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ABSTRACT: In this study, we examine the variation of viscoplastic creep properties of Wolfcamp shale samples from an area in the Permian Basin of West Texas. In this area, multiple horizontal wells are drilled and hydraulically fractured at different depths to exploit the hydrocarbons in different reservoirs. Following Sone and Zoback, (Jour. Petrol. Sci. and Eng., 2014) we are studying the degree to which viscoplastic creep leads to creation of high stress frac barriers. We report here a series of creep experiments on samples with various mineral contents and from different depths to test this concept. The experiments were conducted in a multistage pattern at three different stress levels. A comparison between inelastic time-dependent deformation of shale samples with different mineralogies at high pressures demonstrates that carbonate-rich shales show creep-hardening behavior at higher levels of stress whereas clay rich shales appear to creep more at high pressures and show creep-softening behavior.
Oil and gas production from very low permeability unconventional resources requires hydraulic fracturing. There is a wide range of variations in characteristics within the shale formations; some of the formations are relatively brittle and enabling hydraulic stimulation to enable production, while other shale formations deform ductily and may act as fracture and flow barriers [1, 2, 3]. Predicting the magnitudes of the least principal stress within sedimentary basins has significant practical value in the petroleum industry.
Mineral composition has a direct effect on viscoplastic deformation of shale rocks. Higher amounts of clay and organic matter makes the shale formations deform with time which would likely lead to the formations acting as fracture barriers in the reservoir. In addition, it is important to understand the degree of anisotropy in the rocks and its affect on hydraulic fracturing [4, 5, 6].
In this study, we conducted creep experiments under constant applied differential stress of 30 MPa, 40 MPa and 50 MPa on clay- or carbonate-rich shale samples collected from the Wolfcamp formation in the Permian basin formation in West Texas, USA. Using the results of these experiments, we calculated the creep parameters from a power-law model to study how mineral composition and sample structure change the deformation of samples at different stress levels in longterm. The results help us to better understand the timedependent response of shale formations with various properties at different stress states.
The goal of this study was to compare constitutive parameters derived from short-term (four hour) and long-term (several-week) creep experiments. We conducted a series of creep experiments on clay and carbonate-rich shale samples from unconventional gas and oil reservoirs at room temperature, principally on samples from the Haynesville and Eagle Ford formations. Samples with different mineralogies were subjected to a series of multi-stage loading/creep, unloading/recovery cycles conducted over different time spans. All creep stages of the experiments were performed at a constant differential stress level; only the testing time of each creep/recovery stage was varied. Following Sone and Zoback (Jour. Petrol. Sci. and Eng., 2014), a power-law creep model was used to obtain the creep constitutive parameters. Results of these experiments show that the shale samples follow the same creep trend through time, regardless of the loading history. Also, we show that the simple power-law model is capable of describing creep over multiple time periods. Using this model, we are able to characterize viscoplastic behavior of shale rocks from relatively short-term (1 day) creep experiments.
A common challenge in laboratory creep studies is to know how long laboratory experiments need to be carried out to accurately measure the parameters that make it possible to predict rock deformation over relatively long periods of time. This time-dependent deformation affects mechanical and flow properties of sedimentary rocks [1, 2, 3, 4, 5, 6]. Initial laboratory studies of viscoplatic deformation were carried out via uniaxial and triaxial creep experiments [7, 8, 9, 10]. Because of the time-consuming nature of these experiments, relatively few papers have addressed this topic.
In this paper we carried out a series of creep experiments on shale samples with various amounts of carbonate and clay to investigate the constitutive law and creep parameters obtained in loading steps ranging from several hours to several weeks. We extend the comprehensive short-term (several hour) creep experiments reported by Sone and Zoback (2014) who investigated the role of clay and organic content on viscous deformation of shales from unconventional gas reservoirs . They used a relatively simple (two parameter) power-law model to fit the data.
Several mechanisms have been proposed to explain low temperature creep in sedimentary rocks. Although Peterson and Wong (2004) argue that the deformation of rocks at temperatures below 1200 K can generally be considered brittle , other studies have shown that it is possible to observe inelasic deformation of rocks at lower temperatures due to mechanical compaction processes . For example, Sone and Zoback (2014) argued that the time-dependent deformation they observed was accommodated by changes in the pore volume.
Shale rocks with high clay and carbonate content show time-dependent deformation at different conditions of stress. In this paper, we present a series of time-cycling triaxial deformation creep tests conducted on two samples from the Haynesville formation in east Texas. A power-law model was fit to all creep measurements to find the constitutive parameters of the time-dependent deformation of these samples. We obtained these constitutive parameters from triaxial creep experiments over time periods of 3-4 hours, one day, one week and 3-4 weeks. Although further experiments are needed, the experimental results obtained to date show that short-term creep tests are able to provide constitutive parameters that predict long-term deformation of these clay-rich shales. Both Young’s modulus and the viscoelastic properties are anisotropic, with the samples being more compliant and more viscoplastic when deformed normal to bedding.
Understanding time-dependent deformation in unconventional hydrocarbon reservoirs is one of the most difficult technical problems in the oil and gas industry. Viscous deformation can change both mechanical and fluid flow properties of shales, which are of primary importance in determining production capacity of shale gas reservoirs. Clay minerals may be significantly responsible for the inelastic deformation of shale rocks because of their high viscoelastic behavior [1, 2, 3, 4].
Several techniques have been used to determine the inelastic time-dependent deformation of rocks, among which uniaxial and triaxial creep experiments have been used widely [2, 4, 5, 6, 7]. The main problem with conducting these experiments is that time dependent essences of creep experiments make these tests costly and time consuming. Consequently, predicting the longterm inelastic deformation of rocks from short-time experiments has been a matter of interest to the scholars.
In this study, we performed a series of conventional triaxial creep experiments on two clay-rich samples from the Haynesville shale formation to investigate the inelastic deformation process in shales. The triaxial experiments were performed in a time-cycling pattern on a horizontal and a vertical shale sample to understand the bedding orientation effects. This time-cycling pattern includes a series of multi-stage loading/unloading experiments at different time spans. The main goal of performing this type of experiment was to study the authenticity of the few-hours long creep experiments.
ABSTRACT: The time dependent behavior of a reservoir rock is of outmost importance on the understanding of wellbore stability during drilling and also the long term productivity of a fractured well. For these reasons, the aim of this work is the derivation of a general constitutive model which should capture the creep behavior of Vaca Muerta formation. To achieve this goal, many time cycling triaxial deformation creep tests were performed over many Vaca Muerta rock samples. A phenomenological creep constitutive model from the literature such as the Power Law was tested and evaluated to find the best fit from the data acquired from laboratory testing. Finally, the creep behavior of Vaca Muerta formation is compared briefly with published data from U.S. shales.
Understanding the mechanical properties of reservoir rocks is of outmost importance, not only for defining operational limits regarding wellbore stability, but also for understanding the long-term variations in productivity of a fractured well. As an example, Morales et. al. 2011 suggested that hydraulic fractures could be subjected to long term loss of conductivity due to the intrinsic rheological behavior of the rock at the surfaces were the proppant is in contact with the rock.
A full mechanical characterization of Vaca Muerta shale rock was exposed in Varela and Hasbani, 2017. In that work, an overview of the most relevant research assessments on the understanding of Vaca Muerta's rock mechanics behavior was presented. Furthermore, the different laboratory test parameters that control the laboratory test response were deeply analyzed, but special focus was made in temperature and confining pressure effects in the overall strength of the rock. To complete the characterization of Vaca Muerta's mechanical behavior, some special laboratory studies like Biot's coefficient measurements and creep response characterization were performed on some shale gas core samples; nevertheless, the results of these tests were superficially discussed.
In the study reported here, the effort was set on deeply and accurately investigate the time- dependent deformational behavior of the rock and the constitutive relations between stress, strain and time. Following this main objective, samples from shale gas and shale oil from the Vaca Muerta formation were taken in different locations around the Neuquén Basin and laboratory triaxial creep experiments were conducted on them.
ABSTRACT: The Vaca Muerta formation has been under study as a potential shale reservoir since approximately 2007; however, the first well, targeting this formation, was drilled in 2010. During the last few years, many works showing different discoveries about Vaca Muerta mechanical rock properties have been published. All of them are an invaluable piece of work; nevertheless, they do not give a full characterization of Vaca Muerta mechanical rock properties. Until today, many meters of core samples were recovered from Vaca Muerta formation and a complete set of rock mechanics laboratory tests have been performed on core plugs to accurately characterize all the parameters that governs the mechanical behavior of the rock. In addition, to further understand the variability in the mechanical response of the rock, changes in the setting up of the laboratory tests were done in similar plugs on a same core sample (sister samples). It is the aim of this paper to present an overview of recent Vaca Muerta research works which include mechanical laboratory characterization of the rock and discuss about how different laboratory test parameters such as temperature, confining pressure, time, and fluid saturation among others affects the mechanical response of the rock. To complete the understanding of the Vaca Muerta formation mechanical behavior, special laboratory studies like Biot's coefficient and creep test were performed in some core shale gas samples; nevertheless, in this work, the quality and validity of these results are left as an open discussion. The most important conclusion of the analysis done in this work is that some elastic properties are not mainly controlled by the “rock families or facies”. As a result, a unique correlation between many elastic properties has been found and a simple workflow to fully characterize the rock mechanical properties of Vaca Muerta formation is proposed.
During the exploration, delineation and development of the principal shale oil and gas reservoirs in Argentina, several studies like petrophysics, geochemistry, biostratigraphy and geomechanics haves been done. In order to calibrate the parameters of different models many meters of core has been recovered from the Vaca Muerta formation in the Neuquén basin, Argentina. This core acquisition was fundamental for the correct characterization of the mechanical rock properties. These properties play a fundamental role in every stage in the life of a well i.e. drilling process (wellbore stability), fracturing process and finally production forecasting.