Page 114 - A Practical Companion to Reservoir Stimulation

DESIGN OF MATRIX TREATMENTS

EXAMPLE G-3 EXAMPLE G-4

Calculation of Skin-Effect Reduction Impact of Injection Rate and Acid Volume
in a Limestone Reservoir on Skin-Effect Reduction

What would be the skin-effect reduction if the reservoir in Using the variables in Example G-3, demonstrate the effect of
Example G-2 were 100% limestone and the acid formulation injection rate and volume of acid injected on the skin-effect
a 28% by weight HCl solution? Keep all other variables the reduction. Use a range of injection rates from 0.25 to 5 BPM.
same. Use a range of injection volumes from 25 gal/ft to 300 gal/ft.
Solution (Ref. Section 13-3) Solution (Ref. Section 13-3)
A 28% by weight solution of HCl has adensity approximately From Eq. 13- 13, replacing the Peclet number by its expres-
equal to 1 g/cm3, and thus the acid concentration is sion and grouping variables after substitution,
280/36.5
C= = 7.67 moles/liter, ' ((3-6) (G-9)
I liter 1.6 9
where 36.5 is the molecular weight of HCl. where qi is in cm3/s (1 BPM = 2.648 x lo" cm3/s). Equation
The weight of 1 liter of rock is 2650 g, and since all of it is G-9 is for 150 gal/ft (i.e., a total of 15,000 gal).
CaC03 (MW = loo), its concentration is 26.50 moledliter. Figure G- 1 is a plot of the skin effect decrease vs. injection
The stoichiometric constant of HCl reacting with CaC03 is rate showing a mild reduction in the skin-effect decline with
2. Thus, the acid capacity number can be calculated from increasing injection rate.
Eq. 13-8. Assuming a commonly used injection rate of 1 BPM, then
(0.15) (7.67) from Eq. 13- I3 after grouping variables,
Ac = = 0.02 17.
1
(2) (26.5) As= --ln[l+23.2V], ((3-10)
The Peclet number is the same. However, the fractal 1.6
dimension for mass transfer limited kinetics is equal to 1.6. where V is the injected acid volume in m3. (Note: 1 m3 = 264
Therefore, from Eq. 13-13, gal.)
Figure G-2 is a plot of the skin-effect decrease vs. acid
1 volume injected, showing a significant impact at small incre-
1.6 1 + (0.0217)(8.69 x lo4)-'."'" mental volumes and then flattening out by virtue of the
1
( I .7 x 1 04)( 56.8) logarithmic relationship.

(3.14)( 30.48)( 0.1 5)(0.1) 'A
= -4.5.

G-3