Search

Copyright of Mechanics & Advanced Technologies is the property of National Technical University of Ukraine KPI and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract Diagnostic Fracture Injection Tests (DFITs) have become commonplace in low-permeability (unconventional) reservoirs to obtain parameters used in hydraulic fracture stimulation design and reservoir characterization. However, the time taken to acquire reliable estimates of these parameters may be excessive. As a result, pump-in/flowback tests, as opposed to the more conventional pump-in/shut-in tests, have been applied in the industry to accelerate the closure process and to estimate closure pressure, which is used in hydraulic fracture design. However, this comes at the expense of losing after-closure data, which is required to estimate reservoir pressure and flow properties. The goal of this paper is to accelerate the closure process during a DFIT, without sacrificing the after-closure data and derived parameters. The proposed DFIT procedure includes flowing back the fracturing fluid, after opening and propagation of the fracture, at a very low and constant rate. The flowback process is continued for a few hours after fracture closure. The well is then shut-in and the pressure falloff is monitored. The main purpose of the ultra-low rate flowback is to remove or reduce the afterflow caused by wellbore storage. The new DFIT procedure is applied to two field examples performed in a low-permeability reservoir in western Canada. The data are analyzed using well-established pressure-transient analysis methods. An estimate of closure pressure was obtained in less than two hours for both field examples, despite the low-permeability of the reservoir (several hundred nanodarcies). For one of the field examples, estimates of reservoir pressure and transmissibility were obtained after just 4 days of shut-in following the flowback process. Finally, a conceptual model is presented to analyze the conventional pump-in/flowback tests for determination of reservoir pressure. The conceptual model is validated against synthetic simulation results. Using this method, reservoir pressure is obtainable with just a few hours of flowback data. Highlights • A new DFIT procedure is presented and applied in the field. • A conceptual method is presented for estimation of reservoir pressure in pump-in/flowback tests. • The procedure and analysis method provide reliable estimates of in-situ stress and reservoir pressure in a short period. [ABSTRACT FROM AUTHOR]

• Area of review is the area of injection-zone pressure posing a risk to fresh water. • The size and location of the AoR are key drivers of project risk and cost. • Deep injection, stacked injection zones and pressure barriers can minimize the AoR. • Regulatory focus on CO2 leaves potential for pressure conflict between projects. • Pressure space (pressure times pore volume) is the key subsurface commodity. Elevated pressure from large-volume injection is a key driver of risk and project cost. If transmissive features (e.g., non-isolating wells or fracture systems) are present, increased injection-zone pressure can drive fluids from depth toward protected freshwater resources. In US Carbon Capture and Storage (CCS) law, the area at risk is known as the Area of Review (AoR). The size and number of potentially transmissive features to be evaluated and possibly remediated or managed is a function of the size and location of the AoR. The size of the AoR depends on several variables, including properties of the injection zone, properties of protected resources, and injection rate and duration. Evaluation of the intersection of these variables across a portfolio of sites highlights the injection zone depth and boundary conditions as top-level controls. Deep injection, use of multiple stacked injection zones, reduced injection rate and choice of injection well location can all be used to minimize AoR and the number of potentially transmissive features within it. We introduce the concept of pressure space (defined as connected pore volume times pressure) as the key subsurface commodity for CO 2 storage and we suggest that it forms a more robust basis for leasing and regulation than pore space alone. [ABSTRACT FROM AUTHOR]

Deep coalbed methane (CBM) resource potential is enormous in China, and has become a new field for unconventional natural gas exploration and development. This work discusses the geological conditions (reservoir pressure, formation temperature and ground stress) of deep coal reservoirs in the Eastern margin of the Ordos Basin and their implication on CBM development. Various field test data of CBM wells, including injection/drawdown test data, well temperature test data, and hydraulic fracturing test data were collected from this work and literature to describe the geological conditions of the deep CBM in the study area. From the results, it is found that deep CBM in this area is characterized by high reservoir pressure, high formation temperature, and high ground stress. However, there are diverse geological particularities in the different depth range: (1) Having a wide range of pressure gradient, vast majority of coal reservoirs in the study area are under abnormally low-pressure state, which is more significant in deeper coal seams. (2) Due to the impact of surface runoff, the distribution of geothermal gradient is discrete when the burial depth is less than 700 m, and relatively concentrated when the burial depth is greater than 700 m. (3) In shallow coal reservoirs, ground stress is strongest in the horizontal direction; while in deep coal reservoirs, the strongest ground stress is in the vertical direction. Because of the complex geological conditions associated with deep burial, the balance between CBM adsorption-desorption-seepage and the rheological behavior of coal reservoirs is complex, which has significant influence on the exploration and development of deep CBM in the study area. High pressure in deep coal reservoir often leads a long inefficient desorption stage and a long draining and depressurizing process, which increases production costs. Moreover, the negative temperature effect on gas adsorption indicates that CBM content decreases with increasing depth in deep conditions, and thus the evaluation of deep CBM resources needs to be reconsidered. In addition, different stress states govern fracture patterns, and in deep environments, high ground stress greatly reduces the fracturing improvement of the coal reservoir and significantly affects the deep CBM development. [ABSTRACT FROM AUTHOR]

This paper is based on a talk given at the Arterial Hemodynamics: Past, Present and Future symposium in June 2016. Like the talk it is divided into three different but related parts. Part 1 describes the calculation of reservoir and excess pressure from clinical pressure waveforms measured at 5 different aortic sites in 40 patients. The main results are that the reservoir pressure waveform propagates down the aorta and is effectively constant from the aortic root to the aortic bifurcation. Part 2 describes a low-frequency asymptotic analysis of the input impedance of an arterial tree. Neglecting terms of second order, the results show that the low-frequency component of the pressure waveform is uniform throughout the arterial tree and is delayed by an effective wave travel time that depends on the properties of the network. The low-frequency pressure waveform shares all of the properties of the reservoir pressure waveform, but it is premature to say that they are identical. Part 3 describes the analysis of arterial hemodynamicsusing wave fronts. It shows that every wave front introduced at the root of the aorta generates an exponentially increasing number of reflected and transmitted waves with exponentially decreasing amplitudes. The long-time response of the arterial tree can be described by a number of exponentially decaying eigen-modes, each with a different time constant. The analysis is applied to a 55-artery model of the human circulation and the modes and their time constants are shown. This theory provides an alternative method for studying arterial hemodynamics and helps in the interpretation of reservoir and excess pressure. [ABSTRACT FROM AUTHOR]

The DFIT flowback analysis (DFIT-FBA) method is a new approach for obtaining closure pressure (p c), a proxy for minimum in-situ stress, reservoir pressure (p r), and well productivity index (J) estimates in a fraction of the time required by conventional (pump-in/falloff) DFITs. The time and cost efficiency associated with the DFIT-FBA method provides an opportunity to conduct multiple field tests without delaying drilling and completion programs. The primary novelty of the current study is the first-time demonstration of the implementation and analysis of multiple DFIT-FBA tests along a horizontal well completed in an unconventional reservoir to investigate variations in p c , p r , and J. Furthermore, new corrections are introduced for the DFIT-FBA method to account for perforation friction, tortuosity, and wellbore unloading during the flowback stage of the test. Three DFIT-FBA tests were performed in the studied lateral, including at the toe, at a middle stage and near the heel of the lateral. Minimum in-situ stress, pore pressure, and well productivity index estimates were successfully obtained for all the DFIT-FBA tests and verified with the results of a conventional DFIT, also performed at the toe of the studied well. The new corrections for friction and wellbore unloading improved the accuracy of the closure and reservoir pressures by 4%. Furthermore, the results of flowing material balance analysis demonstrate that wellbore unloading might cause significant over-estimation of the well productivity index. Considerable variation in well productivity index was observed from the toe stage to the heel stage (along the lateral) for the studied well, as determined using DFIT-FBA. This variation has significant implications for hydraulic fracture design optimization, particularly treatment pressures and volumes. • Along-well characterization can be performed without delaying completion program. • Near-wellbore friction may lead to underestimation of minimum in-situ stress. • Wellbore unloading results in overestimation of pore pressure and well productivity. [ABSTRACT FROM AUTHOR]

Abstract: It is commonly assumed that pressure and flow in the arteries are purely the result of forward and backward travelling waves. We will show that various observations of arterial behaviour are difficult to explain using this assumption. In particular we will look at what happens to arterial pressure under different conditions: during ectopic beats, in experimental studies of pressure and flow when the aorta is totally occluded at different locations, in a computational study of the input impedances of randomly generated networks of arteries and when pressure and velocity are measured at different distances along the aorta. We show that all of these observations can be explained using wave intensity analysis. We further show that this analysis suggests that it is useful to separate arterial pressure into a reservoir pressure that accounts for the overall compliance of the arterial system and an excess pressure that is determined by local conditions. Evidence has accumulated in the decade since the introduction of the reservoir-wave hypothesis that the reservoir/excess pressure separation can be useful in interpreting the results of vasoactive drugs on cardiovascular performance and that parameters based on the reservoir/excess pressure are significant predictors of cardiovascular events. The important question that remains to be answered is the usefulness of the concept in the interpretation, physiologically and clinically, of the complex behaviour of the cardiovascular system. We conclude that the evidence suggests that it is a worthwhile topic for future research. [Copyright &y& Elsevier]

Previous studies based on measurements made in the ascending aorta have demonstrated that it can be useful to separate the arterial pressure P into a reservoir pressure P¯ generated by the windkessel effect and a wave pressure p generated by the arterial waves: P  =  P¯ + p. The separation in these studies was relatively straightforward since the flow into the arterial system was measured. In this study the idea is extended to measurements of pressure and velocity at sites distal to the aortic root where flow into the arterial system is not known. P¯ is calculated from P at an arbitrary location in a large artery by fitting the pressure fall-off in diastole to an exponential function and assuming that p is proportional to the flow into the arterial system. A local reservoir velocity U¯ that is proportional to P¯ is also defined. The separation algorithm is applied to in vivo human and canine data and to numerical data generated using a one-dimensional model of pulse wave propagation in the larger conduit arteries. The results show that the proposed algorithm is reasonably robust, allowing for the separation of the measured pressure and velocity into reservoir and wave pressures and velocities. Application to data measured simultaneously in the aorta of the dog shows that the reservoir pressure is fairly uniform along the aorta, a test of self-consistency of the assumptions leading to the algorithm. Application to data generated with a validated numerical model indicates that the parameters derived by fitting the pressure data are close to the known values which were used to generate the numerical data. Finally, application to data measured in the human thoracic aorta indicates the potential usefulness of the separation. [ABSTRACT FROM AUTHOR]

A problem with the conventional diagnostic fracture injection test (DFIT) design, which involves analysis of the pressure falloff after the pump is shut-off, is the long test times required to achieve closure pressure (p c), a proxy for minimum in-situ stress, and after-closure flow regimes that can be analyzed for reservoir pressure in (p r) and transmissibility. An alternative approach to DFIT implementation is to flowback the injected fluid immediately after the pump is shutdown. Historically this approach has been to accelerate closure only, but recently the authors have demonstrated that careful design of the flowback procedure can yield accelerated p c estimates and possibly reservoir information. The new DFIT is referred to as 'DFIT-FBA', where FBA refers to the flowback analysis component to the test. p c is estimated using a Cartesian plot of flowing pressure versus flowback time. p r is determined through flowback data flow regime identification. The well productivity index for the studied reservoir interval is derived using the flowing material balance method. With constraints applied on created fracture properties, reservoir permeability may be determined. The practical application of the DFIT-FBA method is demonstrated with a field trial performed in a western Canadian tight oil reservoir. The resulting DFIT-FBA data quality is excellent yielding both closure and reservoir pressures due to accurate flow rate measurement. Both pressure estimates are in excellent agreement with a conventional DFIT performed on the same zone in the same area, but were obtained in a fraction of the time. In addition, application of the flowing material balance equation to after-closure data yielded an estimate of well productivity index for the tested interval. The DFIT-FBA demonstrated herein allows estimation of properties of interest to completions and reservoir engineers in a fraction of the time of the conventional DFIT. • A problem with the conventional DFIT is the long test times required to achieve closure and reservoir pressures. • The new method, DFIT-FBA, involves analysis of flowback data. • DFIT-FBA estimates minimum in-situ stress, reservoir pressure and well productivity index in less than 2 h. [ABSTRACT FROM AUTHOR]