Whether fracturing a formation or not must be answered before further steps acquired. A comprehensive evaluation of the formation and fluid characteristics is important. Skin build-up evaluation behind casing must be assessed. Sometimes, the fracturing program has been planed before the well is drilled.
Many fracturing technology or techniques have been developed. The fracturing fluid, additives, proppant are vary depend on formation characteristics. Some oil and gas companies use gel fluid for better fracturing and others combine the fracturing fluid with acid. Make sure no precipitation during fracturing and acidizing.
Isolation in multi-zone fracturing is an issue during the fracturing process or operation. After fracturing the first zone, high fluid loss into the first zone is encountered during fracturing the second zone. Some oil companies use a bridge plug to isolate the first zone and open the bridge plug subsequently after fracturing the second zone. It is not easy to do so in high deviated wells or horizontal wells.
In Figure 1, the first zone is perforated by using a jetting tool. Jetting tool perforation is more preferably than shaped charge perforation for some reasons:
- Jetting tool perforation can reach deeper penetration.
- Jetting tool perforation can make a slot or helical perforation instead of the hole only.
- The jetting tool can directly deliver fracturing fluid for fracturing.
- The jetting tool can deliver isolation fluid to isolate the fractured zone.
- The jetting tool can do the same procedures in other zones without any trip.
Subsequently after perforating the first zone, the zone is fractured (Figure 2) by using the jetting tool. Optionally, acid can be mixed with the fracturing fluid to etch the formation or do acidizing separately after fracturing. Other additives can be added to fracturing fluid for additional purposes such as corrosion inhibitor, scale inhibitor, or paraffin depressant. A frac plug or packer is used to contain the fracturing pressure to secure other zones. A high-pressure frac pump and frac blender are installed on the frac site to complete the fracturing process.
In Figure 3, the first zone is isolated by using visco elastic surfactant (VES). Viscoelastic surfactant is more preferably as isolation fluid since the fluid can decrease formation damage, more stable against shear, and no substantial viscosity loss due to temperature increase. By isolating the first zone, there is no fluid loss during any operation on the second zone.
Perforating and fracturing or any other treatments can be repeated in the second zone (Figure 4). If there is no other zone to be perforated or fractured, the viscoelastic surfactant is unloaded by means of circulation or reverse circulation (Figure 5). If the reservoir pressure is strong enough, the well can be flowed back through the jetting tool or help the well to flow-back by decreasing the hydrostatic fluid within the well using Nitrogen.
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