**4. Re-fracturing technology**

Unconventional reservoirs show significant decline rates after few months of production compromising the economics and imposing the need for increasing or stabilizing production. The decline in production from the unconventional reservoirs is attributed to the closure and damage of the fracture networks within the formations. Hence, re-fracturing as an emerging technology has become a viable option for sustaining production and increasing reserves. Re-fracturing is a preferred option over drilling and completing new horizontal wells as it can be carried at only a fractional cost of up to 25–40% [37], thus minimizing the related financial and safety risks.

Production decline rates from unconventional reservoirs are more rapid than those in conventional reservoirs because of the ultralow permeability, limited reservoir contact, and the original completion strategy. The ability of re-fracturing technology provides a potential to extend the productive life of the unconventional reservoirs beyond the normal and up to an additional 20–30 years [38]. Re-fracturing restores production from underperforming formations by increasing fracturing networks, replacing damaged proppant, bypassing skin zones, and connecting old and new fractures [39]. Successful re-fracturing can increase the estimated ultimate recovery (EUR), shorten the capital return time, and increase the net present value (NPV) of the unconventional reservoirs. Decline curve analysis (DCA) showed that re-fractured wells achieved an average of 60% increase in NPV [40]; therefore, re-fracturing application helps reduce the variability in the unconventional reservoir performance and considered the best option for tackling production declines.

#### **4.1 Re-fracturing process**

Re-fracturing literally means a second hydraulic fracturing through same or new perforations to repair or recreate fracture networks within the same formation. If a re-fracturing treatment was carried out after a re-fracturing, then it would be considered a tri-fracturing [41].

Practically, re-fracturing is carried out when the initial hydraulic fracturing treatment was undersized or when suspected skin damage exists [42]. It is possible to use the existing fractures for the re-fracture and still generate a new fracture network sufficient to increase production. In a formation with its low in situ stress anisotropy, pressure can be created within the fracture itself to cause the reservoir
