Evaluation of damage to horizontal wells through equivalent horizontal well length

The skin factor and additional pressure drop which are used to evaluate the formation damage of vertical wells can not accurately be applied to horizontal wells. A new model for evaluating formation damage of horizontal wells is presented. This method is derived by utilizing and modifying the concept of equivalent horizontal well length presented by Malekzadeh, where the permeability anisotropy is considered and a new calculation method of the equivalent productivity ratio is proposed. The results show that for a damaged horizontal well, the equivalent horizontal well length is shorter than the completed horizontal interval, while for a stimulated horizontal well the equivalent horizontal well length is longer than the completed horizontal interval. The fact that the longer the horizontal well is, the smaller the equivalent vertical well skin factor indicates that the horizontal well is more advanced than the vertical well in oilfield development. A longer horizontal well is more subjected to productivity loss when the well is damaged and it is more productive after stimulation. The application in the Bohai Oilfield indicates that the new method can directly and efficiently determine the magnitude of formation damage around horizontal wells and its impaction on productivity. It also sheds light on reservoir development and its evaluation standard.