Some oil and gas companies maintain many oil wells routinely as part of operational risk management. Replacing defect tubing, cleaning the solid deposit, stimulation, installing gravel pack are common activities in oil field operation. Stimulation changes the behavior of fluid flow into the well. Physically, the ultimate goal of stimulation is to decrease skin around the well. The skin is quantified by skin factor where it increases over time as depicted in Figure 1. If oil company does not maintain the flow of fluid from reservoirs into the well, the well eventually will produce below the economic rate where the company will suffer loss to operate the well instead of creating profit. Suppose the economic rate is 9 BOPD, the well must be shut in after producing for 9 months where the skin factor around 15.
There are many things that could increase the skin factor such as fine migration and interfacial tension related skin. Altering interfacial tension could decrease skin factor. By altering interfacial tension to the point more favorable to oil relative permeability will increase the oil production of the well. Furthermore, increasing oil relative permeability will unclog residual oil and allowing the oil to flow into the wellbore. Surfactant is best known in the oil and gas industry to alter the interfacial tension.
Every reservoir fluid has unique characteristics. It is a good practice to check the compatibility of injected chemicals with reservoir fluid before doing stimulation or continuous injection. Incompatibility between fluids will precipitate solid that can damage the reservoir.
Chemical to rock adsorption is also important. High adsorption is an indication of deposition and permanent damage.
Chemical stability to reservoir temperature is suspected to be the reason for unsuccessful chemical flooding. The most stable chemical can be found in the lab by running trial tests.
Treatment facilities are up-front costs which are sometimes a high burden for a small company. Untreated fluid can plug the reservoir. Produced fluid also must be treated before being disposed of.
To decrease risks above, a custom chemical composition is formulated in the lab. In the present invention, the main chemical is amphoteric visco elastic surfactant (VES) gels with the structure depicted in Figure 2. An amphoteric surfactant carries the cationic and anionic group in its structure which is N+ and CH2COO- respectively in Figure 2. The visco elastic feature is beneficial since the surfactant fluid can be easily pumped to the reservoir but stay strong in the reservoir with higher viscosity.
R is C12 to C30 alkane or alkylene, R1 and R2 are alkyl or hydroxy alkane and n are 2 to 6.
Additives must be added to the main chemical to get desired interfacial tension (IFT) lowering and desired viscosity. Alkali can be used as additives to enhance interfacial tension and reduce the surfactant adsorption. Xanthan gum (polymer) can be added to control the injection fluid mobility and provide enough residual viscosity to make better sweeping. Other surfactants (anionic, cationic, or nonionic) or nano particle can improve the main chemical stability.
Recommended reading, surface tension meter and an example of nano particles: