Search

Corrosion is a common problem in offshore oil and gas wells, especially in thermal recovery wells, which will greatly affect the residual strength of casing strings. Field data show that the corrosion pattern of casing is mainly pitting, local corrosion and uniform corrosion, of which pitting is the most common. For this reason, based on the engineering practice of thermal recovery well casing corrosion, the dynamic changes of wellbore temperature and pressure during the gas production process of thermal recovery wells are considered, and the numerical model of casing corrosion under transient temperature and pressure coupling is established. The influence of corrosion morphology and corrosion depth on the residual strength of the casing is analyzed. The transient stress variation law of the corroded casing is studied. The results show that: (1) Under the same corrosion depth, both pitting and uniform corrosion will reduce the residual internal pressure and external extrusion strength of the casing string, and the degree of influence is similar. The effect of uniform corrosion on the residual tensile strength of the casing string is larger than pitting and local corrosion; (2) the residual strength of the casing decreases linearly with the increase of the corrosion depth of the casing; (3) the stress of the corrosion part of the casing shows a significant dynamic change, in the initial stage of steam injection, casing stress rises rapidly at the corrosion position, and then the rate at which the casing stress rises is slowed, which tends to cause fatigue damage to the casing. The research results have reference significance for the assessment of the service status of offshore thermal recovery well casings.

The torque impactor can effectively solve the stick-slip vibration of PDC bit in the deep formation, but the wear of the pendulum seriously restricted the service life of the impactor. In order to reduce the wear of the pendulum under complicated working conditions, the polycrystalline diamond composite material was used as the wear-resistant material to design the pendulum wear-resistant belt. Based on the analysis of the feasibility of using polycrystalline diamond as pendulum wear-resistant material, the finite element model of dynamic contact interaction between pendulum wear-resistant belt and impact cylinder was established, the influence of the arrangement of wear-resistant blocks on the stress of the contact area between the pendulum wear-resistant belt and the impact cylinder was studied. The results show: Polycrystalline diamond composite material has stronger wear resistance and better anti-friction performance than conventional wear-resistant materials, and can reduce the wear of the inner wall of the impact cylinder, which is feasible on the surface of the pendulum hammer. The use of the wear-resistant belt structure is beneficial to improve the variation of the shear stress of the area where the pendulum and the impact cylinder contact each other. The wear-resistant belt structure is beneficial to improve the shear stress variation amplitude of the pendulum and impact cylinder in contact with each other. When the wear-resistant block is arranged in a circular block staggered arrangement, the shear stress change amplitude of the interaction contact area is the largest, the triangular block staggered arrangement mode is second, and the trough block shape staggered arrangement is the smallest. The research results have important reference significance for the design and application of polycrystalline diamond composite pendulum wear-resistant belt.

The well integrity issues in oil and gas wells have a long history, especially in the shale gas development, which has a direct impact on improving single well production of shale gas horizontal wells. Reviewing the domestic and foreign literature researches, the development status of major shale gas blocks at home and abroad were introduced. Well integrity failure characteristics of major shale gas blocks at home and abroad were counted and analyzed in detail. Finally, considering the shale gas development blocks in China as an example, the structural integrity (casing deformation) and seal integrity (Sustained Casing/Annulus pressure, SCP/SAP) were analyzed by clarifying the failure mechanism that well integrity issues affected shale gas production and that shale gas well integrity issues in China are a new problem was thought. The failure mechanism of shale gas well integrity needs to be further clarified, and a complete set of failure prediction and control methods has not yet been formed. Therefore, based on the concept of shale gas geology-engineering integration, taking into account the actual conditions of shale gas development blocks, it is proposed to scientifically and reasonably study shale gas well integrity failure mechanisms and their control methods, which has a reference and guidance to improve shale gas development.

Casing pressure test is an important link of the oil and gas well cementing, but excessive casing pressure test may cause stress failure or plastically deformation of the cement sheath, and generate micro-annulus on the casing-cement sheath cemented surface, then lose seal integrity of the cement sheath. According to the basic theory of elastoplastic mechanics and considering the volume invariance of plastic strain and the influence of elastic strain on volume change, the Mohr-Coulomb criterion is used to establish the casing-cement sheath-surrounding rock combination model. The paper also derived the formula for calculating the micro-annulus of the casing pressure test, and analyzed the influence of the internal pressure of the casing during the pressure test and the elastic modulus of the cement sheath on the radial contact stress of the cement sheath cementation interface after the cementing operation is completed. Results show that: (1) The generation of micro-annulus of casing pressure test is determined by the pressure increase process and the pressure relief process. The pressure increase process may lead the cement sheath into plasticity, and the radial stress at the interface turns into tensile stress and the micro-annulus could appear at the first interface by the decrease of internal pressure during pressure relief process. (2) The varying internal pressure has a great influence on the fatigue failure of the cement sheath, so the continuous multiple casing pressure test should be avoided after the cementing is completed. (3) Under the condition of maintaining the integrity of the cement sheath, increasing the tensile strength of the cement stone and reducing the elastic modulus of the cement stone can improve the pressure bearing capacity of the first interface during the casing pressure test. The model can provide a theoretical basis for the mechanical parameter design of the cement sheath, and provide guidance for on-site construction to reduce or avoid the risk of failure of the first interface seal integrity for the casing pressure test.

Currently, most of the shale gas development in China's Sichuan region adopts the large-displacement staged fracturing technology. Some of the shale gas wells undergo casing deformation during stimulation of the reservoir, where the trapped pressure of cementing section has a non-negligible effect on the casing stress in the multi-stage fracturing process. To this end, this paper constructs a finite element model of casing-cement sheath-formation assemblage under shale anisotropy based on the step-by-step finite element method by taking into account the whole construction process of drilling, completion and fracturing, in order to analyze the effect of trapped pressure on the casing stress under varying missing degrees, temperature field variations and cement sheath-formation property variations of cementing section in the multi-stage fracturing process. The results show that: 1) the larger the temperature drop in the wellbore during fracturing, the greater the effect of trapped pressure on casing stress during instantaneous pumping termination. 2) The smaller the missing degree of cementing section, the greater the effect of trapped pressure on casing stress under variations of cement sheath and formation. Therefore, optimization of cementing quality and reasonable optimization of relevant fracturing operation parameters are necessary in the future shale gas fracturing stimulation, so as to ensure the normal proceeding of subsequent well completion works.

The stick-slip vibration is a kind of serious vibration which could cause serious damage to downhole tools and decrease the rate of penetration. The root cause of the stick-slip vibration has been analyzed, which indicate that the high reactive torque is the main cause of the bit-related stick-slip vibration, while the nonlinear friction force between the drill string and the borehole is the main cause of the string-related stick-slip. In order to mitigate the stick-slip and improve the ROP, a new type of composite impact drilling tool has been designed, this tool can transfer part of the hydraulic energy to the high frequency axial impact energy and the torsional impact energy to auxiliary to fail the formation. The composite impact could break the stick phase of the bit and decrease the friction force between the drill string and the borehole, which could effectively improve the drilling efficiency and ROP.

The deep marine carbonate formation is one of the key areas of oil and gas resource exploration and development for the future, and the basic characters of these resources are HTHP, high content of sour gas and narrow mud density window. Sour gases (H2S or CO2) are prone to invade into wellbore, leading to serious drilling accidents such as kick and blowout. Considering the sour gas solubility, phase transition and effects of temperature and pressure on the physical parameters of drilling fluid, a multiphase flow model for sour gas invasion is established, and a transient heat transfer and pressure coupling solving method is proposed to ensure the accuracy of numerical calculation. The results indicate that the annulus circulating temperature with H2S invasion is the highest, temperature with CO2 comes second and CH4 is the least. As gas solubility decreases and its phase transition near the wellhead, the volume of gas is expanded sharply and the gas phase volume fraction changes dramatically, which may lead to kick and blowout accidents instantly. When gas solubility is considered, a larger annular pressure could be got, and H2S has a larger influence on the annular pressure than CO2. Furthermore, the dissolved gas and annular pressure varying with different content of H2S or CO2 in sour gas are calculated and analyzed. The research results may provide valuable theoretical guidance for the deep sour gas reservoir exploration and development.

More cuttings producing in higher penetration rate of gas drilling are usually in the larger non-uniform size at the bottom and in the smaller uniform size back to the wellhead. It is indicating that cuttings in the well should be appeared in the successive breaking in the process of upward migration. According to the particle broken theories, the successive breaking phenomenon were analyzed, and given the calculating method for the probability of secondary impact crushing, and applied the breaking process matrix and the particle size distribution function to the cuttings migration crushing in the whole wellbore. The calculating results show that the cuttings in the wellbore are broken by the uneven size volumetric fracture progressively turned into the surface fracture of the uniform process, and ultimately stabilized the small size distribution.