Deli Gao

A review of down-hole tubular string buckling in well engineering

Down-hole tubular string buckling is the most classic and complex part of tubular string mechanics in well engineering. Studies of down-hole tubular string buckling not only have theoretical significance in revealing the buckling mechanism but also have prominent practical value in design and control of tubular strings. In this review, the basic principles and applicable scope of three classic research methods (the beam-column model, buckling differential equation, and energy method) are introduced. The critical buckling loads and the post-buckling behavior under different buckling modes in vertical, inclined, horizontal, and curved wellbores from different researchers are presented and compared. The current understanding of the effects of torque, boundary conditions, friction force, and connectors on down-hole tubular string buckling is illustrated. Meanwhile, some unsolved problems and controversial conclusions are discussed. Future research should be focused on sophisticated description of buckling behavior and the coupling effect of multiple factors. In addition, active control of down-hole tubular string buckling behavior needs some attention urgently.

Buckling Analysis of Tubular Strings in Horizontal Wells

A new buckling equation in horizontal wells is derived on the basis of the general bending and twisting theory of rods. The boundary conditions of a long tubular string are divided into two categories: the sum of the virtual work of bending moment and shear force at the ends of tubular strings is equal to zero, and the sum of the virtual work of bending moment and shear force at the ends is not equal to zero. Buckling solutions under different boundary conditions are obtained by solving the new buckling model. For the boundary conditions of the first category, the buckling solutions are identical with previous results. For the boundary conditions of the second category, the buckling solutions are different from the results under the boundary conditions of the first category. The results indicate that buckling behaviors depend on both the axial force and the boundary conditions. Compared with previous results, buckling solutions of the new model provide a more comprehensive description of tubular-buckling behaviors.

The Significance of Fracture Face Matrix Damage to the Productivity of Fractured Wells in Shale Gas Reservoirs

The significance of fracture face matrix damage in fractured shale gas wells is not clear. A new mathematical model was developed to predict the effect of fracture face matrix damage on productivity of fractured gas wells in shale gas reservoirs. Results of the model analyses were sensitized to reservoir properties and facture face matrix skin properties determined by the fracturing fluid properties and treatment conditions. The authors conclude that fracture face matrix damage is strongly affected by the properties of fracturing fluids. Using the lowest possible leak-off coefficient and spurt-loss coefficient can significantly reduce fracture face matrix damage and retain gas well productivity.

New Method for Predicting Casing Wear in Horizontal Drilling

Abstract With the development of horizontal drilling engineering, casing wear is becoming increasingly prominent. Based on the effect of contact pressure instead of contact force on the casing wear rate caused by tool joint rotations, the new casing wear model is established. The casing wear factor is not constant in the new model, but decreases with the increasing of the casing wear groove depth, and it is affected by the previously worn casing geometry and the good agreement between the numerical predictions and the experimental results showed that the new casing wear model is validated.

A method for calculating tubing behavior in HPHT wells

During well testing and production, the helical buckling often occurs near the lower end of tubing and friction appears. When loading continues or temperature increases, the tubing deformation will develop under post buckling condition, and it is difficult to describe the axial force with analytic method, that is why numerical method is needed. In this paper, plastic incremental theory in plastic mechanics is used to calculate the axial force distribution of buckled tubing strings, according to which the final axial force and deformation are determined by both the initial state and the loading process. It is found that the axial force distribution of tubing with friction is changed significantly and is strongly dependent upon the operation steps. The friction can prevent the large loads caused by high pressure and/or high temperature from spreading so that only a small part of tubing deforms severely. This calculation method was verified by experiments and has been used in oil fields.

Effect of Fluid Temperature on Rock Failure in Borehole Drilling

AbstractDrilling fluids injected into geothermal, oil, and gas borehole create a temperature gradient in rock near the bottom hole. This temperature gradient is more pronounced in underbalanced drilling (UBD) with gaseous fluids because of the Joule–Thomason cooling effect at the drill bit. The effect of the temperature gradient on rock failure was investigated experimentally and interpreted by an analytical model in this study. The result indicates that the temperature gradient can significantly promote rock failure during drilling. The rate of penetration (ROP) increased by 22.4% on average as the temperature differential increased from 30 to 180°C in the Chagan Sandstone from the Tamuchage Basin, Mongolia. This explains high ROP, hole enlargement, and hole deviation in gas UBD operations. The analytical model developed in this work can be used for optimizing drill bit and fluid injection rate in geothermal, oil, and gas well drilling.

Research into magnetic guidance technology for directional drilling in SAGD horizontal wells

SAGD horizontal wells are used to enhance oil recovery from heavy oil reservoirs. This technology requires precise separation between the production well and the injection well to ensure the efficient drainage of the reservoir. By studying the attitude of the downhole probe tube and the production well trajectory, an algorithm is proposed for eliminating ferromagnetic interference while drilling injection wells. A high accuracy filter circuit has been designed to correct the detected magnetic signals, which are ultra-weak, frequency-instable, and narrow-band. The directional drilling magnetic guidance system (DD-MGS) has been developed by integrating these advanced techniques. It contains a sub-system for the ranging calculation software, a magnetic source, a downhole probe tube and a sub-system for collecting & processing the detected signals. The DD-MGS has succeeded in oilfield applications. It can guide the directional drilling trajectory not only in the horizontal section but also in the build section of horizontal injection wells. This new technology has broad potential applications.

Water-Based Drilling Fluid Technology for Extended Reach Wells in Liuhua Oilfield, South China Sea

Abstract To reduce the dependence of oil-based drilling fluids for extended reach well drilling, a newly developed water-based drilling fluid system has been used to drill the extended reach wells in Liuhua oilfield located in the east of the South China Sea. Based on numerous experiments, the formulations of this water-based system for various sections of extended reach wells were determined, and several key characteristics, such as lubricity, inhibition, and rheological properties, were evaluated. Using the well Liuhua 11-1-21 (A4) ERW3 as an example, the field applications of this water-based drilling fluid are elaborated in this article. Laboratory experiments and field tests indicate that this kind of specially formulated KCl/polymer system not only possesses excellent shale inhibitive character, but also has proper rheological parameters, good lubricity, and strong ability of anti-contamination for calcium ions. Hence, it can meet the demands of extended reach drilling operations in this area. A unique and integrated technology of water-based drilling fluids for extended reach well drilling in Liuhua oilfield has been formed.

A Comprehensive Prediction Model of Hydraulic Extended-Reach Limit Considering the Allowable Range of Drilling Fluid Flow Rate in Horizontal Drilling

Hydraulic extended-reach limit (HERL) model of horizontal extended-reach well (ERW) can predict the maximum measured depth (MMD) of the horizontal ERW. The HERL refers to the well’s MMD when drilling fluid cannot be normally circulated by drilling pump. Previous model analyzed the following two constraint conditions, drilling pump rated pressure and rated power. However, effects of the allowable range of drilling fluid flow rate (Qmin ≤ Q ≤ Qmax) were not considered. In this study, three cases of HERL model are proposed according to the relationship between allowable range of drilling fluid flow rate and rated flow rate of drilling pump (Qr). A horizontal ERW is analyzed to predict its HERL, especially its horizontal-section limit (Lh). Results show that when Qmin ≤ Qr ≤ Qmax (Case I), Lh depends both on horizontal-section limit based on rated pump pressure (Lh1) and horizontal-section limit based on rated pump power (Lh2); when Qmin < Qmax < Qr (Case II), Lh is exclusively controlled by Lh1; while Lh is only determined by Lh2 when Qr < Qmin < Qmax (Case III). Furthermore, Lh1 first increases and then decreases with the increase in drilling fluid flow rate, while Lh2 keeps decreasing as the drilling fluid flow rate increases. The comprehensive model provides a more accurate prediction on HERL.

Assessment of Wellbore Integrity of Offshore Drilling in Well Testing and Production

AbstractIn this paper, a mechanical model and analysis method have been established to analyze the characteristics of casing-cement-formation of offshore wells in testing and production. The model, which can be simplified to a plane strain problem, has considered the combined actions of temperature and nonuniform in-situ stress with appropriate boundary conditions. Subsequently, the corresponding equations have been deduced to obtain the mechanical responses of the model under the two kinds of actions mentioned previously. Then, a finite-element analysis has been carried out in ABAQUS framework to verify the correctness and accuracy of the theoretical derivation. Meanwhile, the specific stress distribution and its degree of nonuniformity, as well the casing safety factor, have been obtained through a case study. On this basis, the influences of cement elastic modulus, Poisson’s ratio, casing grade, and wellbore temperature on the mechanical properties have been presented. Finally, some significant conclus...