Summary
The importance of produced water reinjection (PWRI) is unquestionable. It is
in many cases the cheapest and most environmentally friendly solution for
wastewater disposal. It is also a feasible method for enhanced oil recovery
(EOR) as a waterflooding mechanism.
PWRI, however, suffers from a major limitation, which is the current
inability in accurately predicting the lifespan and performance of its
injection wells. This is because of the multitude of parameters that affect it.
Current models1,2 that incorporate the thermal effects3 of PWRI leading to
fracture growth exist. However, the leakoff pattern of this injection differs
from that of clean water (seawater) injection because of the damage caused by
the produced water to the formation, especially to the fracture faces. Thus,
static filtration experiments with refined post-mortem analysis have been
conducted to obtain quantitative deposition profiles along the core. This
allows for the testing and verification of existing models.4–8
The postmortem analysis introduced in this paper will be used for future
dynamic filtration experiments as well as experiments specifically devised to
simulate the fracture tip area. A unified model that will accurately reproduce
the permeability decline and deposition profile for all three sets of
experiments will follow, thus advancing the predictability of injectivity
decline associated with PWRI.
The purpose of this paper is to provide a detailed description of the
post-mortem analysis, while rigorous testing of the existing heuristic models
(for instance, Wennberg and Sharma6 and Bedrikovedski et al.7) will be
published in the near future.9
Introduction
PWRI, when first introduced, was seen as a breakthrough solution for water
disposal. It is both environmentally friendly and economically among the
cheapest options. Thus, it garnered much interest. However, the associated
injectivity decline remains a major issue.
In order to simulate and predict the extent of formation damage
(permeability decline) inflicted by PWRI, it is necessary to have a model of
the damage as a function of injection flow rate and particle concentration
among other parameters. Typically, such a model is achieved by coupling a
filtration model with a formation damage model. 4,8,10 The classical deep-bed
filtration model was introduced by Iwasaki in 1937 11 and contained a
phenomenological function---the filtration function GREEKlambda ---which
describes the deposition intensity of the suspended particles and depends on
previous deposition GREELsigma only. Variations of the filtration function
GREEKlambda ( GREEKsigma ) were proposed by subsequent researchers; see Herzig
et. al. 4 and Bai and Tien. 12
Forward-solution simulations were conducted by researchers to generate
effluent profile predictions and compare them to experimental measurements. 12
Consequently, inverse-solution simulators emerged in which the experimentally
quantified effluent concentration profiles were used to extract the filtration
function. 13,14 Furthermore, alternative models were proposed in which the
permeability decline over the core, characterized by 3 or more pressure points,
was used to extract an approximation for the filtration coefficient. 9,15 It
should be noted that inverse solutions based on the effluent profile are
ill-posed, while the three-point pressure method incorporates a simplified
formation model, which itself needs to be verified. In any case, the successful
extraction of a filtration function does not verify the model until predictions
of other variables such as the deposition profile or the suspension
concentration profile along the core have been compared to the experimental
values.
Measurement of the suspension concentration profile has been conducted by
Kau and Lawler 16 in sand columns. No attempts have been made to measure the
deposition profile along a core prior to this work, as far as the authors are
aware. This article presents a novel experimental technique for the
quantification of the deposition profile along a sandstone core post-mortem,
and is the foundation of further work 17 in which multiple online measurements
of the deposition profile can be obtained. Such data, along with the effluent
concentration profile and pressure measurements along the core can be used to
verify and test the different models in a far more rigorous manner than has
been done previously. The experiments conducted can also be used to draw
analogies with field cases. However, care should be taken, because these
experiments are simplified cases in which the true complexity of the real PWRI
processes is not captured.
Six static filtration experiments (Exp. 1 through Exp. 6) were conducted
using a five-port sleeve, to obtain six pressure-drop data channels over a
5.0-in. Bentheim sandstone core of 1 in. diameter. Bentheim sandstone is
homogeneous, with a porosity of 22%, permeability of approximately 1.4 D and
pore throat diameter of 10 to 15 GREEKmum. Distilled water containing 0.1 to 5
GREEKmum hematite (Fe 2 O 3 ) particles (65% of which were less than 1 GREEKmum
in diameter) was injected at different concentrations (20, 40, and 80 mg/l) and
flow rates (5.4 l/hr [ approximately 2.9E-3 m/s] and 10 l/hr \[approximately
5.4E-3 m/s]) into the core---each experiment having only one injection
concentration and one injection flow rate. The concentration of the effluent
solution of the experiment was either measured online using a laser diffraction
unit (Exp. 6 illustrated in Fig. 1) or by collecting samples and quantifying
the concentration at a later stage using chemical analysis (Exp. 1 to Exp. 5,
of which an example is given in Fig. 2). The data gathered used the guideline
suggested by Wennberg 18 as a compliance criterion.
Synthetic produced water consisting of distilled water and hematite
particles was utilized for these experiments for the following reasons: (1)
hematite is a common component of real produced water, being present in the
tubing of the injection wells; (2) hematite is an ideal tracer for these
experiments because it is chemically stable, not present in Bentheim sandstone,
and easily detectable chemically and visually; and (3) hematite has been used
previously by researchers for filtration experiments. 19
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
12 August 2003
- Revised manuscript received:
25 January 2005
- Manuscript approved:
16 March 2005
- Version of record:
15 June 2005
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