Tuesday, November 28, 2017

Matrix Acidizing - Diverting with Foam case study

Foam, a stable mixture of liquid and gas, has been used as a diverter in sandstone acidizing since 1969. By the usual criteria, it is almost perfect. It is cheap to produce; it does a decent job diverting; it does not interact adversely with the formation and formation fluids; and it cleans up rapidly. Foam is produced by injecting nitrogen into soapy water- typically, nitrogen occupies 55 to 75% of foam volume. The soapy water is a mixture of water and small amount of surfactant, or foamer. Injected downhole, foam penetrates the pore space where the cumulative viscous effect of the bubbles blocks further entry of the treating fluid.

 Foam's only drawback is that with time the bubbles break and diversion ceases. This can be seen in laboratory experiments, in which foam is injected simultaneously through two long sand packs, one with high permeability mimicking a thief zone, the other with low permeability mimicking a damaged zone. The cores are preflushed and then injected with foam. Then, acid is injected. At first, diversion works fine, with the low-permeabilty sand pack taking an increasingly greater proportion of the acid. But after about one hour, the foam has broken and the thief zone starts monopolizing the treatment fluid.

Researcher at the Dowell Schlumberger engineering center at Saint-Etienne, France discovered that this breakdown can be postponed by saturating the formation with a preflush of surfactant before injecting the foam and also injecting surfactant with every subsequent stage in the acid process. The surfactant adheres to the rock surface and minimizes adpsorption of surfactant contained in foam, preserving the foam. 

As before, the foam progressively diverts treatment fluid to the damaged zone, but now the diversion holds for at least 100 minutes. If necessary, damaged formation can first be cleaned with mutual solvent to remove oil in the near wellbore region - oil destroys foam - and to ensure the rock surface is water wet and receptive to the surfactant. 

Yet further improvement to foam diversion can be achieved by halting injection for about 10 minutes after foam injection. The diversion of treatment fluid to the damaged sand pack now takes effect almost immediately, rather than almost 50 minutes. It seems that given a 10-minute quiescent period, foam in low-permeabiity sand prematurely breaks down - scientist are not sure why. The combination of surfactant injection and 10-minute shut-in comprises the new FoamMAT diversion service that has been successfull application in the Gulf of Mexico and Africa.

 

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