Modeling of Low-Frequency Downhole Electrical Measurements for Mapping Proppant Distribution in Hydraulic Fractures in Casedhole Wells

SPE Journal ◽  
2018 ◽  
Vol 23 (06) ◽  
pp. 2147-2157 ◽  
Author(s):  
Peng Zhang ◽  
Mrinal K. Sen ◽  
Mukul M. Sharma ◽  
Jeff Gabelmann ◽  
David Glowka
Keyword(s):  
Electrical Potential ◽  
Synthetic Data ◽  
Low Frequency ◽  
Electrical Current ◽  
Hydraulic Fractures ◽  
Single Fracture ◽  
Electrical Measurements ◽  
Multiple Fractures ◽  
Electrically Conductive ◽  
Simulation Results

Summary A tool concept using downhole electrical measurements for mapping electrically conductive proppant in hydraulic fractures is presented in this paper. The method relies on direct excitation of the casing, which is expected to overcome the severe limitations of induction tools in casedhole wells. An array of insulating gaps is installed and cemented in place as a permanent part of the casing string. The envisioned electrical measurements are performed by imposing a voltage across each insulating gap, one at a time, before and after hydraulic-fracture operations. The voltages across other insulating gaps near the transmitter gap are recorded. The proposed tool's response to the geometry of a single fracture was modeled by solving for the electrical potential with a finite-volume method. Previous simulation results have shown that the electrically conductive proppant alters the path of the electrical current in the formation, and this is recorded as differential signals by the string of insulating gaps surrounding the source gap. The simulated differential signals are highly sensitive to a fracture's location, length, and orientation, and less sensitive to the fracture's aspect ratio. However, to enable the implementation of such a practical system, various aspects of the tool concept must be investigated further through simulations. Following our previous work, this paper focuses on the forward modeling of the tool's response to multiple fractures, which demonstrates the influence of these fractures on the signals, and provides important guidance for inverse modeling. Parametric inversion of fractures from synthetic data, generated by exciting various insulating gaps, is solved with very fast simulated annealing (VFSA). Simulation results show that, when multiple hydraulic fractures are present, the voltages measured at the receiver gaps are determined primarily by the fracture that is in direct contact with the excited section of casing. When two fractures touch the same casing section, they induce voltages very similar to those from a single fracture with the same conductivity and volume. Preliminary inversion results that use synthetic data computed from circular fractures indicate that the proposed VFSA can solve for the multiple fractures’ widths and radii at the same time, without requiring numerous forward simulations. Even with noisy synthetic data, VFSA can make good estimates of the fractures’ parameters. This indicates that the VFSA technique is a proper and robust inversion technique for the measured voltages at various receiver gaps.

scholarly journals Sensitivity analysis for the appraisal of hydrofractures in horizontal wells with borehole resistivity measurements

Geophysics ◽  
2013 ◽  
Vol 78 (4) ◽  
pp. D209-D222 ◽  
Author(s):  
David Pardo ◽  
Carlos Torres-Verdín
Keyword(s):  
High Frequency ◽  
Horizontal Well ◽  
Low Frequency ◽  
Horizontal Wells ◽  
Hydraulic Fractures ◽  
Electrically Conductive ◽  
Resistivity Measurements ◽  
Electromagnetic Measurements ◽  
Open Hole ◽  
Thin Disks

We numerically evaluate the possibility of using borehole electromagnetic measurements to diagnose and quantify hydraulic fractures that have been artificially generated in a horizontal well. Hydrofractures are modeled as thin disks perpendicular to the well and filled with either sand-based or electrically conductive proppant. The study focuses on the effect of thickness and length (radius) of hydrofractures to assess their effects on specific configurations of borehole-resistivity instruments. Numerical results indicate that several measurements (e.g., those obtained with low- and high-frequency solenoids) could be used to assess the thickness of a fracture. However, only low-frequency measurements performed with electrodes and large-spacing between transmitter and receivers (18 m) exhibit the necessary sensitivity to reliably and accurately estimate the length of long hydrofractures (up to 150 m) in open-hole wells. In the case of steel-cased wells, the casing acts as a long electrode, whereby conventional low-frequency short-spaced, through-casing measurements are suitable for the accurate diagnosis of long hydrofractures (up to 150 m in length).

Optimizing the Number of Fractures in a Horizontal Well

SPE Journal ◽  
2019 ◽  
Vol 24 (03) ◽  
pp. 1364-1377 ◽  
Author(s):  
Vyacheslav Guk ◽  
Mikhail Tuzovskiy ◽  
Don Wolcott ◽  
Joe Mach
Keyword(s):  
Horizontal Well ◽  
Horizontal Wells ◽  
Optimal Number ◽  
Hydraulic Fractures ◽  
Single Fracture ◽  
Multiple Fracture ◽  
Multiple Fractures ◽  
Type Curves ◽  
Fracture Width ◽  
Technical Potential

Summary Horizontal wells with multiple hydraulic fractures have become a standard completion for the development of tight oil and gas reservoirs. Successful optimization of multiple-fracture design on horizontal wells began empirically in the Barnett Shale in the late 1990s (Steward 2013; Gertner 2013). More recently, research has focused on further improving fracturing performance by developing a model-derived optimum. Some researchers have focused on an economic optimum on the basis of multiple runs of an analytical or numerical model (Zhang et al. 2012; Saputelli et al. 2014). With such an approach, a new set of model runs is necessary to optimize the design each time the input parameters change significantly. Running multiple simulations for every optimization case might not always be practical. An alternative approach is to develop well-performance curves with dimensionless variables on the basis of the performance model. Such an approach was the basis for unified fracture design (UFD) for a single fracture in a vertical well (Economides et al. 2002). However, a similar systemized method to calculate the optimum for a horizontal well with multiple hydraulic fractures was missing. The objective of this study was to develop a rigorous and unified dimensionless optimization technique with type curves for the case of multiple transverse fractures in a horizontal well—an extension of UFD. The mathematical problem was solved in dimensionless variables. Multiple fractures include the proppant number (NP), penetration ratio (Ix), dimensionless conductivity (CfD), and aspect ratio (yeD) for each fracture, which is inversely proportional to the number of fractures. The direct boundary element (DBE) method was used to generate the dimensionless productivity index (JD) for a given range of these parameters (28,000 runs) for the pseudosteady-state case. Finally, total well JD was plotted as a function of the number of fractures for various NP. The effect of minimum fracture width was studied, and the optimization curves were adjusted for three cases of minimum fracture width. The provided dimensionless type curves can be used to identify the optimized number of fractures and their geometry for a given set of parameters, without running a more complicated numerical model multiple times. First, the proppant mass (and hence, NP) used for the fracture design can be selected on the basis of economic or other considerations. For this purpose, a relationship between total JD and NP, which accounts for the minimum fracture width requirement, was provided. Then, the optimal number of fractures can be calculated for a given NP using the generated type curves with minimum width constraints. The following observations were made during the study on the basis of the performed runs: For a given volume or proppant, NP, total JD for multiple fractures increases to an asymptote as the number of fractures increases. This asymptote represents a technical potential for multiple fractures and for high proppant numbers (NP≥100), with a technical potential of 3πNP. Below this asymptote, the more fractures that are created for a fixed NP, the larger the JD. In practice, minimum fracture width constrains the fracture geometry, and therefore maximum JD. For the case when 20/40 sand is used for multiple hydraulic fracturing of a 0.01-md formation with square total area, the optimal number of factures is approximately NP25. Application of horizontal drilling technology with multiple fractures assumes the availability of high proppant numbers. It was shown mathematically that the alternative low proppant numbers (NP≤20 for the previous case) are impractical for multiple fractures, because total JD cannot be significantly higher than JD for an optimized single fracture in the same area. This means that low formation permeability and/or high proppant volumes are needed for multiple fracture treatments.

A goal-oriented framework for rapid integral-equation-based simulation of borehole resistivity measurements of 3D hydraulic fractures

Geophysics ◽  
2017 ◽  
Vol 82 (2) ◽  
pp. D123-D133 ◽  
Author(s):  
Kai Yang ◽  
Ali E. Yılmaz ◽  
Carlos Torres-Verdín
Keyword(s):  
Integral Equation ◽  
Method Of Moments ◽  
Low Frequency ◽  
Hydraulic Fractures ◽  
Approximate Methods ◽  
Electrically Conductive ◽  
Resistivity Measurements ◽  
Integral Equation Methods ◽  
Approximate Integral ◽  
Quantities Of Interest

We have developed a goal-oriented framework for fast integral-equation-based simulation of low-frequency borehole resistivity measurements of 3D arbitrarily shaped hydraulic fractures. The framework explores the possibility of applying various approximate integral-equation methods to simulate borehole electromagnetic (EM) measurements acquired in the vicinity of 3D hydraulic fractures generated with electrically conductive proppant. It includes four approximate methods that are progressively more accurate, costly, and rigorous. Each method is used to approximate the method-of-moments solution of the integral equation and to evaluate/extract quantities of interest, e.g., bucked signals detected at receivers. When compared with rigorous fast Fourier transform (FFT)-accelerated method-of-moments solutions, the numerical results obtained with the four methods indicate the following (1) All of the approximate methods capture the main features of the quantities of interest, e.g., the shape of detected signals. (2) Different approximate methods exhibit different accuracies and efficiencies in the simulation of EM scattering from various 3D fractures. (3) The identified approximate method achieves accurate results (error [Formula: see text]) while reducing the simulation time by a factor of 2–1000 compared with the FFT-accelerated rigorous method. Thus, our approximate simulation framework is a promising candidate for evaluating the Jacobian matrix in the fast inversion of borehole EM measurements to detect and assess the geometry of 3D hydraulic fractures generated with electrically conductive proppant.

Mapping Proppant Distribution in Hydraulic Fractures in Cased Wellbores Using Low Frequency Downhole Electrical Measurements

2018 ◽  
Author(s):  
Peng Zhang ◽  
Mrinal K. Sen ◽  
Mukul M. Sharma ◽  
Jeff Gabelmann ◽  
David Glowka
Keyword(s):  
Low Frequency ◽  
Hydraulic Fractures ◽  
Electrical Measurements

Inversion of downhole electrical measurements for proppant mapping using very fast simulated annealing

Geophysics ◽  
2020 ◽  
Vol 85 (1) ◽  
pp. D13-D22
Author(s):  
Peng Zhang ◽  
Javid Shiriyev ◽  
Mrinal K. Sen ◽  
Mukul M. Sharma
Keyword(s):  
Simulated Annealing ◽  
Synthetic Data ◽  
Divide And Conquer ◽  
Electrical Measurements ◽  
Multiple Fractures ◽  
Direct Excitation ◽  
Before And After ◽  
Very Fast Simulated Annealing ◽  
Fractured Horizontal Wells ◽  
Volume Method

Proppant mapping is critical for optimizing fracture treatment design and improving wells’ productivity. An electrode-based resistivity tool concept was developed earlier for proppant mapping in cased-hole wells. An array of insulating gaps is installed and cemented in place as a permanent part of the casing string. The electrical measurements are performed by imposing a voltage across each insulating gap, one at a time, before and after hydraulic fracture operations. The voltages across other insulating gaps near the transmitter gap are recorded. The method relies on direct excitation of the casing, which is expected to overcome the severe limitations of induction tools in cased-hole wells. A forward model based on a finite volume method has been developed to simulate the tool’s response to one or multiple fractures. To enable the implementation of such a practical system in multistage fractured horizontal wells, a fast and robust inversion approach is required. To that end, we have developed a divide-and-conquer approach based on a global optimization algorithm very fast simulated annealing (VFSA). Specifically, the original inverse problem is divided into subproblems and each subproblem can be solved separately using VFSA. The results indicate that our approach can invert the data and output widths and radii of multiple fractures without requiring a large number of forward simulations. The robustness of the inverse solver is also tested by adding Gaussian noise to the synthetic data. We tested example cases that demonstrate that when up to 5% noise is introduced, VFSA still provides very accurate inversion results with moderate uncertainties. Inversion results with some more realistic conditions, e.g., tilted fractures, complex fractures, and so on, are also presented.

Effect of Multiple Fracture Initiation on the Accuracy of Hydraulic Fracturing Simulation

2020 ◽  
Author(s):  
Ziad Bennour ◽  
Walid Mohamed Mahmud ◽  
Mansur Ermila
Keyword(s):  
Hydraulic Fracturing ◽  
Simulation Model ◽  
Fracture Model ◽  
Single Fracture ◽  
Multiple Fractures ◽  
Actual Treatment ◽  
Fracture Simulation ◽  
Fracturing Process ◽  
Simulation Results ◽  
Actual Fracture

Abstract Hydraulic fracturing is a stimulation technique in which the formation is fractured using high pressure exerted by a fluid. The induced fracture increases the permeability of the formation by providing conductive channels to the formation which results in improved fluids productivity. Hydraulic fracturing is a common practice in oil and gas, particularly in the development of unconventional low porosity and low permeability reservoirs. However, as the hydraulic fracturing technique is costly, considerable preparations efforts must be made before executing the fracturing operation including simulating the intended fracture model. A simulation model of a hydraulic fracturing assists in forecasting and controlling the intended fractures that are to be induced. Although the simulation model can be helpful, it may not exactly mimic or predict the actual initiated fractures due to the complex nature of the actual fracturing process. Thus, the simulated model and the actual fracture might differ in many ways which results in an uncertainty in the simulated fracture model. Therefore, in order to reduce uncertainty, initial data input and assumptions made before and during the fracturing simulation need to be precise in order to obtain accurate simulation results. The growth of a single fracture is often assumed during the simulation of hydraulic fracturing which maybe incorrect as multiple fractures may initiate at the start or middle of the actual fracturing treatment and can have significant effect on the simulated fracturing results. This paper proposes a method to minimize the difference between fracturing simulation and actual fracture treatment results by utilizing sensitivity tests to the main fracturing parameters. Thus, the initial actual fracturing results were used to detect the occurrence of multiple fractures where the latter was considered to enhance the upcoming simulation accuracy of the proposed treatments. The analysis of high net pressure data during the actual treatment indicates the possible presence of multiple fractures where history matching between actual treatment and simulation results data can give an estimate on when and how many multiple fractures were initiated during the fracturing treatment. As a result, the data analysis showed that multiple fractures initiation had a significant effect on the fracture simulation results and the assumption of a single fracture during hydraulic fracturing should be discarded unless it is confirmed to be the case. Geological settings of the reservoir and the presence of natural fractures were also found to cause multiple fractures initiation during the treatments, and therefore, the reservoir data and description need to be determined properly before attempting the simulation of a fracturing treatment.

Detection and quantification of 3D hydraulic fractures with vertical borehole induction resistivity measurements

Geophysics ◽  
2016 ◽  
Vol 81 (4) ◽  
pp. E259-E264 ◽  
Author(s):  
Kai Yang ◽  
Carlos Torres-Verdín ◽  
Ali E. Yılmaz
Keyword(s):  
Electrical Conductivity ◽  
Low Frequency ◽  
Hydraulic Fractures ◽  
Electrically Conductive ◽  
Resistivity Measurements ◽  
Effective Electrical Conductivity ◽  
Shale Formation ◽  
Logging Tool ◽  
Conductivity Contrast ◽  
Vertical Borehole

We have investigated the ability of low-frequency induction resistivity measurements to detect and appraise hydraulic fractures induced near vertical boreholes. Integral-equation-based simulations indicate that coplanar measurements can detect fractures when they are injected with electrically conductive proppant to increase their conductivity contrast with the shale background. Specifically, when a logging tool consisting of one transmitter and two receivers that are 1.2 and 1.5 m away is used to detect fractures with the effective electrical conductivity of [Formula: see text] in a homogeneous shale formation of [Formula: see text] conductivity, the measurements (1) can indicate the boundary of fractures intersecting with vertical boreholes by the signal spikes generated only when the tool enters/exits fractures, (2) can detect fractures as small as approximately 0.15 m and differentiate fractures up to approximately 10 m in width, (3) can detect fractures with height as small as 0.3 m, (4) can differentiate elliptical and rectangular fractures from each other if they exhibit the same width; e.g., they can discriminate if their widths are within approximately 0.4–10 m, and (5) are sensitive to the effective electrical conductivity of the fracture. A second logging tool consisting of one transmitter and two receivers that are 18 and 19.2 m away is found to be less useful in the detection and appraisal of hydraulic fractures induced near vertical boreholes.

Integration of Multi-geophysical Approaches to Identify Potential Pathways of Heavy Metals Contamination - A Case Study in Zeida, Morocco

2020 ◽  
Vol 25 (3) ◽  
pp. 415-423
Author(s):  
Ahmed Lachhab ◽  
El Mehdi Benyassine ◽  
Mohamed Rouai ◽  
Abdelilah Dekayir ◽  
Jean C. Parisot ◽  
...  
Keyword(s):  
Heavy Metals ◽  
Seismic Velocity ◽  
Seismic Refraction ◽  
Geophysical Methods ◽  
Low Frequency ◽  
Electrical Current ◽  
P Wave ◽  
Low Resistivity ◽  
Refraction Tomography ◽  
Geophysical Surveys

The tailings of Zeida's abandoned mine are found near the city of Midelt, in the middle of the high Moulouya watershed between the Middle and the High Atlas of Morocco. The tailings occupy an area of about 100 ha and are stored either in large mining pit lakes with clay-marl substratum or directly on a heavily fractured granite bedrock. The high contents of lead and arsenic in these tailings have transformed them into sources of pollution that disperse by wind, runoff, and seepage to the aquifer through faults and fractures. In this work, the main goal is to identify the pathways of contaminated water with heavy metals and arsenic to the local aquifers, water ponds, and Moulouya River. For this reason, geophysical surveys including electrical resistivity tomography (ERT), seismic refraction tomography (SRT) and very low-frequency electromagnetic (VLF-EM) methods were carried out over the tailings, and directly on the substratum outside the tailings. The result obtained from combining these methods has shown that pollutants were funneled through fractures, faults, and subsurface paleochannels and contaminated the hydrological system connecting groundwater, ponds, and the river. The ERT profiles have successfully shown the location of fractures, some of which extend throughout the upper formation to depths reaching the granite. The ERT was not successful in identifying fractures directly beneath the tailings due to their low resistivity which inhibits electrical current from propagating deeper. The seismic refraction surveys have provided valuable details on the local geology, and clearly identified the thickness of the tailings and explicitly marked the boundary between the Triassic formation and the granite. It also aided in the identification of paleochannels. The tailings materials were easily identified by both their low resistivity and low P-wave velocity values. Also, both resistivity and seismic velocity values rapidly increased beneath the tailings due to the compaction of the material and lack of moisture and have proven to be effective in identifying the upper limit of the granite. Faults were found to lie along the bottom of paleochannels, which suggest that the locations of these channels were caused by these same faults. The VLF-EM surveys have shown tilt angle anomalies over fractured areas which were also evinced by low resistivity area in ERT profiles. Finally, this study showed that the three geophysical methods were complementary and in good agreement in revealing the pathways of contamination from the tailings to the local aquifer, nearby ponds and Moulouya River.

Radio and Electrical Measurements on Glacial Streams

1987 ◽  
Vol 33 (114) ◽  
pp. 239-242
Author(s):  
M. E. R. Walford
Keyword(s):  
Electrical Conductivity ◽  
Water Channel ◽  
Water System ◽  
Low Frequency ◽  
Water Channels ◽  
Radiative Loss ◽  
Radio Waves ◽  
Electrical Measurements ◽  
Water Conductivity ◽  
Glacier Surface

AbstractWe discuss the suggestion that small underwater transmitters might be used to illuminate the interior of major englacial water channels with radio waves. Once launched, the radio waves would naturally tend to be guided along the channels until attenuated by absorption and by radiative loss. Receivers placed within the channels or at the glacier surface could be used to detect the signals. They would provide valuable information about the connectivity of the water system. The electrical conductivity of the water is of crucial importance. A surface stream on Storglaciären, in Sweden, was found, using a low-frequency technique, to have a conductivity of approximately 4 × 10−4 S m−1. Although this is several hundred times higher than the conductivity of the surrounding glacier ice, the contrast is not sufficient to permit us simply to use electrical conductivity measurements to establish the connectivity of englacial water channels. However, the water conductivity is sufficiently small that, under favourable circumstances, radio signals should be detectable after travelling as much as a few hundred metres along an englacial water channel. In a preliminary field experiment, we demonstrated semi quantitatively that radio waves do indeed propagate as expected, at least in surface streams. We conclude that under-water radio transmitters could be of real practical value in the study of the englacial water system, provided that sufficiently robust devices can be constructed. In a subglacial channel, however, we expect the radio range would be much smaller, the environment much harsher, and the technique of less practical value.

scholarly journals Measuring implanted patient response to tone pips

2021 ◽  
Vol 20 (1) ◽  
Author(s):  
Juan M. Cornejo ◽  
Agar K. Quintana ◽  
Nohra E. Beltran ◽  
Pilar Granados
Keyword(s):  
Auditory Nerve ◽  
Dynamic Range ◽  
Test Procedure ◽  
Electrical Potential ◽  
Electrical Current ◽  
Intensity Level ◽  
Electromyographic Activity ◽  
Variable Intensity ◽  
Electrode Current ◽  
Auditory Behavior

Abstract Background An electrical potential not previously reported—electrical cochlear response (ECR)—observed only in implanted patients is described. Its amplitude and growth slope are a measurement of the stimulation achieved by a tone pip on the auditory nerve. The stimulation and recording system constructed for this purpose, the features of this potential obtained in a group of 43 children, and its possible clinical use are described. The ECR is obtained by averaging the EEG epochs acquired each time the cochlear implant (CI) processes a tone pip of known frequency and intensity when the patient is sleeping and using the CI in everyday mode. The ECR is sensitive to tone pip intensity level, microphone sensitivity, sound processor gain, dynamic range of electrical current, and responsiveness to electrical current of the auditory nerve portion involved with the electrode under test. It allows individual evaluation of intracochlear electrodes by choosing, one at the time, the central frequency of the electrode as the test tone pip frequency, so the ECR measurement due to a variable intensity tone pip allows to establish the suitability of the dynamic range of the electrode current. Results There is a difference in ECR measurements when patients are grouped based on their auditory behavior. The ECR slope and amplitude for the Sensitive group is 0.2 μV/dBHL and 10 μV at 50 dBHL compared with 0.04 μV/dBHL and 3 μV at 50dBHL for the Inconsistent group. The clinical cases show that adjusting the dynamic range of current based on the ECR improved the patient’s auditory behavior. Conclusions ECR can be recorded regardless of the artifact due to the electromyographic activity of the patient and the functioning of the CI. Its amplitude and growth slope versus the intensity of the stimulus differs between electrodes. The relationship between minimum ECR detection intensity level and auditory threshold suggests the possibility of estimating patient auditory thresholds this way. ECR does not depend on the subject’s age, cooperation, or health status. It can be obtained at any time after implant surgery and the test procedure is the same regardless of device manufacturer.

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