Hydrate plug in the drainage line is a serious flow assurance problem for the pilot production of offshore natural gas hydrates. Current research focuses on hydrate deposition in the annular flow and the oil-dominated system. The multiphase flow system in the drainage line is a water-dominated system which is normally a bubbly flow. In this work, a new model is developed to study the temperature and pressure field in the drainage line considering that the flow pattern is bubbly flow. Combining with the methane hydrate phase equilibrium curve, the hydrate formation region in the drainage line can be established. The hydrate formation region is enlarged with the ESP pressure increasing and the water production rate decreasing, since the ESP can supply extra pressures in the drainage line and the heat transfer phenomenon is enhanced between the drainage line and environment under the low water production rate condition. The model pointed out that the risk of hydrate formation rises up as the hydrate concentration increases beyond 6%. This study can lay a theoretical foundation for the efficient prevention of gas hydrates in the drainage line during offshore natural gas hydrate pilot or long-term production.
The subsea production system is presently widely adopted in deepwater oil and gas development. The throttling valve is the key piece of equipment of the subsea production system, controlling the safety of oil and gas production. There are many valves with serious throttling effect in the subsea X-tree, so the hydrate formation risk is relatively high. In this work, a 3D cage-sleeve throttling valve model was established by the numerical simulation method. The temperature and pressure field of the subsea throttling valve was accurately characterized under different prefilling pressure, throttling valve opening degree, and fluid production. During the well startup period, the temperature of the subsea pipeline is low. If the pressure difference between the two ends of the pipeline is large, the throttling effect is obvious, and low temperature will lead to hydrate formation and affect the choice of throttling valve material. Based on the analysis of simulation results, this study recommends that the prefilling pressure of the subsea pipe is 7–8 MPa, which can effectively reduce the influence of the throttling effect so that the downstream temperature can be kept above 0°C. At the same time, in regular production, a suitable choke size is opened to match the production, preventing the serious throttling effect from a small choke size. According to the API temperature rating table, the negative impact of local low temperature caused by the throttling effect on the temperature resistance of the pipe was considered, and the appropriate subsea X-tree manifold material was selected to ensure production safety. The hydrate phase equilibrium curve is used to estimate the hydrate formation risk under thermodynamic conditions. Hydrate inhibitors are injected to ensure downstream flow safety.
A vug porosity system, in addition to a matrix, is also the target rocks for the acidizing. In this work, the acidizing process in two typical core-scale separate-vug porosity systems is studied in detail. Numerous cases are conducted to discuss a parametric study on the acidizing process and hydraulic behavior. Results indicate that the presence of vug reduces the pore volume of acid solution consumed to achieve a breakthrough, which is consistent with experimental observations. Increasing the vug diameter and porosity decreases pore volume to breakthrough both for a vugular carbonate rock and isolated vug carbonate rock. In comparison, the acid mass does not change a lot. Typical dissolution patterns can also be observed in the acidizing process when a vug exists. Compared to matrix dissolution patterns, the presence of vug induces wormhole to pass through the vug region.
Carbonate reservoirs are one of the most important fossil fuel sources, and the acidizing stimulation is a practical technique for improving the recovery of carbonate reservoirs. In this study, the improved two-scale continuum model, including the representative elementary volume (REV) scale model and the upscaling model, is used to study the acidizing process with an isolated fracture. Based on this model, a comprehensive discussion is presented to study the effect of the physical parameters of the isolated fracture on the acidizing results and dissolution images, including the isolated fracture geometry, location, and morphology. Results show that the isolated fracture system is still the target system for the acidizing stimulation. The isolated fracture provides a limited contribution to the core porosity. The permeability of the core sample with fracture can be obviously increased only when the fracture penetrates through the whole sample. The existence of the isolated fracture reduces the consumption of acid solution to achieve a breakthrough. The acidizing curve is sensitive to the change of the length, aperture, and position of the isolated fracture. The acidizing curve difference corresponding to different rotation angles has not changed significantly for clockwise rotation and anticlockwise rotation groups.
Ferrofluid is a kind of magnetic fluid, the flow of which is controlled by an external magnetic field. Owing to this property, ferrofluid, as a new function material, has raised extensive concern in the oil industry. In this paper, the issue of ferrofluid flow in complex porous media has been studied by numerical simulation, and the validity and accuracy of the numerical algorithm are demonstrated through a 1-D horizontal tube example. Later, the effects of the magnetic force on ferrofluid flow in complex porous media, such as heterogeneous and fractured porous media, are investigated. The results show that there is basically no flow in low permeability or secondary fracture without a magnetic field, due to the characteristics of porous media and displacement pressure distribution. However, the ferrofluid flow velocity and domain can be changed by applying an external magnetic field. This novel phenomenon may provide a new idea to enhance oil recovery based on controllable flooding technology or meet other industrial needs by using ferrofluid.