Buildup Section Research Articles

Downhole vibration characteristics of 3D curved well under drillstring–bit–rock interaction

Understanding the characteristics of downhole vibration helps optimize the drilling parameters and improve the rate of penetration (ROP). This work establishes a nonlinear drillstring–bit–rock model to predict the downhole vibration behavior of 3D curved wells. The main novelty of this model is to consider the complex nonlinear bit–rock interaction and well trajectory. The model is solved by the Newmark method and verified by an indoor experiment and field data. The drillstring dynamics and effects of the main working parameters on the vibration characteristics are obtained and analyzed. Results show that, in the horizontal section, the drillstring motion and shape deformation are stable, and radial vibration is dominant. In the buildup section, the drillstring motion is irregular and messy, shape deformation becomes large, and radial vibration is significant but tangential vibration starts to become intense. In the vertical section, the drillstring motion is small in wellbore center, shape deformation is stable, and vibration severity reduces. The weight on bit (WOB) only affects drillstring vibration near the bit, and radial, tangential, and axial accelerations intensify as WOB increases. RPM has little effect on the drillstring vibration in 3D curved wells. In the buildup and horizontal sections, as the coefficient of friction (COF) increases, the radial and tangential accelerations rise while the axial vibration obviously weakens. When drilling in 3D curved wells, small WOB and large RPM can be adopted to reduce downhole vibration and increase ROP. The safety of drillstring and downhole tool also needs more attention when drilling in formations with large COF, such as gravel stone and sandstone. Large oil/water ratio or lubricant is recommended to improve the lubricity of drilling fluid. The research results could provide a valuable and useful guidance for drilling engineers to improve ROP and reduce downhole accidents.

Technology Of Drilling Of Slant Well On The Field Of Western Siberia

The technology of drilling slant exploitation well in the oilfield of Western Siberia has been studied. Development of well construction has taken into account the next geological features of the section: permafrost rocks occur to the depth of 260 m, and iciness also takes place. Well construction is selected on the base of intervals of incompatible drilling conditions. The designed profile includes several intervals: one vertical interval, one zenith angle buildup section, two hold sections and one zenith angle drop section. A check calculation of the setting depth of the intermediate casing for the condition of preventing hydraulic fracturing of rocks near the shoe was carried out, and the safety factor of rocks for hydraulic fracturing was found. Drill pipes calculation includes determination of their length and diameter as well as allowable setting depth for drill pipes. This article demonstrates drilling methods, borehole under-reaming methods and the main parameters for the performance of technological operations of directional drilling. A drilling rig has been selected according to the allowable hook load. The density of the used drilling muds has been substantiated for the intervals of compatible drilling conditions, types and parameters of muds.

Well Trajectory Measurement Error Based on Gyro Survey

Gyroscopic survey technology is one of the key technologies to promote the progress of directional drilling technology. It is of great practical significance to study the error of well trajectory measurement based on gyro surveys and identify the main error sources. This paper introduces the error sources which lead to the uncertainty of borehole position in gyroscopic survey, six gyroscopic survey error models, and the calculation model of the contribution rate of the error sources to the error ellipse. The differences between gyro and MWD survey errors and the main error sources of gyro surveys are investigated with an example. The results show that gyroscopic surveys cannot reduce the uncertainty of borehole position compared to MWD surveys in the environment without magnetic interference. The static measurement by a three-axis gyro inclinometer is more beneficial for the improvement of the measurement accuracy of the borehole position. The error sources that contribute to the measurement error of the well trajectory are also different in the vertical section, build-up section, and horizontal section. Unlike the main error source of the MWD survey, the g-dependent error sources of the gyro sensor significantly contribute to the measurement error of the borehole trajectory. Therefore, the measurement error of the well trajectory can be significantly reduced by improving the gyroscope survey tool and reducing the influence of spin mass unbalance.

Stability analysis of seabed strata and casing structure during the natural gas hydrates exploitation by depressurization in horizontal wells in South China Sea

Natural gas hydrates (NGHs) are a new type of clean energy with great development potential. However, it is urgent to achieve safe and economical NGHs development and utilization. This study established a physical model of the study area using the FLAC^3D software based on the key parameters of the NGHs production test area in the South China Sea, including the depressurization method, and mechanical parameters of strata, NGHs occurrence characteristics, and the technological characteristics of horizontal wells. Moreover, this study explored the law of influences of the NGHs dissociation range on the stability of the overburden strata and the casing structure of a horizontal well. The results are as follows. With the dissociation of NGHs, the overburden strata of the NGHs dissociation zone subsided and formed funnel-shaped zones and then gradually stabilized. However, the upper interface of the NGHs dissociation zone showed significant redistribution and discontinuity of stress. Specifically, distinct stress concentration and corresponding large deformation occurred in the build-up section of the horizontal well, which was thus prone to suffering shear failure. Moreover, apparent end effects occurred at the end of the horizontal well section and might cause the deformation and failure of the casing structure. Therefore, it is necessary to take measures in the build-up section and at the end of the horizontal section of the horizontal well to prevent damage and ensure the wellbore safety in the long-term NGHs exploitation.

Is Titanium Drillpipe Applicable to Offshore Drilling? A Question from a Corrosion Fatigue Perspective

Summary In offshore horizontal well drilling, one of the major challenges is the large dogleg severity in the buildup section due to the shallow depth of reservoirs. In such a case, the drillstring has to bend itself to fit the borehole trajectory and suffers greatly from the high alternating stress while rotary drilling. This could lead to fatigue fracture of the drillpipe within a short period. The corrosion from drilling mud may exacerbate the failure risk. Titanium alloy, as a new drillpipe material, has the characteristics of excellent corrosion resistance and low elastic modulus, which can theoretically extend the fatigue life. To study the performance of titanium alloy drillpipe quantitatively, titanium alloy material, and G105 steel of the same grade are compared to obtain the microscopic characteristics and macroscopic properties through experimental tests. Moreover, the mechanism of passive film formation of the two materials is analyzed in the corrosion fatigue (CF) environment. Then, the experimental results are extended to modeling the actual drillstring assembly and simulating its service life in the field practice of offshore drilling. Our numerical results indicated that the titanium alloy drillpipe has much better corrosion resistance but only half the stress level of G105 steel under the same dogleg severity and axial force, which makes its fatigue life over 23 times longer. Eventually an economic appraisal is given by considering the full-service life of the drillpipe. The research results of this paper can provide a detailed theoretical basis and reference for field application and can popularize the use of titanium alloy drillpipe.

Completion Study Aims To Extend Life of Electrical Submersible Pumps

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 20130, “Practical Approach for Solid Production Prediction and Completion Strategy Decisions in Horizontal Wells: A Case Study From a Cretaceous Carbonate Reservoir, North Oman,” by Mohammed Al-Aamri, Sandeep Mahajan, SPE, and Nair Sujith, Petroleum Development Oman, et al., prepared for the 2020 International Petroleum Technology Conference, Dhahran, Saudi Arabia, 13–15 January. The paper has not been peer reviewed. Copyright 2020 International Petroleum Technology Conference. Reproduced by permission. The carbonate reservoir fields in Oman discussed in the complete paper are produced by several horizontal wells from long openhole sections. The wells are completed by barefoot (openhole) completion with electrical submersible pumps (ESPs) located in the wells’ buildup section. The field has experienced significant ESP failures, so a study aimed to provide input for well-completion-strategy design and operational parameters, which could minimize solids production and lower intervention/operating expenditure (OPEX). Based on the study results, recommendations were provided for a drawdown-management strategy, which potentially will benefit from increasing ESP run life and reducing field OPEX. Field Background Problem Statement and Motivation Petrophysical rock typing for the studied reservoir is detailed in the complete paper. The primary understanding of the root cause of these ESP failures was argillaceous rock typing along the horizontal section. The decision was made to recomplete the wells by isolating equipment from such rock typing. As a result, ESP run lives improved, but failures continued. Several wells featured an isolation process from the first day, for example, but run life did not improve. The field team subsequently analyzed a sample of fines taken from the ESP, and their mineralogy was examined. The main finding was that almost 50% of the sample included calcite mineral content with some quartz (Fig. 1). However, the question remained as to which part of the reservoir the sample belonged. All rock types potentially consist of such calcite minerals because of the marine-deposition environment. Hence, investigating and characterizing the possible root causes of the ESP failures, as well as providing effective completion mitigations for upcoming wells, was critical. The key objectives of the study were to understand the mechanisms and causes of the observed solids from a geomechanical standpoint and to provide recommendations to minimize the risk of near-wellbore failure.

Estimating the Catenary Parameter for Catenary Shape Using the Newton Raphson Iterative Method as a Tool for Extended Reach Well Buildup Section Design

The catenary well profile was first used to design extended reach well buildup section in 1985, ever since then a number of wells have been constructed using this profile. It towers over the constant build (circular arc) profile in terms of advantages. Prominent of which is the reduction of torque and drag. This advantage was restrained by the catenary parameter “a”, as “a” determines the shape or trajectory of a catenary shape in any given circumstance. We proffered a solution to estimating the catenary parameter “a” using the Newton-Raphson iteration method for linear equations. This method makes it easy to calculate “a” for almost every desired catenary shape. Also after estimating the values for “a” we went ahead to calculate tensions that will produce that calculated catenary parameter “a” as the parameter is the product of tension at the end of the string and inverse of unit weight. This method of catenary parameter prediction has been used to design overhead electrical cables and predict the extent of sagging of this cables. We used it here to design the path or trajectory of an extended reach well with satisfactory success. Results from the study shows that the use of the Newton-Raphson iterative method for the estimation of catenary parameter “a” for buildup section design of extended reach wells makes it easy for engineers to plan catenary trajectories with available materials and also removes the uncertainties that are inherent in predicting the shape of the trajectory. Keywords : Caternary, String, well, Newton-Raphson, Extended Reach well DOI: 10.7176/JETP/11-5-04 Publication date: October 31 st 2021

Fatigue failure mechanism of 3D tubing strings used in high-pressure, high-temperature and high-yield curved gas wells

In view of the fatigue failure of tubing strings by flow-induced nonlinear vibration (FINV) in high-temperature, high-pressure and high-yield (3H) curved gas wells, a three-dimensional (3D) FINV model of the tubing string is established using the micro-finite method, energy method and Hamilton variational principle, and verified by a similar experiment of tubing vibration. Meanwhile, the cumulative damage theory is used to establish the fatigue life prediction method of tubing string combined with the stress response which was determined by the proposed FINV model and the S-N curve of the tubing material (13Cr-L80) which was measured by fatigue test. Thus, the influences of production rate, well trajectory parameters (inclination angle and well section length), as well as the installation position of downhole tools (packer and centralizer) on the fatigue life of the tubing string are systematically analyzed. The results obtained demonstrate that, first, with the increase of production rate, the fatigue life of the tubing string decreases, and the field production allocation should be far away from the sudden production rate which can be determined using the proposed analysis method. Second, in the well trajectory design, the vertical section length should be reduced, while that of build-up section and the angle of stable inclined section should be increased. Third, the packer and centralizer should be placed in the optimal position, which depends on the well structure, downhole tool size, etc. and can be determined using the proposed analysis method. The research results provide a theoretically sound guidance for designing and practically sound approach for effectively improving the fatigue life of tubing string in 3H curved gas wells.

Friction-wear failure mechanism of tubing strings used in high-pressure, high-temperature and high-yield gas wells

This study establishes a flow-induced nonlinear vibrations (FINV) model of tubing string using micro-finite element and energy methods along with the Hamilton variational principle. This model helps address the wear failure of tubing strings caused by FINV in high-pressure, high-temperature, and high-yield (3H) gas wells and considers the changes in wellbore trajectory, wellbore temperature/pressure, and contact/collision of the tubing-casing. To validate the proposed model, a simulation experiment of tubing string vibration was conducted. Based on the White-Fleisher wear theory, a method is proposed for calculating wear amount and depth of tubing string in 3H gas wells; the friction coefficient and wear rate are determined via a wear experiment. Thus, the influences of production rate, inclination angle, well section length, packer position, and centralizer position on the wear characteristics of tubing string are systematically analyzed. The results obtained demonstrate that, first, with the increase of production rate, the wear life of the tubing string decreases, and there exists a sudden value of production rate, which can be determined using the proposed analysis method, such that the field production allocation should be far away from the sudden production rate. Second, in the well trajectory design, the vertical section length should be small, while that of build-up section and the angle of stable inclined section should be large. Third, the optimal positions for the packer and centralizer in the field depend on the well structure, downhole tool size, etc. and can be determined using the proposed analysis method; these positions help design field tools. The research results provide a theoretically sound guidance for designing and practically sound approach for effectively improving the service life of tubing string in 3H gas wells.

Dynamic wavy distribution of cuttings bed in extended reach drilling

This study investigates the wavy distribution of the cutting layer in an entire wellbore considering a dune mechanism. First, a fully transient two-layer model of cuttings transport is improved via two aspects: integrating the diffusion equation to calculate entrainment and deposition rates between two layers; and considering the buoyancy of cuttings motion in deviated sections. The new model is solved via a Stability Enhancing Two-Step (SETS) method. Next, an index is proposed, namely the ratio of the length of intervals with a height greater than 10% of the wellbore diameter to the entire wellbore length—to estimate the dynamic cleaning effect. Finally, the model is applied to an extended reach well, in which the wavy distribution of a cuttings bed as well as effects of buoyancy, mass exchange, and circulation stoppage are studied; further, the range of controllable parameter R (the time ratio of washing and pure drilling) is recommended.The results demonstrate that, in the iterative process of pure drilling and washing, the cuttings bed is no longer of equal height, but distributed in waves. The height of a cuttings bed decreases with an increase in R. This parameter should be greater than 0.5; otherwise, the gathered pile will severely threaten the subsequent drilling. When R reaches the upper limit of 1.2–2.5, the height of the cuttings bed is less than 10% the wellbore diameter, thus saving extra circulations. Moreover, the calculated bed height in the deviated section is lower when considering the buoyancy than when it is not. Thus, the middle section (inclination of 30°–60°) is not necessarily the most challenging area to clean if appropriate values for density and buoyancy of drilling fluids are considered. Furthermore, the cuttings bed slides downward after circulation is stopped, amassing on the lower side of the buildup section (70°–90°), which poses a significant risk of pipe sticking. The maximum stack height is nearly 1.2–2.5 times higher than that of the initial bed. With the increase of the inclination of the holding section, the stacking bed height increases rapidly when the angle is greater than 85°. The results serve as an essential guide to predict and control the tight spots more precisely for hole cleaning.

Calculation and analysis of dynamic drag and torque of horizontal well strings

In recent years, shale gas production horizontal well types mainly include conventional horizontal well, highly deviated well and scoop-shape horizontal well. For the sake of construction decision making, it is necessary to study these three types of wells from the aspects of drag and torque characteristic, weight on bit transferring efficiency, key hole sections or links and extreme penetration length. In this paper, these three types of horizontal wells were taken as the study objects. Their drag and torque, load transfer and extended reach drilling were explored based on the dynamic model of full hole drilling string and the simulation calculation of dynamic characteristics of full hole drilling string system. And the results were applied and tested on site in three shale gas wells (a conventional horizontal well, a highly deviated well and a scoop-shape horizontal well) in the Changning area, Sichuan Basin. And the following research results were obtained. First, the contact friction strength of the deeper part of the buildup section in the scoop-shape horizontal well is very high, and it is 1.67 times that of the hold section. The total contact force of buildup section in the scoop-shape horizontal well is 1.62 times that in the highly deviated well. Second, the contact friction strength of the hold section in highly deviated well is not only higher than that of its buildup section, but also higher than any characteristic section in the three well types of the same depth. Third, the operating stress of drilling strings during the drilling of three well types is not high, but during the extended reach drilling in the curved section of the scoop-shape horizontal well, it is necessary to focus on the twist off of drilling string in the hold section. Fourth, the weight on bit transferring efficiency of scoop-shape horizontal well is lower than that of conventional horizontal well and highly deviated well. In conclusion, the research results preliminarily reveal the drag and torque characteristics and active load transferring mechanisms of conventional horizontal well, highly deviated well and scoop-shape horizontal well for shale gas production, and present important hole sections for safety assessment. They can be used as references for efficient and safe construction of shale gas horizontal wells.

Key technologies for directional well drilling in high-pressure anhydrite salt layers

The Jeribe–Kirkuk reservoir is one of the major pay zones in the Halfaya Oilfield, Iraq, and its overlying 500 m thick Lower Fars anhydrite salt layer with a formation pressure coefficient of 2.25 acts as the caprock sealing. That's just why the first Jeribe–Kirkuk directional well experienced multiple drill pipe stickings and two side trackings, and the first horizontal well had to be completed with casing ahead of the schedule. Obviously, the drilling in the Lower Fars high-pressure anhydrite salt layer is faced with severe challenges. In this paper, the Lower Fars anhydrite salt layer was analyzed from the aspects of mechanical property, pore pressure, ground stress and rock mineral characteristics. Then, a mathematical model and criterion rules were established for borehole stability and wellbore deformation in the Lower Fars formations with different lithologies and the wellbore deformation in the process of well drilling was simulated. Finally, the high-density saturated salt water drilling fluid was specially developed for directional drilling in anhydrite salt layers, and the well trajectory and drilling program of directional wells were optimized. The following results were obtained. First, the most unstable zone in the Lower Fars anhydrite salt layer is composed of mudstones. As to the main reason for drill pipe sticking, the increase of rheological property after mudstone hydration leads to tight holes, so the neighboring anhydrites and salt rocks are pulled to collapse, which ultimately results in drill pipe sticking. Second, borehole deformation and instability in the Lower Fars layer intensify with the increase of inclination angle and wellbore opening time, and the risk of borehole instability increases sharply with the increase of wellbore opening time. Third, polyamine inhibitor BZ-HIB and polymer thinner JNJ are selected to optimize the formula of the high-density saturated salt drilling fluid, thus making its inhibition and rheology during the drilling in anhydrite salt layer improved effectively. Fourth, the kick off point of the Jeribe–Kirkuk directional well is moved upward to Upper Fars from Lower Fars. Specifically, the buildup section is in Upper Fars, the angle holding section is in Lower Fars, and the inclination angle and directional section length in Lower Fars are decreased, so the risk of drill pipe sticking is reduced. So far, it has been practically applied to 22 wells, showing that the drilling complexities in Ø311.2 mm hole have been effectively controlled. In 2016, the average drilling cycle was 35.8 days, 52.8% shorter than that in 2015, and the average ROP was increased by 122% from 3.3 m/h to 7.33 m/h. In conclusion, technological optimization has achieved good application results.

Simplified Analytical Model and Shaking Table Test Validation for Seismic Analysis of Mid-Rise Cold-Formed Steel Composite Shear Wall Building

To develop the cold-formed steel (CFS) building from low-rise to mid-rise, this paper proposes a new type of CFS composite shear wall building system. The continuous placed CFS concrete-filled tube (CFRST) column is used as the end stud, and the CFS-ALC wall casing concrete composite floor is used as the floor system. In order to predict the seismic behavior of this new structural system, a simplified analytical model is proposed in this paper, which includes the following. (1) A build-up section with “new material” is used to model the CFS tube and infilled concrete of CFRST columns; the section parameters are determined by the equivalent stiffness principle, and the “new material” is modeled by an elastic-perfect plastic model. (2) Two crossed nonlinear springs with hysteretic parameters are used to model a composite CFS shear wall; the Pinching04 material is used to input the hysteretic parameters for these springs, and two crossed rigid trusses are used to model the CFS beams. (3) A linear spring is used to model the uplift behavior of a hold-down connection, and the contribution of these connections for CFRST columns are considered and individually modeled. (4) The rigid diaphragm is used to model the composite floor system, and it is demonstrated by example analyses. Finally, a shaking table test is conducted on a five-story 1:2-scaled CFS composite shear wall building to valid the simplified model. The results are as follows. The errors on peak drift of the first story, the energy dissipation of the first story, the peak drift of the roof story, and the energy dissipation of the whole structure’s displacement time–history curves between the test and simplified models are about 10%, and the largest one of these errors is 20.8%. Both the time–history drift curves and cumulative energy curves obtained from the simplified model accurately track the deformation and energy dissipation processes of the test model. Such comparisons demonstrate the accuracy and applicability of the simplified model, and the proposed simplified model would provide the basis for the theoretical analysis and seismic design of CFS composite shear wall systems.

In-depth study of the mechanical properties for Fe3Al based iron aluminide fabricated using the wire-arc additive manufacturing process

An innovative wire-arc additive manufacturing (WAAM) process is used to fabricate iron aluminide alloy in-situ, through separate feeding of pure Fe and Al wires into a molten pool that is generated by the gas tungsten arc welding (GTAW) process. This paper investigates the morphologies, chemical compositions and mechanical properties of the as-fabricated 30at% Al iron aluminide wall components, and how these properties vary at different locations within the buildup wall. The tensile properties are also measured in different loading orientations; as epitaxial growth of large columnar grains is observed in the microstructures. Fe3Al is the only phase detected in the middle buildup section of the wall structure, which constitutes the majority of the deposited material. The bottom section of the structure contains a dilution affected region where some acicular Fe3AlC0.5 precipitates can be observed, induced by carbon from the steel substrate that was used for fabrication. The microhardness and chemical composition indicate relatively homogeneous material properties throughout the buildup wall. However, the tensile properties are very different in the longitudinal direction and normal directions, due to epitaxial growth of large columnar grains. In general, the results have demonstrated that the WAAM process is capable of producing full density in-situ-alloyed iron aluminide components with tensile properties that are comparable to powder metallurgy methods.

A magnet ranging calculation method for steerable drilling in build-up sections of twin parallel horizontal wells

In drilling technology, adequate control over the distance between adjacent sections of twin parallel horizontal wells has a significant impact on the final recovery rate from unconventional oil and gas resources, such as heavy oil and natural gas hydrates. For this distance to meet design requirements and to reduce the duration of the drilling period, it is necessary to use a rotating magnet ranging system that helps drill an injection well starting from the build-up section. Usually, the build-up sections of the twin parallel horizontal wells are not parallel. Therefore, the spatial relationship between the build-up sections of the two wells can be divided into two types, namely, converging coplanar and non-coplanar. In this paper, we propose a method based on an algorithm for finding extreme values to calculate the radial distance between the build-up sections of the twin parallel horizontal wells. In addition, we propose and discuss a method for calculation of the direction between the magnetic sub and the probe tube in the build-up sections using coordinate transformations combined with the rotating magnet ranging calculation for the horizontal sections of twin parallel horizontal wells. The simulation results indicate that the magnet ranging algorithm for the build-up sections of twin parallel horizontal wells has a precision of a few percent and can meet the actual needs of the drilling site.

Dynamic Analysis of Solid Expandable Tubular and Its Applications in Tahe Oilfield

For ultra-deep horizontal wells in Tahe Oilfield in China, the solid expandable tubular (SET) technology was applied in the buildup section to isolate unstable mud stone with clays above the pay zone. This technology satisfies the requirements of geological water avoidance, and a larger hole size is adopted in the pay zone. This paper proposed a finite element dynamic model to analyze the expansion of the SET, and the impacts of the following factors on the expansion force required for the expandable tubular were analyzed: yield strength of the tubular material, solid tubular expansion ratio, the coefficient of friction between the expansion cone and the tubular, the angle of the expansion cone, movements velocity of the expansion cone, and DLS (dogleg severity). The calculation results indicate that a higher expansion ratio, a higher yield strength, and a higher friction coefficient of the SET result in a larger expansion force to different degrees, whereas the other factors have little effect on the expansion force. The dynamic expansion force calculation model established in this paper is consistent with the actual conditions, and the simulation calculation results can be applied to provide theoretical guidance for SET expansion operations.

Long Horizontal Section of Horizontal Well Section Optimization Design Research

The aim of this study is to optimize the long horizontal section of horizontal well section in extended- reach well. Extended-reach drilling technology has rapidly developed during the past three decades. It requires improved models and technology to extend to greater depth. Well profile design is one of the critical technologies for success in horizontal well because optimum well profile design is an effective means for reducing torque, drag and cost. This study presents long horizontal section of horizontal well section optimization design method based on the two buildup section well profile, and this method is analyzed in detail.

Analysis of the External Pressure on Casings Induced by Salt-Gypsum Creep in Build-up Sections for Horizontal Wells

To alleviate the problems of casing collapse induced by the coupling effect of salt creep movement, as it relates to the curved sections of horizontal wells, an experimental approach was taken to determine the creep (visco-elastic) property of the salt-gypsum formation, using finite-difference software to establish a creep model with curved casing, wellbore and salt formation. With this model, the effects of borehole curvature, drilling direction and casing thickness on external pressure on casing were analyzed, and casing deformation in the non-uniform in situ stress field was simulated. For horizontal wells drilled through salt-gypsum formations, this analysis led to the following conclusions: (1) the casing tends to be much safer when drilling is undertaken along the direction of the minimum horizontal stress; (2) casing deformation occurs and stress increases as the borehole curvature increases; (3) for the same curvature and drilling direction, thicker casings are safer; (4) as the creep time increases, the external pressure on the casing rapidly increases until it reaches its maximum value, whereupon it stays at that value; (5) under the effect of non-uniform in situ stress, a larger non-uniform coefficient would result in greater external pressure on the casing. This model has been successfully applied to analyze the external pressure on curved casings in a build-up section for horizontal wells drilled through salt-gypsum formations in an oilfield in Northwest China. The model has helped to improve the casing design and reduce drilling downtime with greater wellbore stability in salt-gypsum formations.

Multi-target wells: a new concept to improve well economics

The Vienna Basin is a sedimentary basin more than 10 km in thickness and composed of more than a dozen hydrocarbon layers. Since the area has been intensively explored in the past 50 years, future exploration can expect to find only small reservoirs or compartments of already existing reservoirs. However, the existing infrastructure, such as pipelines, facilities and a nearby refinery, would make even small finds attractive. In this light it is of vital importance to reduce risk and costs in hydrocarbon exploration and production. Multi-target wells were found to be an adequate answer to these problems: the chance of discovery increases more or less directly with the number of targets included in the well trajectory; and several small reservoirs can, in total, provide positive well economics. Moreover, by including exploration, appraisal and production targets into one well path a substantial contribution to improve economic hydrocarbon recovery from existing fields can be made. To further reduce drilling costs a new casing philosophy was established. By skipping the intermediate casing across the build-up section substantial savings could be made. To plan and execute these challenging projects efficiently a multidisciplinary team has to be set up at an early stage of the drilling project. In this way the number of targets, the actual well path and well construction with respect to production requirements can be optimized. An example of a recently drilled multi-target horizontal well, including one primary and five secondary targets, is given. By applying this new concept drilling costs could be reduced by more than 30%.