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Abstract In this case study, we apply a novel fracture imaging and interpretation workflow to take a systematic look at hydraulic fractures captured during thorugh fracture coring at the Hydraulic Fracturing Test Site (HFTS) in Midland Basin. Digital fracture maps rendered using high resolution 3D laser scans are analyzed for fracture morphology and roughness. Analysis of hydraulic fracture faces show that the roughness varies systematically in clusters with average cluster separation of approximately 20' along the core. While isolated smooth hydraulic fractures are observed in the dataset, very rough fractures are found to be accompanied by proximal smoother fractures. Roughness distribution also helps understand the effect of stresses on fracture distribution. Locally, fracture roughness seems to vary with fracture orientations indicating possible inter-fracture stress effects. At the scale of stage lengths however, we see evidence of inter-stage stress effects. We also observe fracture morphology being strongly driven by rock properties and changes in lithology. Identified proppant distribution along the cored interval is also correlated with roughness variations and we observe strong positive correlation between proppant concentrations and fracture roughness at the local scale. Finally, based on the observed distribution of hydraulic fracture properties, we propose a conceptual spatio-temporal model of fracture propagation which can help explain the hydraulic fracture roughness distribution and ties in other observations as well.
Abstract This paper is a contribution to failure prediction of unconsolidated intervals that could have a negative impact on injection efficiency because of susceptibility to structural changes under fluid injection processes. In unconsolidated formations, formation fines may be subjected to drag forces by injected water because of poor cementation. This results in small grain moments, and continuation can result in a gradual increase in permeability and eventual development of washed-out or thief zones. This paper presents a new modeling approach using information from profile surveys and grain and pore size distribution to model the process of injection and the induced particle movement. The motivation came from field observations and realization of permeability increase from profile surveys and substantial fines movement, leading to an increase in rock permeability. A series of case studies based on realistic published data on pore and grain size distribution are included to demonstrate the estimated increases in formation permeability. In our modeling approach, once we establish the range of grain sizes that fits the criterion for particle movement, a probabilistic algorithm, developed for the study, is applied to track changes in porosity and associated variations in permeability. This algorithm, presented for the first time, considers a stochastic approach to monitor the reservoir particle movements, pore size exclusion by particle accumulation and their resultant changes in rock properties. For this methodology, we ignored potential effects of wettability and clay swelling, and considered perfect spheres to represent the various grain sizes. Predictions made using various realizations of channel formation and petrophysical alterations show the significance of having access to three sources of information; pore size distribution, grain size distribution, and profile surveys. Through inverse modeling using these pieces of information for a particular formation, we demonstrate how we can predict realistic changes and map rock transport properties.
Badrouchi, F. (University of North Dakota) | Scott, N. (University of North Dakota) | Feilen, H. (University of North Dakota) | Badrouchi, N. (University of North Dakota) | Tomomewo, O. S. (University of North Dakota) | Benouadah, N. (University of North Dakota) | Rasouli, V. (University of North Dakota)
ABSTRACT Drilling long horizontal wells is common in development of unconventional reservoirs. Effective cuttings transportation for better hole cleaning during drilling operations can increase the rate of penetration (ROP) and mitigate various drilling associated problems such as high drag and torque and pipe sticking. A large-scale Slurry Loop Unit (SLU) was used in this study for simulation purposes. The objective of this study was to investigate the cuttings size, density, and fluid properties; coupled with wellbore deviation and circulation rate on hole cleaning efficiency. The analytical models used to predict critical velocities for lifting and rolling the cuttings particles were based on the equilibrium cuttings bed height model and forces acting on a cuttings bed. The analytical model results could predict, with some degree of accuracy, the effective injection rate to clean the annulus. Also, experimental results showed that at angles higher than the repose angle of the sand, only rolling and lifting mechanisms ensure the bed movement and effective hole cleaning. Similarly, at the range of 0° to 60° inclination, the only major forces acting on the cuttings is gravity which can be overwhelmed by increasing the fluid carrying capacity and/or flow rate. 1. INTRODUCTION One of the main functions of the drilling fluid is the efficient removal of the cuttings from the bottom hole to the surface. Poor hole cleaning results in the deposition of drill cuttings in different wellbore locations, possibly leading to several drawbacks in the drilling operation and well completion, such as stuck pipe, high torque and drag, and faulty cementing jobs. Pigott (1942) pioneered the extensive study of hole cleaning in vertical and near-vertical wellbore geometries, which was followed by several other studies discussed later in this section with a focus on the particle's settling velocity of the cuttings as a major factor influencing the hole cleaning. The settling velocity is dependent upon cuttings density, size, and shape, as well as fluid rheology and flow rate. Chien (1993) has introduced a correlation between the settling velocity and irregularly shaped cuttings by introducing a factor to account for the non-sphericity and apply it to a fictive spherical particle with an equivalent volume. His findings can expand the work from spherical to non-spherical particles.
Langbauer, Clemens (Montanuniversitaet Leoben) | Hartl, Manuel (Montanuniversitaet Leoben) | Gall, Sergej (OMV Austria Exploration & Production GmbH) | Volker, Lukas (OMV Austria Exploration & Production GmbH) | Decker, Christian (OMV Austria Exploration & Production GmbH) | Koller, Lukas (OMV Austria Exploration & Production GmbH) | Hönig, Stefan (OMV Austria Exploration & Production GmbH)
Summary Tense economic situations push the demand for low‐cost oil production, which is especially challenging for production in mature oil fields. Therefore, an increase in the meantime between failure and the limitation of equipment damage is essential. A significant number of wells in mature fields are suffering under sand by‐production. The objective of this paper is to show the development process and the testing procedure of an in‐house-built, effective downhole desander for sucker rod pumps on the basis of a sophisticated analytical design model. In weak reservoir zones, often the strategy to prevent equipment damage due to sand by‐production is the sand exclusion method using a gravel pack. Nevertheless, a certain amount of small sand grains still enter the wellbore and may damage the sucker rod pumping system over time. In early 2018, various types and sizes of downhole desander configurations were tested at the pump testing facility (PTF) at the University of Leoben (Montanuniversitaet Leoben). In a period of about 4 months, testing took place under near field conditions to find the optimum and most efficient design. The design optimization was focused on the geometry of the swirl vanes and the sand separation distance at the sucker rod pump intake. An analytical model provided the basis for geometric optimization. Concurrently, field tests of the in‐house downhole desander were performed in the Vienna Basin that confirmed the findings of the tests at the PTF. The test results have shown that the downhole desander design and the pumping speed are the most influencing parameters on sand separation efficiency. Poor design in combination with a wrongly selected pumping speed can reduce the sand separation efficiency to lower than 50%, while if all parameters are chosen correctly, the sand separation efficiency can be 95% or higher. The grain size distribution is the additional parameter that enables a decision and ranks the performance. The sensitivity analysis, performed for several downhole desander types, has shown the high dependency of the sand separation efficiency on the major desander design parameters. Proper selection of the components and operating parameters will contribute to an increase in the meantime between failures. This paper will present the testing configurations, the development of the high‐efficiency in‐house downhole desander, and the sensitivity analysis performed on the design.
In this paper, we introduce a novel fracture imaging method which uses high resolution 3D laser scanning to develop detailed surface maps of the core fracture faces. The digital maps are then used to analyze fracture surface characteristics wherein observed variations provide us with meaningful insights into the fractures. We share a mathematical approach for roughness evaluation to identify morphological properties for individual fractures within rock samples. The approach is tested on core extracted at the Hydraulic Fracturing Test Site (HFTS - 1) in the Permian Basin. We characterize the roughness variations with depth across the cored section. In addition, we compare results obtained previously from core sampling and analysis to demonstrate that proppant entrapment observed within the cored interval is strongly correlated with the changes in fracture morphology. We also use calculated roughness along with the the changing behavior of roughness radially away from the center of fracture faces to predict roughness "types" such as propagational features or textural roughness characteristics. Based on the specific fracture characterization work shared here as well as other potential uses, our paper highlights significant advantages such scanning and digital imaging of fractures may have over traditional cataloging using photographic imaging. Furthermore, as demonstrated in this study, data sampled from these detailed maps can be used to further characterize and analyze these features in a more systematic and robust manner when compared with the more traditional geological analysis of cores.
Rock is typically a heterogeneous material composed of different types of inherent microstructures. The microstructures of a rock at the grain scale are usually associated with different mineral aggregations and microdefects such as joints, voids, and cleavage planes. In the present manuscript, a discrete element grain-based model featuring three-dimensional random Voronoi tessellations is proposed to study the deformation and fracturing of brittle rocks. Different grains, representing different rock-forming minerals, and the contacts between the grains were assigned different mechanical and physical properties, accounting for the grain scale heterogeneity observed in natural granular rocks. The simulations showed that rock specimens characterized by a wide grain size distribution are weaker than those with a narrow grain size distribution, but that spherical grains do not significantly influence the strength of specimen. Moreover, the simulations show that increasing the pre-existing porosity reduces specimen strength, due to the concentration of stress around the voids, and that intra-granular cracking is the dominant contact damage mechanism during uniaxial compression.
The rock is a heterogeneous polycrystalline material in natural conditions. It composes different mineral compositions and microstructures such as grain boundaries, micro-cracks, cleavages (Eberhardt, Stead, Stimpson, & Read, 1998; Martin & Chandler, 1994). Microstructure and mineral grains of rock is known to control the complex macroscopic mechanical response and fracture pattern. Grain boundaries act as the predominant source of stress concentrating flaws. Once the local stress near the flaws exceeds the local strength of rock, the initiation of microcrack starts from existing flaws. The density of cracks increases as the load increases. Propagation and coalescence of the cracks eventually cause macroscopic failure of rock. This paper aims to simulate crack damage evolution of brittle rock at mesoscale (grain scale). Mesoscale is between macroscale (phenomenological) and microscale (atomic or molecular). We used a three-dimensional discrete element grain-based model (3DEC-GBM), consisting of an assemblage of Voronoi polyhedra grains, to represent rock by a dense packing of bonded mineral grains of non-uniform size and shape. We explored the influence of grain size distribution, grain shape and porosity on the mechanical behavior and strength of granular rock.
Nepop, Roman (PetroGM LLC) | Smirnov, Nikolay (PetroGM LLC) | Molodtsov, Roman (PetroGM LLC) | Khomenok, Ivan (PetroGM LLC) | Abalian, Artur (Repsol) | Atanes, Andrea Martin (Repsol) | Maltsev, Andrey (JV Eurotek-Yugra) | Nemirovich, Gennady (JV Eurotek-Yugra)
Abstract This paper presents the results of studying petrophysical and geomechanical properties of rocks by conducting thick-walled cylinder core tests (TWCT) - triaxial compression with flushing by various fluids. Experiments were carried out for a limited number of core samples from the Ourinskoe Field, which is a part of the Western Siberia oil and gas-bearing basin. In the framework of the study, various core testing schemes were applied, the results of experiments using different fluids for flushing were analyzed, and the optimal testing parameters (including loading rate, washing time, sample relaxation time, etc.) were selected. The result of the research is a detailed analysis of rock fragments removed by different fluids as a result of triaxial compression and subsequent destruction of a thick-walled cylinder. The paper presents the qualitative relations of the particle size distribution on the prevailing deformation patterns (elastic or plastic), analyzes some petrophysical properties of the strata that control the behavior of the system "rocks – fluid", and consider typical scenarios for TWCT experiments. The obtained experimental data served as an important source of additional information for assessing the draw-down pressure that controlled sand inflow into the wellbore, to determine its particle size distribution, as well as to calculate the size of the wire-wrapped filter liner completion for sand preventing.
Kong, Hailing (College of Civil Engineering / Yancheng Institute of Technology / School of Civil, Environmental & Mining Engineering / The University of Adelaide) | Wang, Luzhen (College of Civil Engineering / Yancheng Institute of Technology / School of Civil, Environmental & Mining Engineering / The University of Adelaide) | Xu, Guizhong (College of Civil Engineering / Yancheng Institute of Technology) | Qiu, Chengchun (College of Civil Engineering / Yancheng Institute of Technology) | Xu, Bing (College of Civil Engineering / Yancheng Institute of Technology) | Zhang, Dan (College of Civil Engineering / Yancheng Institute of Technology)
ABSTRACT: Excavation in the complex geological structure often accompanies with water-inrush accidents, which is considered to be resulted from mass migration and loss in fractured rock in the seepage process. When granular matters migrate and lose, the grain size distribution (GSD) changes over time, it will in turn affect the migration and loss of fine matters. In this paper, the authors carried out seepage tests for rock granular matters considering mass migration and loss, discussed the GSD variation and derived a new GSD expression. The results indicate that (1) all the rock matters with different sizes change in the seepage process, no matter what the samples’ Talbot power exponent (TPE) is and what the compression condition is; (2) the residual mass ratios of samples without initial compression and the ones with initial compression of 10mm have the same variation tendency for rock matters with sizes of 0∼10mm and 15∼25mm, while they have the different variation tendencies for samples of 10∼15mm; (3) the GSD varies from mass migration and loss in the seepage process; (4) the varied GSD obeys the like-Talbot continuous grading expression. The work can be applied as a technical reference for a potential seepage instability or water inrush disaster.
With the gradual depletion of the shallow resources, mineral resources development in deep become common (Qian 2012). Deep coal seams usually contain complex geological structure, e. g. Karst collapse columns, faults, fracture zones, etc. (Fig. 1). Excavation in complex geological structures often accompanies by water-inrush accidents. Either Karst collapse columns, faults, or the fracture zones are heterogeneous mixture composed of rock granular matters, they have different shapes, sizes, and arrangements in fractured rock mass.
In the deep mining, water often coexists with the rock granular matters, flows in the gaps and spaces of the fractured rock under the pore pressure, and drives fine matters migrate and loss. Some fine granular matters migrate with water flow, part of them loses, which alters the structure and strength of the fractured rock, the grain size distribution (GSD), local stress and seepage fields. When increasing granular matters are lost, porosity and permeability increase, seepage system becomes unstable, which may cause seepage catastrophe.
ABSTRACT: Geomechanics provides vital information on the in-situ stresses within a basin, reservoir stability and the impact on fluid flow. This information is pivotal when assessing the viability of a reservoir. This study aims to quantify the geomechanical properties of key reservoir horizons in the Maui-Maari Fields, Taranaki Basin, to refine stress data for modelling, and determine empirical relations, for correlation between laboratory and regional scale. Despite development in the resources hosted in the Maui-Maari Fields, industry currently relies on non-site specific geomechanical properties of reservoir rocks. Initial results indicate significant under-estimation of UCS using existing models, not calibrated to the Taranaki Basin. This has a direct impact on borehole stability and regional stress field calculations, thus downstream implications for any decisions regarding well design and field planning. Laboratory testing is undertaken on outcrop analogues samples to generate the geomechanical data, including porosity, permeability and UCS. Following a practical approach, empirical relations will be determined between the geomechanical properties, specific to the region. In-depth investigation of the reservoir petrophysical properties provides an insight into the role the rock fabric plays on the geomechanical behaviour and the controls on the ability of fluid to flow.
Around the world, extensive work has been undertaken to understand the geomechanical properties of siliciclastic rocks, for the application to modelling of petroleum reservoirs. Geomechanics provide vital information on the in-situ stresses acting within a basin. This information is pivotal when considering all aspects of field production including borehole stability, reservoir compaction, fault reactivation and petroleum migration.
Unconfined Compressive Strength (UCS) provides an input parameter for regional stress models. Direct measurements of UCS across reservoirs are restricted based upon drill core availability and laboratory studies undertaken. The relationship between UCS and other physical rock properties e.g. porosity, can provide a method for rock strength prediction across a reservoir interval, acquired from geophysical logs e.g. NMR or Sonic. Generic relationships are used in sedimentary basins around the world to predict UCS. However, the assumption of replicating reservoir properties across different basins can lead to significant under/over-estimation of rock strength. Empirical relationships developed for rock strength prediction need to be calibrated to the specific region being assessed. This calibration is developed through experimental laboratory tests on core samples from the local reservoir intervals.
Suffusion is the process by which finer soil particles are moved through constrictions between larger soil particles by seepage forces. It is known as a major cause of dam failure. Researchers have proposed the method of evaluating the internal stability of soils based on suffusion tests. The tests, however, are mostly conducted on artificial soil specimens and thus may not be generally applicable. In this study, seepage tests were conducted on natural well-graded clay-silty sand, which has the representative grain size distribution of the earth-fill dams in Korea, and the suffusion sensitivity was evaluated with the influential factors.
Hydraulic earth structures, such as levees and dams, can undergo different kinds of damages. Among these, internal erosion, overtopping and slope instability are the major possible failure modes. Foster et al, 2000b reported that 57 of the 126 cases of the dam failure abroad were caused by internal erosion, accounting for 45% of the total number of failures. The two other major failure modes, overtopping and slope instability accounting for 44 and 4% respectively. Hence, internal erosion is one of the two main causes of damage in hydraulic earth structures.
Suffusion is the process by which finer soil particles are moved through constrictions between larger soil particles by seepage forces (Wan and Fell, 2008) and it is the phenomenon responsible for internal erosion. If suffusion takes place, the permeability of the soil will change sharply and this can induce a reduction of the shear strength (Chang and Zhang, 2013). It can also lead to clogging by the fine particles in the downstream side of the dam and can develop excessive pore pressure causing slope instability or it can render the filter less effective in protecting the core materials (Wan and Fell, 2008).
The phenomenon of suffusion has been studied by a number of researchers (USACE, 1953; Kenny and Lau, 1985; Sun, 1989; Burenkova, 1993; Skempton and Brogan, 1994; Wan, 2006; Wan and Fell, 2008; Fernanda and Spitia, 2017), and based on the analysis of laboratory suffusion tests results, they have proposed criteria on grain size distribution to evaluate the internal stability of soils. Generally, the suffusion test is conducted by using a 1-dimensional seepage cell and introducing water across the specimens. Through seepage tests, the internal stability of soil is evaluated based on the amount of soils discharged, the color of the effluent and measured hydraulic gradient and flow rate. The suffusion tests conducted by the aforementioned researchers, however, were different in terms of the details, such as the soil that were used in the test, the relative density, the shape of the filter, the flow direction, the hydraulic gradient and the vibration. The details of the suffusion tests that were conducted by the previous researchers are summarized in Table 1.