Borehole Environment Research Articles

Fast Induction Logging Modeling With Hierarchical Sudoku Meshes Based on DGFD

This letter extends the discontinuous Galerkin frequency domain (DGFD)-based domain decomposition method (DDM) to model the open borehole environment accurately with a newly developed hierarchical sudoku mesh. Both the borehole and mud invasion effects can be modeled by this framework efficiently. Furthermore, deep reading measurements can be modeled conveniently by inserting arbitrarily shaped objects into the hierarchical sudoku mesh; the capability to distinguish a cavity saturated with either oil or water with different borehole-object distances are then studied for a deep reading tool in an open borehole carbonate environment. The DGFD with the hierarchical sudoku mesh shows six times faster speed than the traditional finite-element method (FEM) for the deep reading case.

Application of Dipole Array Acoustic Logging in the Evaluation of Shale Gas Reservoirs

In order to effectively evaluate shale gas reservoirs with low porosity, extra-low permeability, and no natural productivity, dipole array acoustic logging, which can provide various types of information including P-wave slowness (DTC) and S-wave slowness (DTS), is widely used. As the dipole array acoustic logging tool has a larger investigation depth and is suitable for complex borehole environments, such as those with a high wellbore temperature, high drilling fluid column pressure, or irregular borehole wall, it has been mainly applied to the evaluation of lithology, gas potential, fractures, and stimulation potential in shale gas reservoirs. The findings from a case study of the Sichuan Basin in China reveal that the acoustic slowness, S-P wave slowness ratio (RMSC), and S-wave anisotropy of the dipole array acoustic logging can be used to qualitatively identify reservoir lithology, gas potential, and fractures. Using the relationship between DTC and the total porosity of shale gas reservoirs, and combined with the compensated neutron (CNL) and shale content (Vsh) of the reservoir, a mathematical model for accurately calculating the total porosity of the shale gas reservoir can be established. By using the relationship between the RMSC and gas saturation in shale gas reservoirs and tied with density log (DEN), a mathematical model of gas saturation can be established, and the determination of gas saturation by the non-resistivity method can be achieved, delivering a solution to the issue that the electric model is not applicable under low resistivity conditions. The DTS, DTC, and DEN of shale can be used to calculate rock mechanic parameters such as the Poisson’s ratio (POIS) and Young’s modulus (YMOD), which can be used to evaluate the shale stimulation potential.

Gamma-Ray-Tool Characterization Avoids Traditional Limitations

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 191717, “Is API Enough for Gamma Ray Logs, or Do We Need More?” by Feyzi•Inanc, SPE, and Andreas Vogt, SPE, Baker Hughes, a GE Company, prepared for the 2018 SPE Annual Technical Conference and Exhibition, Dallas, 24–26 September. The paper has not been peer reviewed. A major issue with gamma-ray (GR) logs is that the API definition is valid only if the tool is run in a 4.89-in. borehole filled with fresh water. Although all GR tools are intended to provide the same results for such a well, in the field there is no single one-size-fits-all concept. In this paper, the authors discuss the characterization process for GR tools and how they behave in boreholes different from the one used in the University of Houston (UH) GR characterization pit. Proposals for developing correction strategies so that GR logs become quantitative logs, rather than the qualitative logs of the past, also are outlined. Introduction One of the issues inherent in ensuring repeatability of GR logs starts with tool characterization. For decades, no common industry standard existed that defined the scale for measuring radioactivity. The chaos created by different tools providing radioactivity in different units in the same environments ended with the creation of the API unit in 1959. The definition of the unit was based on work done with the UH GR characterization pit. This specific pit has a 4.89-in. borehole and a casing to ensure stability within the surrounding synthetic formation. One downside of the API definition is that different combinations of radioactive isotopes, environmental conditions, and tool designs may yield comparable count rates. UH GR pit characterizations of GR tools lose their validity as the wellbore diverges from this previously described specific borehole size and environment. Thus, environmental corrections are needed to bring readings to a reference borehole used for tool characterization. Another issue with the UH GR-pit-based characterizations became evident with the emergence of logging-while-drilling (LWD) GR tools. The LWD tools were supposed to provide natural GR values comparable with those of wireline tools. Recommended practices for wireline tools did not have a provision for calibration of LWD GR tools that did not fit into the UH GR pit. Consequently, characterizations of LWD tools to provide results comparable with those of wireline tools became a topic of interest. Background Although the UH GR characterization pit still exists, it suffers from many issues. The facility is in a state of disrepair, with significant corrosion. In addition, its activity level is decreasing because of the presence of short-lived Ra-226 as one of the main ingredients in the high-activity zone. The radionuclide distribution is not even, causing gamma readings to change with the height, and rock dimensions do not correspond to those of an infinite rock environment.

A Hybrid 3-D Electromagnetic Method for Induction Detection of Hydraulic Fractures Through a Tilted Cased Borehole in Planar Stratified Media

As one of the most important nondestructive characterization techniques, electromagnetic (EM) methods can be used in the subsurface fracture detection, especially for hydraulic fracture evaluation in unconventional petroleum exploration and development. The multiscale nature of long but extremely thin 3-D fractures is difficult for conventional EM modeling methods such as the finite element method (FEM) in numerical simulation. The problem becomes even more challenging when the effects of tilted borehole, casing, and planar stratified media need to be considered. So far, modeling a tilted borehole in layered media is still a major challenge for conventional methods. In this paper, we present the hybrid numerical mode-matching method with the stabilized biconjugate gradient fast Fourier transform method as the forward modeling algorithm that can efficiently model 3-D fractures in planar stratified media with a cased borehole environment. Numerical results validate the accuracy of the hybrid forward method and show orders of magnitude higher efficiency of this forward solver than the FEM.

Estimation of Fracture Compliance From Attenuation and Velocity Analysis of Full‐Waveform Sonic Log Data

AbstractIn fractured rocks, the amplitudes of propagating seismic waves decay due to various mechanisms, such as geometrical spreading, solid friction, displacement of pore fluid relative to the solid frame, and transmission losses due to energy conversion to reflected and transmitted waves at the fracture interfaces. In this work, we characterize the mechanical properties of individual fractures from P wave velocity changes and transmission losses inferred from static full‐waveform sonic log data. The methodology is validated using synthetic full‐waveform sonic logs and applied to data acquired in a borehole penetrating multiple fractures embedded in a granodioritic rock. To extract the transmission losses from attenuation estimates, we remove the contributions associated with other loss mechanisms. The geometrical spreading correction is inferred from a joint analysis of numerical simulations that emulate the borehole environment and the redundancy of attenuation contributions other than geometrical spreading in multiple acquisitions with different source‐receiver spacing configurations. The intrinsic background attenuation is estimated from measurements acquired in the intact zones. In the fractured zones, the variations with respect to the background attenuation are attributed to transmission losses. Once we have estimated the transmission losses associated with a given fracture, we compute the transmission coefficient, which, on the basis of the linear slip theory, can then be related to the mechanical normal compliance of the fracture. Our results indicate that the estimated mechanical normal compliance ranges from 1 × 10−13 to 1 × 10−12 m/Pa, which, for the size of the considered fractures, is consistent with the experimental evidence available.

Long Short-Term Memory and Variational Autoencoder With Convolutional Neural Networks for Generating NMR T2 Distributions

Downhole nuclear magnetic resonance (NMR) logs acquired in the borehole environment are valuable for subsurface characterization because they contain information about the pore size distribution, fluid composition, fluid saturation, fluid mobility, formation permeability, and porosity. NMR log acquisition can be challenging due to operational and financial constraints. Recently, NMR T2 distributions of the subsurface were generated by processing conventional well logs using deeplearning neural-network (NN) models. This improves the accessibility to subsurface pore size distributions. We implement two neural-network models, variational autoencoder-based NN with convolutional layers and long short-term memory (LSTM), to generate NMR T2 distributions from formation mineral content and fluid saturation logs. Prediction performance is evaluated for the entire NMR T2 spectrum ranging from 0.3 to 3000 ms as well as for T2 spectra within four bins obtained by dividing the entire spectrum into four equal parts. Each bin represents a specific pore size range. In terms of R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , both the models have prediction performances above R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of 0.75 for the entire spectrum. The best prediction performance is achieved for the bin of size ranging from 2.7 to 28 ms, representing pore diameters from 10 to 100 nm. The performance of this bin in terms of R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is 0.78. The LSTM model is highly sensitive to noise in T2 distribution used during the training, and both the models are robust to noise in the conventional input logs.

БЫСТРЫЕ АЛГОРИТМЫ ТРЕХМЕРНОГО ЧИСЛЕННОГО МОДЕЛИРОВАНИЯ ПОКАЗАНИЙ ЗОНДОВ ВИКИЗ И БКЗ, УЧИТЫВАЮЩИЕ НЕРАВНОКОМПОНЕНТОЕ ПОЛЕ НАПРЯЖЕНИЙ В ОКРЕСТНОСТИ СКВАЖИНЫ

The paper deals with fast algorithms for three-dimensional mathematical modeling of the Russian lateral log (BKZ) and data of high-frequency induction isoparametric logging tool (VIKIZ), performed on the basis of the finite difference method. Numerical experiments are presented for formation models taking into account unequal component stress-strein state in the borehole environment.

БАЗА МНОГОФИЗИЧНЫХ МОДЕЛЕЙ ПЛАСТОВ АТЛАС МФМ

ATLAS MPhM is collection (database) of multiphysical models of typical reservoirs of the Western Siberia oilfields. A systematic set is constantly updated with models that are built for the received practical data. ATLAS MPhM can be used to estimate the influence of model parameters on the mud invading into the reservoir, electromagnetic logs and interpretation of geophysical and geotechnological well data based on multiphysical simulation. The ATLAS MPhM includes visualization tools that allow to observe evolution of the borehole environment and well logs.

Evaluating Brine Deposits Using Borehole Magnetic Resonance

Brine mining is an important source of elements such as potassium, iodine, lithium, and bromine that occur in solution in groundwater, typically in shallow brines occurring beneath saline or dry lakes or in deep brines in sedimentary basins. Where feasible, brine mining is an attractive alternative to conventional mining due to lower surface and environmental impact and lower OPEX than conventional mining operations. As with any resource, evaluating brine deposits requires developing an understanding of how much resource is present and how it can be most economically produced. How much resource is present is a function of the bulk aquifer volume, the specific yield, and the brine composition, while the primary subsurface control on economic production is hydraulic conductivity, which dictates the rate at which the brine can be produced to surface. Specific yield and hydraulic conductivity are analogous to the free fluid volume and permeability quantities that are of interest in oil and gas resource assessment. Borehole magnetic resonance has been applied in the oil and gas industry for the evaluation of bound and free fluid volumes and permeability for over twenty years. These same methodologies are equally applicable in the evaluation of brine deposits, however the hypersaline brines that are targets for commercial development cause highly conductive borehole environments that can be extremely challenging for magnetic resonance measurements. Nevertheless, use of borehole magnetic resonance measurements to help evaluate a sulphate of potash brine deposit currently under assessment shows that such measurements can be employed successfully in these environments.

Using Borehole Seismic Data in Designing of Development and Further Exploration of Oil Fields

Borehole seismology allows studying the specific features of the geological structure and physical properties of deposits in the borehole environment using waves of various kinds based on the analysis of characteristics of these waves, their rates propagation, attenuation, space polarization, and the nature of rock anisotropy. The research is intended to study the wave fields of various types of waves taking into account their dynamic, speed and polarization characteristics and the features of geological structure both in the vicinity to the borehole and laterally in the crosshole space. Using the borehole seismology method, the geological structure of reservoirs was specified; a prediction was given as to the development of enhanced reservoir units in the crosshole space.

Electromagnetic Forward and Inverse Algorithms for 3-D Through-Casing Induction Mapping of Arbitrary Fractures

This letter extends the axisymmetric version of the efficient forward and inverse algorithms to characterize and reconstruct arbitrary 3-D fractures in a cased borehole environment. We improve our previous hybrid distorted Born approximation and stabilized biconjugate gradient fast Fourier transform method as the 3-D forward modeling algorithm that can significantly reduce the computational cost in forward modeling. The bounding constraints and model parameter transformation functions are introduced to our previous axisymmetric variational Born iterative inverse method to effectively reconstruct 3-D fractures. Numerical results show orders of magnitude higher efficiency of the forward algorithm than the finite-element method, and the effectiveness of the inverse algorithm for through-casing 3-D fracture reconstruction.

High‐resolution multi‐component distributed acoustic sensing

ABSTRACTDistributed acoustic sensing uses an optical fibre together with an interrogator unit to perform strain measurements. The usage of distributed acoustic sensing in geophysics is attractive due to its dense spatial sampling and low operation cost if the optical fibre is freely accessible. In the borehole environment, optical fibres for distributed acoustic sensing are often readily available as a part of other sensing tools, such as for temperature and pressure. Although the distributed acoustic sensing system promises great potential for reservoir monitoring and surface seismic acquisition, the single axial strain measurement of distributed acoustic sensing along the fibre is inadequate to fully characterise the different wave modes, thus making reservoir characterisation challenging. We propose an acquisition system using five equally spaced helical optical fibres and a straight optical fibre to obtain six different strain projections. This system allows us to reconstruct all components of the 3D strain tensor at any location along the fibre. Analysing the condition number associated with the geometry of the optical fibre, we can systematically search for the optimum design parameters for our configuration. Numerical examples demonstrate the effectiveness of our proposed method to successful reconstruction of the full strain tensor from elastic wavefields of arbitrary complexity.

Guided wave sensor for simple digital communication through an oil industry multi-wire cable

This work presents the proprietary design and implementation of a lead zirconate titanate (PZT) sensor for simple digital transmission and reception of guided waves using a high-temperature and corrosion-resistant multiple-wire 1K22 MP-35N cable as a communication channel. These cables are commonly used in the hostile oil and gas borehole environments of the petroleum industry. The sensor design involves electrical/mechanical coupling, instrumentation and casing. The PZT guided wave transmitter instrumentation includes a microcontroller-based pulse position modulator, a signal booster, a PZT-5H crystal and a 9 V battery. The receiver sensor consists of a PZT-5H crystal, an amplifier and a real-time digital signal processing (RT DSP) and data acquisition module connected to a personal computer. Dispersion curves were obtained using Disperse software and a simplified model of the cable. Dynamic linear 3D finite element modelling of the sensor was performed to support the correct frequency selection. To evaluate the sensor design, the transmitter generated 60 kHz guided wave pulse position modulation (PPM) energy packages. Experimental tests were conducted in the laboratory using 1 m and 10 m cables. The results showed that, although there was significant dispersion and there were multiple mode excitations of the transmitted pulses, the system correctly identified 10-bit frames of guided wave PPM-encoded information.

Системы наблюдений при трехмерном вертикальном сейсмическом профилировании для уточнения геологического строения околоскважинного пространства

Системы наблюдений при трехмерном вертикальном сейсмическом профилировании для уточнения геологического строения околоскважинного пространства

Direct and inverse problems of gas emission and the sorptive deformation of coal beds

Using the equations of state for fractured-porous media that describe the sorptioninduced deformation of coal, we develop a geomechanical model for radial gas influx to a borehole drilled in a coal bed with the concurrent evolution of stress field in the borehole environment. A numerical-and-analytical method is proposed for solving the corresponding system of equations for poroelastic media. A relation is found between the volume of slack withdrawn in the borehole (when opening up the gas-bearing seams), the sorption-and-storage capacities of coal, the permeability k, and the horizontal component σ h of the natural stress field. We demonstrate the solvability of the inverse boundary-coefficient problem of determining k and σ h on the basis of pressure in the closed borehole. We substantiate an express-method for estimating the permeability by the measurements of pressure in the borehole operating in the “pressure drop” mode.

Through-Casing Hydraulic Fracture Evaluation by Induction Logging I: An Efficient EM Solver for Fracture Detection

Hydraulic fracturing is an essential way to improve the production of unconventional shale oil and gas. It is important to characterize the produced fractures using either acoustic or electromagnetic (EM) methods. Conventional EM solvers in the low-frequency range face significant challenges by such multiscale problems where the fracture width is orders of magnitude smaller than its diameters. Furthermore, the cased borehole environment is extremely difficult to simulate with conventional EM solvers due to meshing difficulties and the multiscale nature of the problem. In this paper, we develop a hybrid distorted Born approximation and 3-D mixed ordered stabilized biconjugate gradient fast Fourier transform (DBA-BCGS-FFT) method to simulate the very challenging 2-D, 2-D-axisymmetric, and 3-D hydraulic fracture models under both open and cased borehole environments. Numerical examples show that this method has orders of magnitude higher efficiency than the finite element method. The capabilities of the DBA-BCGS-FFT method for the induction tool fracture mapping are demonstrated by comparing with laboratory experimental results and other reference results.

Significance of Time-Lapsed Hydrocarbon Saturations From Pulsed-Neutron-Capture (PNC) Logs and the Impact of Borehole-Environment Parameters on PNC-Derived Saturations

Summary Reservoir surveillance with pulsed-neutron-capture (PNC) logs has shown its importance historically in providing reservoir hydrocarbon saturations behind the casing. However, there is little in the literature to show the importance of PNC logs for contact tracking and accessing bypassed/moved hydrocarbons. In addition, no one has ever attempted to characterize the various borehole environments and subsequently understanding their impact on the PNC-derived hydrocarbon saturations. This paper is divided into two parts. The first part shows the importance of time-lapse PNC logs in understanding reservoir-fluid movement and chasing migrated hydrocarbons behind the casing. A case study from the UK Central North Sea is presented to highlight the importance of the PNC sigma data. The second part of the paper shows the computation of borehole-environment parameters from simple to complex completions including multiple casing/tubing strings, varying hole sizes, and annuli fill. For the first time, the work presented in this paper shows the importance of having correct environmental parameters and the impact of using incorrect parameters on the PNC-derived hydrocarbon saturations. Not making such detailed checks may result in misinterpretation of the PNC data, leading to incorrect conclusions about fluid dynamics, and risky decisions in adding perforations and water-shut off (WSO) opportunities.

Carrier-interleaved orthogonal multi-electrode multi-carrier resistivity-measurement tool

This paper proposes a new carrier-interleaved orthogonal multi-electrode multi-carrier resistivity-measurement tool used in a cylindrical borehole environment during oil-based mud drilling processes. The new tool is an orthogonal frequency division multiplexing access-based contactless multi-measurand detection tool. The tool can measure formation resistivity in different azimuthal angles and elevational depths. It can measure many more measurands simultaneously in a specified bandwidth than the legacy frequency division multiplexing multi-measurand tool without a channel-select filter while avoiding inter-carrier interference. The paper also shows that formation resistivity is not sensitive to frequency in certain frequency bands. The average resistivity collected from N subcarriers can increase the measurement of the signal-to-noise ratio (SNR) by N times given no amplitude clipping in the current-injection electrode. If the clipping limit is taken into account, with the phase rotation of each single carrier, the amplitude peak-to-average ratio can be reduced by 3 times, and the SNR can achieve a 9/N times gain over the single-carrier system. The carrier-interleaving technique is also introduced to counter the carrier frequency offset (CFO) effect, where the CFO will cause inter-pad interference. A qualitative analysis and simulations demonstrate that block-interleaving performs better than tone-interleaving when coping with a large CFO. The theoretical analysis also suggests that increasing the subcarrier number can increase the measurement speed or enhance elevational resolution without sacrificing receiver performance. The complex orthogonal multi-pad multi-carrier resistivity logging tool, in which all subcarriers are complex signals, can provide a larger available subcarrier pool than other types of transceivers.

Introduction to this special section: Advances in seismic sensors

Seismic detectors have been around for centuries and seismic prospecting for many decades. The variety of seismic sensors in modern surveys is dazzling; they can measure particle velocity and acceleration, pressure and pressure gradient, strain and strain rate, and multiple components (Z, H1, H2, yaw, pitch, roll), from near DC to hundreds or thousands of hertz, in land, marine, borehole, and extraterrestrial environments.

Improved in situ mineral and petrophysical interpretation with neutron-induced gamma-ray spectroscopy elemental logs

Neutron logs are routinely expressed as apparent neutron porosity based on the assumption of a freshwater-saturated homogeneous formation with solid composition equal to either sandstone, limestone, or dolomite. Rock formations are often extremely heterogeneous and consist of different minerals and fluids in varying proportions, which cause simultaneous matrix and fluid effects on neutron logs. Detailed quantification of formation mineral composition enables the correction of matrix effects on measured neutron logs to unmask fluid effects; this in turn enables accurate quantification of porosity and water saturation. Neutron-induced gamma-ray spectroscopy is one of the most direct means available to quantify in situ formation mineralogy but available spectroscopy-based interpretation methods are usually tool dependent and incorporate empirical correlations. We have developed a new interpretation method to quantify mineral concentrations through the joint nonlinear matrix inversion of measured spectroscopy elemental weight concentrations and matrix-sensitive logs, such as gamma ray, matrix photoelectric factor, matrix sigma (neutron capture cross section), and matrix density. The estimated mineralogy was used in the correction of matrix effects on porosity logs and subsequent calculation of true formation porosity. The water saturation was quantified through joint petrophysical interpretation of matrix-corrected porosities and resistivity measurements using an appropriate saturation model. The developed inversion-based interpretation method is applicable to a wide range of formation lithologies, well trajectories, and borehole environments (including open and cased hole environments), and it is independent of tool and neutron source type. Verification results with synthetic and field cases confirm that the spectroscopy-based algorithm is reliable and accurate in the quantification of mineral concentrations, matrix properties, porosity, and hydrocarbon saturation.