Page 206 - Introduction to Petroleum Engineering

HYDRAULIC FRACTURING 193

Example 10.5 HCl Volume for Removing Calcium Carbonate
Estimate the volume of 15% HCl needed to remove calcium carbonate in the
perforated zone around a vertical well in sandstone formation with the follow-
ing properties: 18% porosity and 12% of grain volume is CaCO . The treatment
3
is to extend 1 ft beyond well radius (0.33 ft).
Answer
First, use volumetric relations to find the volume of CaCO within the treatment
3
radius per foot of formation:
2
π
Volume CaCO /ft = ( r t 2 −r w )( 1− ) φ f CaCO 3
3
= ( 133 − . 2 ft 10.118 012) = 0 51ft /ft
)(
0 33 ) (
π
2
3
2
−
.
.
.
Second, use the dissolving power of calcium carbonate from Table 10.1 for
15% HCl to find the acid treating rate:
3
051 ft /ft
.
Volume HCl/ft = = 626 ft /ft = 47 gal/ft
.
3
0 082
.
There are several complicating factors that should be considered in design of acid
treatments, especially in sandstone formations. For example, dissolution of iron rust in
the casing can lead to ferric hydroxide Fe(OH) precipitate in the formation. Acids can
3
interact with asphaltenes in the oil of the formation to produce sludges. Tests with
formation oil samples before an acid treatment are essential. If sludges form, then injec-
tion of an aromatic solvent to displace oil away from the near‐well region is an option.
In sandstone formations, precipitation of calcium fluoride can be avoided by a preflush
of hydrochloric acid to dissolve and displace any calcium carbonate in the near‐well
region. Precipitation of colloidal silica Si(OH) is another complication of sandstone
4
acidizing. To prevent significant precipitation of colloidal silica, some engineers recom-
mend injection of HF at a high rate followed by rapid production of the spent acid.
Historically, a typical acid treatment for sandstones was as follows: 50 gal/ft 15%
HCl preflush, 50–200 gal/ft of 3/12% HF/HCl (“mud acid”), and 200 gal/ft brine
postflush; then immediate flow back. More recently, lower acid concentrations are being
used. It is believed that lower concentrations will lead to less precipitates and reduced
risk of unconsolidating the formation. In principle, testing of acids with reservoir rock
samples should help with design; but that probably does not happen very often.
10.5 HYDRAULIC FRACTURING
The goal of hydraulic fracturing or fracking is to increase the number and capacity of
flow paths between the wellbore and the surrounding formation. Fracking increases
oil or gas production to a well in the same way that a freeway increases traffic flow