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.

Many strategies have been evaluated and applied by CNPC in the Sulige Gas Field that typically features in low permeability, low pressure, low quality and low productivity. Cluster well technique was supposed to be most appropriate to deal with the “four-low” reservoir related problems, especially in the vulnerable ecologic environment of desert area. The early pilot test, however, severely restricted its extension in larger scale due to the longer operation cycle and higher drilling cost compared to vertical wells. The Sulige development team was driven to re-design and optimize the cluster drilling project, with the principal objectives to further improve the rate of penetration and reduce the unit gas development cost. This paper will focus on the optimized cluster drilling concept that has been successfully applied in recent two years within Sulige, which is the largest gas field in China to date. A “Cluster” usually consists of a reference “Mother” well and several “Daughter” wells from a single pad location. In Sulige, the “Daughters” are mostly directional ones and sometimes with auxiliary horizontal ones. Based on the optimized wellbore configuration, controlled trajectory and improved PDC bits, bottom hole assemblies and circulation fluids, a complete set of cluster well technique in a safe and faster drilling mode have been formed. Its application has resulted in an average ROP improvement of more than 36% and a dramatic increase of short- and long-term well economics, compared with previous cluster wells. The mature “Cluster” pattern triggers its complete extension in Sulige and promotes its trial applications in the Hailaer-Tamtsag Basin and Sichuan Basin. This paper will present the optimized measures of cluster drilling and operational results within the Sulige field, and will discuss the cluster well design challenges, relevant drilling operations and subsequent post well evaluation. Recommendations on the helicoid hydraulic motor, directional wellbore profile and drilling fluid will also be presented.