A higher-precision predicting method for discrete fracture network based on cross-hole hydraulic pressure test

Hydraulic tests are used in various sectors of the national economy to check the strength and tightness of vessels and pipelines, their parts and assembly units loaded with pressure. The hydraulic test pressure is a calculated value and is determined on the basis of regulatory documentation for the relevant control objects. Analysis of current trends in the field of hydraulic testing both in the Russian Federation and abroad has shown a trend to reduce the damageability of test objects by choosing the most optimal hydraulic test pressure. The main purpose of the research was to reduce the operational damage to equipment and pipelines of nuclear power plants by reducing the pressure of hydraulic strength tests during maintenance. The research is based on probability theory, mathematical statistics, strength physics and fracture mechanics, taking into account the possibility of occasional discontinuities or defects that might be observed in operation. The method is based on a risk-based approach, which makes it possible to determine the permissible risk variation based on recommendations on the safety of nuclear power plants in the Russian Federation. The objects of research are the equipment and pipelines of nuclear power plants. The result of the research is the development of a method substantiating the reduction of the pressure of hydraulic strength tests for equipment and pipelines of nuclear power plants based on a risk-based approach. The developed method can be used by organizations involved in the production of electricity (at nuclear or thermal power plants), transportation of oil, gas or other substances, at facilities where pipelines are an essential part of their structures, affecting the reliability and efficiency of operation of these facilities.

Faults in some deep mudstones have poor hydraulic connectivity owing to high normal stress on the fault planes. Designing a method for modelling solute transport pathways in such faults/fractures using available data is a critical issue vis-à-vis the safety assessment of radioactive waste disposal. In this study, faults in deep siliceous mudstones with low swelling capacity are investigated using cross-hole hydraulic and tracer tests between two boreholes. The water pressures observed during the hydraulic test are reproduced via hydraulic simulations, assuming a one-dimensional (1D) flow channel with a length 8–80 times the linear distance (4.5 m) between the two borehole test sections. This flow channel length indicates a tortuosity remarkably higher than that previously reported in discrete fracture network simulations and laboratory experiments using a single fracture (i.e. 1–5). Advection–dispersion simulations using a pipe model with a tortuosity of 8–80 reproduces a time series of tracer concentrations observed during the tracer test. The radius of the pipe simulated in the tracer test was 7–39 times the flow channel radius estimated via the hydraulic test, which is in agreement with the ratio of 5–20 previously reported based on in situ tracer and hydraulic tests of faulted/fractured rocks. The simulated longitudinal dispersivity is 1/9–1/20 of the estimated flow path lengths, in agreement with the laboratory and field tracer study values. These results indicate that transport pathways in faults with low hydraulic connectivity can be modelled using a highly tortuous 1D pipe flow path.

A deterministic-stochastic identification and modelling method of discrete fracture networks using laser scanning: Development and case study

Shale reservoirs usually show the characteristics of low porosity and low permeability, and the internal natural fractures are generally developed, and their reservoir space and fluid seepage capacity are mainly controlled by complex fracture networks. As the dominant channel for fluid migration in the reservoir, the distribution of natural fractures not only directly determines the accumulation and production of oil and gas, but also has a key impact on the expansion and final transformation of fracture network morphology in the subsequent hydraulic fracturing process. It should be pointed out that such fracture systems often show heterogeneous and self-similar fractal characteristics in terms of spatial distribution. Therefore, the degree of development of the fracture network can be characterized by fractal dimensions. In this paper, the relationship between fractal dimension and permeability of fracture networks is studied by simulating fracture networks of different complexity through the simulation method of discrete fracture networks. The results show that the fractal dimension of shale reservoir fracture network is linearly positively correlated with the number of fractures and the maximum length of fractures. There is also a linear positive correlation between the fractal dimension corresponding to the maximum length and number of fractures and the equivalent permeability of different fractures, and the correlation is good. This discrete fracture network simulation of the natural fracture system of shale reservoirs is not only an important way to understand its reservoir characteristics and seepage mechanism, but also a key link to improve the efficiency of shale oil and gas development and optimize the fracturing design of fracturing.

Low permeability reservoirs are currently being produced using horizontal wells and massive hydraulic fracturing operations. The design of stimulation jobs requires an integrated knowledge of the reservoir (lithology, mechanical properties, fracture properties, PVT, etc), needing calibration and scenario simulation capacities. Current tools permitting such a workflow exist, yet rarely fully integrated within a single package. In this paper we aim to show the advantages of using two new tools, presently developed as prototypes, namely an unstructured fracture model and a multiphysics coding platform designed to integrate all concepts currently under research pertaining to unconventional reservoirs. Characterization of unconventional reservoirs implies the conciliations of several scales, demanding the integration of a potentially large fracture information database. Within this "rich" database, today's models have difficulties integrating all this information. Thus, improving upon current Discrete Fracture Networks (DFN) would require many fractures to be accounted for (up to 500,000). Coupling of such DFN's to reservoir modeling packages often use up-scaling methods, resulting in models which in turn are simulated using extensions of classical dual continuum models. Current reservoir models do not integrate all physical pertinent phenomena though as being important for gas or multiphase production such as dynamic permeability varying with pressure, nonequilibrium effects, multicomponent adsorption models, diffusion effects or proper transfer functions between matrix and fractures. Using a realistic example inspired from field data, we show how the construction of a fracture model using a consistent Discrete and Deformable Fracture Networks (DDFN), tractable for multiphase flow reservoir simulations, can help describing a complex fracturing case. The use of a coding platform tailored to pertinent unconventional physics is discussed, through examples of developed multiphysics Geoscience applications. The example shows how the integration of the representation of a multistage operations through a DDFN model, using the joint characterization of a field natural fracture system and a propagating fracture network corresponding to the hydraulic fracturing process, calibrated on the BHP and microseismic cloud, is input as a specific unstructured dual discretization into a reservoir model. This explicit description of the fracture geometry is coupled to a non-discretized matrix refinement function accounting for matrix heterogeneities, well-adapted to the dynamic pressure behavior observed in such reservoirs. A generalized multiple interacting continua formulation (named "transient transfer influence function") is used within the matrix medium, allowing the simulation of a longer transition period, typical of many unconventional reservoirs, thus improving matrix contribution during hydraulic fracturing. Because the full process may include several hundred thousand fractures and approximately the same number of cells for the matrix medium, we show how run time performance is improved through a preconditioning technique which reduces the condition number of the matrix associated to the linear system, and speed up the iterative parallel linear solver convergence. The discussion of results obtained by using the integrated DDFN is extended to the potential use of an adapted computational platform which could be used for the inclusion of specific physics pertinent to unconventional reservoirs. This DDFN approach is able to computationally handle 100,000's of fractured coupled to a fluid flow simulator. The platform on which it was implemented could be extended to multiphysics problems, essential for unconventional resources.

This study proposes a new fractal permeability model for fractured rocks that comprehensively accounts for the geometric fracture characteristics and the fluid transport mechanism. Then, the permeability changes of fractured rocks are analyzed using discrete fracture networks (DFNs) with or without roughness and different geometry parameters in the DFN modeling and finite element simulation. The results show that the proposed permeability model well agrees with the experimental data, and the established DFN numerical model more realistically reflects the fracture network in fractured rocks. Fluctuation of tortuous fracture lines (rough fractures) increases the fracture intersection probability, consequently increasing the fracture intersection area or connecting adjacent fractures. Moreover, permeability increases with the fractal dimension Df, porosity ϕ, maximum fracture length lmax, and proportionality coefficient β, and it decreases with increasing fractal dimension DTf of fracture tortuosity. When the fracture proportionality coefficient is 0.001 ≤ β ≤ 0.01, different DFNs yield similar simulation results for permeability. However, with increasing fracture network complexity, the predictive model created using conventional DFN (C-DFN) increasingly overestimates the fractured rock permeability. Thus, building a permeability model for a fractured rock using rough DFN (R-DFN) is more effective than that using C-DFN. Our findings are helpful for real permeability predictions via DFN and analytical modeling.

To guarantee the reliability and performance of hydraulic systems, it is of vital importance to do hydraulic impulse testing for hydraulic components. However, the testing time is usually so long that the energy consumption is very large. An energy regeneration system with an accumulator is proposed and used for hydraulic impulse testing equipment. Mathematical analysis and simulations show that the hydraulic impulse testing system with an accumulator can reduce energy consumption by 15% compared with that of the system without an accumulator in the cycle. Furthermore, the energy efficiency of the hydraulic impulse testing system is increased from 62.82 to 75.71% by the use of an accumulator. Both simulation results and experimental results show that the control performance of the hydraulic impulse testing equipment with an accumulator is better than that of the equipment without an accumulator, and has a faster response speed and a larger pressure rise rate.

Our regular inspection during the use of cylinders is an important means to ensure the safe use of cylinders, and reasonable periodic inspection efficiency is a necessary means to ensure the normal circulation of cylinders and maintain the stability of the cylinder industry. As a bottleneck in the testing process of the hydrostatic test, the efficiency of periodic inspection of gas cylinders has been affected for a long time. In order to solve the above problems, this thesis mainly researches and solves the bottleneck problems that affect the efficiency of hydraulic pressure tests, in order to achieve rapid detection of the hydraulic pressure tests of cylinders, and develops a kind of gas cylinders to dial, rotate, flip, water, hoist, weigh It can accurately and accurately locate the water jacket, pour water, purge, and dry the cylinder logistics conveying device, and realize the fast and accurate cylinder hydraulic pressure test, eliminating the bottleneck problem in the detection process and achieving the effect of rapid cylinder detection.

With large specific strength and outstanding corrosion resistance and erosion resistance in sea water, titanium and titanium alloy are widely used in heat exchanger production. In particular, pure titanium demonstrates outstanding molding performance and may be considered optimal for production of heat exchanger. Since titanium is very vulnerable to oxidation and embrittlement during welding, processes with less heat input are widely used, and laser welding is widely applied by considering production performance and shield etc in atmosphere. So far, 1st report and 2nd report compared and analyzed embrittlement degrees by bead colors of weldment through oxygen and nitrogen quantitative analysis and hardness measurement, and evaluated welding performance and mechanical properties of butt welding. This study evaluated field applicability of lap welding to heat exchange plate of LPG re-liquefaction device for ships through tensile stress test, hardness test and internal pressure test etc after deducing optimal weding condition and applying to actual heat exchange plate. In bead overlap area, the experiment produced sound welds with no porosity or defect by increasing and decreasing laser power, and tensile-shear test results indicated virtually the same tension and yield strength as base metal. As a result of measuring hardness at lateral cross section and bead overlap zone of actual heat exchanger welds, hardness difference within 20Hv was produced at base metal, HAZ and weldment, and as a result of pneumatic and hydraulic pressure test, no leakage occurred.

Microseismicity is a physical phenomenon which allows us to estimate the production capability of the well after hydraulic fracturing (HF) in a naturally fractured (NF) reservoir. Some of the microseismic events are reactivations of NFs induced by a direct hit of HF, while others are induced by the fluid leak-off from the previous stages or by elastic waves emitted into the reservoir with hydraulic fracture plane propagation. The former NFs have a chance to be propped there as the latter will not significantly increase their contribution to the production. Identification of such microseismic events helps to reduce uncertainty in the description of fracture network geometry. Based on inferred data from core analysis NF densities and orientations, we generated multiple realizations of the semi-stochastic Discrete Fracture Network (DFN). In order to constrain them, we used time evolution of microseismic cloud in addition to results of core analysis. Fluid and proppant pumping schedule is used to identify such microseismic events because they should be located close to the pressure diffusion front generated by hydraulic fluid. Events outside of proposed region may be triggered by other factors, such as stress-strain relaxation from other stages and correspondent fractures. In most cases, they are not wide enough to take proppant from the main HF. This approach was used to reduce range of production for DFN realizations. This workflow is implanted to a 15-stage hydraulic fracture treatment on a horizontal well placed in a siltstone reservoir with intrinsic fractures. The spatio-temporal dynamics of microseismic events are classified into two groups by the front of nonlinear pressure diffusion caused by 3-dimensional hydraulic fracturing, considered as effective and ineffective events. DFNs with only effective microseismicity and with all the induced events are generated. Then, two types of DFN related uncertainties on production are performed to evaluate the impact of filtration. Results of aleatory uncertainty quantification caused by the randomness of DFN modeling indicate the filtered events can generate a production DFN with a more consistent connected fracture area. Moreover, sensitivity analysis caused by lack of accuracy in natural fracture characterization shows the production area of DFN with filtration process is more insensitive to the variation of fracture parameters. Finally, a history match with production data and pressure data indicates this DFN model properly represents the reservoir and completion. Our methodology characterizes well the conductive fracture network utilizing core data, microseismic data, and pumping schedule. It could restore the true productivity of each fractured stage from a massive microseismic cloud, which helps understand the contribution of fracturing job right after the treatment.

A review of the literature on hydraulic testing and interpretive methods, particularly in low‐permeability media, indicates a need for a comprehensive hydraulic testing interpretive capability. Physical limitations on boreholes, such as caving and erosion during continued drilling, as well as the high costs associated with deep‐hole rigs and testing equipment, often necessitate testing under nonideal conditions with respect to antecedent pressures and temperatures. In these situations, which are common in the high‐level nuclear waste programs throughout the world, the interpretive requirements include the ability to quantitatively account for thermally induced pressure responses and borehole pressure history (resulting in a time‐dependent pressure profile around the borehole) as well as equipment compliance effects in low‐permeability intervals. A numerical model was developed to provide the capability to handle these antecedent conditions. Sensitivity studies and practical applications are provided to illustrate the importance of thermal effects and antecedent pressure history. It is demonstrated theoretically and with examples from the Swiss (Nationale Genossenschaft für die Lagerung radioaktiver Abfälle) regional hydrogeologic characterization program that pressure changes (expressed as hydraulic head) of the order of tens to hundreds of meters can result from 1° to 2°C temperature variations during shut‐in (packer isolated) tests in low‐permeability formations. Misinterpreted formation pressures and hydraulic conductivity can also result from inaccurate antecedent pressure history. Interpretation of representative formation properties and pressures requires that antecedent pressure information and test period temperature data be included as an integral part of the hydraulic test analyses.

Fractures are amongst the most important factors determining the behavior of rock masses. The lack of direct access to the real fractures’ distribution leads to the use of indirect methods for their study. The generation of the discrete fracture network is one of the most common indirect methods. Fractures’ shape is one of the most important properties that the simulation of which is of great help in studying the fracture network. A wide range of shapes which include from infinite plates to elliptical shape have been suggested for the simulation of fractures. The present paper makes use of the data collected from the pumped-storage power plant and dam project in Roudbar, Lorestan Province, to provide circular and elliptical network of discrete fractures; moreover, the parameters as density, intensity and the real mean value of the length of the fracture traces have been used to evaluate the effect and accuracy of the selected shapes. Following the implementation of the proposed method, it was made clear that the elliptical discrete fracture network, with a mean error value of 13.5 %, outperforms the circular disk discrete fracture network, with a mean error value of 27.5 %; hence, it was found providing more accurate results for the description of the fractures network in the aforementioned region.

Summary Unconventional shale–gas and tight oil reservoirs are commonly naturally fractured, and developing these kinds of reservoirs requires stimulation by means of hydraulic fracturing to create conductive fluid–flow paths through open–fracture networks for practical exploitation. The presence of the multiscale–fracture network, including hydraulic fractures, stimulated and nonstimulated natural fractures, and microfractures, increases the complexity of the reservoir simulation. The matrix–block sizes are not uniform and can vary in a very wide range, from several tens of centimeters to meters. In such a reservoir, the matrix provides most of the pore volume for storage but makes only a small contribution to the global flow; the fracture supplies the flow, but with negligible contributions to reservoir porosity. The hydrocarbon is mainly produced from matrix/fracture interaction. So, it is essential to accurately model the matrix/fracture transfers with a reservoir simulator. For the fluid–flow simulation in shale–gas and tight oil reservoirs, dual–porosity models are widely used. In a commonly used dual–porosity–reservoir simulator, fractures are homogenized from a discrete–fracture network, and a shape factor based on the homogenized–matrix–block size is applied to model the matrix/fracture transfer. Even for the embedded discrete–fracture model (EDFM), the matrix/fracture interaction is also commonly modeled using the dual–porosity concept with a constant shape factor (or matrix/fracture transmissibility). However, in real cases, the discrete–fracture networks are very complex and nonuniformly distributed. It is difficult to determine an equivalent shape factor to compute matrix/fracture transfer in a multiple–block system. So, a dual–porosity approach might not be accurate for the simulation of shale-gas and tight oil reservoirs because of the presence of complex multiscale–fracture networks. In this paper, we study the multiple–interacting–continua (MINC) method for flow modeling in fractured reservoirs. MINC is commonly considered as an improvement of the dual–porosity model. However, a standard MINC approach, using transmissibilities derived from the MINC proximity function, cannot always correctly handle the matrix/fracture transfers when the matrix–block sizes are not uniformly distributed. To overcome this insufficiency, some new approaches for the MINC subdivision and the transmissibility computations are presented in this paper. Several examples are presented to show that using the new approaches significantly improves the dual–porosity model and the standard MINC method for nonuniform–block–size distributions.

ABSTRACT: Hydraulic fracture trajectory near the wellbore directly influences the efficiency of fluid flow into and out of the well. Modeling the near-wellbore hydraulic fracture behaviors has been challenging due to the combined impacts of wellbore perforation placement, stress concentration around the wellbore and perforation tunnels, in-situ stress conditions, as well as pre-existing fractures in the subsurface that interact with hydraulic fractures. In this paper, we have built numerical models to take into account the aforementioned factors and investigate near-wellbore fracture growth and fluid pressure characteristics. The numerical models use a distinct element method (DEM) approach that explicitly represents the well casing, cement sheath, and perforation tunnels, with natural fractures represented by Smooth Joint Model. The results show that natural fractures intersecting the perforation tunnels at a favorable angle with respect to in-situ stress directions can facilitate fracture initiation in hard rocks. Multiple initiation points may occur, while one fracture becomes dominant at a later time in the simulated case. In addition, the simulation results show the termination of hydraulic fracture growth in the highly fractured zone, as well as hydraulic fractures crossing natural fractures that have small effective sizes at the interaction location. This study also demonstrates the efficacy of the near-wellbore model to represent detailed wellbore elements, consider complex discrete fracture networks, and capture the fundamental mechanisms that influence near-wellbore hydraulic fracture behaviors. 1. INTRODUCTION The trajectory of hydraulic fractures near the wellbore directly influences the efficiency of fluid flow into and out of the well, making it a crucial factor that can impact several critical aspects in unconventional reservoirs and enhanced geothermal systems, such as reservoir connectivity, fluid circulation, proppant transportation, and energy production efficiency. Modeling near-wellbore fracture initiation and propagation has been challenging due to complex influencing factors that need to be considered, including the casing-cement sheath-perforation tunnel-rock structure, stress concentrations resulting from the wellbore and perforation tunnels, interference, competition, and merging of hydraulic fractures, interactions within the hydraulic discrete fracture network, and in-situ stress conditions. A model must span multiple scales to accurately capture different fracture behaviors, from fracture initiation along perforation tunnels at a millimeter scale to fracture propagation near the wellbore at a meter scale. In this study, a numerical model was established to investigate hydraulic fracture behaviors in the near-wellbore region. Utilizing a distinct element method (DEM) lattice approach, we analyzed hydraulic fracture initiation and propagation behaviors under the combined impacts of wellbore perforation and discrete fracture network (DFN). The model is based on hydraulic fracturing tests conducted at the Utah FORGE Site. Well casing, cement sheath, and perforation tunnels were explicitly represented in the simulation, and fluid pressure characteristics and corresponding fracture behaviors are discussed in detail.

Deep saline aquifers are target for carbon sequestration since these geological structures abound in many areas worldwide. Hydraulic characterization tests are focused on site feasibility assessment to inject CO2 in an efficient and safely manner. For this, it is necessary to carry out both laboratory and field tests to determine hydraulic properties and operating parameters such as permeability and injectivity in the reservoir, and the trapping degree of the structural complex. CO2 injection experiences usually come from projects conducted in aquifers composed by sandstones and similar rocks, unlike those carried out in carbonates that are quite limited. Sometimes carbonates are porous mediums, but in other cases, primary permeability is really poor being the fluid transmissivity mainly through the fracture network. Moreover, geochemical reactivity produced by the acidification of the mixture of CO2 and resident brine plays a key role in these cases. This chapter address the innovative on-site hydraulic characterization tests conducted in the deep saline aquifer of Hontomin Technology Development Plant (Spain), which is composed of naturally fractured carbonates with low primary permeability. The impacts of artificial brine and CO2 migration through the fracture network are described, analyzed and discussed, considering that produces hydrodynamic, mechanical and geochemical effects different from those caused by the injection in mediums with a high matrix permeability.