Search

It is reported that supercritical carbon dioxide (SC-CO2) jet can efficiently erode rocks at a comparably low threshold pressure with high penetration rate. However, the influences of bottom-hole pressure and borehole irregularities on SC-CO2 jet cutting efficiency have not been studied. Therefore, in this research, comprehensive methods of numerical simulations and lab experiments were carried out to investigate the influences of the ambient pressure and the nozzle-to-target distance on the jet impinging pressure and perforation performance. Results show that, both the effective jet impinging pressure and the eroded depth of perforation hole notably decrease with the increase of the ambient pressure, when jet inlet pressure is constant. When the pressure difference between inlet pressure and ambient pressure is kept constant, the effective impinging pressure hardly changes with ambient pressure, but eroded depth increases at first and then decreases, bounded by the critical pressure of CO2. As the nozzle-to-target distance extends, both the depth and volume of the perforation hole decrease, and diameter of the perforation hole increases at first and then decreases. Under the research condition of certain pressure and temperature, a distance of 8 times the nozzle diameter is the critical distance that clarifies the different effect of nozzle-to-target distance on the hole diameter. Different features between using SC-CO2 jet and water jet, including the optimal distance for rock-erosion efficiency, are attributed to the difference in fluid properties. Further simulation results show that, unlike water, SC-CO2 fluid is compressible and it leads to specific variations of jet structure, namely the increase of length of SC-CO2 jet potential core, and flow type from non-submerged jet to submerged jet. As a consequence, under the simulated bottom-hole conditions, SC-CO2 jet is proven to be able to acquire high rock-erosion efficiency and the jet-assisted drilling rate at a larger application ranges. This research can promote the future application of SC-CO2 jets.

Low rate of penetration has been recognized as a major problem affecting the cost of operation in drilling ultra-deep wells (UDW) in the oil and gas industry. This study investigated hydraulic pulse jet (HPJ) technique for increasing rate of penetration in UDW drilling. Results of full-scale testing, numerical simulation, and field applications in the UDW conditions are presented in this paper. The full-scale testing indicates that the HPJ has characteristics of fluid-flow rate dependent amplitude and frequency of pressure fluctuation. This performance was confirmed by numerical simulation of fluid mechanics inside the tool. Field applications in the Tarim and Tahe oilfields, China, show that the rate of penetration (ROP) was increased by 22.2%–54.9% in drilling well sections ended at up to 6162 m. It has been concluded that the HPJ technique is suitable to drilling ultra-deep wells in China.

Hydro-jet fracturing is a unique hydraulic fracturing method, and has been recognized worldwide as an effective well-stimulation method for oil and gas production. However, because of controversy about the method's impact on water consumption and environmentally quality, hydraulic fracturing faces increasing restrictions. The demand for decreased water usage has promoted research on the use of non-aqueous fracturing fluids. In particular, the idea of using supercritical carbon dioxide (SC-CO2) fluid has attracted a lot of interest. Using SC-CO2 as a fracturing fluid is more effective and has significant advantages compared with water-based fluids. The unique properties of the SC-CO2 fluid facilitate fracture propagation, formation of complex fracture networks, and elimination of flow blockage in the rock matrix and pores. Therefore, the SC-CO2 jet fracturing technique may effectively enhance oil and gas recovery. Similar to hydro-jet fracturing, the pressurization effect in the perforation tunnel is a key precondition for successful jet fracturing with SC-CO2. This study investigated these effects using numerical simulations and experimental studies. Our results show that the pressurization effect during SC-CO2 jet fracturing improves with an increase in pressure difference, ambient pressure, nozzle diameter, and fluid temperature. However, pressurization becomes weaker with increasing casing hole diameter and annulus spacing. Comparing SC-CO2 and water jet fracturing pressurization effects suggests that using SC-CO2 jetting has advantages over water jet fracturing under in situ reservoir conditions. As the working depth increases, SC-CO2 jet fracturing achieves better pressurization than the water jet. The novel approach in our preliminary study begins to prove the feasibility of applying SC-CO2 jet fracturing to leverage unconventional resources.

Pulsating hydro-fracturing (PHF) technology, based on hydraulic fracturing and pulsating injection, is extremely applicable for coal seams because of the remarkably low mechanical strength of coal rock. Consequently, PHF is currently under development, and new applications are being explored in China. Current research studies on PHF mainly focus on coal rock damage via experimental methods; however, to the best of our knowledge, none of these studies consider stress disturbances, which significantly impact the stimulation effect. This paper presents a stress-disturbance numerical model in an infinite elastic formation that uses a high-order, staggered grid finite-difference scheme and boundary conditions that combine a perfectly matched layer with a quasi-static confining pressure load. We define the effective stress disturbance zone as an area where the principal tensile stress exceeds the tensile strength of the coal rock and investigate the influences of the technical parameters (frequency, amplitude, and mean stress) on the PHF stress disturbance effect. Several unique behaviors were observed. First, the effective disturbance zone caused by PHF is much larger than the zone caused by quasi-static fracturing. Second, although the disturbance zone created by PHF can be improved by increasing the source frequency, limitations in the stress parameters likely result in a threshold frequency. Third, in contrast with frequency, the disturbance zone created by PHF is positively correlated with the amplitude and mean stress, which have no threshold values. Finally, the impacts of the pulsating stress parameters are greater than those of the time parameter. As a result, the amplitude and mean stress should be maximized, while an excessively high frequency is unnecessary. These research results will provide guidance for optimizing the technical parameters for using pulsating hydro-fracturing technology in a coal seam.