Page 246 - Petroleum Production Engineering, A Computer-Assisted Approach

Guo, Boyun / Computer Assited Petroleum Production Engg 0750682701_chap16 Final Proof page 244 21.12.2006 2:30pm

16/244 PRODUCTION ENHANCEMENT
(1)(100:1)
16.1 Introduction b 15 ¼ (0:15)
(2)(36:5)
Matrix acidizing is also called acid matrix treatment.Itis a
technique to stimulate wells for improving well inflow per- ¼ 0:21 lb m CaCO 3 =lb m 15 wt% HCl solution:
formance. In the treatment, acid solution is injected into the The dissolving power on a volume basis is called volumet-
formation to dissolve some of the minerals to recover per- ric dissolving power and is related to the gravimetric dis-
meability of sandstones (removing skin) or increase per- solving power through material densities:
meability of carbonates near the wellbore. After a brief
introduction to acid–rock interaction, this chapter focuses X ¼ b r a , (16:2)
on important issues on sandstone acidizing design and r m
carbonate acidizing design. More in-depth information where
can be found from Economides and Nolte (2000).
X ¼ volumetric dissolving power of acid solution,
3
3
ft mineral=ft solution
r a ¼ density of acid, lb m =ft 3
16.2 Acid–Rock Interaction 3
r m ¼ density of mineral, lb m =ft
Minerals that are present in sandstone pores include mont-
morillonite (bentonite), kaolinite, calcite, dolomite, sider-
ite, quartz, albite (sodium feldspar), orthoclase, and 16.2.3 Reaction Kinetics
others. These minerals can be either from invasion of The acid–mineral reaction takes place slowly in the rock
external fluid during drilling, cementing, and well comple- matrix being acidized. The reaction rate can be evaluated
tion or from host materials that exist in the naturally experimentally and described by kinetics models. Research
occurring rock formations. The most commonly used work in this area has been presented by many investigators
acids for dissolving these minerals are hydrochloric acid including Fogler et al. (1976), Lund et al. (1973, 1975), Hill
(HCl) and hydrofluoric acid (HF). et al. (1981), Kline and Fogler (1981), and Schechter (1992).
Generally, the reaction rate is affected by the characteristics
of mineral, properties of acid, reservoir temperature, and
16.2.1 Primary Chemical Reactions rates of acid transport to the mineral surface and removal of
Silicate minerals such as clays and feldspars in sandstone product from the surface. Detailed discussion of reaction
pores are normally removed using mixtures of HF and kinetics is beyond the scope of this book.
HCl, whereas carbonate minerals are usually attacked
with HCl. The chemical reactions are summarized in
Table 16.1. The amount of acid required to dissolve a 16.3 Sandstone Acidizing Design
given amount of mineral is determined by the stoichiom-
etry of the chemical reaction. For example, the simple The purpose of sandstone acidizing is to remove the dam-
reaction between HCl and CaCO 3 requires that 2 mol of age to the sandstone near the wellbore that occurred dur-
HCl is needed to dissolve 1 mol of CaCO 3 . ing drilling and well completion processes. The acid
treatment is only necessary when it is sure that formation
damage is significant to affect well productivity. A major
16.2.2 Dissolving Power of Acids formation damage is usually indicated by a large positive
A more convenient way to express reaction stoichiometry skin factor derived from pressure transit test analysis in a
is the dissolving power. The dissolving power on a mass flow regime of early time (see Chapter 15).
basis is called gravimetric dissolving power and is defined as
n m MW m 16.3.1 Selection of Acid
b ¼ C a , (16:1)
n a MW a The acid type and acid concentration in acid solution used
in acidizing is selected on the basis of minerals in the
where
formation and field experience. For sandstones, the typical
b ¼ gravimetric dissolving power of acid treatments usually consist of a mixture of 3 wt% HF and
solution, lb m mineral=lb m solution 12 wt% HCl, preceded by a 15 wt% HCl preflush. McLeod
C a ¼ weight fraction of acid in the acid solution (1984) presented a guideline to the selection of acid on the
n m ¼ stoichiometry number of mineral basis of extensive field experience. His recommendations
n a ¼ stoichiometry number of acid for sandstone treatments are shown in Table 16.2.
MW m = molecular weight of mineral McLeod’s recommendation should serve only as a starting
MW a ¼ molecular weight of acid. point. When many wells are treated in a particular forma-
tion, it is worthwhile to conduct laboratory tests of the
For the reaction between 15 wt% HCl solution responses of cores to different acid strengths. Figure 16.1
and CaCO 3 , C a ¼ 0:15, n m ¼ 1, n a ¼ 2, MW m ¼ 100:1, shows typical acid–response curves.
and MW a ¼ 36:5. Thus,

Table 16.1 Primary Chemical Reactions in Acid Treatments
þ
þ
Montmorillonite (Bentonite)-HF/HCl: Al 4 Si 8 O 20 (OH) 4 þ 40HF þ 4H $ 4AlF þ 8SiF 4 þ 24H 2 O
2
þ
þ
Kaolinite-HF/HCl: Al 4 Si 8 O 10 (OH) 8 þ 40HF þ 4H $ 4AlF þ 8SiF 4 þ 18H 2 O
2
þ
þ
þ
Albite-HF/HCl: NaAlSi 3 O 8 þ 14HF þ 2H $ Na þ AlF þ 3SiF 4 þ 8H 2 O
2
þ
þ
þ
Orthoclase-HF/HCl: KAlSi 3 O 8 þ 14HF þ 2H $ K þ AlF þ 3SiF 4 þ 8H 2 O
2
Quartz-HF/HCl: SiO 2 þ 4HF $ SiF 4 þ 2H 2 O
SiF 4 þ 2HF $ H 2 SiF 6
Calcite-HCl: CaCO 3 þ 2HCl ! CaCl 2 þ CO 2 þ H 2 O
Dolomite-HCl: CaMg(CO 3 ) 2 þ 4HCl ! CaCl 2 þ MgCl 2 þ 2CO 2 þ 2H 2 O
Siderite-HCl: FeCO 3 þ 2HCl ! FeCl 2 þ CO 2 þ H 2 O