Summary
This paper describes results from a series of comparative corefloods and
static compatibility tests examining the differences in laboratory-test
procedure, scale-inhibitor (SI) returns, and modeling approaches for nonaqueous
and aqueous SI treatments. Two types of nonaqueous systems, one ethylene
glycol (EG) -based and two oil-soluble products, each containing
penta-phosphonate SIs, were investigated. Detailed compatibility and
injectivity tests were carried out before coreflooding, and a carefully
designed application/treatment process was required as a result of the
hydrophobic nature of these products. To understand the SI-transport and
-retention mechanisms for these nonaqueous systems, comparisons were made with
the corresponding aqueous applications. These comparisions were made in terms
of SI-return performance, flowback permeability, possible formation damage, and
changes in the wettability conditions that might account for any post-treatment
differences. In addition, approaches to mathematically modeling these
corefloods were studied. This paper focuses on the application of a
partitioning model in a standard reservoir simulator. An alternative
two-phase mathematical model for such systems, which includes both interphase
partitioning and adsorption, has been described in detail in another recent
publication (Guan et al. 2004).
All corefloods were performed with outcrop Clashach sandstone material
rather than reservoir cores, and hence, the advantages of deploying nonaqueous
treatments over the conventional aqueous treatments might not be evident.
However, the experimental and modeling results help to capture the main
transport and retention mechanisms of these nonaqueous systems in an
understandable way. Results confirmed the existence of complex phase
behavior during the corefloods with the two oil-soluble products. Examination
of the core after flooding with an environmental scanning-electron microscope
(ESEM) indicated decreased water wetness following the two oil-soluble SI
treatments compared with the aqueous treatment. Numerical modeling
results show that the behavior of this system is most consistent with the
assumption that the SI in the nonaqueous system is only slightly soluble in the
oil phase during the oil post-flush.
Introduction
In oil fields where seawater injection has been used for pressure
maintenance and hydrocarbon sweep, scale formation has often been experienced
in producer wells. Mineral scale may lead to significant production
decline, and its removal once deposited is both difficult and expensive. The
application of SI, often in a squeeze treatment into the near-wellbore
formation, is regarded as a good method for preventing this problem.
Downhole scale prevention generally is carried out by use of aqueous-based SIs
(e.g., phosphonates, polyacrylates, and sulfonated copolymers). However,
in situations in which relative permeability effects, water blocking, fluid
lifting, or deep chemical penetration of the near-well formation are of major
concern, aqueous SI squeeze treatments may not be desirable. Indeed,
aqueous treatments may lead to impaired productivity, extended cleanup times,
and process upsets during the flowback of the treatment fluids. Various
researchers have developed an alternative treatment philosophy through the use
of nonaqueous SIs, proposed initially in the late 1990s. Although the SI
component in nonaqueous packages is usually based on a conventional phosphonate
or polymeric species, their delivery systems are quite different, thus leading
to transport and retention mechanisms that are also different, as reviewed in a
recent publication (Guan et al. 2004). For example, (a)
oil-soluble/miscible inhibitors (Guan et al. 2004; Wat et al. 1998a, 1998b,
1999a, 1999b; Jordan et al. 2000; Scott and Littlewood 2000; Jenvey et al.
2000) contain conventional SI products that have been manufactured to be
inherently oil soluble before application; (b) in invert-emulsion systems
(Collins et al. 2000, 2001; Lawless and Smith 1998; Jordan et al. 2006; Smith
et al. 2000), the aqueous SI is deployed in a water-in-oil emulsion; (c)
microencapsulated SIs (Scott and Littlewood 2000; Jenvey et al. 2000; Collins
et al. 2000, 2001; Lawless and Smith 1998; Jordan et al. 2006; Smith et al.
2000; Bourne et al. 2000) involve the separation of chemical solution from the
external environment by a wall or membrane, which is usually a polymer.
One of the difficulties in applying nonaqueous SIs is that their detailed
transport and retention mechanisms in porous media are much less well
understood. This is true for each of these major types of nonaqueous
products. Therefore, although several companies have performed successful
field applications for their nonaqueous products, squeeze models that allow us
to design such treatments in a systematic and routine manner are not yet
routinely available.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
29 July 2004
- Meeting paper published:
26 May 2004
- Revised manuscript received:
18 January 2006
- Manuscript approved:
27 January 2006
- Version of record:
20 November 2006
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