A major challenge in acidizing treatments is to achieve optimum coverage of the reservoir interval, particularly in long wellbore sections. The most common method is to use chemical diverters, such as oil based resins, rock salts, gelling agents or particulate diverters, which can be added to certain stages of the acid treatments. Though more expensive, more reliable methods are mechanical diverting methods, in which a reservoir section may be split into zones, and each stimulated separately. Multi-stage stimulation methods also allow for customized treatments for each zone if required, depending on the reservoir characteristics.
In new wells, downhole completions may be installed during completion, including a series of packers to isolate between different zones, and either ball or dart mechanisms are used to isolate each zone, from the lowest section/toe of the well, upwards. In wells already on production, the following methods may be used:
- Plug and perf – more time consuming than other methods, however well-known and practised. The plug and perf method stimulates through open perforations, and thereafter a plug is set, before the next perforations are made at the next zone for stimulation. After all zones have been treated, plugs are milled out using coiled tubing tools, and the clean-up operations carried out, followed by production.
- Coiled-tubing – treatments can be delivered downhole via coiled tubing, starting at the bottom/toe of the well, moving upwards or towards the heel. Operational footprint may be smaller, but extra care must be taken for protecting the coil from acid corrosion.
- Radial jetting – according to log data, certain zones are selected in which to create lateral conduits, reaching up to 100m into the reservoir, up to 2” wide. Ideal areas for stimulating in this manner are low permeability streaks. Integrated reservoir studies will help to select the optimum locations, number of jets and jet azimuth.