A Robust Torque and Drag Analysis Approach for Well Planning and Drillstring Design

IADC/SPE 39321 A Robust Torque and Drag Analysis Approach for Well Planning and Drillstring Design Opeyemi A. Adewuya, SPE, and Son V. Pham, SPE, Baker Hughes INTEQ Copyright 1998, IADC/SPE Drilling Conference This paper was prepared for presentation at the 1998 IADC/SPE Drilling Conference held in Dallas, Texas 3–6 March 1998. This paper was selected for presentation by an IADC/SPE Program Committee followin g review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the International Association of Drilling Contractors or the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the IADC or SPE, their officers, or members. Papers presented at the IADC/SPE meetings are subject to publication review by Editorial Committees of the IADC and SPE. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract This paper presents a novel torque and drag analysis approach and demonstrates its robustness when used with a versatile computer program. Torque and Drag analysis remains an important evaluation process for assessing drilling feasibility of directional wells, minimizing the occurrence of catastrophic drill string failures and avoiding premature termination of the drilling operation before reaching planned target depth. From a draft well plan, the drilling engineering analysis is initiated with the development of a representative analytical model using selected entries in a Torque and Drag computer program. Several parameters and instances of evaluation are needed to capture the physical behavior of modeled systems and to produce technically sound results. The availability of computational tools have not necessarily improved the drilling engineering process or enhance the quality of recommendations without a methodical approach and application of results. To minimize the iterative steps required to reach an interpretable result, the analytical process as presented in this paper is accelerated with a directed search and a convergence to the determinant drilling variables. The novel approach narrows - the design search domain and tests sensitivities of well-plan characteristics, simulates drilling conditions and applicable drillstring - to the dominant operating factors that determine the boundaries of application. A record extended reach well (MD/TVD ratio of 2.9) with a lateral displacement of approximately 6,000 ft. was drilled in

the GOM using this approach to select tubulars and their position in the well with respect to dogleg severity, inclination and target objectives. Introduction Suppose we define Drilling Mechanics analysis as consisting of a number of well-established activities, including Wellpath planning, Torque and Drag analysis, Drillstring design and the selection of Drilling Systems. The subject of this paper - well-path design and, torque and drag analysis - maintains a strong interest in the petroleum industry. The process of well-path planning and drillstring design for given geological targets are subject to Bottom Hole Assembly (BHA) directional performance, torque and drag analysis, Hydraulics analysis and mechanical strength of drillstring components has seen progressive development, the current surge in Extended Reach Drilling (ERD) operations, Horizontal re-entries and other complex drilling programs is an excellent testimonial. Torque and drag analysis comprises well-path description and drillstring load modeling process aimed at simulating the same mechanics and characteristics of a real-life drilling operation. Torque and Drag analysis is now considered a valuable tool used primarily for design, planning and application screening of drilling and completion systems. However, evolution of successful approaches has been dogged by heuristic concepts and rules of thumb which are not effective when non-linear situations exist or when decisions become sensitive to quantitative measures rather than qualitative indicators. It is our belief that the evident complexity of run-time problems does not permit solutions based only on the experience of the drilling group. The design and troubleshooting ability of those who undertake such analysis should not be limited to historical experiences and performance of the applied drilling system if the proper use of computational tools and methodical approaches enable thorough and concise evaluation of the drilling program. The implementation of model-derived analytical solutions should be brought about by providing a framework for system behavior dynamics to evaluate the possible system states created in the modeling process.

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A robust modeling process is based on the logical representation of system states as functions of interval objectives at the modeling stage, providing solutions for extremely complex interplay of variables without necessarily simplifying the system model. The approach we propose uses available theoretical foundations and analyses, combined with the extensions to conservative criterion offered by practice to arrive quickly at feasible parameters for hole Dogleg Severity (DLS), optimum tubular properties, and scope of drilling feasibility. This paper is presented in two sections, in the first section beginning with Well Planning Considerations we discuss the Torque and Drag implications of the Well-path Trajectory Method used in survey calculations for the well design. Completing this first section is a discussion on the attributes that makes this proposed modeling approach robust and the steps demonstrating its value - minimized iteration time and ease of implementation - using an example well is outlined. In our conclusion we summarize, with emphasis, the most valuable components of that process. Well-planning Considerations Well-path Trajectory Method: Many methods for calculating well-path trajectory have been formulated to represent a suitable plan to reach geological objectives. There are basically six different methods, which have been widely used in the directional drilling applications, are the Tangential, Average Angle, Balanced Tangential, Mercury, Minimum Curvature and Radius of Curvature method. All except for the Tangential method demonstrates relative accurate representation of the well-bore trajectory [16]. Readily available computational tools naturally leads to the use of the more demanding Minimum Curvature Method in order to maximize on survey calculation accuracy. While the variation in survey calculation methods plays a minor role in the overall torque and drag analysis, it does contribute to the overall accuracy and thoroughness of the well-path design. Therefore Minimum Curvature Method is the formulation of choice and is consistently utilized in the well planning process. Constraints Definition and Management: Most engineering systems are designed to operate within specified set of constraints which may be limitations on operating load levels, modes or overall system response. The constraints define the lower and upper bounds of selected design variables and in terms of design performance becomes a yardstick for measuring compliance. To allow for efficient processing of design steps, a mechanism for defining constraint properties at each design stage is necessary [see Figure 1]. 1.Structural (Surface location and Target coordinates) Geophysicists and geologists work together to select take points and target intersection requirements. Candidate surface

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locations are chosen based on proximity, logistic requirements, criteria to maximize slot recovery opportunities and minimization of drilling costs for trajectory and ancillary resources required to complete a well. The choice of surface location relative to target coordinates define the design space for the trajectory of the well. The geometric elements of the well are prescribed by other factors which include drag and allowable curvature for drilling tools in applicable hole size. 2.Geometric specifications: The variables which shape the geometry of directional well plans are Kick-off Point (KOP), Build-up Rate (BUR), hole inclination and casing program. Rehashing what is common knowledge today, have been the subject of much research, it is clear that the depth of kick-off has a significant contribution on the torque and drag characteristics and horizontal reach of a well. Build-up rates are a matter of connecting points along the wellbore to intersect target coordinates, but the choice of an optimal BUR is determined by hole size, drilling tool capability, anticipated drag effects and an over-all evaluation of the drilling objectives. 3.Casing Program: The casing design process requires the selection of a casing program to meet at the minimum design requirements such as imposed mechanical stress (hoop, radial and tri-axial) and loads (burst, collapse, tensile) among other prerequisites which include estimated life-cycle of well, future re-entry work, formation isolation and casing wear tolerance. Strategic casing placement to extend drilling assembly performance, although an opportunity cost issue, can be justified by using the example well presented later in this paper. For the work on which this paper is based, the casing program was specified for inclusion in the well-plan. 4.Geological obstacles: Crooked well-paths or 3-D trajectories are not wellprofiles of choice. Furtive views of local geology obtained from seismic data provides information on enroute geological obstacles such as sensitive shales, unstable sandstone stringers, dips, faults and the prominent water or gas sands subtended by the oil bearing reservoir. 5.Drilling system operational compatibility: From an automated well design tool, BUR necessary to connect geometric markers (End of Build, End of Hold, etc.) is obtained routinely, optimization of the well-design is achieved when consideration is given to the interval hole size and applicable performance drilling system. Top hole sections necessarily are large holes requiring the use of large diameter tools. The mechanical constraints of the large diameter tools limits the degree of curvature that can be used in the top hole section. In addition, the lower bending capability leads to high lateral loads and the attendant drag and torque effect. The use of collar-based Measurement While Drilling (MWD) tools introduces even greater rigidity which places further limitation on planned well-bore curvature. Conventionally, most top hole sections are drilled

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A ROBUST TORQUE AND DRAG APPROACH FOR WELL PLANNING AND DRILLSTRING DESIGN

vertically to a selected kick-off depth, to allow drilling large hole sections and setting conductor casing. However when trajectory efficiency requires early directional work, relatively smaller hole size i.e. 12-1/4” can be drilled out of large casing, enabling the use of higher BUR. When the interval TD is reached, the hole is opened up to 17-1/2”, as was done in the example well, to accommodate a 13-3/8” casing string. In smaller hole sizes, BUR ranges cover a wider spectrum allowing flexibility in trajectory geometric properties. At the high end of this wide spectrum is a drilling assembly limitation posed by push-through radius. That discussion is beyond the scope of this work. So far we have discussed constraints as defined in the preamble to this subsection, suppose a constraint were to be used to advantage, for instance, designing a well path to maximize drilling assembly rotation and exploiting the drop tendency of the drilling assembly from gravitational effects to track the well into the target location. Drillstring Torque and Drag Modeling and Design Drilling engineering algorithm developers are constantly striving to produce sophisticated computational engines from mathematical representations of drill string dynamics which offer greater accuracy and more realistic results. While the computational engines improves, the results produced are more intricate and refined. The impressive developments in areas such as trajectory simulation are to be immensely appreciated but each step brings its own problems for the enduser. Software Tools: Robust analysis of modeled multivariate systems require considerable computational processing before meaningful results are obtained. The Torque and Drag analysis tool used in this work is one of the seven module suite of Baker Hughes INTEQ proprietary drilling engineering software tools. In the Torque and Drag calculation mode the software computes the surface-to-bit load, stress and lateral force information for rotary and oriented drilling operations at userspecified evaluation depths. Operating load cases including magnitude, location and mode of occurrence (e.g. drilling, rotating-on-bottom, tripping, etc.) The computational engine allows fast and rigorous engineering mechanics analysis of the modeled well-trajectory and casing configuration, drillstring and drilling parameters, based on a continuous elastic beam column theory. From the vast array of state-of-the-art analytical solutions, the relevant solutions for Euler, sinusoidal, helical buckling and post buckling behavior, drillstring torsion and load displacement hysteresis in buckling mode transition was the focus in this application [2]. Availability of computational tools facilitate fast and accurate iterations which will naturally be incorporated into drillstring optimization processes. Well-plan Drillstring Optimization - Supplementary Issues:

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In Extended Reach directional wells, what remains a protracted optimization issue is not simply DLS minimization but the effect of the inter-play between inclination, azimuthal change, drag and buckling. Micro-loading studies into sensitivities of drillstring to varying well-profile pursue the quantitative indexing of dominant load factors towards achieving optimization. As reported by Payne and Abbassian [4], critical well-bore inclination i.e. angle at which pipe no longer falls at own weight, is one of the several factors that shape ERD well-bore design [see Figure 2]. Logically, lower inclination angles produces less drag, but lacks the well-bore support (cradling effect) needed to manage the severity of buckling. An interesting observation also presented by Payne and Abbassian though empirical, identifies the sensitivity of hole inclination to type of operation and briefly stated, a high KOP well profile is favorable to a 121/4” hole by 9-5/8” casing/coiled tubing run, while a low KOP well profile is preferred for an 8-1/2” hole by 5-1/2” liner/pipe runs. Steering in well-bores with azimuthal and inclination changes combined with long tangent sections present a challenge to the transmission of mechanical forces. Precise orientation of tool-face in the presence of significant torque couples (normal/contact force, circular frictional drag) aggravates the uncertainty of heading and achieving geometric drilling objectives. Process for achieving Analytical Robustness with Well-path and Torque and Drag Modeling Well-path Modeling: To account correctly for the degree of variation of dogleg severity in finite course lengths two approaches was examined by the authors. One approach uses a user-specified maximum relative noise amplitude based on a scale of 0 - 10 to produce random net well-path tortuosity nominally ranging from 0.0 2.0 deg/100 ft. [1], while the definition given by Dr. Rapier Dawson suggest the correction of the well-path by the addition of a sinusoidal variation to the inclination and azimuth angle over 1,000 ft. course lengths [17]. Different methods of applying tortuosity to a well-plan may result in the same average dogleg severity [see Figure 3] but from our observation, of the drilling operation, applying a random noise factor is more representative of a tortuous well path compared to a cyclic factor applied by the tortuosity equation [see Figure 3, Equation 3-A] The Tubular Buckling Theories Compared: The available theoretical foundations on which tubular buckling has been developed can be grouped into two categories, namely conservative and extended models. The models that can be classified as conservative criteria consists of the combined work of Lubinski, Dawson/Paslay, Chen/Cheatham and, Sextro and He/Kylinstad. The recent developments by Wu/Juvkam-Wold qualifies as an extended criteria model [1]. The differences between the

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two classes of criterion is enumerated in terms of scope of application and impact on modeling. Conservative Theories: Critical buckling loads predicted by the Dawson/Paslay equation are much lower than actual or operating critical loads. In addition, the equation represents the mechanical behavior of long finite tubular elements and produces erroneous results for short elements [15]. The critical buckling load limits predicted by Chen indicated a 40% increase in load during sinusoidal to helical buckling transition and an 18% increase in the magnitude of critical load required to initiate buckling. Extended Criteria Theories: A full understanding of the premises on which the buckling theories proposed by Wu and Juvkam-Wold is important to recognizing their relevance to resulting load behavior of modeled drilling assemblies. Directional wells with long tangent sections and hole inclination approaching critical angles with respect to friction are stereotypical of the parameters which validate the suitability of the extended buckling theories. At low inclination angles where the contribution of tubular weight to axial compressive force is greatest, the Wu and Juvkam-Wold buckling equation [12] suffices with the critical length and axial load term. The He & Kyllinstad work contributed the effect of wellbore curvature to the development of mathematical basis for assessment of critical buckling loads. In essence, the normal forces due to curvature as an additional resistance modulus is added to the force term [15]. A broad comparison of the two classes of criteria can be summarized in terms of common factors namely the normal force and the stiffness terms. Invoking the conservative buckling criteria assesses buckling loads based a quotient of unit stiffness and normal force, while the extended buckling criteria assumes higher indices for modifiers to stiffness and normal force terms. In summary, the reason for enumerating the differences between the conservative and extended buckling assessment approaches is to draw our attention to the quantitative quality of analytical work based on these models. In practice, factors such as hole friction, wellbore inclination and curvature affect the initiation of buckling and un-buckling discriminatorily contributes to the torque and drag analysis. Extended-reach wells by virtue of design and required tubular configuration manifest loads at higher thresholds and are best analyzed with models based on extended criteria. Frequent occurrences of drillstring failures or completion string collapse would have dogged ERD save that there are favorable interplay of influencing factors which make current theories poor predictors. The Genealogy of a Robust Torque and Drag Modeling Approach Multiple Analytical stations: Traditionally the drillstring design process tended to focus on meeting minimum safety requirements in the string, for example design based on mechanical ratings, size, drilling mode, casing points and relative component function. Also,

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emphasis was placed on drillstring applicable only at TD, whereas in most cases stations such as KOP, casing points, whipstock exits and build-turn sections present greater drilling challenges. A common assumption is that the analysis at TD of the well-plan will yield the limiting parameters for the drilling applications of the entire well-path - which neglects the varying tool size utilized and changing geometry of the wellbore. Due to the weight and complex load bearing characteristics of different size drillstring components it is necessary to perform computational analysis for each hole interval to better understand and optimize on the wellbore/drillstring interactions. Correct interpretation of the drilling program enables effective drilling mechanics analysis of the drillstring and the quality of results approach close approximations. Reflective of Actual / Changing Hole Conditions: The initial torque and drag modeling allows us to systematically develop a thorough understanding of the interaction between well-bore, drillstring components and operational parameters [see Figure 4]. By discretely selecting modeling stations or evaluation intervals, drilling parameters (ROP, WOB, RPM) that best describe hole conditions (lithology, temperature, hole cleaning, mud properties, etc.) can be applied for a representative model that approaches actual drilling condition. It also enables narrowing the drillstring design search domain and improves ability to test sensitivities of drillstring well-bore interaction in the following modes: rotary drilling, slide drilling and tripping. By closely evaluating the different drilling modes we can determine safe drilling limits. Operational limits consist of the applicable WOB without buckling the drillstring, tripping capabilities and frictional tolerences. On a scale of significance, friction factor and WOB are dominant contributions to the torque and drag effects on drillstring application. Trade-Offs: Traditionally, the objective of Heavy Weight Drillpipe (HWDP) application is to contribute to string weight as a mean to transfer weight to the bit. However well curvature impose a limit on the functional relevance of the HWDP. String weight in the curve produces greater normal loads and contact forces. As hole friction changes, the ability to maximize HWDP functional performance is affected by the rate at which a compressive state or a tension state is approached or maintained. In cased hole, when friction becomes a property of the contacting materials and imposed loads, the function of HWDP can be exploited to a higher degree. A secondary mechanical characteristics of HWDP is the inherent capacity to withstand relatively higher compressive loads [12]. By strategically utilizing this load characteristics of HWDP we can meet complex and challenging drilling objectives which would not otherwise be successful with

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normal drillpipe application. Optimization requires an evaluation of the load and drag distribution of the well based on the selected drillstring. In addition, optimal use of HWDP requires correct assessment of required length, location in the borehole and balance of performance in mitigating buckling while maximizing transmission of the weight to bit. In drilling work of horizontal wells with long laterals, the effective application of HWDP is becoming more of a science than a convention. This emerging functional use of HWDP for sustainable transfer of weight to the bit is becoming critical to achieving lateral length target displacements and reach target depth. The “inverted drillstring” configuration is now an established arrangement of drillstring assemblies. An inverted drillstring arrangement places the HWDP above regular drillpipe. The common belief supported by static force analysis of weight-derived axial force indicates that half the amount of this force is available at hole inclinations greater than 60° i.e. the weight of HWDP element x cos(60) ≡ weight of HWDP x 0.5. Although this guideline is generally acceptable for noncritical applications advance well-bore construction requires methodical computational drillstring analysis which takes into account friction factor, trip analysis, WOB and other drilling optimization and constraining parameters. Presentation of Model Analysis: Graphical representation and summary tables simplifies complex data sets for quick and accurate interpretations and serve as an invaluable communications tool. The extensive knowledge captured from the modeling process needs to be communicated to all team members. When used as a monitoring or look ahead tool on the field, deviation from predicted outcome can be flagged early and corrective measures taken. In the next section the application of this approach on the field is discussed using the example well. Logical presentation of data allows the operational team to easily and quickly assemble feedback information facilitating easy understanding of complex relationships between modeled and output variables. Execution of results and recommendations is straightforward and less prone to misinterpretation by field or implementation staff because of the graphical highlights that limit additional processing. Example Step-through Modeling Process This methodology was first used in an extended reach well with a MD/TVD ratio of 2.9 and a lateral displacement of 6000 ft. In this section the application of the components of the robust modeling thesis enumerated thus far as it applies to the different phases involved in the design and eventual successful drilling of the well is presented. Wellpath Planning of Example Well Preliminary well design requirements was developed by a

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multidisciplinary team composed of the operator and service personnel. The example well [see Figure 5] consist of a 20” drivepipe set at ± 300 ft., an initial drill-out 12-1/4” hole kicks-off beginning at 3°/100 ft. and end-of-build reached at 1,000 ft.,MD with final heading of 341.23°. The 12-1/4” hole is reentered and opened to a 17-1/2” conductor hole to be drilled with a 5°/100 ft. build rate, building to a 40° inclination at 1,500 ft.,MD. Beyond the planned 13-3/8” conductor set depth, curve building would be continued at 5°/100 ft. to an intermediate end-of-build inclination of 83° at 2,373 ft.,MD. The 83° inclination is held to the end-of-hold depth at 6,317 ft.,MD. A two section drop was designed to intersect a target sand for which the complete coordinates (orientation and depth) definition was unknown. The two section drop would facilitate a slide and search drilling operation and a controlled drop rate of 1.5°/100 ft. to reach the bottom hole location. Fit for Purpose Well Design: the combination build-rate of 3°/100 ft. and 5°/100 ft. used in the kick-off after drivepipe is installed was chosen after careful evaluation of its potential torque and drag implications. The curvature produced by the strategically chosen combination build rates is intended to provide a less aggressive trajectory thereby reducing normal forces and lateral loads that affect the drag distribution in the bore-hole drillstring interface. The magnitude of build-up rate used in curved sections follow a scheme that locates the smaller BUR, 3°/100 ft. at the beginning of the curve section and the larger BUR, 5°/100 ft. at the end of the curve. By following this scheme the ability to maintain WOB and stable string is ensured and lateral load in the curved section of the hole is evenly distributed. Surface Casing Location: the choice of set depth for the surface casing was informed by the following reasons: • provide a cased hole to place HWDP for effective transmission of WOB in the drilling of the target. • provide a reduced friction channel to minimize drag and torque. • to enable rotation of the drillstring in the tangent section and smooth bore-hole with less dogleg severity and the ability to drill to target depth. • introduce a significant hole-cleaning advantage is offered by placing the surface casing about midway of the tangent section, since half the section is cased-off, the hole cleaning requirement for the tangent section is halved. Production Hole Design for Maximum Rotary drilling and expected drop tendency: this hole section consist of a long tangent section and a drop of inclination in the end in order to search for the target sand. Both the tangent and angle drop section allows us to employ the natural tendency of the drilling assembly and maximize drilling in the rotary mode. The challenging aspect of ERD is the ability to place sufficient WOB in the sliding mode in order to maintain directional control. By planning on the natural drop tendency of the drilling assembly we can minimize the need for slide

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drilling and therefore maximizing operational success. Torque and Drag Analytical Stations: The analysis for the example well was performed for all hole intervals (surface, protective and production). For each scenario defined by well-bore design, select BHA and drillstring, drilling parameters and imposed loads, the analyst must model all possible configurations and must take into account the combination of several interacting or related factors. The reality of such undertaken is that modeling is a study of non-deterministic events, a phenomenon amplified by the number of cases required to test the influence of each factor. Unarguably, a guided search to test sensitivities of factors is indispensable, providing a precise experimental delineation to reduce the number of iterations. Due to space constraints we will only use the analysis of the TD point of the production interval to highlight the robust methodology. The selection of the 8-1/2” production interval clearly demonstrates slide drilling and tripping concern inherent in all ERD. For this cycle of evaluation, we will closely scrutinize the results in the form of Summary Data Tables and the Drilling, Tripping and Frictional Sensitivity Analysis. As will be demonstrated, the format of the data presentation leads to a thorough and logical interpretation of the modeled results. Drilling Sensitivity Analysis: in this scenario we will isolate WOB to determine its affect on the drillstring during the drilling of specific intervals. The modeling consist of varying the WOB while constraining to the same frictional factor, drilling assembly and other rig parameters. Since the operation calls for the use of a water based fluid system the frictional values of .25 and .30 was chosen, based on a historical database, for casing and open-hole sections respectively. The model results will therefore lead to an operational WOB boundary based on a realistic frictional estimation. Selection of WOB is based on tools specifications as well as operational parameters. The operational WOB expected for an 8-1/2” hole section will range from 0-25 klbs. The modeling take points will analyzed at 0, 15, 25, and 50 klbs WOB in order to view the dynamic condition reflective of the operational performance. We will now interpreted the actaul data grouped in a table and graphical format. The result summary table allows us to easily compare the models results with the specification of the 5” drillpipe. The initial analysis was performed on a drillstring consisting solely of drillpipe [see Table 1]. We can quickly learn that any WOB above 10 klbs will result in a negative Hook Load at surface and enter into the helical buckling regime at the top 500 ft. of the well-bore, as seen from the graphical representation [see Figure 6]. A comparison of the analysis using the preliminary versus the same sensitivity analysis of the modified drillstring [see Table 2] will demonstrate the value of the simple method in integrating complex variables. The placement 4,000 ft. of HWDP in the modified drillstring was not derived from only the Drilling Sensitivity

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Analysis but the Tripping Sensitivity Analysis was a major contributing factor. Tripping Sensitivity Analysis: these sets of analysis uses similar parameters as set for the Drilling Sensitivity Analysis which was performed using the preliminary drillstring. The main objective for this type of analysis to determine the location and amount of HWDP needed (if any) in order to trip to TD while still having sufficient WOB available to overcome any ledges. The graphical results [see Figure 7] indicates, by interpolating between the 0 and 15 klbs trip curve, that any ledge requiring WOB over approximately 5 klbs cannot be applied with the current drillstring. Contingency plan requires the force of at least 15 klbs for any ledges encounter during the drilling process and therefore modifications must be made to the preliminary drillstring. The information capture from the trip analysis lead to the replacement of the drillpipe with 4,000 ft. of HWDP at the top section of the preliminary drillstring. Subsequently, the selection of the amount of HWDP leads to the strategic decision to set the 13-3/8” casing string at 4,000 ft.,MD to encompass the heavier weight drillpipe in a stable will-bore and therefore reducing the overall torque and drag affects. Friction Sensitivity Analysis: this final sensitivity analysis completes the torque and drag evaluation of the 8-1/2” production hole interval. Determining the tolerable frictional operating range assist in making the decision to incorporate the type of mud system, lubricious additives, drillpipe rubbers, hole cleaning equipment, stringent fluid parameters, etc. into the drilling program. The graphical results in this case [see Figure 8] shows that adequate load transfer and helical buckling can be feasibly mitigated by modification of the drillstring design rather than upgrade to the more costly oil-based mud system. The economical decision, from a frictional perspective, is a contingency plan to incorporate the use of the water-based fluid system with a stringent solids removal program and the addition of lubricious additives as contingency. Interpretation and Field Implementation: In recent publications and papers it has been implicitly expressed that there are discrepancies in the analytical results and recommendations put forward by drilling service companies, and the expectations of operators during crucial drilling operations or at planning stages. Drilling services companies share objectives for success of drilling projects and would undertake a rigorous drilling mechanics evaluation for an mechanical application match of the prescribed drilling system. The procedure developed to integrate well planning, drillstring design and torque and drag analysis have demonstrates is fruitfulness through field use and observation. On the technical merit, we have seen the torque, drag, and frictional trends distinctly matched with the modeling results. In cases where the trends diverge or differ parameters different from the ones chosen for modeling were identified and accounted for in changes made to the operational parameters.

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A secondary merit of the modeling process in the field communications and implementations. Through the format of the result presentation, explanation and support of the design parameters were clearly understood and plans were carried out as specified. Slide Drilling Limitations Leading Towards State-of-theart Drilling Technology: The frictional drag in the drill ahead direction in the wellbore relative to the string poses a limitation to the ability to slide. The severity of this frictional drag is dependent on wellbore profile, traversed formation type and bore-hole geometry. We can observe the tremendous reactionary load difference between the sliding and rotating drilling model [see Table 1 & 2]. Recently, torque reducers have seen prolific use in solving the problem by isolating would-be contact points between the tool-joints/drill-stem and the well-bore. Torque reducers can be defined as active if they rotate relative to drillpipe or passive if non-rotating. However, in ERD wells the severity of frictional drag is such that contact points become pseudo-fixed points along the drill-string producing increasing sensitivity to WOB. The following approaches have been touted as successful antidotes for excessive drag, • Increased mud lubricity • Low friction drill-pipe protectors • Running DC or Heavy-Weight Drill-Pipe (HWDP) in near vertical well sections • Boost weight transfer with Bumpers, and Thrusters for smooth WOB application • Use extended or double-power section motors to increase stalling resistance. And, recently Rotary Closed Loop Drilling Systems , an advancement over the Variable Gauge Stabilizer emerged as a panacea for overcoming critical drag limitation in ERD wells. Otherwise, drilling mechanics practitioners emphasize qualifying drillstrings and well-sections for rotation, that may otherwise present drag limitation. Conclusion A novel well planning and torque and drag analysis approach and demonstrates its robustness when used with a versatile computer program. The value of using a methodical procedure in the evaluation of a drilling program can clearly appreciated through: ♦ Application of the state-of-the-art theories and computational algorithms ♦ Incorporating the dynamics of the field operation into the planning and modeling process by carrying out drilling, tripping and frictional sensitivity analysis ♦ Multiple points of analysis ensures a thorough and precise understanding of well-bore/drillstring interactions from surface to TD ♦ Advance deployment of HWDP for efficient weight transfer to bit and integration into the drilling

♦ ♦ ♦

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assembly as a load bearing member to mitigate drillstring helical buckling Logical and simple presentation of data through tabular and graphical summaries to represent complex modeling systems Useful communications tools to be incorporated into the drilling program for precise field implementation and appreciation of model optimization Refinement and proven through field usage

Acknowledgments The authors wish to thank the respective management of Baker Hughes INTEQ for permission to prepare and publish this paper. The support of the following people are gratefully acknowledged during the initial stages and final preparation of this work: Thomas Dahl, Steve Dearman, Keith Fisher, David Gaudin, Spencer Harris, Pat Havard, Raymond Jackson and Les Shale. References 1. 2. 3. 4. 5. 6. 7. 8. 9.

10. 11. 12.

13.

Baker Hughes INTEQ Torque and Drag v.4.1. Program, User’s Guide Baker Hughes INTEQ, “Drilling Engineering Software v.3.20, ” Marketing Documentation Batchelor, B. J., and Moyer, M. C., “Selection and Drilling of Recent Gulf of Mexico Horizontal Wells,” OTC 8462 (May 1997) Payne, M. L., and Abbassian, F., “Advanced Torque-and Drag Considerations in Extended-Reach Wells,” SPE 35102 (March 1996) Ruddy, K. E., and Hill, D., “Analysis of BuoyancyAssisted Casings and Liners in Mega-Reach Wells,” IADC/SPE 23878 (February 1992) Guild, G. J., Hill T. H., and Summers, M. A., “Designing and Drilling Extended Reach Wells, Part 2 , Petroleum Engineer International (January 1995) McKown, G. K., “Drillstring Design Optimization for High-Angle Wells,” SPE/IADC 18650 (February 1989) Maurer Engineering Inc., “Horizontal Technology Manual - DEA 44 ” September 1994 Payne, M. L., Duxbury, J. K., and Martin, J. W., “Drillstring Design Options for Extended-Reach Drilling Operations,” PD-Vol. 65, Drilling Technology, ASME ETCE, 1995 Callin, J. K., and Hatton, P., “Drillstring Considerations and BHA Design for Horizontal Wells,” Internal Eastman Christensen Paper(now Baker Hughes INTEQ) Chen, Y. C., Lin, Y. H., and Cheatham, J. B., “Tubing and Casing Buckling in Horizontal Wells,” JPT, p140191, February 1990 Morris, E. R., “ Heavy Wall Drill Pipe A Key Member of the Drill Stem,” Presented at the Joint Petroleum Mechanical Engineering and Pressure Vessels and Piping Conference, Mexico City, Mexico, September, 1976 Wu, J. and Juvkam-Wold, H. C., “Buckling and Lockup

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YEMI ADEWUYA, SON PHAM

of Tubulars in Inclined Wellbores,” PD-Vol. 56, Drilling Technology, ASME ETCE, 1994 14. Brett, J. F., Beckett, A. D., Holt, C. A., and Smith, D. L. “Uses and Limitations of a Drillstring Tension and Torque Model to Monitor Hole Conditions,” SPE 16664, September 1994 15. McCann, R. C. and Suryanarayana, P. V. R., “Experimental Study of Curvature and Frictional Effects on Buckling,” OTC 7568 16. API Bulletin D20, “Directional Drilling Survey Calculation Methods and Terminology,” American Table 1: Result Summary - Analysis of Preliminary Drillstring Table 2: Result Summary - Analysis of Optimized Drillstring

Figure 1: Well Planning and Engineering Analysis Procedure Figure 2: Critical Inclination Curve - Simple Static Analysis Figure 3: Tortuosity Comparison Chart Figure 4: Torque and Drag Analysis Procedure Figure 5: Plot of Plan vs. Actual Well-path Figure 6: Torque and Drag - Drilling Sensitivity Analysis Figure 7: Torque and Drag - Tripping Sensitivity Analysis Figure 8: Torque and Drag - Frictional Sensitivity Analysis

IADC/SPE 39321

Petroleum Institute, December 1985 17. Maurer Engineering “DDRAG8 Torque and Drag User’s Manual 18. Hill, T. H., Summers, M. A., and Guild, G. J. , “Designing and Qualifying Drillstrings for ExtendedReach Drilling,” SPE DRILLING AND COMPLETION, June 1996, Vol. II, No. 2, Pg. 111-117 19. Arora, J. S., “Introduction to Optimum Design,” McGraw-Hill, Inc., 1989

Table 1: Result Summary - Analysis of Preliminary Drillstring

RESULTS

5" S-135 19.5# DP Premium (NC50) - API RP7G

-----

REFERENCE

Friction Factors [csg/oh] Weight on Bit [klbs] Max. Tot. Eqv. Stress (MTES) [psi] Location of MTES [ft, MD] Mode of MTES Yield Stress [psi] Torque - Drilling [ft-lbf] Torque - Rot-Off-Bot. [lbf] Make-Up Torque [ft-lbf] Torsional Yield [ft-lbf] Hook Load - Drilling [lbf] Hook Load - Rot. Off Bot. [lbf] Hook Load - Pick-Up [lbf] Hook Load - Slack-Off [lbf] Max. Allow. Hk Ld @ Min. Yld [lbf] Neutral Point [ft, MD from bit]

--------------------135,000 --------24,645 63,406 ----------------560,764 -----

-----------------------------------------------------------------

8-1/2" Hole Size: 5"DP to Surface

ORIENTED

ROTARY=100rpm

ORIENTED

.25/.30 .25/.30 .25/.30 .25/.30 .25/.30 .25/.30 .25/.30 .25/.30 .35/.40 .45/.50 0 15 25 50 0 15 25 50 25 25 28,044 28,992 30,490 41,126 32,988 31,646 31,458 34,736 34,584 45,021 1,060 1,960 1,630 800 650 530 530 1,210 1,060 90 Pick-up Drilling Drilling Drilling Drilling Drilling Drilling Drilling Pick-up Drilling ----------------------------------------0 708 1,181 2,361 10,335 10,193 10,722 13,943 1,181 1,181 10,360 10,360 10,360 10,360 10,360 10,360 10,360 10,360 14,066 17,772 --------------------------------------------------------------------------------15,322 -4,483 -18,608 -67,353 53,735 38,764 28,762 3,690 -49,464 -99,771 54,091 54,091 54,091 54,091 54,091 54,091 54,091 54,091 54,091 54,091 107,959 107,959 107,959 107,959 107,959 107,959 107,959 107,959 139,324 180,089 15,322 15,322 15,322 15,322 15,322 15,322 15,322 15,322 -6,824 -39,028 ----------------------------------------0 7,659 7,659 7,659 0 3,533 6,066 7,466 7,659 7,659

Table 2: Result Summary - Analysis of Optimized Drillstring

RESULTS

5" K-55 49.3# HWDP (NC50) - Drilco Handbook

8-1/2" Hole Ssize: 5"DP to 9-7/8" Casing Shoe and 5"HWDP to Surface

5" S-135 19.5# DP Premium (NC50) - API RP7G

REFERENCE

Friction Factors [csg/oh] Weight on Bit [klbs] Max. Tot. Eqv. Stress (MTES) [psi] Location of MTES [ft, MD] Mode of MTES Yield Stress [psi] Torque - Drilling [ft-lbf] Torque - Rot-Off-Bot. [lbf] Make-Up Torque [ft-lbf] Torsional Yield [ft-lbf] Hook Load - Drilling [lbf] Hook Load - Rot. Off Bot. [lbf] Hook Load - Pick-Up [lbf] Hook Load - Slack-Off [lbf] Max. Allow. Hk Ld @ Min. Yld [lbf] Neutral Point [ft, MD from bit]

--------------------135,000 --------24,645 63,406 ----------------560,764 -----

----.25/.30 .25/.30 .25/.30 .25/.30 .25/.30 .25/.30 .25/.30 .25/.30 .35/.40 .45/.50 ----0 15 25 50 0 15 25 50 25 25 ----26,183 26,796 27,478 35,219 28,275 27,949 28,207 32,626 28,006 28,614 ----6,797 4,323 4,323 4,563 4,053 4,053 4,053 7,458 4,323 4,323 ----Drilling Drilling Drilling Drilling Drilling Drilling Drilling Drilling Drilling Drilling 55,000 --------------------------------------------0 708 1,181 2,361 13,877 14,063 14,480 14,291 1,181 1,181 ----13,889 13,889 13,889 13,889 13,889 13,889 13,889 13,889 19,008 24,126 29,400 ----------------------------------------51,375 --------------------------------------------45,305 26,547 13,124 -26,884 99,227 84,245 74,246 49,202 -23,972 -77,286 ----99,717 99,717 99,717 99,717 99,717 99,717 99,717 99,717 99,717 99,717 ----165,662 165,662 165,662 165,662 165,662 165,662 165,662 165,662 204,021 253,452 ----45,305 45,305 45,305 45,305 45,305 45,305 45,305 45,305 15,085 -26,911 691,185 --------------------------------------------0 7,019 7,344 7,659 0 3,534 5,544 6,463 7,659 7,659

Note:

ORIENTED

Critical results Reference limits and specifications

ROTARY=100rpm

ORIENTED

Figure 1: Well Planning and Engineering Analysis Flowchart Preliminary Well-Path Design

Constraints Definition - Surface Location and Target Coordinates - Rig Specifications - Geological Specifications and Obstacles - Drilling Systems Operational Compatibility

Anti-Collision Anaysis

No

Initial Well-Path Approval

Completions Program

Yes

Fluids Program

Preliminary Drillstring Design

Bit Program Hydraulics Analysis

Torque & Drag Anaysis No Optimized Drillstring Design

Optimal Well-Path and Drillstring Design Collect Useful Drilling Parameters Torque (surface, down-hole), Weight On Bit (surface, down-hole), Friction Factor (caing, open-hole), Drag (slack-off, pick-up), Rate of Penetration, Rotary Speed, Pump Rates, Fluids Properties, Bit Performance, etc.

Yes Develop Drilling Program

Perform Drilling Operation

Figure 4: Torque and Drag Analysis Flowchart Preliminary Drillstring Design

A thorough evaluation of the drilling program will include this cycle of analysis for each hole interval

Torque & Drag Anaysis

Drilling Sensitivity Anaysis

Oriented Drilling - WOB #1 (No Load) - WOB #2 (Low Oper. Load) - WOB #3 (High Oper. Load) - WOB #4 (Max. Load)

Drill-String Component Limitations and Specifications

Rotary Drilling - WOB #1 (No Load) - WOB #2 (Low Oper. Load) - WOB #3 (High Oper. Load) - WOB #4 (Max. Load)

Tripping Sensitivity Anaysis - WOB #1 (No Load) - WOB #2 (Low Oper. Load) - WOB #3 (High Oper. Load) - WOB #4 (Max. Load)

Evaluation of Summary and Graphical Results

Drillstring Optimization

Friction Factor Sensitivity Anaysis - Friction Factor #1 (Expected) - Friction Factor #2 (High) - Friction Factor #3 (Problematic)

Figure 2: Critical Inclination Curve - Simple Static Anaysis 1.00

Summing forces in the X direction yields the following simple relationship:

0.90

FA

Ff

θ = Inclination [deg] µ = Friction Factor

FN =µ

where:

0.70

+

X

 1 θ = atan   µ

0.80

Friction Factor

Y

θ

FN

FY

0.60

FW

0.50

FX

FA

0.40 0.30 0.20 0.10 0.00 45

50

55

60

65 70 Critical Inclination [deg]

75

80

85

90

Figure 5: Plot of Plan vs. Actual Well-Path 0 Planned

Actual

7,000 Planned TD

20" Drive Pipe @ 375'md

6,000

Actual TD

5,000 4,000 3,000

12-1/4" x 17-1/2" Hole Size

Depth [ft,TVD]

1,000

2,000