Technique for determining the fatigue wear of a drilling tool

RU EN UZ

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A.A. YAVORSKY, Chief Process Engineer 

 

 

 

 A.M.MILENKY, Dr.-Ing. Chief Engineer 

 

 

 

 
 ERIELL NEFTEGAZSERVICE LLC, Moscow, 109028, Russia 

 

 

 
<h3 style="text-align: right;"></h3>
 

 S.S. RUBLEV, Project Manager 

 

 B.P. IVANOV, Director for the commissioning of new facilities at Messoyakhaneftegaz JSC 

 

 Gazpromneft NTC LLC St. Petersburg, 190000, Russia 

 

 

 
<h3 style="text-align: right;">A.A. SOKOLOV, Head of Engineering</h3>
 

 

 

 
 Meretoyakhaneftegaz LLC Tyumen, 625026, Russia

 

 One of the vital tasks during the construction of horizontal wells using modern expensive subsurface equipment is the ability to predict accumulated fatigue wear of drilling tools and prevent their failures associated with washouts and breaks along the drill pipe body. 

 


 The article describes the technique of determining the fatigue wear of drilling tools by the example of horizontal wells, which confirmed its practical application at the facilities of ERIELL NEFTEGAZSERVICE LLC and GAZPROM NEFT PJSC. 

 

 Keywords: fatigue wear, corrosion, stress concentrators, bending stresses, horizontal wells, bottomhole assembly (BHA), rotary steerable system (RSS) 

 


 Statistics show that most failures (washouts, breaks) of drilling tools operated in deviated and horizontal wells are caused by fatigue failure. The extent of accumulated fatigue wear of drill pipe depends on parameters of wellbore curvature, vugular porosity of open borehole, type of flushing fluid used, the value of flushing fluid performance and erosion wear of the outer surface of the drill pipe body, value of stretching load in the interval of intensive wellbore curvature, number of drill pipe rotation cycles in the areas of the maximum spatial intensity of wellbore curvature [1]. 

 


 Fatigue failures along the drill pipe body often have a typical appearance, different from those caused by exceeding the allowable ultimate loads. A fatigue crack is flat and perpendicular to the pipe axis (Fig. 1). When a crack forms, the flow of flushing fluid often erodes the crack, which in turn leads to through-washing of the drill pipe body (Fig. 2). 

 


 A fatigue crack that is eroded by the flushing fluid flow usually maintains its transverse orientation. The fatigue crack propagation process, combined with the application of combined tensile and torsional loads, can lead to a sudden catastrophic failure of the pipe body before the crack becomes large enough (Fig. 3) [2]. 


<img width="473" alt="Статья_рис1.jpg" src="/upload/medialibrary/282/hn7qq8vn1m310kjvcy1zx91kywk8x1qq.jpg" height="173" title="Статья_рис1.jpg"> 



Fig. 1. Fatigue crack in drill pipe under microscope (400 µm) 
 


<img width="385" alt="Статья_рис2.jpg" src="/upload/medialibrary/913/uxqn0be9sc1o6usvv7719fw5kwdnmjym.jpg" height="220" title="Статья_рис2.jpg"> 


 

 Fig. 2. Through-washing of the drill pipe body 
 


<img width="442" alt="Статья_рис3.jpg" src="/upload/medialibrary/4c4/k9sbelzjzluf0e881ftfjn2x1le8y88e.jpg" height="298" title="Статья_рис3.jpg"> 


 

 

 


 Fig. 3. Fatigue failure along the drill pipe body


We encountered a particularly acute problem of premature fatigue wear of drilling tools when drilling horizontal development wells on the PK1 design horizon in the Tazovsky region of Western Siberia. The design of these wells provides for the drilling of a 178-mm production casing shoe into the roof of PK1 formation with the zenith angle of 90 degrees and vertical depth of 1186 m and further drilling of the liner section 114 with 152.4-mm bit and the length of the open borehole up to 2000 m. When drilling two holes, the total length of the sinking along the design horizon may be up to 4,000 m.

 


 The BHA 102x8.38 S-135 drilling tools, RSS (rotary steerable system), and a motorized section in the BHA are used for drilling along the target horizon. The maximum actual spatial intensities of curvature in the production casing section reach 2.6 deg/10m. (Fig. 4). Inhibited salt drilling mud with KCL salinity of 4-5 % is used as flushing fluid. The pay horizon is represented by a heterogeneous reservoir, therefore during the running of production liner section 114 in dense clayed areas, the mechanical sinking speed decreases from 40 m/h down to 5-10 m/h, and the vibration loads on drill string increase. 

 


 The accumulated fatigue wear of drill pipes increases significantly when mechanical sinking speed decreases - due to an increase of rotation cycles and maximum bending stresses in intervals with maximum spatial curvature intensity. Application of low mineralized drilling mud as flushing fluid together with alternating bending stresses leads to the formation of additional stress concentrators in the form of pitting corrosion on the outer wall of the drill pipe body. The corrosion rate in low-salinity drilling fluids is significantly higher than in freshwater drilling fluids. The corrosion rate depends on the mass fraction of salt in the drilling mud. As the salt concentration in the drilling fluid increases up to 5%, the corrosion rate increases and reaches its peak. At salt concentrations above 5%, the corrosion rate decreases due to a decrease in oxygen solubility. For 15 % salinity, the corrosion rate is lower than in freshwater (Fig. 5) [3]. 


<img width="604" alt="Статья_рис4.jpg" src="/upload/medialibrary/c42/ceynvysq13tmptw2ky6jod413fmu632b.jpg" height="908" title="Статья_рис4.jpg"> 



Fig. 4 Parameters of curvature of horizontal development wells on the PK-1 design horizontal


<img width="481" alt="Статья_рис5.jpg" src="/upload/medialibrary/e0d/js2ahm57tjl0iagf1m5afehbt18407cy.jpg" height="262" title="Статья_рис5.jpg"> 


 

 Fig. 5. Relationship of solution mineralization to corrosion rat 



 


 When a crack forms, the flushing fluid flow often erodes the crack, which in turn leads to through-washing of the drill pipe body. 

 


 The operation of drill pipe in these conditions requires drillers to keep track of the set's running time not only in hours of circulation and meters of sinking, but also in % accumulated fatigue wear - to be able to predict the service life and timely replacement of used drill pipes. 

 


 When drill pipes are operated in areas with high spatial curvature intensity, the drill pipe, while rotating, is subjected to alternating bending stresses. The correlation between the maximum variable bending stress and the fatigue limit of the drill pipe in these sections is determined by the equation [4]: 

 


 σ = a × Nb (1) 

 


 σ – maximum bending stress; 

 


 N – drill pipe fatigue limit, a dimensionless value, denotes the maximum possible number of revolutions (cycles) on a certain curved section of the well; 

 


 a and b – experimentally derived values determining the fatigue properties of the material, including the operation of drill pipes in a corrosive environment (Sathuvalli U.B., Payne M.L., Pattillo P.D., Livesay R.B.: “Advanced assessement of drillpipe fatique and application to critical well engineering” paper SPE 92591, prepared for presentation at the SPE/IADC Drilling Conference held in Amsterdam, the Netherlands, 23-25 February 2005). 

 


 The accumulated fatigue wear of the drill pipe D in a certain section of the well - i is defined as the quotient of the actual number of revolutions of the drill pipe ni divided by the corresponding fatigue limit Ni of the given drill pipe in the given section: 

 


 Di = (ni / Ni ) ×100 % (2) 

 


 Drill pipe during different processing operations in the well (drilling, tooling development, forward and back reaming, flushing with rotation) successively pass sections with intensive curvature "top-down" and "bottom-up". The total fatigue D value of the drill pipe is equal to the sum of Di values of each section. The D value, in percent, corresponds to the size of fatigue cracks. If the D value is greater than 100%, it means the crack has propagated through the entire wall of the drill pipe. 

 


 The actual number of drill pipe cycles ni is determined by the results of real data from the drilling parameter sheet or by the following expression: 

 


 ni =(60×RPMхL) / VOP (3) 

 


 RPM – RPM of the rotor or spindle of the USP; 

 


 VOP – the mechanical sinking speed of the borehole section i; 

 


 L – the length of the given section of the wellbore i; 

 


 The practical sequence of calculations is as follows: 

 


 The Landmark software package simulates the drilling, back reaming process every 100 m of the open borehole and displays the maximum bending stresses for each drill pipe based on the actual parameters (Fig. 6). 

 


 Further, knowing the maximum bending stresses on each pipe σ and having initial data about the number of actual rotation cycles ni, we can determine the fatigue limit of pipe N from expression (1), and also accumulated fatigue wear of drill pipe Di on each considered section from expression (2) (Fig. 7). 


<img width="758" alt="Статья_рис6.jpg" src="/upload/medialibrary/bda/cpgw9u0vc35szl8ifv3ccpylbkpdwtc3.jpg" height="530" title="Статья_рис6.jpg"> 



Fig. 6. Calculation of maximum bending stresses in the Landmark software package 
 

 

 <img width="705" alt="Статья_рис7_eng.jpg" src="/upload/medialibrary/bea/geuy4m6ze8l595oli4qhv8s0rdc2dv56.jpg" height="612" title="Статья_рис7_eng.jpg"> 
 

 

 Fig. 7. Form of calculation of accumulated fatigue of drill pipes Di, on each considered section of the wellbore - i 



The total accumulated fatigue wear of each drill pipe D set will be determined by the sum of accumulated fatigue wear Di on each considered wellbore section in relation to actual operations with drill string rotation - drilling, straight and back reaming, rotation with flushing, tooling development, cutting to a new borehole, etc. (Fig. 8).

 


 When the mechanical sinking speed drops, the accumulated fatigue wear of drill pipes increases significantly - due to an increase in rotation cycles and the action of maximum bending stresses in intervals with maximum spatial curvature intensity. 

 


 One of the examples of the given technique proof in the field conditions is shown on the photo of corrosion-fatigue wear of drill pipe at the next inspection with the calculated accumulated fatigue wear - 62 % (Fig. 9). 


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Fig. 8. Total accumulated fatigue wear of HWDP 102x8.38 S-135 set, operated while drilling 2 boreholes of one horizontal well to PK1 design horizon - 5.32 % 


<img width="435" alt="Статья_рис9.jpg" src="/upload/medialibrary/d5e/kfb00i4a2fns621vmt52429x2z638d3j.jpg" height="897" title="Статья_рис9.jpg"> 


 

 

 


 Fig. 9. Photo of corrosion-fatigue wear defects of HWDP 102 x 8.38 S-135 with accumulated fatigue wear - 62 %

 


 The operation of drill pipes with such defects is unacceptable, as there is a risk of drill string failure followed by loss of expensive BHA in a wellbore and liquidation of already drilled wellbore [2]. 

 


 To minimize accumulation of corrosion-fatigue wear of drilling tools with respect to conditions where this problem exists, the following complex processing measures should be constantly adhered to: 

 


 1.Application of water-based corrosion inhibitors and oxygen scavengers in low-salinity drilling muds; 

 


 2.Designing well trajectory parameters and maintaining the actual spatial curvature intensity at the lowest possible values; 

 


 3.When drilling multilateral wells, the cutting time for new wellbores should be reduced as much as possible by selecting optimal process equipment in the BHA, mode parameters of cutting, as well as the forecasting of cutting intervals with the most optimal geological conditions; 

 


 Drill pipe during different processing operations in the borehole (drilling, tooling development, forward and back reaming, flushing with rotation) successively pass sections with intensive top-down and bottom-up curvature. 

 


 The next stage of improvement of the implemented drilling tool fatigue calculation technique by our team is automatic digitalization of the process by applying special radio-frequency tags in the drilling tools together with a digital platform that accumulates actual operating parameters with a real-time calculation of accumulated fatigue for each drill pipe. 

 

The authors express their deep gratitude to V.S. Shabrov (ERIELL NEFTEGAZSERVICE LLC), S.A. Goryachev, and E.S. Khayvarin (Meretoyakhaneftegaz LLC) for their significant contribution to the implementation of this engineering approach to drilling string reliability in complicated mining, geological, and process conditions of well construction in Western Siberia.

 

References 

 

 
 1.Rukovodyashchiye ukazaniya po proyektirovaniyu i rezhimam ekspluatatsii elementov burovoy kolonny API RP 7G [Guidelines for the design and operation of drill string elements API RP 7G]. Amerikanskiy institut nefti Publ., Issue 16 izd, avgust 1998, pp. 71–79. (In Russian). 

 

 
 2.Inspektsiya buril'nykh kolonn [Inspection of drill strings]. Stan- dart DS-1 TH HILL ASSOCIATES, INC., Vol. 3, p 294. (In Russian) 

 

 
 3.Korroziya v neftyanoy industrii [Corrosion in the oil indus- try]. (In Russian). Available at: ht<a href="http://www.slb.com/-/media/files/">tps://www</a>.slb<a href="http://www.slb.com/-/media/files/">.com/-/media/files/</a>
 oilfield-review/p04. (accessed: 07.06.2021). 

 

 
 4.Metodika rascheta predela ustalosti buril'nykh trub [Method- ology for calculating the fatigue limit of drill pipes]. «Shankhayskiy nauchno-issledovatel'skiy institut nefteprovodov kompanii Khay long» Publ., 11.12.2019, pp. 2–18 (In Russian).


Published in "Drilling & Oil", issues 7-8 2021