Complex Structure Wells Research Articles

Effect of Impact Force on the Impact-Sliding Wear of Titanium Alloy Drill Pipes against Steel Casings

Titanium alloys have been widely used for drill pipes in complex structural wells and ultra-deep wells because they can provide great benefits in cost and operation in drilling systems due to their excellent properties such as high strength, low density, exceptional corrosion resistance and inherent flexibility. However, their wear behaviors during drilling have not been well understood by the communities. In this work, the effect of the impact force on the wear performance of titanium alloy drill pipes against steel casings was investigated through using a modified wet impact-sliding pin-on-disk test system, in which ZSA-3 titanium alloy was used for the pins and J55 steel was for the disks. The tests were conducted under simulated drilling mud conditions. As the impact force increases, the coefficient of friction tends to become stable and fluctuates slightly, and thus the wear rate of the tribo-pairs decreases gradually. The wear mechanism of J55 steel disks presents a transition from plastic deformation to plastic deformation accompanied by adhesive wear, while that of the titanium alloy pins changes from the plowing effect with adhesive wear to plastic deformation and adhesive wear. In addition, complex oxides will be formed on the surfaces of the tribo-pairs due to the oxidation of Ti and Fe during the wear process. The present results could provide as a fundamental guide for the drilling engineers in related industries for reducing casing wear in the drilling environment.

Numerical Simulation of Production Behavior with Different Complex Structure Well Types in Class 1-Type Hydrate Reservoir

Enhancing the production capacity of natural gas hydrates (NGHs) is critical for its commercial development. Complex structure wells may efficiently increase drainage areas while enhancing exploitation efficiency. Based on the field data of China’s first offshore NGH test production, the numerical method was used to analyze the production performance of different complex structure well types by continuous depressurization production for 360 days under the preconditions of fixed effective completion length of 300 m and a pressure difference of 6 MPa. Results indicated that the complex structure well types deployed at the three-phase layer demonstrated superior production performance within 240 days of production; the DLW2 and HW2 well types stood out, with an average gas production rate Qg reaching 43,333 m3/d and a specific production index J of 24.1. After 360 days of production, benefiting from multi-layer combined production, the Cluster vertical well deployed at the multi-layer had the best production performance, with an average Qg of 34,444 m3/d and a J-index of 19.1. The research results provided insights into the complex structure well-type selection strategy for NGH depressurization in this sea area.

Enhanced gas production from silty clay hydrate reservoirs using multi-branch wells combined with multi-stage fracturing: Influence of fracture parameters

Natural gas hydrates are typically deposited in low-permeability clayey-silty sediments with low gas production. The two main methods for enhancing gas production are complex structural wells and hydraulic fracturing. However, using only one of those methods does not significantly increase gas production. Herein, a method of combined multi-branch well with multi-stage fracturing was proposed. A novel evaluation method for increased gas production under fracturing was first proposed to reveal the effects of fracturing domain area and number. Then five fracturing distribution models were designed and analyzed. Results indicate that the combined method increased the gas recovery ratio by 829.13% and the hydrate dissociation ratio by 710.34% compared to branching wells because of enlarging pressure propagation and communicating deep thermal fluids. The dissociation of hydrates is enhanced by an increase in the area and the number of fractures, but the rangeability of the increased stimulating effect declines. Different fracture distributions influence production performances at different stages. Short-term production is beneficial when fractures are in the branch well centers. Long-term gas production can be improved when fractures are mainly located at the branch distal end. This novel study has important implications for commercial exploitation at different production stages.

Comparative Analysis of Friction and Wear Properties of Titanium Alloy Drill Pipe and Steel Drill Pipe

During the drilling process of complex structural wells such as horizontal wells, the wear, and tear of the drill pipe can lead to its weakened strength, which is one of the important reasons for the damage accidents of the drill pipe. Titanium alloy drill pipes are widely used in major oil fields as a new type of lightweight drill pipe with high fatigue life. To compare the wear performance of titanium alloy drill pipe with wear-resistant zones and traditional S135 steel drill pipe, this article tested and analyzed the chemical composition, hardness, and metallographic structure of these two drill rod materials and established models for these two materials. In the experiment, the wear morphology and wear rate before and after the experiment were analyzed and compared. Finally, based on the experimental results, an in-depth analysis was conducted on the wear mechanism of the drill pipe, and anti-wear measures were proposed. The results show that the hardness of the two materials decreases from the wear-resistant belt to the inner wall, and the microhardness of the S135 drill pipe is about half of that of titanium alloy drill pipe with wear-resistant belt drill pipe; the wear test shows that the wear loss of the S135 drill pipe is 0.1667 g, the wear rate is 0.042 g/h; through measurement, it was found that the wear loss of titanium alloy drill pipe reached 0.0055 g, and the wear rate is 0.0063 g/h. The wear rate of S135 steel drill pipe under the same conditions is 6.7 times that of titanium alloy drill pipe. Through analysis, it is shown that under the same conditions, titanium alloy wear-resistant belts have excellent wear resistance, which can effectively protect the inner matrix and prevent excessive wear of drill pipes. The research results of this article can provide theoretical references for the on-site use of titanium alloy drill pipe and experimental research on drilling tool wear resistance.

Simulation on a new reverse circulation fishing tool: Design and evaluation of the salvage capacity and efficiency

AbstractTo solve the insufficient energy for reverse circulation fishing tools in complex structure wells, it is necessary to further optimize the design of the tools to improve the impurity salvage efficiency. In this work, a three‐dimensional (3D) flow field simulation model of reverse circulation fishing tool with venturi negative pressure is employed to identify the vortex zone of circulation channel and to investigate the negative pressure and jet velocity distribution around the spray nozzle based on the ANSYS‐CFX. The effects of the spray angle, diameter of the suction nozzle, and the nonstructural parameters on negative pressure and jet velocity are investigated through sensitivity analysis. As revealed from the results, there may be an optimum value for the spray angle, and the nozzle jet direction can indeed affect the propagation and attenuation of the bottom hole flow field greatly. The combination of the inner diameter of the nozzle outlet and that of the nozzle inlet (Φ + Φ′ = 3 mm + 6 mm) is the best design solution when the negative pressure at the nozzle outlet can be the highest. The negative pressure and jet velocity increase with the increasement of the inlet flow rate. The comparative analysis of different bottom hole impurity mass in the cases of the borehole inner diameter 5′ and 8.5″ on the salvage efficiency are conducted numerically. It is observed that the impurity mass salvaged and salvage time are not directly related to its total amount, but only related to the flow rate of the sucked fluid flow. The impurity salvage efficiency can be as high as 95.4%, which meets the engineering requirements in the field. This work improves the working performance of the reverse circulation fishing tool and provides theoretical guidance for well washing.

Subsection and superposition method for reservoir formation damage evaluation of complex-structure wells

Kinds of complex-structure wells can effectively improve production, which are widely used. However, in the process of drilling and completion, complex-structure wells with long drilling cycle and large exposed area of reservoir can lead to the fact that reservoir near wellbore is more vulnerable to the working fluid invasion, resulting in more serious formation damage. In order to quantitatively describe the reservoir formation damage in the construction of complex-structure well, taking the inclined well section as the research object, the coordinate transformation method and conformal transformation method are given according to the flow characteristics of reservoir near wellbore in anisotropic reservoir. Then the local skin factor in orthogonal plane of wellbore is deduced. Considering the uneven distribution of local skin factor along the wellbore, the oscillation decreasing model and empirical equation model of damage zone radius distribution along the wellbore direction are established and then the total skin factor model of the whole well is superimposed to realize the reservoir damage evaluation of complex-structure wells. Combining the skin factor model with the production model, the production of complex-structure wells can be predicted more accurately. The two field application cases show that the accuracy of the model can be more than 90%, which can also fully reflect the invasion characteristics of drilling and completion fluid in any well section of complex-structure wells in anisotropic reservoir, so as to further provide guidance for the scientific establishment of reservoir production system.

Application of the enlarged wellbore diameter to gas production enhancement from natural gas hydrates by complex structure well in the shenhu sea area

The current casing diameters of the production section are about 244 mm in marine natural gas hydrate (NGH) tried tests, which are a small order of magnitude and have the great potential to be enlarged. Expanding the contact areas between the wellbore and the NGH reservoir using complex structure wells has been popularly studied recently. In this study, the enlarged wellbore diameters (casing diameters) of complex structure wells were first proposed to enhance the gas recovery from the NGH reservoir in the Shenhu area of the South China Sea. The gas production characteristics, temperature, and pressure in the NGH reservoir and near the wellbore induced by the depressurization method were analyzed under various wellbore diameters of complex structure wells. Results indicate that using the enlarged wellbore diameter can further expand the contact areas between the wellbore and the NGH reservoir, promoting gas production efficiency, especially in short-term production. The evolutionary trends of temperature and pressure meet the conditions for stable short- and long-term production. Moreover, it was suggested that a relatively significant gas production increase of the enlarged wellbore diameter could be obtained by a slight increase in the wellbore diameter.

Numerical analysis of gas recovery enhancement from natural gas hydrate reservoir by using a large-diameter casing

Multi-well patterns and complex structural wells can enlarge the contact area between the wellbore and natural gas hydrate (NGH) to improve productivity significantly, but the construction cost of drilling is still too high. The previous study has proposed a self-entry exploitation device (SEED) with an enlarged device diameter, allowing more NGHs to dissociate simultaneously. While for conventional wellbore, a large-diameter casing may be applied in NGH development to enlarge the wellbore diameter of the production section. This work continued to study the influences of enlarged device/wellbore diameter on gas production characteristics, temperature-pressure evolutions, and the dissociation front movement pattern. The sensitivity analysis was also carried out by various reservoir and depressurization parameters under different wellbore diameters. Results show that the enlarged wellbore diameter can enlarge the drainage area and increase the dissociation efficiency. The temperature and pressure around the wellbore remain relatively stable, meeting the conditions for long-term production. Also, the dissociation front moves farther away faster as the wellbore diameter enlarges, accelerating NGH dissociation at distant wells. Finally, the sensitivity analysis reveals that the enlarged wellbore diameter enhances gas production significantly in unfavorable production parameters: poor production pressure, low permeability, low NGH saturation, and narrow reservoir thickness.

Wellbore drift flow relation suitable for full flow pattern domain and full dip range

Aiming at the simulation of multi-phase flow in the wellbore during the processes of gas kick and well killing of complex-structure wells (e.g., directional wells, extended reach wells, etc.), a database including 3561 groups of experimental data from 32 different data sources is established. Considering the effects of fluid viscosity, pipe size, interfacial tension, fluid density, pipe inclination and other factors on multi-phase flow parameters, a new gas-liquid two-phase drift flow relation suitable for the full flow pattern and full dip range is established. The distribution coefficient and gas drift velocity models with a pipe inclination range of −90°–90° are established by means of theoretical analysis and data-driven. Compared with three existing models, the proposed models have the highest prediction accuracy and most stable performance. Using a well killing case with the backpressure method in the field, the applicability of the proposed model under the flow conditions with a pipe inclination range of −90°–80° is verified. The errors of the calculated shut in casing pressure, initial back casing pressure and casing pressure when adjusting the displacement are 2.58%, 3.43%, 5.35%, respectively. The calculated results of the model are in good agreement with the field backpressure data.

Numerical simulation of productivity improvement of natural gas hydrate with various well types: Influence of branch parameters

The depressurization method and its modified scheme may be the best way to efficiently recover marine natural gas hydrate (NGH) from the reservoir. However, the capacity of existing engineering tests still poses considerable challenges to the industrial development of NGH. The complex structure wells can effectively promote the NGH contact area to enhance the exploitation efficiency, but the influence of branch parameters on gas production performance is not well sorted out for reference when performing well type optimization. Therefore, a geological model of an ideal hydrate reservoir and 65 wellbore models were established based on the geological data from station AT1 in Nankai Trough to analyze the influence of different branch parameters on the production capacity for various well types. Results indicate that the open hole section length is heavily constrained by the size of the reservoir for a single well. Compared to the corresponding single well, the multi-branch vertical wells can increase productivity by about 2.12%–51.18%, the multi-branch horizontal wells by about 0.99%–29.75%, and the cluster horizontal wells by about 54.36%–250.66% well. Moreover, the correlation coefficients of various branch parameters were analyzed as a reference when optimizing the well type and show that the cluster horizontal well has the highest sensitivity to branch parameters, which can significantly promote gas production efficiency, and is expected to break the threshold of industrial development.

Numerical Simulation of Natural Gas Hydrate Exploitation in Complex Structure Wells: Productivity Improvement Analysis

About 90% of the world’s natural gas hydrates (NGH) exist in deep-sea formations, a new energy source with great potential for exploitation. There is distance from the threshold of commercial exploitation based on the single well currently used. The complex structure well is an efficient and advanced drilling technology. The improvement of NGH productivity through various complex structure wells is unclear, and there is no more complete combing. Thus, in order to evaluate their gas production characteristics, we establish a mathematical model for exploitation of NGH, and then 13 sets of numerical models based on the geological parameters of the Nankai Trough in Japan are developed and designed, including a single vertical well, a single horizontal well, 1~4 branch vertical wells, 1~4 branch horizontal wells, and 2~4 branch cluster horizontal wells. The research results indicate that wells with complex structures represented by directional wells and multilateral wells can significantly increase the area of water and gas discharge, especially cluster wells, whose productivity can be increased by up to 2.2 times compared with single wells. Complex structural wells will play an irreplaceable role in the future industrialization of NGH.

Prospect of marine natural gas hydrate stimulation theory and technology system

The key to realize the commercial production of natural gas hydrate (NGH) is to increase theNGH productivity significantly in the scale of magnitude. Whether NGH production can be commercialized depends on two aspects. The first is whether the in-situ recoverable reserves are large enough to support the basic production period for commercial production. The second is whether the average productivity can reach the standard for commercial production. In this paper, we will analyze mainly about the potential stimulation technologies for NGH development, and discuss about the basic principles, the evaluation methods, and the technical bottlenecks for NGH production and stimulation. The results indicate that the main mechanisms for increasing theNGH productivity are in three respects, namely enlarging the drainage area, increasing the NGH dissociation efficiency, and improving the seepage conditions. With complex-structure wells and multiple-well patterns, combined with novel production methods and/or reservoir stimulation technologies, the NGH productivity can be increased greatly. Particularly, the complex-structure wells and well patterns are very important for increasing NGH productivity. With complex-structure wells and well patterns, combined with heat injection and/or reservoir stimulation, NGH productivity can be increased on a magnitude scale. Currently, in fundamental researches, there are some technical bottlenecks for the studies of NGH production, mainly in sample preparation, simulated reservoir monitoring, and mechanical coupling technologies. Therefore, it is suggested that the study focuses should be on the above technical bottlenecks during the basic research on how to increase the NGH productivity. It is concluded that the combined application of complex-structure wells (horizontal wells and multi-lateral wells), well-pattern production models (with multi-cluster/group well production), the novel production methods (mainly thermal stimulation, together with depressurization), and reservoir stimulation technologies (hydraulic fracturing) are the keys to increase NGH productivity in the scale of magnitude.

Horizontal well workover technology present situation and development trend

In the downhole operation of horizontal Wells, the pipe string is subjected to the combined action of many kinds of loads. The pipe string may bend sinusoidal or spiral under the action of axial compressive stress, which will also increase the friction between the pipe string and the wellbore wall. Because of the existence of friction, to the milling, salvage, unstuck, sand and other construction to bring adverse effects. This paper describes the characteristics of horizontal well in fishing, sand washing, unlocking and casing milling. The paper introduces the development of horizontal well application, such as drilling technology matching for complex structure wells, high-level completion of lateral Wells, radial lateral wells in low permeability reservoirs and supporting technology for completion and production of horizontal wells.

Effect of variable drill pipe sizes on casing wear collapse strength

Casing wear, on the process of drilling complex structure well, has resulted in the casing collapse strength being weaker, which could be one of the important causes of casing damage accidents. For the complex structure wells with variable drill pipe size, a bigger error would occur if the casing wear is calculated by the single size crescent wear model.In this paper, based on the hypothesis of the single size crescent wear and the size of the drill pipe joints being involved in wear, we have established two geometric models, named compound sharp wear and compound blunt wear and deduced the casing collapse strength formula considering the compound wear. Meanwhile, how the compound wear rained influence on casing collapse strength was also analyzed with two methods—analytical and finite element analysis.At the same wear depth, for the collapse strength, there are not many differences between the compound crescent wear casing and the single crescent wear casing with minimum wear radius and the maximum error, less than 5%, would decrease as the wear depth increased. That was why we regarded the latter as an approximation of the former to calculate the collapse strength of compound wear casing in our study, which will provide references for the deformation mechanism and strength analysis of casing section in complex cases.

Design and Development of Horizontal Well and Complex Structure Well Combined with Computer Data Evaluation Technology

This article briefly introduces the completion structure and key support technologies of drilling, conventional horizontal Wells, lateral horizontal Wells and branch horizontal Wells under computer data evaluation. The problems existing in the process of well completion in complex structure Wells and the development direction in the future are put forward.

A Review of Metaheuristic Algorithms for Optimizing 3D Well-Path Designs

Due to a variety of possible good types and so many complex drilling variables and constraints, optimization of the trajectory of a complex wellbore is very challenging. There are several types of well such as directional wells, horizontal wells, redrilling wells, complex structure wells, cluster wells, and extended reach wells etcetera. Over the past few years, the number of unconventional well including deviated well, highly deviated well is steadily increasing. Directional drilling has some advantages over vertical drilling though it is more expensive. In drilling engineering optimization of wellbore plays an important role, which can be optimized based on the minimization of the length of wellbore, minimization of mud pressure, critical pressure etcetera. Till today so many approaches or methods used to optimize this wellbore trajectory. From those methods here in this study we focused on metaheuristic approaches that used to optimize wellbore trajectory. This reduction of the wellbore length helps to establish cost-effective approaches that can be utilized to resolve a group of complex trajectory optimization challenges. For efficient performance (i.e.; quickly locating global optima while taking the smallest amount of computational time) we have to identify flexible control parameters. Later this parameter can be effectively fixed to tune different algorithms. This research will develop a review of the various algorithm used to optimize deviated wellbore trajectories. In the environment of challenging commodity prices, engineers and commercial firms bound to utilize technology to find out a low-cost solution. Technology is also helpful for the automatic solution of a complex problem, which also allows for the better utilization of resources. This review paper helps to find out optimal wellbore trajectory optimization algorithms that can close the technology gap by giving a useful method. This method can generate a solution automatically. Before establishing a different algorithm, those previous authors considered a number of factors and limitations. Safety is one of them. Before drilling depleted zones should be well studied, otherwise, it may be the reason for so many problems. The heuristic approach builds up solutions through the trial and error method. Those methods take a decent amount of time for solving a complex problem. For a single known problem, different metaheuristic approaches take a different amount of time to find the optimal solution. Among different metaheuristic algorithm which was used to find minimum true measured depth (TMD), we try to make a review where we focused on their computational time, minimization of length, method, etcetera.

Finite Element Analysis of Effect of Flexible Pupjoints on Push-the-Bit Rotary Steering Deflecting Force

As one of the most advanced international drilling technologies at present, rotary steering has been extensively applied to large-displacement, long-distance horizontal, and complex structural wells with numerous advantages, such as strong steering deflecting capacity and good borehole quality. Push-the-bit rotary steering is nowadays commercially applied on a large scale. In the bottom hole assembly (BHA) design of push-the-bot rotary steering drilling, flexible pup joints are the main components that exert an important effect on the exertion of steering deflecting capacity of push-the-bit rotary steering tools. The effect of flexible pup joints on steering capacity of BHA used in the push-the-bit rotary steering drilling process was investigated through a finite element analysis and their action laws were summarized. The results indicated that in order to optimize the deflecting capacity of rotary steering drilling tools, flexible pup joints should be installed cooperatively so as to obtain greater steering drilling capacity. In the rotary steering drilling process, as the outer diameter of flexible pup joints increased, the maximum stress of the BHA decreased. Side cutting force of the drill would increase with the length of pup joints, but this would result in degradation of anti-torsion performance of the BHA.

Productivity Models of Infill Complex Structural Wells in Mixed Well Patterns

The mathematical models of productivity calculation for complex structural wells mainly focus on the single well or the regular well pattern. Previous research on the seepage theory of complex structural wells and vertical wells in mixed well pattern is greatly insufficient. Accordingly, this article presents a methodology of evaluating the productivity of infill complex structural wells in mixed well patterns. On the basis of the mirror-image method and source–sink theory, two semi-analytical models are established. These models are applied to the productivity prediction of an infill horizontal well inhorizontal-vertical well pattern and an infill multilateral well inmultilateral-vertical well pattern, respectively, in which the interference of other wells, the randomicity of well patterns, and the pressure drawdown along the horizontal laterals are taken into account. The semi-analytical models’ results are consistent with those calculated by the Eclipse reservoir simulator with the relative error of less than 15%. Results indicate that the bottom hole flowing pressure decreases logarithmically while the wellbore flow rate increases monotonically from the toe to the heel of the horizontal well. Due to the pseudo-hemispherical flow at each endpoint and the pseudo-linear flow at the center of the horizontal well, the drainage area at each endpoint is relatively larger than that at the center. The radial inflow at each endpoint of the horizontal segment is considerably greater than that at the center, which presents the U-shape distribution. The proposed methodology enhances and promotes the theory of productivity evaluation for complex structural wells in mixed well patterns.

A new drilling method-Earthworm-like vibration drilling.

The load transfer difficulty caused by borehole wall friction severely limits the penetration rate and extended-reach limit of complex structural wells. A new friction reduction technology termed “earthworm-like drilling” is proposed in this paper to improve the load transfer of complex structural wells. A mathematical model based on a “soft-string” model is developed and solved. The results show that earthworm-like drilling is more effective than single-point vibration drilling. The amplitude and frequency of the pulse pressure and the installation position of the shakers have a substantial impact on friction reduction and load transfer. An optimization model based on the projection gradient method is developed and used to optimize the position of three shakers in a horizontal well. The results verify the feasibility and advantages of earthworm-like drilling, and establish a solid theoretical foundation for its application in oil field drilling.

FSQGA based 3D complexity wellbore trajectory optimization

Determination of the trajectory of a complex wellbore is very challenging due to the variety of possible well types, as well as the numerous complicated drilling variables and constraints. The well type could be directional wells, cluster wells, horizontal wells, extended reach wells, redrilling wells, and complex structure wells, etc. The drilling variables and constraints include wellbore length, inclination hold angles, azimuth angles, dogleg severity, true vertical depths, lateral length, casing setting depths, and true vertical depth. In this paper, we propose and develop an improved computational model based on Fibonacci sequence to adjust the quantum rotation step in quantum genetic algorithm for achieving cost-efficient complex wellbore trajectories. By using Fibonacci sequence based quantum genetic algorithm (FSQGA) in a complex searching problem, we can find high-quality globally optimal solutions with high speed through a parallel process. The simulation results show that FSQGA can significantly reduce computation complexity, and reach minimum objection values faster. Meanwhile, minimization of the true measurement depth of complex wellbore trajectory in actual gas-oil field shows that the drilling cost can be reduced up to 4.65%. We believe this new algorithm has the potential to improve drilling efficiency, to reduce the drilling time and drilling cost in real-time wellbore trajectory control.