New Closed Blender Reduces Footprint, Gasification of CO2 in Waterless Fracturing
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Carbon dioxide (CO2) waterless fracturing uses liquid CO2 to replace water as the fracturing fluid in reservoir stimulation. The continuity and reliability of the blender are key factors determining performance of the operation. The complete paper proposes a novel closed blender that uses a vertical, rather than horizontal, tanker. This modification can reduce the footprint and effectively suppress CO2 gasification. The field practice suggests that the developed closed blender combines the advantages of both vertical and horizontal blenders and ensures successful implementation of CO2 waterless fracturing operations.
The CO2 waterless fracturing process, as an alternative technology for developing unconventional reservoirs, greatly reduces consumption of water resources in the development of unconventional resources. In addition, compared with traditional fracturing measures, this technology has the advantages of low reservoir damage, complex artificial fractures, good energy-storage effect, and a high degree of recovery because of the unique physical and chemical properties of CO2.
The CO2 waterless fracturing process requires the following equipment: CO2 tank trucks, booster pump trucks, sand blenders, high-pressure pump trucks, and low-and high-pressure manifolds. The blender acts as the core equipment for dry fracturing of liquid CO2, its main function to mix CO2 with proppant. It then feeds the mixed sand fluid to the fracturing pump truck for CO2 waterless fracturing. In the fracturing process, CO2 on the ground is required to be under low temperature and high pressure continuously to maintain its liquefaction. In addition, CO2 fluid has characteristics of low viscosity, high friction resistance, corrosiveness, and poor lubrication. Compared with traditional hydraulic fracturing processes, the sand blender for CO2 waterless fracturing has specific requirements.
Components of the Closed Blender
Drawing lessons from design of internationally available closed blenders for CO2 waterless fracturing, the authors have developed independently a new generation of closed blender, mainly composed of a sand tanker, a sand-mixing system, a blender manifold (a discharge manifold and a suction manifold), a chemical-addition system, a hydraulic system, an electronic control system, and a data-acquisition system (Fig. 1).
Sand Tanker. This key component adopts the structure of a vertical tanker truck. During transportation of the equipment, the tanker can be driven by, and then be placed on, the chassis truck. The tanker adopts a double-layer cylindrical structure, with the inner tube composed of 16MnDR high-quality alloy steel, the outer tube of Q235B high-quality carbon steel, and the pipeline of austenite stainless steel.
Because CO2 waterless fracturing has the characteristics of low temperature, high pressure, an airtight state, and easy corrosion, thermal insulation measures should be taken for the sand tanker. When the ambient temperature is 20°C, the gasification volume should not exceed 5 kg/h, with a 1-mm corrosion margin reserved for tanker-wall thickness. The sand tanker is equipped with a safety valve.
Sand-Mixing System. This component includes a CO2 inlet, a sand inlet, a sand auger conveyor, and a discharge outlet. The contact part of the mixing system consists of corrosion-resistant material. Sand is transported by both mechanical and nonmechanical means, and sand transportation is controlled collectively by the auger conveyor at the bottom of the blender and by adjustment of the opening of the butterfly valve and, thus, the fluid-infusion rate of the sand-mixing tanker. Proppant is added to the liquid CO2 in the main pipeline proportionally with the automatic sand-mixing function to realize the automatic addition of sand at the set ratio. The proppant in the sand tanker enters the sand inlet by gravity and then enters the auger conveyor by a remotely controlled valve.
The sand-supply process is controlled by the difference between the pressure inside the tanker and at the end of the tanker and the rotating speed of the auger conveyor. Through proper adjustment of the fluid-infusion rate, a relatively stable micropositive pressure difference between the inner tanker and the end of the tanker is formed, achieving a stable sand supply.
The auger conveyor is connected structurally with the main pipe at an angle to the main sand-mixing channel so that sand can be added to the main sand-mixing pipeline along the direction of liquid CO2 to achieve real-time mixing. The auger conveyor is driven by a quantitative hydraulic motor, and the speed can be regulated by adjusting the flow of the hydraulic system. Sand is added into the main mixing pipeline in the direction of an obtuse angle formed against the direction of the main mixing pipeline. After being homogenized by a mixing device, sand is supplied to each fracturing truck through the discharge manifold.
Blender Manifold. This mainly consists of a suction manifold and a discharge manifold. The two manifolds are made separately into two modules of pressure-bearing seamless steel tubes that are low-temperature-resistant and corrosion-resistant. A densitometer is mounted obliquely on the discharge manifold to ensure accurate and real-time density measurement of fracturing fluid.
Chemical Additive System. This system is equipped with a hydraulically driven pumping system at a variable speed for liquid chemical additives, which are added to the suction manifold from an external chemical storage tank. The system has two pumps with a maximum discharge pressure of 7 MPa and a maximum displacement of clean water of 95 L/min for each pump. The function of starting under pressure is configured for liquid chemical additives. High-precision flowmeters must be installed in each system to realize automatic proportioning.
Hydraulic System. The hydraulic cylinder, featuring both local and remote control, is equipped with a level gauge capable of quickly leveling the tanker. The hydraulic leg is provided with a hydraulic lock. Two closed hydraulic systems are required to drive two auger conveyors, respectively. A load-sensitive open hydraulic system is provided to drive the hydraulic leg, the turnover oil cylinder, and the liquid-feed system.
Electronic Control System. This system includes remote and local monitoring. Local control is provided with a monitoring meter and a control system. The monitored data mainly include liquid CO2 flow, instantaneous sand ratio, CO2 density, and working pressure.
The remote control is equipped with a portable monitor box that can monitor and control the sand blenders remotely and in real time. It controls all data-display and execution functions of the local control system. A reserved measuring-truck interface can help achieve integration with a measuring truck where all local monitoring and control can be realized.
Data-Acquisition System. This system collects the following data: instantaneous and cumulative flow discharged by the sand blenders, density of sand-carrying fluid, instantaneous and cumulative flows of proppant and additive, and pressure and temperature of the sand tanker. The data-acquisition system can display data in real time, replay historical data, and print data in the form of curve and data tables.
Field Test of CO2 Waterless Fracturing
Between 2014 and 2017, 19 wells in the Jilin oil field were tested successively in tight reservoirs and low-pressure-sensitive oil and gas reservoirs. Therefore, the key components of the featured system—closed blenders, construction technology, and safety control—were further verified. Breakthroughs were achieved in all operation parameters, including in one-time operation displacement (8 m3/min), sand volume (23 m3), and liquid volume (860 m3).
A typical well, R11-xx, was put into development in August 2005, and the water well began injection in May 2005. At the time of writing, the well is at the stage of secondary decline development.
The main contradictions in the targeted block are as follows: no gas production after injection, low single-well production, and poor returns from ordinary fracturing measures. The authors have conducted single-well tests by CO2 waterless fracturing technology in the R11 block for the purpose of verifying the stimulation technology and core equipment process.
Compared with the prefracturing period, oil production has doubled, oil pressure has risen from 0.5 to 12.4 MPa, and an obvious reservoir-storage effect has been noted. Yield of four adjacent wells has increased by 0.2–0.6 tons, with increased production.
New Closed Blender Reduces Footprint, Gasification of CO2 in Waterless Fracturing
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