Stimulation treatments are aimed to enhance and maximize production from the well by creating new flow channels or removing the formation damage near wellbore area and restoring the well to its natural, undamaged inflow performance.
Matrix stimulation, or acidizing treatments are generally injected below fracturing pressure down tubing, drill pipe or coiled tubing, and usually include a sequence of several fluids, referred to as stages. A minimal treatment consists of a pre-flush stage with a non-damaging, non-reactive fluid to establish an injection rate, a stage of the main treating fluid to restore reservoir permeability, and an over-flush stage to clear the main treatment fluid out of the tubing and displace it into the near-wellbore area. In most treatments, other supporting stages are included to enhance the effectiveness of the treatment.
A number of elements are essential in designing the fluid system, selecting additives, and determining the stages for an acidizing treatment:
- Rock Lithology – Carbonate and Sandstone acidizing treatments are vastly different in terms of the chemical approach. Full understanding of the mineral content is required to avoid using incorrect chemistries, and causing insoluble products which could further impact production
- Formation Damage – Damage is characterized using laboratory tests, logging techniques and well history. Detailed study is necessary to develop a list of suspected damages from the available data. Multiple types of damage are normally suspected and each type is considered when selective additives for a matrix acidizing treatment.
- Reservoir Temperature – Since primarily mineral acids are used in matrix acidizing, the temperature must be considered, not only for an effective treatment in the reservoir, but also to ensure that the correct protection is provided for the tubing and subsurface components, which are prone to corrosion with exposure to such fluids.
- Fluid Compatibility – To ensure an effective treatment, all fluids MUST be tested with the hydrocarbon in place, to avoid inducing other damage, such as sludges and acid induced Asphaltenes, by incompatibility issues.
- Delivery Method and Diverters – The most common way to deliver an acid treatment to the formation is to bullhead. However, in cases with long reservoir sections, or long horizontal sections, other deployment methods may be used, such as coil tubing, specialised downhole tools for stimulating one zone at a time, or a “plug and perf” method, to ensure complete coverage of the reservoir interval. Chemical diverters may also be used to divert fluid from “thief” zones – those with higher permeability – to encourage fluid into those with lower permeability, and ultimately access more reserves. Chemicals may include oil soluble resins, gelling agents, and particulate diverters.