Summary
Injectivity decline during produced-water reinjection (PWRI) originates not
only from filter-cake buildup but also from in-depth deposition of oil droplets
or solid particles. Physical modeling of particle-deposition mechanisms in
porous media is thus of key interest for optimizing PWRI operations. The
present work brings new insights on oil-droplet and solid-particle-deposition
mechanisms in porous media.
The experimental conditions were selected such that the ratio between pores
and particle sizes is sufficiently large to ensure in-depth propagation. The
parameters are the nature of the particles injected and a Peclet number
calculated on the size of the collector grains (Pg ) that
encompasses in a nondimensional form the impact of both the flow rate and the
particle size.
The results are analyzed within the framework of the "colloidal
approach." For oil droplets and solid particles, the collection efficiency
(η) shows a transition from a behavior in which η varies as a
power law of Pg , with exponent values −⅔ [(diffusion-limited
deposition (DLD)] to −1 [reaction-limited deposition (RLD)] that are typical of
the convection/diffusion regime, to a behavior characterized by an increase of
η vs. Pg , typical of the hydrodynamic deposition
regime. In the case of oil droplets (slightly charged), the transition occurs
at a critical Pg value, PgC ≈
PgCgeom/10, corresponding to a diffusion-layer
thickness around the collector grain of the same order of magnitude as the
droplet diameter. In the case of electrosterically stabilized solid particles,
the transition takes place at PgC <<
PgCgeom for small particles and at
PgC > PgCgeom for larger
particles.
Introduction
As the environmental regulations for water discharge are becoming
increasingly stringent, PWRI is now recognized as an important issue with
respect to environmental protection and profitability. In many mature zones,
PWRI is becoming an economically attractive option. It enables reduction in
processing costs of the produced water before disposal and allows handling the
ever-increasing volumes of produced water without increasing environmental
risks associated with water discharges. PWRI also can be used for waterflooding
or pressure maintenance in the initial stage of production. Therefore, most of
the produced water has to be reinjected either in a suitable formation for
disposal or in the producing formation for improved-oil-recovery purposes.
However, great uncertainties still remain about the consequences of PWRI and
the actual injectivity behavior. If injectivity decline is severe, sustainable
injection rate for extended periods will become impossible, jeopardizing the
entire reinjection operation. From field observations, it was acknowledged
that:
- With produced water, plugging is important and injectivity is lower than
expected. The solids and the oil do not behave separately.
- With matrix PWRI, a continuous loss of injectivity is obtained, even in
high-permeability formation (soft rock) (Detienne and Po 2005).
- Successful PWRI is likely to require fracturing (Detienne and Po 2005;
Raaen 2005; van den Hoek and Bjoerndal 2005; Sweeney 2005).
Thus, the injectivity performance is viewed increasingly as being dictated
by a dynamic coupling between fracture growth and plugging of fracture faces.
This general scheme raises additional challenges related to the issues of
fracture containment, sweep efficiency, and conformance, especially in
soft-rock formations.
Simulation of PWRI under fracturing conditions is therefore necessary to
optimize the injectivity behavior and to establish a PWRI strategy (Detienne et
al. 2005; Ochi et al. 1999; van den Hoek et al. 1996). However, until now,
development of models with reliable prediction ability suffers from the lack of
a clear insight into the actual physics of the damage process. Understanding
the damaging mechanisms when injecting water containing solid particles and oil
droplets and evaluating the impact of such damage on injector performance are,
thus, important research needs.
PWRI-induced damage results from in-depth particle penetration into the
surrounding formation and from particle accumulation on formation/fracture face
to form a heterogeneous and highly compressible filter cake.
The filter-cake permeability is among the important parameters that have a
great impact on injectivity. Hence, a representative estimation of this
parameter is required to forecast the injection behavior (van den Hoek et al.
1996).
More generally, the impact of particles, including mixtures of oil and
solids, on injectivity has received increasing attention recently (van den Hoek
et al. 1996; Al-Abduwani et al. 2003; Al-Riyamy and Sharma 2002; Bedrikovetsky
et al. 2001; da Silva et al. 2004; Hofsaess and Kleinitz 2003; Sæby et al.
2005). Regarding oil-in-water-emulsion flow and deposition in porous media,
despite significant experimental and theoretical work (Soo and Radke 1984,
1986; Soo et al. 1986), impact on injectivity is still not understood clearly,
especially under severe injection conditions (high flow rate into
low-permeability formation).
The objective of this study was to bring new insights into the impact of
flow rate on in-depth deposition mechanisms of the colloidal particles present
in produced water. The present work was focused on two kinds of particles: (1)
uncharged oil droplets in dilute and stable oil-in-water emulsions and (2)
electrosterically stabilized latex microspheres. For both kinds of particles, a
broad range of velocities was investigated.
In the next section, some useful results regarding the theoretical
background of the colloidal approach are presented. In the section after that,
materials and experimental procedures are described. Experimental results are
presented and discussed in the last section.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
1 March 2007
- Meeting paper published:
30 May 2007
- Revised manuscript received:
1 February 2008
- Manuscript approved:
21 February 2008
- Version of record:
15 November 2008
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