Modeling and Analysis of Hydraulic Fracture Skin and Its Control on Shale Gas Production from Horizontal Wells

Abstract It is widely recognized that hydraulic fracturing creates a region of altered permeability near the fracture/matrix interface. Although we have a limited understanding, it is believed this region plays an important role on the production rates. The objective of this study is to model the region as a fracture skin explicitly including local physical and chemical phenomena and their associated effects, namely: (1) permeability improvement due to unpropped fracture development during the fracturing, (2) stress increase due to slick water invasion and clay-swelling effect developing after the fracturing, and (3) water-saturation buildup due to capillary end effect (CEE) developing during the flowback and production. A sensitivity analysis on the derived skin indicates key parameters controlling production and help in minimizing the damage. The proposed fracture skin increases at a decreasing rate with distance away from the fracture/matrix interface, which mirrors the invaded fracturing fluid (slick-water) near the interface. The sensitivity analysis performed shows that clay swelling is the predominant damage mechanism and is controlled by the geo-mechanical parameters of the permeability model: (i) the normalized effective stress at the fracture matrix/interface when clay swelling develops in the altered zone, and (ii) strength of the unpropped fractures and cracks inside the damage zone to stay open. On the other hand, the contribution of CEE to the skin is mainly controlled by the average water saturation in the damaged zone. CEE can thus be severe in formations with high water saturation. To minimize the damage, the salinity of the injected water needs to be increased, which in turn will reduce the chemical imbalance between the clay-bound water and slick-water, preventing osmosis. In addition, using proppants with a wider particle size distribution could reduce the damage by allowing smaller size proppants to invade into secondary fractures and cracks, hence, keeping them propped open during the production.