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Improving the rate of penetration in deep well is long-term problems for drilling industry. Pulsed jet drilling which has been proved in field, is a cost-effective way to improve the rate of penetration. An Adjustable frequency pulsed jet generating tool was developed in this paper. The main parts of the adjustable frequency pulsed jet generating tool include turbine, valves, and a resonance pulse chamber. Rotary valve is driven by the turbine. Rotation of the rotary valve causes the state of drilling fluid flow from the continuous to the periodic. So, the periodic flow has frequency and amplitude. In order to obtain higher amplitude, a resonance pulse chamber was used to amplify the amplitude of the periodic drilling fluid flow. The pulsed jet is formed when the periodic drilling fluid flow is issued from the nozzles of the bits. The frequency of the pulsed jet can be easily adjusted by changing the parameters of the rotary valves. In this paper the working principle of the adjustable frequency pulsed jet generating tool was analyzed. Theoretical calculations and experiments were carried out to reveal the characteristic of the pulsed jet. The effect of the flow rates on the frequency and amplitude of the pulsed jet were studied in detailed. The experiment results show that the pulsed jet with lower frequency and higher amplitude can significantly enhance the rate of penetration in deep wells. The adjustable frequency pulsed jet generating tools have been tested in several wells. The field test results show that the rates of penetration were improved by 15-45% and the tool can meet the operation requirements. Adjustable frequency pulsed jet generating tool as an effective approach to improve ROP, has great application potential in deep well drilling.

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In the current numerical simulation studies, bottom water in Class II hydrate-bearing layers is represented by grids with high water saturation that significantly extends the calculation time if the volume of the bottom water is large or grid size is small. Moreover, the influence of the bottom water volume on the depressurization performance of Class II hydrate-bearing layers has not been fully investigated. In this study, the Fetkovich analytic aquifer model was coupled with a simulation model of a hydrate reservoir to accelerate the simulation of Class II hydrate-bearing layers. Then the simulation results and calculation time were compared between the coupled model and the model in which the bottom water layer is only represented by grids. Finally, the influence of the bottom water volume on the productivity of gas and water in the depressurization method was investigated and the variation of pressure, temperature, and hydrate saturation during the production process was analyzed. The results show that the coupled model can significantly reduce the simulation time of Class II hydrate-bearing layer while ensuring calculation accuracy. When the pore volume of the aquifer increases to 20 times that of the bottom water layer, the computation time of a single model in which the bottom water layer is represented by grids is 18.7 times that of the coupled model. Bottom water invasion slows down the depressurization, and therefore, the larger the aquifer, the lower the peak value of gas production, and the later it appears. However, the invading bottom water can provide heat for hydrate dissociation; therefore, the gas production rate of the hydrate-bearing layer with bottom water is higher than that of the hydrate-bearing layer without bottom water in the late development stage. Generally, the presence of bottom water reduces the cumulative gas production and increases the cumulative water production; therefore, the larger the aquifer, the more unfavorable the depressurization development of the hydrate-bearing layer.

In this paper, Bottomhole pressure intermittent reducing tool (BPIRT) was introduced. The tools can enhance rate of penetration (ROP) and prolong bit footage, and ultimately improve drilling efficiency. The tool can be used in vertical wells, directional wells and horizontal wells. It can be used either with downhole motor or be used alone.Bottomhole pressure intermittent reducing tool consists of a turbine, rotary sealing valves, and hydraulic shock damping system. The rotary sealing valves are driven by the turbine. Rotation of the rotary valves opens and closes the flow path ways and turns the continuous mud flow into periodic flow. As the periodic mud flow jets from bit nozzles, the bottomhole pressure is reduced periodically, which alter the stress state of the bottomhole rock and reduces the rock breaking energy. Hydraulic shock damping system contains a piston. The upward face of the piston is excreted by periodic mud flow, and the downward face is hydraulically connected with the annulus by 4 pores. Pressure loss of bit and hydraulic shock system provide a constant force on the piston. Typically, this force is designed to be greater than WOB with 20-30KN. The piston also provides additionally a periodically varying force (0-40KN, 10-35HZ) on bit. In normal drilling process, the piston is set at the lower restricted point. When bit jumping occurs, the tool allows the piston to move upward 5-8mm automatically, which can protect bit and drilling string.The advantages of bottomhole pressure intermittent reducing tool as following: 1) reduce the bottomhole pressure intermittently, alter the stress state of the rock, reduce the rock breaking energy; 2) change the distribution of cuttings bed in the bottomhole, improve the cuttings upward velocity from bottomhole rock matrix, and avoid the cuttings of repeated broken; 3) provide bit additional impact energy, improve rock breaking efficiency; 4) decrease bit jumping, protect bit and drilling string; 5) stable directional drilling tool face, improve drilling efficiency.Bottomhole pressure intermittent reducing tool has been used over 30 wells, the total footages over 35000m. The ROP is improved by 26.32%-132.18%, total 200 days drilling cycle and 30 millions drilling costs are saved. The tool has simple structure, reliable performance and long service life. Bottomhole pressure intermittent reducing tool as an effective approach to improve ROP, has great application potential in China.As mentioned in advantages, the tool can reduce the bottomhole pressure intermittently, alter the stress state of the rock, reduce the rock breaking energy, improve the cuttings upward velocity from bottomhole rock matrix, provide bit additional impact energy and also can decrease bit jumping.