Highly-deviated Wells Research Articles

Selection Results of Solid Material for Horizontal and Highly-Deviated Well Completion Gravel-Packing: Experiments, Numerical Simulation and Proposal

Lightweight and ultra-lightweight solid materials are being used in gravel packing for horizontal wells instead of traditional quartz and ceramsite to decrease the risk of premature plugging and improve packing efficiency. Physical and numerical simulation experiments of gravel packing were conducted to assess the effectiveness of reducing solid material density and investigate its impact on packing and sand control. Packed gravel destabilization experiments highlighted the importance of high-compaction degree packing for effective sand control. Further gravel packing experiments examined the packing performance of different solid materials, revealing that lightweight solids have minimal gravitational deposition effect because their density is similar to the gravel slurry, relying primarily on fluid flow for compaction. The numerical simulation indicated that lightweight ceramsite is unsuitable for horizontal and highly-deviated wells because of its poor compaction degree and sand control, especially with high-viscosity slurry. High-density particles enhance gravitational deposition, improving packing compaction and sand control. Lightweight materials are recommended only when advanced plugging of α wave packing cannot be avoided. In highly-deviated wells, high-density materials significantly improve packing stability and sand control. This study provides clear technical guidelines for selecting solid materials for gravel packing in horizontal and highly-deviated wells.

Fracturing parameter optimization technology for highly deviated wells in complex lithologic reservoirs

Highly-deviated wells are the key technology to reduce the risk of drilling accidents and improve the utilization of reservoirs. However, for reservoirs with complex lithology, highly-deviated wells are faced with the problems of geomechanical transformation and fracturing parameter optimization. The research on fracturing parameter optimization technology of high-deviated wells in complex lithologic reservoirs is helpful to the research and application of geomechanics in deep unconventional reservoirs. This paper is based on geological mechanics laboratory experiments and logging interpretation, combined with regional geological background, to clarify the geological and mechanical characteristics of the Fengcheng Formation shale oil region in the Mabei Slope. On this basis, based on the current geostress field and natural fracture distribution pattern of the Mabei Slope, an integrated model of shale oil geological engineering in local well areas was established. Based on the finite element method, optimization design was carried out for the cluster spacing, construction fluid volume, displacement, and sand volume of highly deviated well fracturing, and three-dimensional simulation of fracturing fractures was completed. The research results indicate that: (1) The current dominant direction of the maximum principal stress in the Fengcheng Formation on the Mabei Slope is from northeast to southwest, with the maximum horizontal principal stress generally ranging from 90 to 120 MPa and the minimum horizontal principal stress generally ranging from 70 to 110 MPa. (2) The difference in stress between the two horizontal directions is relatively large, generally greater than 8 MPa. Two sets of natural fractures have developed in the research area, one with a northwest southeast trend and the other with a northeast southwest trend. The natural fracture density of the Fengcheng Formation shale reservoir in the Mabei Slope is 0.32–1.12/m, with an average of 0.58/m, indicating a moderate to high degree of fracture development. (3) The geological model and three-dimensional geo-mechanical model are established according to the actual drilling geological data, and different schemes are designed to carry out single parameter optimization. The optimization results show that the optimal cluster spacing of the subdividing cutting volume pressure of the highly deviated wells in the Fengcheng Formation of the Mabei Slope is 12 m, the optimal construction fluid volume is 1400–1600 m3/section, the optimal construction displacement is 8 m3/min, and the optimal sanding strength is 2.5 m3/m. At the same time, by comparing the fracturing implementation effect with the fracturing scheme design, it is proven that the artificial parameter optimization method for highly deviated wells based on the finite element method based on the regional stress background and the natural fracture development law proposed in this paper is feasible and can provide a scientific basis for the fracturing development of highly deviated wells in complex lithologic reservoirs. This research has been well applied in Mahu area of Xinjiang oilfield.

Stick-slip vibration behaviors of BHA and its control method in highly-deviated wells

Stick-slip vibration is a common phenomenon met in oil gas drilling, especially in the complex trajectory wells such as highly-deviated wells, it will cause the earlier failure of BHA (bottom hole assembly) and lower drilling efficiency. In this present paper, a finite element model of highly-deviated well, which consists of a drill string system, a borehole, a PDC bit (Polycrystalline Diamond Compact bit) and a rock model, is carried out to investigate the stick–slip vibration behaviors of drill bit and BHA. Besides, the torsional impact drilling method is proposed in the context to control the stick–slip vibration of BHA. The research results show that, the phenomenon of stick–slip vibration of BHA gradually weaken and disappear with the distance from the drill bit increases, and the stick–slip vibration mainly occurs at the BHA which near drill bit. The high frequency torsional impact drilling method can reduce the resistance torque of the bit by 34.71%, and increase the drilling depth by about 58.28%. It can effectively improve the drilling efficiency of the bit, making the rotation of drill bit becomes relatively more stable, and reducing or even eliminating the stick–slip vibration. These results lead to an enhanced understanding of stick–slip vibration behaviors of drill bit and BHA in highly-deviated wells, and provide the basis for improving the drilling efficiency.

Key technologies for the efficient development of ultra-deep ancient dolomite karst gas reservoirs: A case study of the Sinian Dengying Formation gas reservoir in the Anyue gas field of the Sichuan Basin

The Sinian Dengying Formation gas reservoir in the Anyue gas field of the Sichuan Basin is an ultra-deep ancient dolomite karst gas reservoir, whose development is of high difficulty, for the reservoir is thin and scattered with strong heterogeneity and is overall characterized by low porosity and low permeability. In order to provide support for the large-scale productivity construction of the gas reservoir of the fourth member of Dengying Formation (Deng 4 Member) in the marginal platform belt of the Anyue gas field, this paper carries out multi-discipline comprehensive researches on the technical bottlenecks based on the geological characteristics of the gas reservoir. In addition, the targeted key technologies for the efficient development of gas reservoirs were developed in the mode of “simultaneous research, application and improvement”. And the following research results are obtained. First, based on the modeling technology of fractured–vuggy dolomite karst reservoirs, accurate modeling of karst reservoirs is realized. Late actual drilling results show that the model's coincidence rate is increased from 63.0% to 90.7% and the “transparency” of gas reservoirs is achieved. Second, based on the reserve mobility evaluation technology of low-porosity and highly-heterogeneous karst reservoirs, the recovery factors of different reservoirs are determined. And consequently, the lower limit of well-controlled reserves and radius for the efficient development of different reservoirs are obtained and type I and II reservoirs are selected as the main development targets. Third, based on the development optimization design technology of highly-heterogeneous karst gas reservoirs, it is recommended to apply highly-deviated wells (the maximum deviation angle of 75°–84°) in the situations with multiple reservoirs crossed and superimposed. If type I and II reservoirs are developed intensively, it is recommended to adopt horizontal well drilling with a horizontal section of 800–1100 m. What's more, the reasonable well spacing in type I and II reservoirs shall be 1.6–3.3 km and 0.9–1.4 km, respectively. Fourth, based on the distinguished drilling technology of highly-deviated wells/horizontal wells in deep carbonate rocks, the fault complexity ratio is decreased from 20.7% to 4.9%, so safe and fast drilling and completion of highly-deviated wells/horizontal wells is realized and well control risks are diminished. What's more, the drilling rates of type I and II reservoirs are increased from 24.2% to 87.3%, indicating an effective improvement of reservoir drilling rates. Fifth, based on the open-hole segmented accurate acid fracturing technology of highly-deviated and horizontal wells, the difficulty in the highly-heterogeneous reservoir stimulation by horizontal well drilling is solved by conducting the differentiated acid fracturing process of “one segment, one strategy”. Based on the application of these innovative development technologies in the Deng 4 Member gas reservoir in the marginal platform belt of the Anyue gas field, the annual natural gas production capacity and cumulative natural gas production by the end of December 2020 are 60 × 108 m3 and 103 × 108 m3, respectively. In this way, the Sinian Dengying Formation gas reservoir of the Anyue gas field, the economic benefit of which is close to the marginal benefit, becomes the main force of conventional natural gas production increase.

Prediction of casing wear depth and residual strength in highly-deviated wells based on modeling and simulation.

Casing wear is a serious problem in highly-deviated wells because serious wear will lead to casing deformation, drilling tool sticking and failure of subsequent operations. The purpose of this paper is to predict casing wear depth and evaluate its effect on casing strength degradation in highly-deviated well drilling operation. Special attention has been given to the algorithm to achieve the prediction and evaluation. The effect of tool joint on contact force distribution is considered in contact force model. Then a wear depth prediction model and its solution method are proposed based on crescent-shaped wear morphology and wear-efficiency model. Besides, strength degradation of worn casing is analyzed in bipolar coordinate system and the model is verified by finite element method. Therefore, the technology of casing wear prediction and residual strength evaluation is completed systematically. Then, to apply casing wear prediction and residual strength evaluation technologies to an actual highly-deviated well, casing wear experiment and friction coefficient experiment are carried out to obtain wear coefficient and friction coefficient. Finally, based on the established models as well as experimental results, the distribution of casing wear is predicted and residual strength is evaluated. The method presented in this paper will contribute greatly to casing wear prediction and evaluation in highly-deviated wells.

Modeling and numerical study on drillstring lateral vibration for air drilling in highly-deviated wells

The combination of directional well technique and air drilling has been regarded as an effective technique for drilling engineering. Given the currently inadequate understanding of drillstring lateral vibration for air drilling in highly-deviated wells, the modeling and numerical study is conducted. Therefore, a new finite element (FE) model with full-dimensional beam elements was built in the paper, which combined the advantage of FE method and beam-column theory to avoid complicated derivation and solution. The model also included contact model, static-kinetic friction model, mass eccentricity, and Rayleigh damping. To verify the stability and applicability of the model, a numerical simulation was conducted to study the effects of rotation speed, weight on bit (WOB), well inclination, friction coefficient, and stabilizer (STB) on lateral vibration for air drilling in highly-deviated wells. Besides, the effects of drilling mud on lateral vibration were discussed. The results show that the FE model can be used effectively to analyze drillstring lateral vibration. Meanwhile, rotation speed and friction coefficient have promoting effects on lateral vibration, full-packed STB has a suppressing effect, while WOB has a relatively small effect. Moreover, well inclination has a suppressing effect on lateral displacement, and a promoting effect on lateral acceleration. Compared with mud drilling, the lateral displacement and lateral acceleration of air drilling are obviously large. When air drilling is used in highly-deviated wells, a smaller rotation speed is proposed as long as it satisfies the ROP demand, and a slightly larger WOB is proposed as long as it does not lead to drillstring buckling, and full-packed STBs are proposed to limit lateral vibration. In addition, the formations with large friction coefficients can make lateral vibration more violent, and the drilling process needs more attention. Furthermore, increasing the lubricity of the circulating medium is recommended to reduce lateral vibration. The study in this paper can help to further develop drillstring dynamics, understand lateral vibration characteristics, and select suitable parameters for air drilling in highly-deviated wells.

Analysis of Gas Flow Behavior for Highly Deviated Wells in Naturally Fractured-Vuggy Carbonate Gas Reservoirs

To improve the carbonate gas reservoir development and production, highly deviated wells (HDW) are widely used in the field. Production decline analysis of HDW is crucial for long-term gas reservoir development. However, it is a new challenge to incorporate the complex pore structure of naturally fractured-vuggy carbonate gas reservoirs and evaluate the production performance of HDW. This paper presents a semianalytical model to analyze the pressure and production behavior of HDW in naturally fractured-vuggy carbonate gas reservoirs, which consist of fractures, vugs, and matrix. The primary flow occurs only through the fracture and the outer boundary is closed. Introducing pseudopressure and pseudotime, the Laplace transformation, Fourier transformation, and its inverse and Stehfest numerical inversion were employed to establish a point source and line source solutions. Furthermore, the validity of the proposed model was verified by comparing a field data from the Arum River Basin in Turkmenistan. Finally, the effects of major parameters on the production decline curves were analyzed by using the proposed model and it was found that they had influences at different stages of gas production history and the sensitivity intensity of each parameter was different. With its high efficiency and simplicity, this semianalytical model will serve as a useful tool to evaluate the well production behavior for the naturally fractured-vuggy carbonate gas reservoirs.

EVALUATING THE EFFECT WELL INCLINATION AND FORMATION ANISOTROPY ON CONING RATE IN DEVIATED WELLS

In oil reservoirs with bottom water drive, coning is a fundamental problem during oil production. Horizontal and deviated wells are often used to reduce the effect of water coning on total oil production. To avoid the premature breakthrough of water, the production of oil should be maximised through effective monitoring of the critical coning rate, and many researchers have developed models to determine the critical coning rate. However, very few studies have been conducted to evaluate the effect of good inclination and formation anisotropy on the critical coning rate. Therefore, we have incorporated well deviation and formation anisotropy into a water coning model to investigate their effects on the critical coning rate. Previous researches have been limited mainly to the calculation of water critical coning rate in the vertical well. In this study, a combination of radial and spherical flows in deviated wells has been adopted to develop a new model for analysing coning in non-vertical wells. The well inclination considered ranged from 20o to 80o, horizontal permeability (kh ) from 100 mD to 200 mD and the vertical permeability (kv ) was varied from 2 mD to 10 mD. Results showed that highly-deviated wells and formations with a high degree of reservoir heterogeneity (kv/kh) result in high critical coning rate and therefore less susceptible to early water breakthrough. This shows that more allowance to increase the production rate is possible for horizontal wells and anisotropic formations than in vertical and homogeneous reservoirs.
 Keywords: Water coning, deviated wells, reservoir heterogeneity, anisotropy permeability, critical coning rate, critical parameters.

Temperature field analysis on the highly-deviated wellbore in backwashing condition

Based on the measured borehole trajectory, a prediction model of temperature field of highly-deviated wells in backwashing condition is established. To verify the model, a test instrument to measure the temperature variations in the wellbore is developed and used to measure the temperature of two typical highly-deviated wells in backwashing condition. The comparative analysis shows that the wellbore temperature predicted by the model is close to that measured in field with the relative error less than 8%. The model was further used to analyze the influence of liquid-injection speed and injection time on the temperature distributions of oil pipe, annulus space and radial direction under the backwashing operation with original-temperature and hot washing liquid. It is found that for the backwashing operation with original-temperature washing liquid, the temperature at the lower part of the shaft is most affected by the displacement rate and backwashing time, whereas the curve section and the lower part of the shaft are most affected by the displacement rate and backwashing time in hot washing operation. Along the depth of the borehole, there is a constant temperature section which is almost unaffected by displacement rate and backwashing time. The radial heat transfer analysis shows that the heat exchange range is not large and near the wellbore.

Key technologies for well drilling and completion in ultra-deep sour gas reservoirs, Yuanba Gasfield, Sichuan Basin

The Yuanba Gasfield is a large gas field discovered by Sinopec in the Sichuan Basin in recent years, and another main exploration area for natural gas reserves and production increase after the Puguang Gasfield. The ultra-deep sour gas reservoir in the Yuanba Gasfield is characterized by complicated geologic structure, deep reservoirs and complex drilled formation, especially in the continental deep strata which are highly abrasive with low ROP (rate of penetration) and long drilling period. After many years of drilling practice and technical research, the following six key drilling and completion technologies for this type reservoir are established by introducing new tools and technologies, developing specialized drill bits and optimizing drilling design. They are: casing program optimization technology for ROP increasing and safe well completion; gas drilling technology for shallow continental strata and high-efficiency drilling technology for deep high-abrasion continental strata; drilling fluid support technologies of gas–liquid conversion, ultra-deep highly-deviated wells and horizontal-well lubrication and drag reduction, hole stability control and sour gas contamination prevention; well cementing technologies for gas medium, deep-well long cementing intervals and ultra-high pressure small space; horizontal-well trajectory control technologies for measuring instrument, downhole motor optimization and bottom hole assembly design; and liner completion modes and completion string optimization technologies suitable for this gas reservoir. Field application shows that these key technologies are contributive to ROP increase and efficiency improvement of 7000 m deep horizontal wells and to significant operational cycle shortening.

Evaluation of casing integrity defects considering wear and corrosion – Application to casing design

Casing integrity is an important category of well integrity in drilling and well operations. Casing integrity defect due to wear and corrosion can cause casing strength degradation, casing deformation and even well abandonment. In this paper, a theoretical model for casing strength degradation due to wear is established in bipolar coordinate system. Another model is established to calculate stress concentration factor of casing with corrosion pit at inner wall. The effects of relevant parameters on residual strength of defective casing are analyzed according to parametric study. According to the stress distribution of casing after wear and corrosion under tri-axial stress, strength check is carried out based on Von Mises yield criterion and Lame thick-walled solution for the pipe. Then, in order to evaluate casing integrity of a real well, casing wear and corrosion experiments are conducted to study mechanisms and obtain relevant parameters for calculation. Finally, according to the established models and experimental results, a modified casing design of this well is proposed considering the effects of wear and corrosion on casing strength degradation. The work presented in this paper can provide a theoretical foundation and technological basis for casing integrity evaluation and casing design of highly-deviated wells or extended-reach wells in sour environment.

A new concentric zonal water injection technique for highly-deviated wells

Abstract Existing eccentric zonal water injection techniques cannot meet the requirements of separated layer water injection, testing and adjustment in highly-deviated wells, because of their low fishing success rate of down-hole nozzles, low efficiency of connection between instruments and nozzles, and low precision of zonal flow rate test. A new concentric zonal water injection technique is proposed for highly-deviated wells. This technique adopts concentric testing and adjustment, i.e., the test instrument is concentrically connected with the down-hole regulator, with high success rate of connection. The down-hole regulator uses bridge-type channel to effectively eliminate inter-layer interference and thereby improving testing and adjustment efficiency. Water flow rate is adjusted with an eccentric valve, which features small loss in full-off state, small adjustment torque and easier water adjustment. This technique is functional for both collected flow test and non-collected flow test, so it is workable for low flow rate test. It can achieve packer seal test on line by cable, the status of seal balls can be monitored and all seal test operations can be completed in one trip. This technique can obtain flow rate, pressure, temperature and other parameters of any layer on line and automatic testing and adjustment of injection allocation rate, without fishing, significantly improving testing and adjustment efficiency. Until now, this technique has been tested in 11 wells (maximum well deviation of 55°), with tested flow rate error of less than 5%, and three-layer seal check and adjustment time of less than one day. Compared with the existing techniques, this new technique has advantages such as high testing and adjustment efficiency, high precision and low cost.

Study on Cuttings Starting Velocity in Air Drilling Horizontal and Highly-Deviated Wells

Air injection rate as the key parameter of drilling air is directly related to the success of drilling procedure. In the previous studies, most of air injection rate models were built based on the vertical wells, which are not appropriate for horizontal and highly-deviated well and limit the wide application of air drilling to some extent. Based on the movement analysis of cuttings, this paper assumed that after the cuttings deposit to the low side of borehole, Air is injected to carry the cuttings out of the wellbore. Then, the starting velocity model in different hole angles is established through the stress analysis of cuttings motion, which can be used to calculate the minimum air injection rate. The numerical simulation results show that starting velocity of cuttings reaches the maximum in the hole angle 60-70 degrees. The larger diameter cuttings require a high starting velocity, which is difficulty to be transported. This means some appropriate methods should be applied for air drilling procedure, like using secondary crush tools. The subsidence degree is one important factor to the calculation model, and it is necessary to study cutting subsidence degree in next step.

Cuttings Transport with Gas in the Highly-Deviated Horizontal Wells

Gas drilling has been widely used due to its advantages of protecting and finding oil and gas reservoir, reducing lost circulation and pipe sticking, extending the bit life, and improving rate of penetration (ROP). Cuttings transport in highly-deviated and horizontal segment is considered as an important part of the design of hydraulic parameters. In this case, poor cuttings transport will lead to stuck pipe, higher drag and torque, slower ROP, bad cementing quality, and other problems. In this study, a two-layer steady model was developed to depict cuttings transport behavior, and the sensitivity analyses were conducted. The simulations show the ratio of bed area and annular area and pressure drop decrease with the increase of pipe eccentricity or inclination, and increase with the increasing gas density or gas viscosity. But the inclination, gas density and viscosity have a minor effect on cuttings transport.

FORMATION EVALUATION IN HIGHLY-DEVIATED, LONG REACH DEVELOPMENT WELLS ON THE NORTH RANKIN GAS FIELD, NW SHELF, AUSTRALIA

A large proportion of the North West Shelf development gas wells are long reach (greater than 3500 m) and highly deviated. For reservoir description and management purposes, comprehensive formation evaluation needs to be carried out in these wells.Considerable difficulties have been encountered with electric log data acquisition due to friction and borehole conditions in these long, highly-deviated wells. As a result, new techniques to log the zones of interest were introduced. A system using the drill pipe to transport the downhole logging tools has been successfully used.Also, low-toxicity oil-based mud (LTM) was introduced in order to ease drilling problems and borehole conditions. However, owing to the non-conductive nature of the oil-based drilling fluid, improvements were required in the vertical resolution of the resistivity measurements and the estimation of the formation porosity.A computer program using a forward deconvolution technique recently developed by Shell's research laboratory in Holland has been successfully applied to enhance the vertical resolution of the resistivity log reading.The large range of uncertainty on the pore volume has been reduced to reasonable level by calibrating the porosity log data against core data obtained in a well drilled with LTM.