Study of the slippage of particle / supercritical CO2 two-phase flow

In this paper, the slippage velocity and displacement between particles and supercritical CO2 (SC-CO2) were studied to reveal the particle-SC-CO2 two-phase flow behavior. Visualization experiments were performed to directly measure the slippage velocity and displacement. Eight groups of experiments involving various pressures (7.89–10.96 MPa), temperatures (38.6–47.5 °C), particle diameters (0.3–0.85 mm), particle densities (2630 and 3120 kg/m3) and SC-CO2 flow rates (0.920–1.284 m/s) were conducted. The measured particle slippage velocities in the flowing direction were approximately 10.3% of the SC-CO2 flow rate. The measured particle slippage displacements were all at the centimeter level, which indicated that SC-CO2 had a superior particle transporting capability that was similar to those of liquids even if it had a low viscosity that was similar to those of gases. A numerical model was built, and analytic slippage calculations were performed for SC-CO2 for additional analyses. The density of SC-CO2 was found to have a greater influence on the slippage than the viscosity. Moreover, a comparison of the slippage between SC-CO2 and water showed that the particle slippage in water was constant, while the particle slippage in SC-CO2 continually accumulated at an extremely slow rate

A new post-frac evaluation method for shale gas wells based on fracturing curves

AbstractPost-fracturing evaluation by using limited data is of great significance to continuous improvement of the fracturing programs. In this paper, a fracturing curve was divided into two stages (i.e., prepad fluid injection and main fracturing) so as to further understand the parameters of reservoirs and artificial fractures. The brittleness and plasticity of formations were qualitatively identified by use of the statistics of formation fracture frequency, and average pressure dropping range and rate during the prepad fluid injection. The composite brittleness index was quantitatively calculated by using the energy zones in the process of fracturing. It is shown from the large-scale true triaxial physical simulation results that the complexity of fractures is reflected by the pressure fluctuation frequency and amplitude in the main fracturing curve, and combined with the brittleness and plasticity of formations, the fracture morphology far away from the well can be diagnosed. Well P, a shale gas well in SE Chongqing, was taken as an example for post-fracturing evaluation. It is shown that the shale beds are of stronger heterogeneity along the extension directions of horizontal wells, and with GR 260 API as the dividing line between brittleness and plasticity in this area, complex fracture systems tend to form in brittleness-prone formations. In Well P, half of the fractures are single fractures, so it is necessary to carry out fine subsection and turnaround fracturing so as to improve development effects. This paper provides a theoretical basis for improving the fracturing well design and increasing the effective stimulated volume in this area

An evaluation method of supercritical CO2 thickening result for particle transporting

This paper aims to propose an evaluation method for measuring the supercritical CO2 thickening result for particle transporting. By analyzing the particle transporting trajectory in supercritical CO2, the cotangent of particle landing angle (ratio of particle horizontal velocity to vertical velocity) was proposed as a new criterion of thickening result. Previous studies of supercritical CO2 thickening were evaluated and drawn in three dimensions using this new criterion. Moreover, the effects of CO2 density and viscosity on particle vertical and horizontal velocities and the cotangent of particle landing angle were analyzed. The cotangent of particle landing angle is more sensitive to supercritical CO2 density than viscosity. Therefore, supercritical CO2 density should be considered for the evaluation of supercritical CO2 thickening for particle transporting. The particle settling velocity was found to determine the particle transporting distance and also the transporting capability of supercritical CO2. According to this conclusion, the apparatus for experimental evaluation of supercritical CO2 thickening will be miniaturized significantly by the simplification from two-dimensional velocities measurement into one direction, particle settling velocity in vertical direction. Additionally, the supercritical CO2 viscosity was found to have an optimum value, exceeding which the effect of viscosity on particle transporting levels off

Calculation on the following performance of proppant in supercritical carbon dioxide

As a new type of fracturing fluid, supercritical carbon dioxide (SC-CO2) has now become the research hotspot in domestic and foreign academia. However, the relevant technique is limited by the unclear sand-carrying mechanism. In this study, the concept of following performance is introduced to investigate the sand-carrying performances of fracturing fluid; the following performance of proppant and the sand-carrying performance of fracturing fluid are characterized by the ratio of the proppant horizontal velocity to the fracturing-fluid horizontal flow velocity. Then the following performance of proppant is used as the criteria for evaluating the horizontal sand-carrying performances of SC-CO2. Based on the classic BBO equation, the expression of drag force has been modified; in combination with the auxiliary equations of drag force coefficients, a calculation model has also been established to analyze the following performance of proppant in SC-CO2, which provides the theoretical foundation for evaluating and optimizing the transportation of proppant in SC-CO2. To verify the correctness of the model, an experiment is conducted on the following performance of proppant using the self-developed equipment, and then the factors that affect the following performance of proppant in SC-CO2 are analyzed. The results show that the following perfomance of proppant is slightly affected within the common range of density, which is consistent with the previous experimental results. In addition, a comparative analysis is performed on the following performance of sands in SC-CO2, slickwater and air. The analysis results indicate that the sand-carrying performance of SC-CO2 is more affected by its high-density property but less affected by its low viscosity. Because of high-density and low-viscosity properties, SC-CO2 is far better than air but worse than or approximately equal with slickwater in terms of sand-carrying performances. Especially at a high flow velocity, the horizontal sand-carrying performance of SC-CO2 is comparable to that of slickwater

Volume fracturing of deep shale gas horizontal wells

Deep shale gas reservoirs buried underground with depth being more than 3500 m are characterized by high in-situ stress, large horizontal stress difference, complex distribution of bedding and natural cracks, and strong rock plasticity. Thus, during hydraulic fracturing, these reservoirs often reveal difficult fracture extension, low fracture complexity, low stimulated reservoir volume (SRV), low conductivity and fast decline, which hinder greatly the economic and effective development of deep shale gas. In this paper, a specific and feasible technique of volume fracturing of deep shale gas horizontal wells is presented. In addition to planar perforation, multi-scale fracturing, full-scale fracture filling, and control over extension of high-angle natural fractures, some supporting techniques are proposed, including multi-stage alternate injection (of acid fluid, slick water and gel) and the mixed- and small-grained proppant to be injected with variable viscosity and displacement. These techniques help to increase the effective stimulated reservoir volume (ESRV) for deep gas production. Some of the techniques have been successfully used in the fracturing of deep shale gas horizontal wells in Yongchuan, Weiyuan and southern Jiaoshiba blocks in the Sichuan Basin. As a result, Wells YY1HF and WY1HF yielded initially 14.1 × 104 m3/d and 17.5 × 104 m3/d after fracturing. The volume fracturing of deep shale gas horizontal well is meaningful in achieving the productivity of 50 × 108 m3 gas from the interval of 3500–4000 m in Phase II development of Fuling and also in commercial production of huge shale gas resources at a vertical depth of less than 6000 m