Reducing surface tension to improve clean up efficiency of hydraulically fractured wells: does it really work?

The main purpose of applying surfactants for hydraulically fractured wells is to reduce fracturing fluid surface tension during the leak-off process and hence improve its cleanup efficiency. Significant research has been devoted to developing such chemicals that can effectively reduce capillary forces between fracture fluid and resident rock and fluids. However, in a recent numerical study (SPE-14414), we have shown that, for some cases, reducing surface tension tends to decrease the cleanup efficiency.

Following our previous study, we have conducted a comprehensive sensitivity study to identify conditions in which reduction of surface tension improves cleanup efficiency. During this exercise, the impacts of matrix permeability (km), fracture permeability (kf) and fracture fluid injection volume were investigated. Over 200 simulation runs were performed covering a wide range of variation of of these pertinent parameters for a single fractured well model.

The results indicate that at the early stage of production the cleanup efficiency is relatively poor and almost independent of IFT, km and kf. At late stages of production and when kf is low, reducing surface tension decreases the cleanup efficiency. For high kf values, on the other hand, cleanup efficiency improves with such a reduction. For the cases with km values greater than 0.001, cleanup efficiency is more effective if IFT increases. Furthermore, as km decreases, the damage due to fracture fluid blockage becomes more severe. It is interesting to note that when km is less than 0.0001, cleanup efficiency always decreases with increasing surface tension, for all different kf values. The amount of gas production loss for such cases is relatively high, indicating the severity of fracture fluid damage for very tight gas reservoirs. Increasing the fracture fluid injection volume did not significantly change the above trend.

The results presented here aim to help the industry in properly evaluating the added value of using surfactant to improve the cleanup process of the hydraulically fractured wells.