Effects of a Nano-Silica Additive on the Rock Erosion Characteristics of a SC-CO2 Jet under Various Operating Conditions
In order to improve the erosion capacity of a supercritical carbon dioxide (SC-CO 2 ) jet, the influence of a nano-silica additive on the rock erosion characteristics was experimentally investigated. By impinging the SC-CO 2 jets with nano-silica mass fractions of 0 wt % (pure SC-CO 2 jet), 3 wt %, 6 wt %, 9 wt %, 12 wt %, 15 wt %, and 18 wt % on specimens of red sandstone, the erosion volumes under various operating conditions were measured and analyzed. Results show that an appropriate amount
... appropriate amount of nano-silica additive can greatly enhance the erosion ability of a SC-CO 2 jet. The effect on the erosion ability largely depends on the operating conditions. For instance, when the other conditions are fixed, 6 wt %, 9 wt %, 12 wt %, and 15 wt % were the optimum mass fractions, successively, with the inlet pressure increasing from 30 MPa to 60 MPa. With the increase in ambient pressure, the optimum mass fraction is unchanged under the constant inlet pressure, while it increases under the constant pressure drop. Additionally, the optimum mass fraction decreases when the fluid temperature increases. In addition, the optimal standoff distances are about five times the nozzle diameter of the nano-silica SC-CO 2 jet, and three times for the pure jet. This research provides a new method for effectively enhancing the rock erosion performance of a SC-CO 2 jet. In the late 1990s, Kolle  first used SC-CO 2 as the drilling fluid in coiled-tubing drilling technology, as shown in Figure 1 , and conducted initiative research on the rock erosion of a SC-CO 2 jet. The results showed that the rock erosion capacity of a SC-CO 2 jet is stronger than that of a water jet, and that the threshold pressure is lower than that of a water jet. They also concluded that the rate of penetration in Mancos Shale, and the drilling specific energy using a SC-CO 2 jet, is 3.3 times and approximately 20% of that when using a water jet, respectively. In recent years, to make better use of a SC-CO 2 jet by maximally increasing the erosion capacity, many researchers have been trying to understand the jet impingement characteristics and optimize the operating parameters. For instance, an attempt was made by Du et al.  to comprehensively investigate the influences of various factors on the rock erosion performance of a SC-CO 2 jet. The experimental data indicated that the erosion capability increases with the increase of the nozzle diameter or the standoff distance, until it reaches the optimum value. The increase of inlet pressure can improve the erosion capacity. Moreover, the SC-CO 2 jet can always provide a better rock erosion performance than the subcritical liquid CO 2 jet. Similarly, with the use of a simulation well bore device, Wang et al.  also conducted a series of experiments on the rock erosion efficiency of a SC-CO 2 jet. The results showed that the rock erosion efficiency decreases with increasing ambient pressure, and initially increases, before decreasing, with the increase of fluid temperature. Additionally, the rotary speed of the core sample has an influence on the average width of the erosion grooves, but no obvious influence on the erosion depth. By employing methods of scanning electron microscopy (SEM) analysis, He et al.  carefully studied the rock failure mechanism and the change in the pore structure of rock specimens, after having been impinged by a SC-CO 2 jet. They found that the SC-CO 2 jet erodes rock substances, mainly in the brittle tensile failure mechanism, accompanied with shear failure mechanism. Furthermore, the SC-CO 2 jet appears to be more efficient and suitable than a water jet forslim-hole radial drilling and hydraulic fracturing, particularly in unconventional reservoirs with low permeability. Simultaneously, the flow field of a SC-CO 2 jet was studied using the computational fluid dynamic method, by Wang et al.  . The results showed that the SC-CO 2 jet has a stronger impact pressure and a higher velocity than a water jet does, when under the same conditions. They also claimed that the maximum velocity and impact pressure of the SC-CO 2 jet, increase with the increasing nozzle pressure drop. Moreover, the increasing fluid temperature has almost no effect on the impact pressure, but can increase the maximum velocity. Also, Long et al.  numerically investigated the impinging flow field in the bottom hole. Their results illustrated that the increasing inlet temperature can increase the axial velocity, and reduce both the mass flow rate and the impingement of the SC-CO 2 jet. To further study the dynamic flow characteristics of a SC-CO 2 jet at the bottom hole, Wang et al.  conducted a series of simulations and experiments to understand the bottom hole temperature and pressure distributions of SC-CO 2 fluid. The results showed that the bottom hole impact pressure and temperature, increase with increasing nozzle diameter. When the standoff distance increases, the bottom hole temperature decreases, while the bottom hole pressure initially rises, before dropping. Additionally, the bottom hole pressure and temperature both increase with the ascent of inlet pressure. Moreover, by combining the method of computational fluid dynamic (CFD) and lab experiments, Tian et al.  investigated the influence of ambient pressure and standoff distance on the impinging pressure and perforation performance of a SC-CO 2 jet. The results showed that, when the inlet pressure is constant, the effective impact pressure and erosion depth notably decrease, with increasing ambient pressure. When the nozzle pressure drop is constant, the erosion depth increases at first, but then decreases with increasing ambient pressure. In spite of the many works that have been completed on the effects of thermodynamic conditions and operating parameters on the rock erosion capacity of a SC-CO 2 jet, a study that associates the rock erosion events with an appropriate amount of proper additive, has not yet been pursued. It is commonly known that various additives have been applied in water jet technology, in order to enhance the cleaning, crushing, and erosion capacity. For instance, experimental studies were performed by Massimilinano et al. , to provide a quantitative assessment of the effect of additives on the erosion capacity of a water jet. The results indicated that both the erosion surface quality, and the erosion depth Appl. Sci. 2017, 7, 153 3 of 17 of the specimen, are greatly enhanced by this method. This suggests that a proper additive is able to alter the jet flow field, and subsequently, changes the erosion characteristics of a water jet. Similarly, Hu et al.  experimentally investigated the effect of additives on the erosion quality of marble cut by an abrasive water jet, concluding that a certain fraction of additive can improve the erosion quality. Therefore, it is reasonable to speculate that a proper additive should also have great effects on the erosion characteristics of a SC-CO 2 jet. Moreover, nano-silica is an appropriate kind of additive for improving the filtration properties of drilling fluid and enhancing the well bore stability, especially under high temperature and pressure conditions     . Also, Li et al. experimentally investigated the effect of CO 2 fracturing, influenced by a nano-silica additive, and concluded that the effect of nano-silica causes a decrease in CO 2 fingering and an increase in the drainage area. Additionally, the argued that the CO 2 fracturing effect can be enhanced by the nanoparticles  .