Using a self-resonated cavitation jet can increase the salt dissolution rate and improve the efficiency of natural gas storage cavern construction, particularly in bedded (vs. domed) salt deposits. The rapid solution-mining tool can increase salt production sharply compared with conventional solution mining, accelerating storage construction. The tool also reduces debris settling in the bottom of a cavern.
Background
China's second West-East natural gas pipeline is under construction. Work includes associated underground gas storage salt caverns. Construction of salt caverns requires finding an appropriate salt dome or salt bed, drilling a well into the formation, and injecting fresh water to dissolve and extract a certain amount of salt to get a specified volume and shape.1 Bedded salt deposits are widespread and used as much or more than diapiric salt bodies to host storage facilities. These deposits are layered and interspersed with non-salt sedimentary materials such as anhydrite, shale, dolomite, and limestone.2
Solution mining bedded-salt formations by conventional methods is difficult. Dissolving and shear stress of circulated fluid break thin, weak interlayers, but thick and hard interlayers are difficult to break efficiently, slowing the dissolution rate and changing the salt cavern's shape. The erosion of interlayers also generates insoluble debris that settle at the bottom of salt cavern.
Too many interlayers will deposit much of the debris at the bottom, reducing the effective dissolution volume of the salt cavern, especially in the pocket construction stage, forcing relocation of the mining pipe to a higher position to achieve rapid solution mining and optimal cavern shape.
This article details a new method of rapid solution mining via numerical simulation and field testing of a new self-resonated cavitation jet.
Self-resonated jet
The self-resonated cavitation jet has strong pressure oscillation and high cavitation inception characteristics.3 A continual jet using the fluid transient theory describing the transition of small turbulent wave in the pipe, and the principle of fluid self-sustained oscillation of hydroacoustics modulates the jet.4 In conditions of ambient and atmospheric pressure, the new jet has stronger cavitation, higher destructive power, and better application effect than the conventional continuous jet.5
The characteristics of self-resonated cavitation jet and the technique of damping rotation control have led to development of a rapid solution-mining tool for bedded salt mining by China University of Petroleum. Fig. 1 shows the schematic of the tool and fluid circulation.
The tool consists of a filter, a rotation damping controller, rotary jetter, and self-resonated cavitation nozzles. The tool connects to the tubing and travels into the salt caverns. Fresh water pumped through the tubing, filter, damper, and jetter generates multiple self-resonated cavitation jets.
Four lateral high-pressure water jets drive the tool and rotate it to impact the salt rock and interlayers uniformly, generating a strong helical flow in the cavern (Fig. 2). The three directional jets pointed downward rotate and impact the debris settlements in the bottom of the cavern to lift it and suspend it in the helical fluid. After salt dissolution and diffusion, the fresh water becomes brine and is circulated out of the cavern with suspended debris through the annulus between tubing and casing.
Physical effects
The rapid solution-mining tool can generate four compound physical effects: self-resonated cavitation jet erosion, supersonic wave, helical flow, and forced circulation.
• Self-resonated cavitation jet erosion. The self-resonated cavitation jet has intense pressure oscillation and better rock erosion effectiveness than standard jets.6 At atmospheric pressure, its rock erosion rate measures two to four times higher than conventional jets.7 Fig. 3 shows the effect of four powerful hydraulic pulses on the wall at every circle. Each self-resonating cavitation jet is a high-frequency oscillating jet.
Repeated powerful pulsed impacts produce microfractures on the wall and sometimes the salt rock and interlayers can be broken directly. The spreading microfracture and penetration increase the rate of salt dissolution.
The self-resonated cavitation jet also has a high cavitation bubble inception ability. The collapsing cavitation bubble produces transient pressure 8.6-124 times higher than the jet impact for a few microseconds and can also break the rock directly.8
• Supersonic wave. The self-resonated cavitation jet can also produce high frequency, high-radiation supersonic waves, creating lower brine viscosity, better flow ability, and a higher mass-transfer rate.9 The noise wave impact on the salt rock surface also causes fatigue fracture which can enhance the salt dissolution rate. The energy from the noise transferring to the salt surface can also potentially create severe vibration and accelerate salt diffusion.
• Helical flow. The action of the lateral water jets rotating as four agitator arms (Fig. 4) rotates the fluid in the cavern as a helical flow with a vertical rotation axis. The fluid velocity on the surface of the salt rock increases and the flow direction can be changed, enhancing the mass transfer of salt molecules near the wall. The process also changes the brine's flow pattern in the cavern, increasing the convective diffusion of saturated brine.
The rotation jets and helical flow also improve efficiency of debris cleanout and reduce settlement in the bottom of the cavern. Debris broken from the interlayers suspends and moves out with the circulated fluid immediately, and debris settled in the bottom is lifted and suspended by the rotary impact of the three downward directional jets, moving out with the helical fluid.
• Forced circulation. The high-pressure water jets force fresh water to the rock surface quickly. This forced circulation changes the horizontal and vertical distribution of concentration of brine and enhances the convective diffusion of saturated brine near the wall, improving the salt-dissolution rate dramatically.
Numerical simulation
A numerical simulation carried out with a 3D cylinder model (5 m high by 2 m diameter) researched the helical flow and efficiency of debris cleanout on the basis of two-fluid Euler-Euler model and RNG κ – ε model. Field operations and hydraulic parameters design led to 177.8 mm (ID) casing and 114.3 mm (OD) tubing being chosen as the solution-mining pipes and 6 mm being set as the optimal orifice diameter for the self-resonated nozzles.
Fig. 5 presents the path lines of high-pressure water jets and contour of tangential velocity of circulated fluid in the salt cavern. The rapid-solution-mining tools easily create helical flow, which could increase the salt-dissolution rate and cleanout efficiency of debris. The maximum tangential velocity (3 m/sec) occurs near the mining pipe and decreases as the radial distance increases.
Fig. 6 shows the contour of debris volume fraction at different cleaning times. The volume of debris settled at the bottom decreases over time, instead spreading all over the cavern as the lift force overcomes gravity.
Flow rate strongly affects the efficiency of debris cleanout (Fig. 7), the cleanout efficiency increasing as flow rate increases and reaching 85% at 57.3 cu m/hr after 10 min. The diameter of debris and rotation speed of the jetter also influence cleanout efficiency.
Fig. 8 shows sharp decreases in efficiency as debris diameters increase. Maximum and minimum cleanout efficiency occurs at 80 rpm and 60 rpm, respectively. In this condition, debris bigger than 0.7 mm becomes difficult to clean out due to inadequate lift, nearly eclipsing any difference in efficiency caused by the jetter rotation speed. In this simulation the jetter optimal rotation speed measures 80 rpm and a maximum efficiency of 79.2% occurs with a diameter of 0.1 mm after 10 min operation.
This simulation suggests that, when constructing a pocket, the rapid-solution-mining tools with a self-resonated cavitation jet can generate helical flow, enhance the efficiency of debris cleanout, and reduce settlements in the bottom of a salt cavern.
Field test
The rapid-solution-mining tool helped build a pocket at well J1, Jintan, Jiangsu province. Salt thickness measured 241 m, while total depth from the surface to the top and bottom of the pocket measured 945 m and 1,186 m, respectively. Material analyses showed the formation contained about 13.4% insoluble materials, consisting of interlayers about 4.2 m thick. A 177.8 mm (7 in.) casing and 114.3 mm (4.5 in.) tubing served as the middle pipe and central pipe, respectively. Well C5, developed by the conventional solution-mining method, served as a control well. The chemical properties of the produced brine, bed thickness, and total depth of the salt formation in both C5 and J1 were identical.
Fig. 9 compares the salt production rates from C5 and J1, showing a much higher rate for J1. Fig. 10 compares accumulated salt production for both wells. Accumulated salt production over the initial 11 days of C5 well was 376.0 tons, while J1 well produced 866.7 tons, a 130.5% greater rate, demonstrating the greater capabilities of the rapid solution-mining tool with self-resonated cavitation jets.
Acknowledgment
The authors express their appreciation to the EU-CHINA Energy and Environment Program (Europe Aid-120723-D-SV-CN) and the PhD Programs Foundation of the Ministry of Education of China (No. 20070425006) for their financial support of this work.
References
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The authors
Li Gensheng ([email protected]) is a professor and director of the department of petroleum engineering at China University of Petroleum-Beijing. He has also served as a professor and director of the water jet technology center at China University of Petroleum-East China. He holds a PhD (1998) from China University of Petroleum-Beijing. He is a member of SPE and the Water Jet Technology Association (WJTA).
Song Xianzhi ([email protected]) is a PhD candidate at China University of Petroleum-Beijing. He holds a bachelors (2004) from China University of Petroleum-East China.
Tian Shouceng ([email protected]) is an assistant professor at China University of Petroleum-Beijing. He has also served as an assistant professor at Shengli Petroleum School, Dongying, China. He holds a PhD (2008) from China University of Petroleum-Beijing.
Wang Haizhu ([email protected]) is a PhD candidate at China University of Petroleum-Beijing. He earned his bachelors (2005) at Xi'an University of Petroleum, Xi'an, China.
Yuan Guangjie ([email protected]) is senior engineer at the Drilling Engineering Research Institute of CNPC, Beijing. He holds a PhD (2004) from Shanghai Jiao Tong University, Shanghai.
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