Optimum Design Of Horizontal Section Length Of The Horizontal Wells In Sebei-2 Gas Field

In recent years, horizontal well technology has developed into a routine technology all over the world, it is also widely used in domestic oil fields. With CNPC's large-scale promotion of this techonology, as of the end of 2008, 45 horizontal wells has been drilled totaly in Sebei gas field of Qinghai Oilfield Company. The overall effect is good, however, some issues such as lower gas production, water problem and so on began to emerge. As the focus of Sebei-2 gas field's productivity construction in 2009, some new horizontal wells is going to be drilled, but the horizontal section length is difficult to determine. In this paper, through literature research combining with theoretical study and computer simulation, an evaluation of the effect of horizontal well drilled since beginning has been made firstly, which demonstrates that optimizing horizontal section length is necessary. Then from the production point of view, a numerical model considering the pressure drop in the horizontal wellbore is built coupling with gas flow through reservoir and in the horizontal wellbore. Using this model, the optimization calculation of horizontal section length is carried out. Then, using a numerical gas reservoir simulation software, relationship curves between the horizontal section length and production capacity and performance is obtained, from which the scale of optimum length is discussed. Afterwards, from the perspective of economic benefits, the optimal point is defined as the greatest point of difference between economic benefits and costs. The result shows that the optimum length in Sebei-2 gas field is 400-600m, preferably not more than 600m, but not limited to this range. When designing a specific horizontal well, we should firstly target layer selection, well planning, horizontal direction, then demonstrate optimum horizontal section length using various methods. When the horzontal well is implemented, well trajectory should be strictly controlled, especially when start drilling into the target because of the important role of heel section to the well producton, to ensure the effectiveness of the horizontal length. While in the previous study, a computer software called GasHoriOffice is developed, the functions include productivity formula calculation, IPR curve prediction, sensitivity analysis of factors on well production, statistical regression of costs of horizontal wells, calculation of optimal horizontal section length and so on. The sofware has been applicated with good results in Sebei gas field, but still need to be improved considering the actual situation and more methodology.