Page 247 - Petroleum Production Engineering, A Computer-Assisted Approach

Guo, Boyun / Computer Assited Petroleum Production Engg 0750682701_chap16 Final Proof page 245 21.12.2006 2:30pm

MATRIX ACIDIZING 16/245
16.3.2 Acid Volume Requirement and
The acid volume should be high enough to remove near- 2 2
a
wellbore formation damage and low enough to reduce cost V m ¼ p r r ð 1 fÞC m , (16:4)
w

2
2
of treatment. Selection of an optimum acid volume is V P ¼ p r r f, (16:5)
a
w
complicated by the competing effects. The volume of acid
needed depends strongly on the depth of the damaged where
zone, which is seldom known. Also, the acid will never r a ¼ radius of acid treatment, ft
be distributed equally to all parts of the damaged forma- r w ¼ radius of wellbore, ft
tion. The efficiency of acid treatment and, therefore, acid f ¼ porosity, fraction
volume also depends on acid injection rate. To ensure that C m ¼ mineral content, volume fraction.
an adequate amount of acid contacts most of the damaged
formation, a larger amount of acid is necessary. Example Problem 16.1 A sandstone with a porosity of
The acid preflush volume is usually determined on the 0.2 containing 10 v% calcite (CaCO 3 ) is to be acidized with
basis of void volume calculations. The required minimum HF/HCl mixture solution. A preflush of 15 wt% HCl
acid volume is expressed as solution is to be injected ahead of the mixture to dissolve
the carbonate minerals and establish a low pH
V m
V a ¼ þ V P þ V m , (16:3) environment. If the HCl preflush is to remove all
X carbonates in a region within 1 ft beyond a 0.328-ft
where radius wellbore before the HF/HCl stage enters the
formation, what minimum preflush volume is required in
V a ¼ the required minimum acid volume, ft 3 terms of gallon per foot of pay zone?
V m ¼ volume of minerals to be removed, ft 3
V P ¼ initial pore volume, ft 3
Table 16.2 Recommended Acid Type and Strength for Sandstone Acidizing
HCl Solubility > 20% Use HCl Only
High-perm sand ( k > 100 md)
High quartz (80%), low clay ( <5%) 10% HCl-3% HF a
High feldspar ( >20%) 13.5% HCl-1.5% HF a
High clay ( >10%) 6.5% HCl-1% HF b
High iron chlorite clay 3% HCl-0.5% HF b
Low-perm sand ( k < 10 md)
Low clay ( <5%) 6% HCl-1.5% HF c
High chlorite 3% HCl-0.5% HF d
a
Preflush with 15% HCl.
b
Preflush with sequestered 5% HCl.
c
Preflush with 7.5% HCl or 10% acetic acid.
d
Preflush with 5% acetic acid.
300
8 wt% HF
250
4 wt% HF
Percent of Original Permeability 150
2 wt% HF
200

100

50
Berea
sandstone
80 F-100 psi
0.1 1 10 100 1,000
Pore Volumes of Acid
Figure 16.1 Typical acid response curves (Smith and Hendrickson, 1965).