structure and composition change across the thickness and the micro inhomogeneity observed in analyzing the microstructure of a transverse micro-sample and confirmed by the data of coating microhardness measurements are characterized by the formation of a honeycomb type structure. Consequently, the coating may be considered as a composition material with uniformly distributed hard islands surrounded by softer veins. Tables 1 and 2 shown chemical composition and mechanical characteristics of aluminum alloy 2024 [5].

Designation (NF A03104)	Chemical composition (%)
2024	AL	Cu	Mn	Mg	Si	Fe
	The complement	3.8–4.5	0.3–4.5	1.2–1.8	0.5	0.5

Table 1.

Chemical composition of the superficially treated alloy.

Tube sections	Ultimate tensile strength; σr [MPa]	Stress; σ0,2 in [MPa]	Elasticity limit; σpr [MPa]	Relative extension to rupture; δ en %	elastic range slope; α [°]	Young’s modulus; E [MPa]
2 × 30	Before superficial treatment (OMA)
	491	382	360	12	80	71,805
	After superficial treatment (OMA)
	468	360	337	13	81	8763

Table 2.

Results of tensile testing (micro-tests) on test-tubes in the superficially treated alloy, carried out on the machine “ALA-TOO”.

3. Stress loads

The Decision Space Well Engineering Software Torque & Drag outputs can be used to predict and analyze the torque and axial forces generated by drill strings, casing strings, or liners while running in, pulling out, sliding, backreaming, and/or rotating in a three-dimensional wellbore. The effects of mud properties, wellbore deviation, weight-on-bit (WOB), and other operational parameters can be studied.

In Decision Space Well Engineering, many stress calculations are performed using the following equations. These calculations include the effect of [6]:

• Axial stress (tension + bending + buckling)

• Bending stress approximated from wellbore curvature

• Bending stress due to buckling

• Hoop stress due to internal and external pressure

• Radial stress due to internal and external pressure

• Torsional stress from twist

• Von Misses force.

3.1 Drilling for 8½″hole section with steel drill pipe

The root causes for different problems encountered on the 8½ section in well drilled in Algeria are;

1. Hole geometry (wellbore tortuosity 0.2032 m).

2. Inharmonic in drilling string and parameters: weight-on-bit (WOB), rotational speed (RPM), bottom hole assembly (BHA).

3. High torque peaks, while drilling and back reaming (37,962–48,809) Nm recorded at the surface [3].

3.2 Load summary

According to Table 3, the drill pipe is exposed to fatigue failure during all operations, which is naturally related to the well trajectory, which has some tortuosity as shown in Table 4, which displays doglegs per unit length in the string during drilling operations and located points along the well that may be subjected to high severe doglegs leading to a high degree of severity of the moment bending on the mentioned areas. For that reason; the solution proposed in this paper consists in simply replacing joints of standard steel drill pipe with lighter aluminum drill pipe while keeping the same bottom hole assembly (BHA).

Load condition	Stress failure	Measured weight (tonne)	Total stretch (m)	Axial stress = 0
				Distance from surface (m)	Distance from bit (m)
Tripping in	Fatigue	123	6.60	3793	208
Tripping out		161	7.63	3793	208
Rotating on bottom		137	6.74	3793	208
Backreaming		149	7.36	3793	208
Rotating off bottom		147	7.25	3793	208

Table 3.

Load summary for steel drill pipe.

- Tripping in: when running in hole (RIH)

- Tripping out: when pull out hole (POOH)

- Rotating on bottom: means that the pipe rotates with axial movement.

- Rotating off bottom: means that the pipe rotates without any axial movement

- Backreaming: the practice of pumping and rotating the drill string while simultaneously pulling out of the hole.

MD (m) measured depth	Inclination (°)	AZ (°)	TVD (m); Vertical depth	DLS; (°/30 m); Dog leg	Absolut tortuosity (°/30 m)	Reel tortuosity /(°30 m)	Vertical section(m)	Build (°/30 m)	Walk (°/30 m)
2905.00	1.00	63.67	2905.22	0.010	0.010	0.000	11.25	0.010	0.000
2912.00	1.14	57.89	2912.37	0.741	0.012	0.000	11.31	0.587	−24.252
2921.00	1.03	66.98	2921.51	0.669	0.014	0.000	11.39	−0.361	29.836
2930.80	1.05	116.65	2930.65	2.868	0.023	0.000	11.39	0.066	163.031
2939.00	1.63	161.92	2939.70	3.852	0.035	0.000	11.23	1.923	150.066
2949.00	3.42	180.29	2948.93	6.306	0.055	0.000	10.83	5.812	59.643
2958.00	5.52	183.53	2958.05	6.932	0.076	0.000	10.11	6.885	10.623
2967.38	6.72	183.90	2967.14	3.941	0.088	0.000	9.14	3.939	1.214
2976.52	7.50	185.52	2976.21	2.643	0.096	0.000	8.01	2.560	5.317
2985.67	7.92	186.26	2985.27	1.415	0.100	0.000	6.79	1.377	2.426
2994.81	8.17	186.80	2994.32	0.857	0.102	0.000	5.52	0.821	1.772
3003.96	8.44	187.19	3003.38	0.904	0.104	0.000	4.21	0.885	1.279
3013.10	8.44	189.80	3012.42	1.257	0.108	0.000	2.88	0.000	8.567
3022.24	8.55	191.52	3021.46	0.909	0.110	0.000	1.56	0.361	5.646
3031.39	8.31	193.54	3030.51	1.250	0.114	0.000	0.25	−0.787	6.623
3040.53	8.33	194.91	3039.55	0.654	0.115	0.000	−1.03	0.066	4.497
3049.68	8.34	196.13	3048.61	0.581	0.117	0.000	−2.31	0.033	4.000
3058.82	7.82	193.58	3057.66	2.073	0.123	0.000	−3.55	−1.707	−8.370
3067.96	7.22	203.80	3066.72	4.803	0.137	0.000	−4.68	−1.969	33.545
3077.11	9.00	206.56	3075.78	5.972	0.154	0.000	−5.85	5.836	9.049
3086.25	5.96	209.55	3084.84	10.056	0.183	0.000	−6.90	−9.978	9.814
3095.40	5.24	213.98	3093.94	2.752	0.191	0.000	−7.66	−2.361	14.525
3113.68	4.23	223.24	3112.16	2.074	0.202	0.000	−8.85	−1.658	15.197
3122.83	4.09	225.14	3121.29	0.644	0.203	0.000	−9.32	−0.459	6.230
3131.97	4.90	228.33	3130.40	2.781	0.211	0.000	−9.81	2.659	10.470
3177.69	1.70	243.22	3176.04	2.156	0.239	0.000	−11.42	−2.100	9.770
3195.98	0.87	265.04	3194.32	1.557	0.246	0.000	−11.55	−1.361	35.790
4000.65	0.94	232.89	3998.90	0.019	0.200	0.000	−16.06	0.003	−1.199

Table 4.

Variation in wall trajectory via tortuosity.

Thus; A torque and drag optimization study has been run to determine the optimum number of ADP joints along the drill string to minimize friction and reduce compression along the well trajectory and limit the maximum depth which can aluminum drill pipe reached without any failure, accordingly, 150 joints of standard steel DP have been replaced by aluminum DP above the BHA. Table 5 represented below give the main characteristics of ADP.

Hole section; 0.2032 m	Steel drill pipe/aluminum
	Drill pipe (0.127 m)-G105	DP drill pipe (0.127 m) -AL2024
Depth	1350 (m)	2592.86 (m)
Elastic limit σe (MPa)	724	52127.5
Tensile strength σ m (MPa))	931	359
Young’s modulus of elasticity E (MPa)	206896.55	88,763
σ endurance limite (MPa)	137.89	160

Table 5.

Details in each specific drilling string and section.

Note: The tool-joints for aluminum drill pipe are manufactured from steel.

3.3 Results and interpretation

The results exposed in Table 6 indicate that aluminum drill pipe gives good results in tortuous intervals compared to steel drill pipe but in total depth do not exceed the 4000 m.

Load condition	Stress failure	Measured weight (tonne)	Total stretch (m)	Axial stress = 0
	Fatigue			Distance from surface (m)	Distance from bit (m)
Tripping in		121,560	7.31	3793	207
Tripping out		160,340	8.52	3793	207
Rotating on bottom		135,690	7.34	3793	207
Backreaming		147,680	8.21	3793	207
Rotating off bottom		14,680	8.07	3793	207

Table 6.

Load summary for aluminum drill pipe.

- Tripping in: when running in hole (RIH)

-Tripping out: when pull out hole (POOH)

- Rotating on bottom: means that the pipe rotates with axial movement.

-Rotating off bottom: means that the pipe rotates without any axial movement

- Backreaming: the practice of pumping and rotating the drill string while simultaneously pulling out of the hole.

This result is confirmed by the output obtained in the torque drag effective tension graph which has included the graphical curves on tension vs. distance along with string (tension limit, helical buckling, sinusoidal buckling, in all operations (rotate off bottom, rotate on bottom, tripping out, tripping in).

Accordingly; it is obviously seen that all operations curves do not cross the Tension Limit curve; the drill string is located into a safe window; therefore, there is no danger of exertion the aluminum drill pipe in the tortuous interval, as depicted in Figure 1.

Figure 1.

Effective tension limit curve.

Thus we can assume that the 0.2032 m drilling phase benefited from ADP for the reason that; First of all aluminum drill pipe has significantly good fatigue, which is confirmed by the fatigue ratio value which is 0.642 less than 1. As shown on Figure 2.

Figure 2.

Aluminum drill pipe fatigue ration curve.

$$\text{Fatigue ratio}{\mathrm{R}}_{\mathrm{f}}=\frac{\left(\left|{\mathrm{\sigma }}_{\text{bending}}\right|+\left|{\mathrm{\sigma }}_{\text{buckling}}\right|\right)}{{\mathrm{\sigma }}_{\text{fatigue Limt}}}=1\text{is equal to the safety limit}$$E1

where ${\mathrm{\sigma }}_{\text{bendiing}}={\mathrm{\sigma }}_{\text{flexion}}$ [6].

Secondly; the 0.2032 m drilling phase benefited from (aluminum drill pipe) ADP since there was a considerable reduction of the surface torque and hook load as illustrated in Figure 3, for the reason that there was a reduction in side force as represented in Table 7.

Figure 3.

Hook load for aluminum drill pipe (ADP).

	Rotating on bottom; ADP side force	Rotating on bottom; SDP side force
(m)	(N)	(lbf/length)
2908	3114	3172
2917	5805	5907
2926	10,814	11,009
2935	958	9751
2944	8723	8874
2953	6135	6326
2962	3177	3230
2971	2532	2572
2981	2736	2785
2990	3759	3822
2999	3906	3968
3008	3501	3559
3017	2024	2059
3026	1940	1975
3035	2296	2335
3044	4622	4706
3062	4422	4493
3071	3684	3750
30,791	9662	9831

Table 7.

Side force aluminum drill pipe (ADP)/steel drill pipe (DP).

As a final point; analysis of stresses in drill pipe and their results on each one, are shown on the graph cited, in Figure 4 which gives us the different results for the Stresses (psi), the critically of Failure will depend on the value of these stresses. The Von Mises yield condition, states initial yield limit is based on the combination of the three principal stresses axial stress, radial stress, and hoop stress [3].

Figure 4.

Aluminum drill pipe tripping out stress detail.

$${\sigma }_{\mathit{VM}}=\sqrt{\frac{\left({\sigma }_r-{{\sigma }_h}^2\right)+{\left({\sigma }_a-{\sigma }_r\right)}^2+{\left({\sigma }_h-{\sigma }_a\right)}^2+6{\sigma }_s^2+6{\sigma }_t^2}{2}}$$E2

σ_VM: Von misses stresses, σ_a: axial stress, σ_r: radial stress, σ_h: hoop stress, σ_t: torsion stress $\sigma$_S: shear stress [6].

Thus; we can be well observed that the Von-Misses Stress is under the tension limit line (red color), due to our model simulation is located into the safety window.

4. Conclusion

In synopsis; aluminum drill pipe is subjected to less strain than steel drill pipe. This is due to their lower modulus of elasticity and lighter density than steel drill pipe. Which leads to a good fatigue resistance even at the simultaneous presence of high applied torque and axial load, and severe doglegs, which makes the use of other options Steel drill pipe impractical from the fatigue failure standpoint. The limiting factor for using aluminum drill pipe in such severe conditions is not the fatigue resistance of the material that the pipe is made of, but the high contact forces between the wellbore and aluminum drill pipe.