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Evaluation of the relationship between fracture conductivity, fracture fluid production, and effective fracture lengthLolon, Elyezer P. 12 April 2006 (has links)
Low-permeability gas wells often produce less than predicted after a fracture treatment. One of the reasons
for this is that fracture lengths calculated after stimulation are often less than designed lengths. While
actual fracture lengths may be shorter due to fracture growth out of zone, improper proppant settling, or
proppant flowback, short calculated fracture lengths can also result from incorrect analysis techniques. It is
known that fracturing fluid that remains in the fracture and formation after a hydraulic fracture treatment
can decrease the productivity of a gas well by reducing the relative permeability to gas in the region
invaded by this fluid. However, the relationships between fracture fluid cleanup, effective fracture length,
and well productivity are not fully understood.
In this work I used reservoir simulation to determine the relationship between fracture conductivity,
fracture fluid production, effective fracture length, and well productivity. I simulated water saturation and
pressure profiles around a propped fracture, tracked gas production along the length of the propped
fracture, and quantified the effective fracture length (i.e., the fracture length under single-phase flow
conditions that gives similar performance as for multiphase flow conditions), the "cleanup" fracture length
(i.e., the fracture length corresponding to 90% cumulative gas flow rate into the fracture), and the
"apparent" fracture length (i.e., the fracture length where the ratio of multiphase to single-phase gas entry
rate profiles is unity).
This study shows that the proppant pack is generally cleaned up and the cleanup lengths are close to
designed lengths in relatively short times. Although gas is entering along entire fracture, fracturing fluid
remains in the formation near the fracture. The water saturation distribution affects the gas entry rate
profile, which determines the effective fracture length. Subtle changes in the gas rate entry profile can
result in significant changes in effective fracture length. The results I derived from this work are consistent
with prior work, namely that greater fracture conductivity results in more effective well cleanup and longer
effective fracture lengths versus time. This study provides better explanation of mechanisms that affect
fracturing fluid cleanup, effective fracture length, and well productivity than previous work.
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Investigation of the effect of gel residue on hydraulic fracture conductivity using dynamic fracture conductivity testMarpaung, Fivman 15 May 2009 (has links)
The key to producing gas from tight gas reservoirs is to create a long, highly
conductive flow path, via the placement of a hydraulic fracture, to stimulate flow from the
reservoir to the wellbore. Viscous fluid is used to transport proppant into the fracture.
However, these same viscous fluids need to break to a thin fluid after the treatment is over
so that the fracture fluid can be cleaned up. In shallower, lower temperature (less than
250oF) reservoirs, the choice of a fracture fluid is very critical to the success of the
treatment. Current hydraulic fracturing methods in unconventional tight gas reservoirs
have been developed largely through ad-hoc application of low-cost water fracs, with little
optimization of the process. It seems clear that some of the standard tests and models are
missing some of the physics of the fracturing process in low-permeability environments.
A series of the extensive laboratory “dynamic fracture conductivity” tests have
been conducted. Dynamic fracture conductivity is created when proppant slurry is
pumped into a hydraulic fracture in low permeability rock. Unlike conventional fracture
conductivity tests in which proppant is loaded into the fracture artificially, we pump
proppant/ fracturing fluid slurries into a fracture cell, dynamically placing the proppant
just as it occurs in the field.
Test results indicate that increasing gel concentration decreases retained fracture
conductivity for a constant gas flow rate and decreasing gas flow rate decreases retained
fracture conductivity. Without breaker, the damaging effect of viscous hydraulic
fracturing fluids on the conductivity of proppant packs is significant at temperature of
150oF. Static conductivity testing results in higher retained fracture conductivity when
compared to dynamic conductivity testing.
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Investigation of the effect of gel residue on hydraulic fracture conductivity using dynamic fracture conductivity testMarpaung, Fivman 10 October 2008 (has links)
The key to producing gas from tight gas reservoirs is to create a long, highly
conductive flow path, via the placement of a hydraulic fracture, to stimulate flow from the
reservoir to the wellbore. Viscous fluid is used to transport proppant into the fracture.
However, these same viscous fluids need to break to a thin fluid after the treatment is over
so that the fracture fluid can be cleaned up. In shallower, lower temperature (less than
250°F) reservoirs, the choice of a fracture fluid is very critical to the success of the
treatment. Current hydraulic fracturing methods in unconventional tight gas reservoirs
have been developed largely through ad-hoc application of low-cost water fracs, with little
optimization of the process. It seems clear that some of the standard tests and models are
missing some of the physics of the fracturing process in low-permeability environments.
A series of the extensive laboratory "dynamic fracture conductivity" tests have
been conducted. Dynamic fracture conductivity is created when proppant slurry is
pumped into a hydraulic fracture in low permeability rock. Unlike conventional fracture
conductivity tests in which proppant is loaded into the fracture artificially, we pump
proppant/ fracturing fluid slurries into a fracture cell, dynamically placing the proppant
just as it occurs in the field.
Test results indicate that increasing gel concentration decreases retained fracture
conductivity for a constant gas flow rate and decreasing gas flow rate decreases retained
fracture conductivity. Without breaker, the damaging effect of viscous hydraulic
fracturing fluids on the conductivity of proppant packs is significant at temperature of
150°F. Static conductivity testing results in higher retained fracture conductivity when
compared to dynamic conductivity testing.
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A study of the effect of stress and fluid sensitivity on propped fracture conductivity in preserved reservoir shalesPedlow, John Wesley 07 November 2013 (has links)
A sizable amount of literature exists analyzing the effect of confining stress on fracture conductivity in sandstones. This thesis attempts to answer similar questions with regard to shale formations. The low Young’s Moduli and Brinell hardness values characteristic of many prospective shale formations may lead to a great deal of embedment and fines production which can drastically reduce fracture conductivity. Furthermore, shales exhibit sensitivity to aqueous fluids which may cause them to be weakened in the presence of certain fracturing fluids. Previous work analyzing shale fluid sensitivity has failed to preserve the shales’ formation properties by allowing the shale to dry out.
This paper presents a study of propped fracture conductivity experiments at reservoir temperature and pressure using various North American shale reservoir cores. Exposure to the atmosphere can alter the mechanical properties of the shale by either drying or hydrating the samples, so care was taken to preserve these shales in their native state by maintaining constant water activity (relative humidity). Variations in applied closure stress and aqueous fluid exposure were analyzed and in certain cases altered the propped fracture conductivity by crushing proppant, embedding the proppant into the fracture face, and producing fines. The damage to fracture conductivity is correlated to mineralogy for the various shale samples. These findings show that a one-size-fits-all frac design will not work in every shale formation, rather a tailored approach to each shale is necessary.
In the future, the results of this work will be analyzed alongside easier to perform Brinell hardness tests, swelling tests, and other characterization techniques incorporated into the UT Shale Characterization Protocol. Correlations were developed to relate the simpler tests to the fracture conductivity experiments which yield a straight forward method to determine the role embedment and fluid sensitivity have on post treatment fracture conductivity in shales. The UT Shale characterization Protocol can then be used to optimize the design and execution of fracing treatments. / text
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Development, setup and testing of a dynamic hydraulic fracture conductivity apparatusPongthunya, Potcharaporn 02 June 2009 (has links)
One of the most critical parameters in the success of a hydraulic fracturing treatment is to have sufficiently high fracture conductivity. Unbroken polymers can cause permeability impairment in the proppant pack and/or in the matrix along the fracture face. The objectives of this research project were to design and set up an experimental apparatus for dynamic fracture conductivity testing and to create a fracture conductivity test workflow standard. This entirely new dynamic fracture conductivity measurement will be used to perform extensive experiments to study fracturing fluid cleanup characteristics and investigate damage resulting from unbroken polymer gel in the proppant pack. The dynamic fracture conductivity experiment comprises two parts: pumping fracturing fluid into the cell and measuring proppant pack conductivity. I carefully designed the hydraulic fracturing laboratory to provide appropriate scaling of the field conditions experimentally. The specifications for each apparatus were carefully considered with flexibility for further studies and the capability of each apparatus was defined. I generated comprehensive experimental procedures for each experiment stage. By following the procedure, the experiment can run smoothly. Most of dry runs and experiments performed with sandstone were successful.
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Development, setup and testing of a dynamic hydraulic fracture conductivity apparatusPongthunya, Potcharaporn 02 June 2009 (has links)
One of the most critical parameters in the success of a hydraulic fracturing treatment is to have sufficiently high fracture conductivity. Unbroken polymers can cause permeability impairment in the proppant pack and/or in the matrix along the fracture face. The objectives of this research project were to design and set up an experimental apparatus for dynamic fracture conductivity testing and to create a fracture conductivity test workflow standard. This entirely new dynamic fracture conductivity measurement will be used to perform extensive experiments to study fracturing fluid cleanup characteristics and investigate damage resulting from unbroken polymer gel in the proppant pack. The dynamic fracture conductivity experiment comprises two parts: pumping fracturing fluid into the cell and measuring proppant pack conductivity. I carefully designed the hydraulic fracturing laboratory to provide appropriate scaling of the field conditions experimentally. The specifications for each apparatus were carefully considered with flexibility for further studies and the capability of each apparatus was defined. I generated comprehensive experimental procedures for each experiment stage. By following the procedure, the experiment can run smoothly. Most of dry runs and experiments performed with sandstone were successful.
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Evaluation and Effect of Fracturing Fluids on Fracture Conductivity in Tight Gas Reservoirs Using Dynamic Fracture Conductivity TestCorrea Castro, Juan 2011 May 1900 (has links)
Unconventional gas has become an important resource to help meet our future
energy demands. Although plentiful, it is difficult to produce this resource, when locked
in a massive sedimentary formation. Among all unconventional gas resources, tight gas
sands represent a big fraction and are often characterized by very low porosity and
permeability associated with their producing formations, resulting in extremely low
production rate. The low flow properties and the recovery factors of these sands make
necessary continuous efforts to reduce costs and improve efficiency in all aspects of
drilling, completion and production techniques. Many of the recent improvements have
been in well completions and hydraulic fracturing. Thus, the main goal of a hydraulic
fracture is to create a long, highly conductive fracture to facilitate the gas flow from the
reservoir to the wellbore to obtain commercial production rates. Fracture conductivity
depends on several factors, such as like the damage created by the gel during the
treatment and the gel clean-up after the treatment.
This research is focused on predicting more accurately the fracture conductivity,
the gel damage created in fractures, and the fracture cleanup after a hydraulic fracture treatment under certain pressure and temperature conditions. Parameters that alter
fracture conductivity, such as polymer concentration, breaker concentration and gas flow
rate, are also examined in this study. A series of experiments, using a procedure of
“dynamical fracture conductivity test”, were carried out. This procedure simulates the
proppant/frac fluid slurries flow into the fractures in a low-permeability rock, as it
occurs in the field, using different combinations of polymer and breaker concentrations
under reservoirs conditions.
The result of this study provides the basis to optimize the fracturing fluids and
the polymer loading at different reservoir conditions, which may result in a clean and
conductive fracture. Success in improving this process will help to decrease capital
expenditures and increase the production in unconventional tight gas reservoirs.
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The Effects of Initial Condition of Fracture Surfaces, Acid Spending, and Type on Conductivity of Acid FractureAlmomen, Ali Mansour 16 December 2013 (has links)
Fracture conductivity and the effects of treatment variables can be studied in the laboratory. We conducted experiments based on scaling down the field conditions to laboratory scale by matching Reynold’s and Peclet numbers. Experiments conducted were comprised of three stages: dynamic etching, surface characterization of etched cores, and conductivity measurement. The effect of initial condition of fracture surfaces on the etching pattern and conductivity were investigated in this study. Another area of interest is the variation of conductivity along the fracture due to acid spending. We also investigated the contact time, acid system type, and treatment temperature effects on conductivity using San Andres dolomite cores.
The results from these studies showed that rough-surface fractures generate higher conductivity by an order of magnitude compared with a smooth-surface fracture at low-closure stress. Also, conductivity generated on rough-surface fractures by smoothing peaks and deepening valleys which widen the gap between the fracture surfaces after closure and acid creates conductivity on smooth-surface fractures by differential etching that creates asperities.
The results suggest that an increase in acid spending does not automatically result in lower conductivity; and etched volume alone is not adequate to predicate the conductivity. Conductivity results from a combination of etching pattern, etched volume, and rock compressive strength after etching.
In-situ crosslinked acid was found to be more effective in etching rock and controlling acid leakoff compared with linear-gelled acid. Also, crosslinked acid reduces the number of pits and the pit diameters. Based on conductivity tests, linear-gelled acid is more favorable at higher temperatures while in-situ crosslinked acid showed higher conductivity at lower temperatures. For a rough-surface fracture, shorter contact time created high conductivity compared to longer contact while injecting the same volume of acid, suggesting the existence of an optimum contact time.
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Modeling Acid Transport and Non-Uniform Etching in a Stochastic Domain in Acid FracturingMou, Jianye 2009 August 1900 (has links)
Success of acid fracturing depends on uneven etching along the fracture surfaces
caused by heterogeneities such as variations in local mineralogy and variations in leakoff
behavior. The heterogeneities tend to create channeling characteristics, which provide
lasting conductivity after fracture closure, and occur on a scale that is neither used in
laboratory measurements of acid fracture conductivity, which use core samples that are
too small to observe such a feature, nor in typical acid fracture simulations in which the
grid block size is much larger than the scale of local heterogeneities. Acid fracture
conductivity depends on fracture surface etching patterns. Existing acid fracture
conductivity correlations are for random asperity distributions and do not consider the
contribution of channels to the conductivity. An acid fracture conductivity correlation
needs the average fracture width at zero closure stress. Existing correlations calculate
average fracture width using dissolved rock equivalent width without considering the
effect of reservoir characteristics. The purpose of this work is to develop an intermediate-scale acid fracture model
with grid size small enough and the whole dimension big enough to capture local and
macro heterogeneity effects and channeling characteristics in acid fracturing. The model
predicts pressure field, flow field, acid concentration profiles, and fracture surface
profiles as a function of acid contact time. By extensive numerical experiments with the
model, we develop correlations of fracture conductivity and average fracture width at
zero closure stress as a function of statistical parameters of permeability and mineralogy
distributions.
With the model, we analyzed the relationships among fracture surface etching
patterns, conductivities, and the distributions of permeability and mineralogy. From
result analysis, we found that a fracture with channels extending from the inlet to the
outlet of the fracture has a high conductivity because fluid flow in deep channels needs a
very small pressure drop. Such long and highly conductive channels can be created by
acids if the formation has heterogeneities in either permeability or mineralogy, or both,
with high correlation length in the direction of the fracture, which is the case in
laminated formations.
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Dynamic Fracture Conductivity—An Experimental Investigation Based on Factorial AnalysisAwoleke, Obadare O 02 October 2013 (has links)
This work is about fracture conductivity; how to measure and model it based on experimental data. It is also about how to determine the relative importance of the factors that affect its magnitude and how to predict its magnitude based on these factors. We dynamically placed the slurry hereby simulating the slurry placement procedure in a field-scale fracture. We also used factorial and fractional factorial designs as the basis of our experimental investigation. The analysis and interpretation of experimental results take into account the stochastic nature of the process. We found that the relative importance of the investigated factors is dependent on the presence of outliers and how they are handled.
Based on our investigation we concluded that the investigated factors arranged in order of decreasing impact on conductivity are: closure stress, polymer loading, flow back rate, presence of breaker, temperature and proppant concentration. In particular, we find that at high temperatures, fracture conductivity was severely reduced due to the formation of a dense proppant-polymer cake. Also, dehydration of the residual gel in the fracture at high flow back rates appears to cause severe damage to conductivity at higher temperatures. This represents a new way of thinking about the fracture cleanup process; not only as a displacement process, but also as a displacement and evaporative process. In engineering practice, this implies that aggressive flow back schemes are not necessarily beneficial for conductivity development. Also, we find that at low proppant concentrations, there is the increased likelihood of the formation of channels and high porosity fractures resulting in high fracture conductivities.
The uniqueness of this work is a focus on the development of a conductivity model using regression analysis and also the illustration of a procedure that can be used to develop a conductivity model using dimensional analysis. We reviewed both methodologies and applied them to the challenge of modeling fracture conductivity from experimental studies.
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