Chan, K.S., Dowell Schlumberger (France)
A new non-polymer in-situ gelling system has been developed for reservoir water control treatments. This paper includes the results of a series of key laboratory tests in characterizing this system for practical well treatment designs.
The system comprises two to three chemicals which are readily soluble in most field mix waters and seawater. The fluid has a low viscosity close to that of the water, and can be pumped at any rate. The treatment is a simple single slug injection process using ordinary field mixing and injection equipment. In the rock formations, the injected fluid system forms a solid gel which will transform into a solid phase after a time period. Typically, the treated formation permeability reduction is 50 folds or 98%.
The microscopic pore blocking efficiency as tested by core flow experiments can be correlated with the gel strength which is expressed in term of gel extrusion pressure gradient (psi/ft) as measured by a core plug test. This enables the treatment fluid penetration requirement calculations using the near wellbore pressure gradient values during the production or injection as input data instead of a blind assumption on a certain radial penetration.
Excellent fluid flow selectivity in the water-out layer has been demonstrated by many laboratory bi-core flow experiments modelling a typical two-layer reservoir.
Many materials and techniques have been used to plug water thief zones in a multi-layered reservoir. Cement and silicate based techniques are used only for the very special purpose of permanently plugging certain zones near the wellbore. Polymer, especially polyacrylamide, based techniques are largely used because of their relatively low cost and their supposed selectivity, i.e., only the water permeability is reduced.
These systems are sensitive to traces of oxygen, wellbore shearing, and salinity as well as the hardness of field water. Polymers are also subjected to thermal and biological degradation. These problems have been well discusses in the literature. Injectivity of the polymer fluids is poor as they have viscosities many times higher than that of water. They may not preferentially flow into the water channels and so there can be placement problems. Because of these fluid flow limitations, many water thief zone treatments using polymer systems were implemented for a relatively short penetration. The success of field applications are more fortuitous rather than the result of the design.
Many alternative treatment fluid systems having good injectivity have also been developed. Lignosulfonate gels have been shown to be cost effective systems but they require a long time to form gels and are still sensitive to temperature and salts. Furfuryl alcohol resins and polyisocyanurate salts cause problems in controlling the in-situ reactions and are not removable once the gels are formed. A low pH aluminium chloride solution can form a good gel when the pH of the fluid is increased. But the dissolution of this chemical in water generates a lot of heat and toxic acid fumes.
In summary, an ideal reservoir water control treatment chemical system should possess the following characteristic properties:
High salinity and hardness tolerance. Compatible with any mix water including seawater.
Gel formation process controllable in all possible range of near wellbore and reservoir temperatures.
Low viscosity, capable of achieving deep formation penetration.
Insensitive to shear and can be pumped at any rate.
Chemical readily soluble in any mix water without using any elaborated technique or special equipment.