Producing natural gas from shale gas presents a unique challenge to the petroleum engineers because of the low permeability nature of this type of gas reservoirs. Large scale hydraulic fracturing operations are often required for enhancing gas well productivity. Due to the shally characteristics of the reservoir rock, the productivity of the hydraulically fractured gas wells is very vulnerable to the damage by the fracturing fluids. The damaging mechanisms have been recognized to include fluid invasion, proppant embedment, gel filter cake at the fracture face, and the gel residue in the proppant pack. The relative significances of the well damage due to different mechanisms are not clear. It is highly desirable to have an analytical method for predicting the relative importance of different types of damage and optimizing fracturing treatment accordingly. This paper presents such a model and observations from model analyses.
A new mathematical model was developed in this study to predict the effects of fracture fluid filtration, proppant embedment, gel filter cake at the fracture face, and the gel residue in the proppant pack on the productivity of fractured gas wells in shale gas reservoirs. A unique feature of the new model is that it considers reservoir-fracture cross-flow in wells with long fractures and non-linear influx to wells with short fractures. Sensitivity analyses with the model show that, among the investigated four mechanisms, the fluid invasion should have the most detrimental effect, while the formation of proppant-embedment layer should have a negligible effect on the productivity of fractured wells in shale gas reservoirs.