Optimization of Fracturing Fluid and Retarded Acid for Stimulating Tight Naturally Fractured Bedrock Reservoirs.

In tight naturally fractured bedrock reservoirs, hydrocarbons are typically stored in fractures, where hydraulic fracturing is needed to connect these fractures to the wellbore. The cross-linked gel is used as the fracturing fluid to reduce the fluid leak-off through natural fractures; however, it can cause formation damage due to its high content of residues after breaking. A synthetic polymer is introduced and evaluated that can maintain a high viscosity to minimize the leak-off , while having a low residue content after breaking. To further enhance the conductivity of the created fracture network, acid is applied to etch and roughen the created fracture faces. Because the target reservoir has a complex mineral composition, a three-step coreflood sequence using reservoir rock samples with controlled fracture widths is established to quantify the enhancement of different retarded acids and to reveal the mechanism behind it. The results indicate the synergy effect of reducing the acid concentration and surfactant adsorption on rock surfaces can lead to an obvious enhancement of the fracture permeability after acidizing, while the mud acid or hydrofluoric acid is not suitable for the target reservoir where concentrations of silicates and clays are relatively high.