Summary
In the application of hydraulic fracturing of oil and gas reservoirs, the
objective is to create a conductive pathway for hydrocarbons to flow into the
wellbore. This is accomplished by placing a proppant into the created fracture
that will prevent fracture closure and maintain a high conductivity for an
extended time period. A number of mechanisms have been identified that can
degrade the fracture conductivity, including mechanical failure of the proppant
grains, liberation of formation fines, proppant embedment, formation spalling,
damage from the fracturing fluid, stress cycling, asphaltene deposition,
proppant dissolution, and others. These factors in combination can reduce the
effective conductivity by orders of magnitude as compared with the typically
published conductivity data measured under reference conditions.
Another mechanism for degradation of proppant over time has recently been
postulated. This mechanism has been labeled "diagenesis" and refers to a
dissolution and reprecipitation process that may reduce the porosity,
permeability, and strength of the proppant pack as precipitants are deposited.
Factors that were believed to control the occurrence and degree of diagenesis
include closure stress, reservoir temperature, proppant type, and mineralogy of
the rock formation. While much work has gone into evaluating this phenomenon by
the industry, there remains uncertainty as to the prevalence of this mechanism
and the prediction of its occurrence.
This paper will summarize results from high-temperature static tests,
extended-duration flow tests at reservoir conditions, detailed analyses of
precipitates, the effects of this environment on the mechanical properties of
proppants, chemical and mineralogical analysis of various reservoir shales, and
evaluation of actual proppant samples that have been retrieved from producing
wells.
The paper will conclusively demonstrate that crystalline precipitates can be
formed on the surface of all proppant types, including ceramic, sand,
resin-coated materials, and even inert steel balls or glass rods. The nature of
these precipitants indicate that they may be classified as zeolites. The
chemistry and environment leading to the formation of zeolite precipitates will
be reviewed. Testing indicates that zeolites can be formed without the presence
of alumina-bearing proppant and appear to be largely dependent on the
characteristics of the formation material and fluid. Conductivity tests
carefully simulating reservoir conditions with actual reservoir shale core
samples indicate that if diagenetic precipitation does occur it does not
appreciably affect conductivity performance in the flowing conditions
evaluated. Furthermore, other conditions known to occur in oil and gas
reservoirs would naturally prevent the formation of zeolites.
The results of this work will aid the stimulation engineer in proppant
selection and treatment design. While there are many mechanisms contributing to
the degradation of proppant performance, these studies indicate that zeolite
precipitation is unlikely to be a dominant concern in most wells.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
17 February 2011
- Meeting paper published:
25 January 2011
- Revised manuscript received:
19 July 2011
- Manuscript approved:
27 September 2011
- Published online:
29 March 2012
- Version of record:
1 May 2012
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