Experimental Study of the Effect of Stress and Fluid Sensitivity on Propped and Un-propped Fracture Conductivity in Preserved Reservoir Shale PDF Download

Author: Pratik Kakkar
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Languages : en
Pages : 130

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A good amount of work has been done on analyzing the effect of stress and fluid sensitivity on fracture conductivity in sandstones. This thesis tries to answer similar questions with regard to shale formations. Shales are very sensitive to aqueous fluids and their mechanical properties change when exposed to it. This mechanical property change in shale is mainly caused due to clay swelling. Some of the previous researchers working on shale fluid sensitivity failed to use preserved reservoir cores for their experiments and allowed them to dry out. This study has been conducted on preserved Utica and Eagle Ford core samples. Experiments were conducted to study the effect of effective stress on propped and un-propped fracture conductivity. These experiments were conducted at reservoir temperature and pressure conditions to mimic field conditions. Different fluids were flowed through the fracture to compare the effect of different fluids on fracture conductivity. To prevent clay swelling various clay stabilizers are used in the field during drilling and fracturing operations. Experiments were conducted to test the effectiveness of different clay stabilizers in preventing fracture conductivity reduction. Some of the clay stabilizers were more effective than others but all of them were unable to prevent fracture conductivity reduction when fracture was flowed with a high pH fluid.

Author: John Wesley Pedlow
Publisher:
ISBN:
Category :
Languages : en
Pages : 180

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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.

Author: Weiwei Wu (Ph. D.)
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ISBN:
Category :
Languages : en
Pages : 0

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A large proportion of the hydraulic fractures created during a hydraulic fracturing treatment remain unpropped after hydraulic fracturing despite the significant quantities of proppant injected in the process. These fractures either have a fracture width smaller than the size of the proppants, or are too far away from the wellbore where proppant cannot reach. Their presence has been demonstrated and corroborated by multiple independent sources of evidence such as flowback, production and microseismic data. These unpropped fractures present a huge potential for production enhancement, since they possess a very large contact area with the reservoir. Unfortunately, this potential flow area is closed by the closure stress during production. Without the presence of proppants, unpropped fractures are expected to behave differently from propped fractures. In this study, fracture conductivities of unpropped fractures in shales are measured with preserved Eagle Ford and Utica shale cores to better understand their closure behavior, in particular those after exposure to fracturing fluids. The unpropped fractures exhibit fracture conductivities 2 to 4 orders of magnitude lower than those of propped fractures, and are more sensitive to closure stress. Plastic deformation is found to dominate the closure process, and strong hysteresis occurs in unpropped fracture conductivity with a 70-80% reduction after a loading-unloading cycle of closure stress. Exposure to water-based fracturing fluids reduces unpropped fracture conductivity by shale softening or fines production. Unpropped fracture conductivities also appear to be sensitive to shale mineralogy, which affects the shale mechanical properties and shale-fluid interaction. A numerical model is developed to simulate the closure of unpropped and natural fractures, and to compute their corresponding fracture conductivity. A conjugate gradient algorithm and fast Fourier transform technique are incorporated to dramatically enhance the computation efficiency. Plastic deformation and deformation interaction among asperities, ignored in some previous models, are considered and shown to play an important role in the closure process. The model is validated against analytical solutions and experiments, for both elastic-only and elastoplastic scenarios. The compliance of unpropped fractures is demonstrated to be sensitive to the roughness and hardness of fracture surfaces, while less affected by Young's modulus. The new model is also capable of simulating closure of heterogeneous fracture surfaces. More plastic deformation and lower fracture conductivity is measured when surfaces with high clay content are used. Given the same mineralogy, the mineral distribution pattern shows a smaller impact on the closure behavior. The possibility of employing acid fracturing to stimulate unpropped fractures is also explored. The acid-etched topography of shale fracture surfaces is found to be dependent on both the content and distribution of the carbonate minerals. Shales with a high carbonate content (over 60 wt%) generally tend to develop rougher acid-etched surfaces. However, more carbonate content does not always necessarily lead to increased etched roughness. High etched roughness is more likely developed from a blocky, rather than scattered, distribution of carbonate minerals. A new experimental method, the "half-core approach", is formulated to address the challenge caused by shale heterogeneity in experimentally evaluating and comparing fracture performance. The half-core approach splits one shale core into two half cores, which are then subjected to treatments of interest independently, followed by assemblage into individual full cores with a spacer for fracture conductivity measurement. The half-core approach is effective in creating a baseline with reduced sample variation among shales to improve evaluation of fracturing fluids. Similar mineralogy and mechanical properties are found between half-cores among preserved shale samples spanning a wide range of mineralogy from Barnett, Eagle Ford, Haynesville and Utica shales. By applying the half-core approach, acid fracturing is systematically benchmarked against brine with Eagle Ford shales categorized into low (below 40 wt%), medium (40-70 wt%) and high (over 70 wt%) carbonate content. Compared to brine exposure, non-uniform acid fracturing enhances unpropped fracture conductivities for shales for a wide range of carbonate contents, while uniform acid fracturing generally leads to lower fracture conductivities due to shale softening. The unetched zone in non-uniform etching reduces shale softening and creates a surface topography that enhances fracture flow. Channels are more likely to form in carbonate-rich shale (over 70 wt%). Development of channels substantially increases the unpropped fracture conductivity, and reduces the hysteresis of unpropped fracture conductivities to closure stress. The presence of carbonate veins is found to promote the development of non-uniform etching

Author: Jose Domingo Romero Lugo
Publisher:
ISBN:
Category :
Languages : en
Pages :

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Hydraulic Fracturing stimulation technology is used to increase the amount of oil and gas produced from low permeability reservoirs. The primary objective of the process is to increase the conductivity of the reservoir by the creation of fractures deep into the formation, changing the flow pattern from radial to linear flow. The dynamic conductivity test was used for this research to evaluate the effect of closure stress, temperature, proppant concentration, and flow back rates on fracture conductivity. The objective of performing a dynamic conductivity test is to be able to mimic actual field conditions by pumping fracturing fluid/proppant slurry fluid into a conductivity cell, and applying closure stress afterwards. In addition, a factorial design was implemented in order to determine the main effect of each of the investigated factors and to minimize the number of experimental runs. Due to the stochastic nature of the dynamic conductivity test, each experiment was repeated several times to evaluate the consistency of the results. Experimental results indicate that the increase in closure stress has a detrimental effect on fracture conductivity. This effect can be attributed to the reduction in fracture width as closure stress was increased. Moreover, the formation of channels at low proppant concentration plays a significant role in determining the final conductivity of a fracture. The presence of these channels created an additional flow path for nitrogen, resulting in a significant increase in the conductivity of the fracture. In addition, experiments performed at high temperatures and stresses exhibited a reduction in fracture conductivity. The formation of a polymer cake due to unbroken gel dried up at high temperatures further impeded the propped conductivity. The effect of nitrogen rate was observed to be inversely proportional to fracture conductivity. The significant reduction in fracture conductivity could possibly be due to the effect of polymer dehydration at higher flow rates and temperatures. However, there is no certainty from experimental results that this conductivity reduction is an effect that occurs in real fractures or whether it is an effect that is only significant in laboratory conditions. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/148364

Author: Anton Nikolaev Kamenov
Publisher:
ISBN:
Category :
Languages : en
Pages :

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Hydraulic fracture conductivity in ultra-low permeability shale reservoirs is directly related to well productivity. The main goal of hydraulic fracturing in shale formations is to create a network of conductive pathways in the rock which increase the surface area of the formation that is connected to the wellbore. These highly conductive fractures significantly increase the production rates of petroleum fluids. During the process of hydraulic fracturing proppant is pumped and distributed in the fractures to keep them open after closure. Economic considerations have driven the industry to find ways to determine the optimal type, size and concentration of proppant that would enhance fracture conductivity and improve well performance. Therefore, direct laboratory conductivity measurements using real shale samples under realistic experimental conditions are needed for reliable hydraulic fracturing design optimization. A series of laboratory experiments was conducted to measure the conductivity of propped and unpropped fractures of Barnett shale using a modified API conductivity cell at room temperature for both natural fractures and induced fractures. The induced fractures were artificially created along the bedding plane to account for the effect of fracture face roughness on conductivity. The cementing material present on the surface of the natural fractures was preserved only for the initial unpropped conductivity tests. Natural proppants of difference sizes were manually placed and evenly distributed along the fracture face. The effect of proppant monolayer was also studied. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/149386

Author: Jiayi Yu
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ISBN:
Category :
Languages : en
Pages :

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We investigate the evolution of` fracture conductivity as a function of proppant loading concentration under varying effective stresses as an analog to reservoir drawdown. In particular, we define the relative impacts and interplay between proppant crushing, proppant embedment, compaction and particle rearrangement and their impacts on fluid transport. Proppant of realistic concentrations is sandwiched between split core-plugs of Marcellus shale that accommodates embedment as well as rigid steel that excludes it. Impacts of proppant crushing and embedment and roles of particulate transport in fracturing-fluid clean-up are defined. Experiments are performed under triaxial stresses with independent control on confining stress and pore pressure. Normal loading is incremented to represent reservoir drawdown with conductivity evolution recorded continuously via flow-through of brine (20,000 mg/L KCl). Proppant embedment is characterized pre- and post-test by white light optical profilometry with pre-and post-test particle size distributions of the proppant defining the impact of proppant crushing. The conductivity of propped fractures decreases by up to 95% as effective stress is increased by 50 MPa (7000 psi). This reduction is broadly independent of whether the fracture walls are rigid or deformable. The stress-sensitivity of conductivity is generally muted with increasing proppant loading concentration. We normalize fracture conductivities to equivalent permeabilities of the proppant pack to directly compare pack permeabilities. Low proppant concentrations return higher permeability at low effective stresses but lower permeability at high effective stress, relative to high proppant concentrations. This results since proppant crushing and embedment are both mitigated with increasing proppant loading concentration, as more displacement degree of freedom are added to the system and provide accommodation for interior compaction and rearrangement. Extended effective stress holding times (24h vs

Author: Kathryn Elizabeth Briggs
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ISBN:
Category :
Languages : en
Pages :

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Hydraulic fracturing is the primary stimulation method within low permeability reservoirs, in particular shale reservoirs. Hydraulic fracturing provides a means for making shale reservoirs commercially viable by inducing and propping fracture networks allowing gas flow to the wellbore. Without a propping agent, the created fracture channels would close due to the in-situ stress and defeat the purpose of creating induced fractures. The fracture network conductivity is directly related to the well productivity; therefore, the oil and gas industry is currently trying to better understand what impacts fracture conductivity. Shale is a broad term for a fine-grained, detrital rock, composed of silts and clays, which often suggest laminar, fissile structure. This work investigates the difference between two vertical zones in the Fayetteville shale, the FL2 and FL3, by measuring laboratory fracture conductivity along an artificially induced, rough, aligned fracture. Unpropped and low concentration 30/70 mesh proppant experiments were run on samples from both zones. Parameters that were controllable, such as proppant size, concentration and type, were kept consistent between the two zones. In addition to comparing experimental fracture conductivity results, mineral composition, thin sections, and surface roughness scans were evaluated to distinguish differences between the two zones rock properties. To further identify differences between the two zones, 90-day production data was analyzed. The FL2 consistently recorded higher conductivity values than the FL3 at closure stress up to 3,000 psi. The mineral composition analysis of the FL2 and FL3 samples concluded that although the zones had similar clay content, the FL2 contained more quartz and the FL3 contained more carbonate. Additionally, the FL2 samples were less fissile and had larger surface fragments created along the fracture surface; whereas the FL3 samples had flaky, brittle surface fragments. The FL2 had higher conductivity values at closure stresses up to 3,000 psi due to the rearrangement of bulky surface fragments and larger void spaces created when fragments were removed from the fracture surface. The conductivity difference between the zones decreases by 25% when low concentration, 0.03 lb/ft2, 30/70 mesh proppant is placed evenly on the fracture surface. The conductivity difference decrease is less drastic, changing only 7%, when increase the proppant concentration to 0.1 lb/ft2 30/70 mesh proppant. In conclusion, size and brittleness of surface fracture particles significantly impacts the unpropped and low concentration fracture conductivity. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/152755

Author: Mark John McGinley
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ISBN:
Category :
Languages : en
Pages :

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Production of hydrocarbons from low-permeability shale reservoirs has become economically feasible thanks in part to advances in horizontal drilling and hydraulic fracturing. Together, these two techniques help to create a network of highly-permeable fractures, which act as fluid conduits from the reservoir to the wellbore. The efficacy of a fracturing treatment can best be determined through fracture conductivity analysis. Fracture conductivity is defined as the product of fracture permeability and fracture width, and describes both how much and how easily fluid can flow through fractures. It is therefore directly related to well performance. The goal of this work is to explore fracture conductivity of Marcellus shale samples fractured in both horizontal and vertical orientations. The Marcellus shale, located primarily in Pennsylvania, Ohio, West Virginia, New York, and Maryland, is the largest gas-bearing shale formation in North America, and its development has significant implications on regional economies, the northeast United States' energy infrastructure, and the availability of petrochemical plant feedstock. In this work, a series of experiments was conducted to determine the propped fracture conductivity of 23 different samples from Elimsport and Allenwood, Pennsylvania. Before conductivity measurements were taken, the pedigree of samples was verified through XRD analysis, elastic rock properties were measured and compared against literature values, and fracture surface contours were mapped and measured. Fracture conductivity of both horizontally and vertically-fracture samples was determined by measuring the pressure drop of nitrogen gas through a modified API conductivity cell. Results show that fracture conductivity varies as a function of fracture orientation only when anisotropy of the rock's mechanical properties is pronounced. It is hypothesized that the anisotropy of Young's Modulus and Poisson's Ratio play a significant role in fracture mechanics, and therefore in the width of hydraulically-induced fractures. Ultimately, the experiments conducted as part of this work show that fracture conductivity trends are strongly tied to both proppant concentration and the rock's mechanical properties. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/155300

Author: Songyang Tong
Publisher:
ISBN:
Category :
Languages : en
Pages : 0

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During hydraulic fracturing treatments, proppants - usually sand - are placed inside fractures to improve fracture conductivity. However, a large portion of the generated hydraulic fractures often remain unpropped after fracturing treatments. There are two primary reasons for this poor proppant placement. First, proppants settle quickly in common fracturing fluids (e.g., slickwater), which results in unpropped sections at the tip or top of the fracture. Second, a large number of the microfractures are too narrow to accommodate any common commercial proppant. Such unpropped fractures hold a large potential flow capacity as they exhibit a large contact area with the reservoir. However, their potential flow capacity is diminished during production due to closing of unpropped fractures because of closure stress. In this study, fractures are categorized as wider fractures, which are accessible to proppant, and narrower fractures, which are inaccessible to proppant. For wider fractures, proppant transport is important as proppant is needed for keeping them open. For narrower fractures, a chemical formulation is proposed as there is less physical restriction for fluids to flow inside across them. The chemical formulation is expected to improve fracture conductivity by generating roughness on fracture surfaces. This dissertation uses experiments and simulations to investigate proppant transport in a complex fracture network with laboratory-scale transparent fracture slots. Proppant size, injection flow rate and bypass fracture angle are varied and their effects are systematically evaluated. Based on experimental results, a straight-line relationship can be used to quantify the fraction of proppant that flows into bypass fractures with the total amount of proppant injected. A Computational Fluid Dynamics (CFD) model is developed to simulate the experiments; both qualitative and quantitative matches are achieved with this model. It is concluded that the fraction of proppant which flows into bypass fractures could be small unless a significant amount of proppant is injected, which indicates the inefficiency of slickwater in transporting proppant. An alternative fracturing fluid - foam - has been proposed to improve proppant placement because of its proppant carrying capacity. Foam is not a single-phase fluid, and it suffers liquid drainage with time due to gravity. Additionally, the existence of foam bubbles and lamellae could alter the movement of proppants. Experiments and simulations are performed to evaluate proppant placement in field-scale foam fracturing application. A liquid drainage model and a proppant settling correlation are developed and incorporated into an in-housing fracturing simulator. Results indicate that liquid drainage could negatively affect proppant placement, while dry foams could lead to negligible proppant settling and consequently uniform proppant placement. For narrower fractures, two chemical stimulation techniques are proposed to improve fracture conductivity by increasing fracture surface roughness. The first is a nanoparticle-microencapsulated acid (MEA) system for shale acidizing applications, and the second is a new technology which can generate mineral crystals on the shale surface to act as in-situ proppants. The MEA could be released as the fracture closes and the released acid could etch the surface of the rock locally, in a non-uniform way, to improve fracture conductivity (up to 40 times). Furthermore, the in-situ proppant generation technology can lead to crystal growth in both fracking water and formation brine conditions, and it also improves fracture conductivity (up to 10 times) based on core flooding experiments

Author: Di Zhang
Publisher:
ISBN:
Category : Hydraulic fracturing
Languages : en
Pages : 70

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