Borehole Tool Research Articles

Mitigation of installation-related effects for small-scale borehole-to-surface ERT

Small-scale resistivity inhomogeneities can result from the local distribution of water and the water and nutrient uptake of plants. Measuring small-scale Electrical Resistivity Tomography (ERT) in the field comes with a set of particularities, especially when including borehole electrodes for a better resolution with depth. We apply small-scale borehole-to-surface ERT over a palaeochannel. Combining surface ERT with detailed borehole-to-surface ERT profiles along the measurement line allows a delineation of finer layering within the coarser lithology. Our field setup includes a borehole electrode tool with 20 ring electrodes, electrically coupled to the ground via a conductive mud. Two main points are addressed in this publication: (1) In the field, we electrically coupled the borehole electrodes to the ground by filling the cavities around the tool with a soil mud, i.e., we need to account for the unknown conductive borehole filling in the inversion. If not incorporated, the mud has a considerable influence on the resistivities close to the borehole tool, but also on the region around the surface electrodes. Consequently, alongside with a 3D inversion scheme representing the electrodes with the Complete Electrode Model (CEM), we include the mud as a separate and uncoupled region. We model the geometry of the mud layer around the tool and do not allow an influence of this region on the rest of the model. (2) Due to the small electrode distances and the overall small-scale nature of the array, the depth of installation of the borehole electrode tool must be known accurately in the inversion model. However, it is not easy to measure the tool depth in the field with the required accuracy, due to small-scale surface roughness, e.g., from a weathered loose soil layer at the surface or from vegetation. We also investigate the influence of a tilted tool installation and optimise for the depth and installation angle of the borehole tool before inverting for resistivities. An accurate knowledge of the borehole electrode positions is crucial for a reliable and precise inversion result. The surface electrodes establish a coordinate system around the borehole tool on the surface, with an angle φ describing the direction around the tool in the top view. The sensitive plane (in-plane) is defined as the x-z plane cutting through φ=0° and φ=180°. A tilting of the tool from the vertical direction is described by a tilting angle θ. A tilting of the borehole tool within the sensitive plane manifests in an increased misfit between data points on both sides of the tool, i.e., at φ=0° and φ=180°. We use this difference to optimise on the tool angle. The true depth of the borehole tool is found by searching for a minimum of the objective function, describing the goodness of the found model, while assuming different tool depths in each inversion. We see a minimum of the objective functionwhich can be attributed to the correct depth range, as shown by a synthetic study. Through our optimisations, we can determine a tilting of the tool, i.e., the angle θ, with an accuracy of 2° to 3° and the tool depth with an accuracy of a few centimetres, depending partially on the subsurface resistivities, i.e., our optimisation works mainly in predominantly horizontally layered soils. A tilting in directions out of the sensitive plane (out-of-plane) can be projected onto the sensitive plane, since the out-of-plane tilt has a negligible influence on the data. After this optimisation, we can determine layer resistivities from our field data.

Feasibility study of a surface-borehole NMR method

We present results of a feasibility study of a borehole induction-coil sensor for surface-borehole NMR (SBNMR) investigations. This sensor of 7 cm diameter and 180 cm length is connected to a standard MRS (Magnetic Resonance Sounding) instrument. Thus, SBNMR is a cost-effective extension of the MRS method. Using a downhole sensor increases the depth of investigation and the resolution of MRS. In the near-horizontal Earth's magnetic field, the sensitive area of the sensor is represented by a cylinder of a few meters in diameter. A blind zone of 0.5 to 1 m around the borehole is due to the disturbance of the Earth's magnetic field by the magnetic core of the sensor. The relatively large volume investigated with SBNMR and the blind zone around borehole may represent an advantage of SBNMR over the NMR (nuclear magnetic resonance) borehole tool investigating a narrow zone around the borehole. However, using the Earth's magnetic field renders the SBNMR performance site dependent with an inherently low signal-to-noise ratio. Our first results show a good correspondence between SBNMR, MRS and borehole data.

Considerations regarding small-scale surface and borehole-to-surface electrical resistivity tomography

Our synthetic study focuses on small-scale Electrical Resistivity Tomography (ERT) used for high resolution imaging of the very near surface region, e.g., the first meter of the subsurface. Corresponding arrays employ an electrode spacing of a few centi- or decimetres, i.e., the electrode spacing is in the same length range as the active electrode length - the part of the electrode in contact with the ground. When simulating such ERT data, electrodes cannot be approximated as point electrodes but have to be considered with their real geometry, i.e., 3D modelling is required. Using 3D modelling, we confirm a change of the configuration factor but observe also a change of the sensitivity and coverage distributions compared to point electrode configurations. Sensitivity and coverage are high at the electrode surfaces but drop rapidly below them. This difference means a reduction in both the depth of investigation of the array and the resolution in the region below the electrodes. The effect increases with increasing active electrode length-to-spacing ratios. One way to compensate for the sensitivity reduction with depth is to add borehole electrodes. We study a borehole-to-surface (b2s) electrode array with the borehole electrodes situated in the middle of the array. As for the surface electrodes, we observe strongly increased sensitivities close to the borehole electrodes. Thus, we obtain a T-shaped region with high coverage and a good model recovery. For our synthetic models, the b2s array outperforms both the pure surface array and a pure borehole measurement in terms of model recovery. Hence, small-scale b2s ERT can improve the limited imaging capabilities of small-scale surface ERT. However, b2s is best suited to a layered subsurface as a good model recovery is limited to a region close to the electrodes. We deduce the following main recommendations for small-scale ERT from our synthetic study. For surface ERT, we found that: (i) Long electrodes (compared to their separation) decrease the model recovery and depth of investigation. For b2s ERT, the following main points can be stated: (ii) b2s ERT gives a high resolution of a layered subsurface, but the model resistivities are only well recovered in the near electrode regions. (iii) High resistivity contrasts between layers lead to artefacts, which can be suppressed by introducing a separate borehole region. Overall, we can confirm: (iv) Small-scale ERT requires 3D modelling with accurate representation and placement of the electrodes. In addition to the synthetic study, we developed a borehole tool with 20 ring electrodes and applied it in a back-filled trench using a b2s array. The inverted resistivity distribution reproduces the artificial subsurface structure and thus confirms our synthetic results.

Stress state analysis in the vicinity of the hydraulic fracturing tool located near the bottom of the well

ABSTRACT Hydraulic fracturing is a widely used method in the mining industry. In the case of underground workings, the formation of parallel fractures across the borehole axis is of great particular interest for many geotechnical issues, for example, improving the production of coal seam gas or making an impermeable screen in massif to eliminate a water inflow. The article presents a hydofrac tool’s scheme with one inflatable packer promoting the creation of transverse crack. We observe in the laboratory the behaviour of the packer to estimate the displacement of its sliding end when the pressure was applied. These data were used to assess the possibility to form a fracture in the desired direction. In addition, numerical studies of the stress state caused by loading, created with the help of the downhole tool, have been carried out. Analysis of parameters necessary for the formation of an initial crack on the borehole wall is presented. Results show that the using of the borehole tool equipped with an anchoring module allows to create conditions for the appearance of a transverse initiating crack. It has been established that a decrease in the length of the anchor leads to an increase in the tensile axial stresses.

A Comparison of a Conventional Borehole Tool and Distributed Acoustic Sensing at a Dedicated Field Laboratory

Distributed Acoustic Sensing (DAS) uses standard telecommunication fibre optic cables to detect acoustic and seismic signals. The technique utilises optical time-domain reflectometry; a “light-box” measures the light backscattered from a series of laser pulses emitted into the fibre. As seismic waves impinge on the cable, the fibre is strained, causing variations in the time taken for the backscattered light to travel back up the fibre. The acoustic signal can then be reconstructed by analysing phase differences in the backscattered light. DAS is especially suited for VSP applications, as it offers significant efficiency advantages when compared to conventional borehole acoustic sensors. Conventional VSP surveys usually take an extended period to acquire as the tools need to be placed in multiple positions in the well to record data. With DAS, the complete fibre is a sensor, and thus all levels are acquired simultaneously, reducing the cost considerably. In this work, we compare the results of acquiring a VSP survey using a conventional 3-component geophone tool, a cemented fibre-optic cable, and a suspended fibre-optic cable deployed loosely in the well. The VSP data was acquired at both near and far offset points. Results show that the cemented DAS approached the quality of a conventional geophone VSP survey. The suspended DAS data had smaller SNR, while still clearly acquiring up-going and down-going PP-wave reflections.

Hydraulic-fracturing-induced strain and microseismic using in situ distributed fiber-optic sensing

Hydraulic fracturing operations in unconventional reservoirs are monitored using distributed fiber-optic sensing through which physical effects such as temperature, strain, and microseismic activity can be measured. When combined with treatment curves and other reservoir information, these measurements give engineers more data to understand the effectiveness of a treatment program and make future reservoir management decisions. Distributed fiber-optic data are acquired within a borehole that is actively being fractured and is subsequently used as an observation well for treatments of a neighboring well. The large-aperture and finely sampled data acquisition provides a variety of measurements at different resolution scales as changes are induced in the reservoir. Distributed acoustic sensing microseismic events are observed with magnitudes above −2. The spatial distribution of events, up to 500 m from the observation well, allows us to estimate diffusivity and fracturing trends for nearby treatment wells. Microseismic events acquired using a standard three-component borehole tool provide insight into the type of events that can be observed only on one system or simultaneously on both systems. We correlate very low frequency (well below 1 Hz) strain-front behavior with the onset of microseismic events, as they are triggered by pore pressure and fracturing progress throughout the reservoir.

Numerical sensitivity test of three-electrode laterolog borehole tool

Finite element numerical simulation has been carried out to investigate quantitatively the response of the three-electrode laterolog borehole tool (LL3) on radial and vertical heterogeneity of the rock. In order to calculate the apparent resistivity from the electric potential and the current discharge of the measurement electrode the probe coefficient of the LL3 tool with finite electrode extent was determined. Two independent methods, a finite element modeling and a semi-analytical solution, resulted in the probe coefficient of approx. 0.15 m with a relative deviation of 2.4% due to the different geometry, resolution and electronics of the models. It was established that LL3 is only slightly sensitive to the presence of mud when the borehole diameter is d ≤ 30 cm and the ratio of the resistivity of rock and the borehole mud is 1 ≤ R t/R m ≤ 1000. Vertical heterogeneity test pointed out that the layer boundaries can be localized exactly even for thin bedded layer (with a thickness of 1 m) and the presence of low-resistive borehole mud. Correction factors were suggested to decrease the biasing effect of the low-resistive borehole mud and the shoulder beds on the apparent resistivity observed by LL3. Finally, it was verified that the probe has large penetration depth with excellent vertical resolution, what explains the enduring popularity of the LL3 tool in well logging.

Borehole tool for studies in coalbed degasification wells

The paper presents a downhole tool designed for gas-dynamic research to be carried out in coalbed methane drainage holes. Structurally, the tool has a twin-packer design. The tool allows hydraulic fracturing, gas-dynamic investigations using indicator diagrams and pressure drop and recovery curves, and local destressing of coal.

Gravity monitoring at oil and gas fields: data inversion and errors

The present paper considers certain problems of gravity monitoring at oil and gas fields arising due to inversion of repeat measurement data, when finding the positions of water-oil and water-gas contacts. The main sources of noise in gravity data are errors in vertical positioning of the tool, changes in atmospheric pressure, and variations in groundwater level and soil moisture. An algorithm based on using a multisensor borehole tool is proposed for a more accurate inversion. Examples of locating successfully the water front while solving model problems with the help of this algorithm are provided.

Stress Analysis in Boreholes Drag Bh and Leknes Bh, Nordland, North Norway

Nordland in northern Norway is characterized by enhanced seismicity and uplift that makes it the most tectonically active area in Norway. This study is part of a project entitled Neotectonics in Norway—Implications for Petroleum Exploration, which aims at enhancing the understanding of regional-scale stress and strain dynamics in Nordland, and to impact risk and hazard assessment and petroleum exploration. This paper attempts to constrain the orientation of in situ horizontal stress using high-resolution acoustic televiewer logging data. The Geological Survey of Norway has drilled two 0.8 km deep near-vertical boreholes on opposite sides of the Vestfjord in Nordland, the open bight of sea that separates the Lofoten archipelago from the Norwegian mainland. Both boreholes are drilled just North of 68°N, with borehole Leknes Bh located near the geographic center of the Lofoten archipelago, and borehole Drag Bh located on approximate equal distance from the shore, on the Norwegian mainland. The results of this study are in most practical aspects inconclusive, mainly due to poor data quality. The data analysis has revealed erroneously high-borehole diameter, and several artifacts such as eccentric logging tool, rugose borehole wall, spiral hole, tool sticking and missing data. Four intervals with passive in situ stress indicators (borehole breakout and drilling-induced fractures) were found in travel time and amplitude images of the Drag Bh, suggesting approximately N–S orientation of maximum horizontal stress. However, these intervals are not found in cross-plots. Either result yields the lowest World Stress Map ranking quality (E).

Simulation of Logging While Drilling Tools Based on Amorphous Si:H and LaBr3:Ce Detectors

Bore hole logging data represent the properties of rocks, such as porosity, density and constitutiveelements of formation, as a function of depth in a well. Properties of rocks are obtained from thermal neutron counting and gamma ray radiation due to neutron activation. Transport of neutrons, from an Am-Be source situated in a bore-hole tool, through rock media to detectors, has been simulated using a GEANT4 radiation transport code. The high precision GEANT4 cross section library was used to gain better analyses about well formation. In this paper we present the results of simulation of logging tools based on boron coated amorphous silicon detector for thermal neutron detection and LaBr3:Ce detector for gamma spectroscopy. Key words: Well logging; Spectroscopy; GEANT4

Gamma–gamma density and lithology tools simulation based on GEANT4 advanced low energy Compton scattering (GALECS) package

Geophysical bore-hole data represent the physical properties of rocks, such as density and formation lithology, as a function of depth in a well. Properties of rocks are obtained from gamma ray transport logs. Transport of gamma rays, from a 137Cs point gamma source situated in a bore-hole tool, through rock media to detectors, has been simulated using a GEANT4 radiation transport code. The advanced Compton scattering concepts were used to gain better analyses about well formation. The simulation and understanding of advanced Compton scattering highly depends on how accurately the effects of Doppler broadening and Rayleigh scattering are taken into account. A Monte Carlo package that simulates the gamma–gamma well logging tools based on GEANT4 advanced low energy Compton scattering (GALECS).

The effect of starting hole geometry on borehole quality and tool life of twist drills

This paper reports about the relationship between starting hole geometry and twist drill performance, which is determined by the location error and diameter of the drilled borehole as well as the tool life of the employed twist drills. The experimental results lead towards the conclusion that the type of starting hole has a significant effect on borehole quality. This, however, is not as commonly assumed attributed to a variation in the location error of the different starting holes themselves, but to how a starting hole allows the subsequent twist drill to engage in the cutting process. Although twist drills that drill straight into the workpiece material achieved the longest tool life due to a gentle engagement, a deep starting hole (‘pilot hole’) results in the most satisfying overall twist drill performance.

Reliability and limitations of surface NMR assessed by comparison to borehole NMR

ABSTRACTThe measurements of nuclear magnetic resonance (NMR) parameters to investigate petrophysical properties related to fluid (e.g., water) storage and transport processes provide unique insights compared to other geophysical methods and have become a very useful tool for geophysicists during the last decades at the laboratory scale and as a borehole tool.We investigated, at a groundwater test site in the desert of Abu Dhabi, the reliability and limitation of surface NMR, a new but establishing technique that measures the NMR parameter from the surface by comparing its results to borehole NMR logs. Surface NMR or magnetic resonance sounding measurements (MRS) were conducted along a profile, close to several boreholes. The available borehole NMR logs were used to i) evaluate the potential of surface NMR derived results comparing them with borehole NMR measurements and to ii) extend the hydrogeological knowledge of the groundwater site.Firstly, we show how to carefully handle short relaxation signals of surface NMR data. The most significant steps during this process are: i) broad‐band filtering to preserve the short decaying NMR signals, ii) correction for relaxation during pulse effects and iii) QT‐inversion to extract reliable subsurface parameter distribution.By comparing surface NMR results with borehole NMR logs we found the following limitations: i) surface NMR is not able to detect borehole NMR measured water content related to decay times lower than decay time. This reduced detectable water content of surface NMR is due to an instrumental dead time of 40 ms, measured relaxation times and a lower Larmor frequency of 2 kHz and ii) borehole NMR has significantly higher vertical resolution. Taking this into account, surface NMR is in good agreement with borehole NMR.Secondly, on a profile of 1.3 km length 11 MRS measurements were carried out to map the lateral aquifer structure. The obtained results show that surface NMR provides unique lateral information of demanded aquifer properties complementary to e.g., transient electromagnetic.

Numerical modeling of the gamma–gamma density tool responses in horizontal wells with an axial asymmetry

A signal of a spectrometric gamma-gamma density tool in specific borehole conditions has been numerically calculated. Transport of gamma rays, from a point (137)Cs gamma source situated in a borehole tool, through rock media to detectors, has been simulated using a Monte Carlo code. The influence of heterogeneity of the rock medium surrounding the borehole on the signal of the detectors has been examined. This heterogeneity results from the presence of an interface between two different geological layers, parallel to the borehole wall. The above conditions may occur in horizontal logging, when the borehole is drilled along the boundary of geological layers. It is possible to assess the distance from the boundary on the basis of the responses of the gamma-gamma density tool, using the classic interpretation "spine & ribs" procedure. The effect of different densities of the bordered layers on the tool response has been analyzed. The presented calculations show the wide possibilities of numerical modeling of the complex borehole geometry and solving difficult interpretation problems in nuclear well logging.

Capacitive conductivity logging and electrical stratigraphy in a high-resistivity aquifer, Boise Hydrogeophysical Research Site

We tested a prototype capacitive-conductivity borehole tool in a shallow, unconfined aquifer with coarse, unconsolidated sediments and very-low-conductivity water at the Boise Hydrogeophysical Research Site (BHRS). Examining such a high-resistivity system provides a good test for the capacitive-conductivity tool because the conventional induction-conductivity tool (known to have limited effectiveness in high-resistivity systems) did not generate expressive well logs at the BHRS. The capacitive-conductivity tool demonstrated highly repeatable, low-noise behavior but poor correlation with the induction tool in the lower-conductivity portions of the stratigraphy where the induction tool was relatively unresponsive. Singular spectrum analysis of capacitive-conductivity logs reveals similar vertical-length scales of structures to porosity logs at the BHRS. Also, major stratigraphic units identified with porosity logs are evident in the capacitive-conductivity logs. However, a previously unrecognized subdivision in the upper portion of one of the major stratigraphic units can be identified consistently as a relatively low-conductivity body (i.e., an electrostratigraphic unit) between the overlying stratigraphic unit and the relatively high-conductivity lower portion — despite similar porosity and lithology in adjacent units. The high repeatability and resolution and the wide dynamic range of the capacitive-conductivity tool are demonstrated here to extend to high-resistivity, unconsolidated sedimentary aquifer environments.

A comparative study of borehole size and tool effect on dispersion curves

Sonic wave dispersion characteristics are one of the most important targets of study, particularly in estimating shear wave velocity from borehole sonic logging. We have tested dispersion characteristics using monopole and dipole sources. Theoretical dispersion curves were computed for tool-absent and tool-included models having the same physical properties but different diameters (including Φ520 mm, Φ150 mm, and Φ76 mm). Comparisons were made between boreholes of different sizes and between tool-absent and tool-included models. Between the tool-included and the tool-absent boreholes, a close similarity in dispersion curve shape was revealed for the monopole source, and a significant difference was shown for the dipole source. However, for the cut-off frequency, particularly in the engineering boreholes (Φ76 mm and Φ50 mm), a significant difference was observed for signals from the monopole source, but approximately the same cut-off frequencies were found with the dipole source. This indicates the need of careful choice of source frequency in monopole-source sonic logging, particularly in an engineering borehole.The results of numerical experiments show that cut-off frequency is exponentially proportional to the inverse of borehole radius, irrespective of the mode type and the presence of a tool, and that the cut-off frequencies for each borehole environment could be expressed as an exponential function, rather than the inversely proportional relationship between the cut-off frequency and the borehole radius that was previously generally recognised. From the direct comparison of dispersion curves, the effects on the dispersion characteristics of borehole size and the presence of the tool can be revealed more clearly than in previous studies, which presented the dispersion curve and/or characteristics for each borehole environment separately.

Non‐contact infrared temperature measurements in dry permafrost boreholes

While planning the COAST Expedition to the Siberian Laptev Sea in 2005, the question of how to make a short equilibrium temperature measurement in a dry borehole arose. As a result, an infrared borehole tool was developed and used in three dry boreholes (up to 60.2 m deep) in the coastal transition zone from terrestrial to sub‐sea permafrost near Mamontovy Klyk in the western Laptev Sea. A depth versus temperature profile was acquired with equilibration times of 50 × 10−3 s at each depth interval. Comparison with a common resistor string revealed an offset due to limitations of accuracy of the infrared technique and the influence of the probe's massive steel housing. Therefore it was necessary to calibrate the infrared sensor with a high precision temperature logger in each borehole. The results of the temperature measurements show a highly dynamic transition zone with temperature gradients up to −0.092°C/m and heat flow of −218 mW/m. A period of submergence of only 600 years the drilled sub‐sea permafrost is approaching the overlying seawater temperature at −1.61°C with a temperature gradient of 0.021°C/m and heat flow of 49 mW/m. Further offshore, 11 km from the coastline, a temperature gradient of 0.006°C/m and heat flow of 14 mW/m occur. Thus the sub‐sea permafrost in the Mamontovy Klyk region has reached a critical temperature for the presence of interstitial ice. The aim of this article is to give a brief feasibility study of infrared downhole temperature measurements and to present experiences and results of its successful application.

Excellent synthetic seismograms through the use of edited logs: Lake Borgne Area, Louisiana, U.S.

Information of great value for seismic processing and interpretation can be gleaned from synthetic seismogram ties and their associated well-based wavelet extractions, but only if the well data are excellent. If the well data are not very closely and correctly edited, wrong conclusions are likely. Synthetic seismograms often fail to tie with surface seismic data because they are made from well logs distorted by borehole washouts, formation damage, tool miscalibrations, digitizing or database errors, and many other difficulties. These difficulties may be overcome by editing the logs, especially if the editing method is multizone and multiwell, and uses regression rather than neural nets or other curve-fitting techniques. Box and Lowrey show that one such method, called log purification (described below) detects most of the errors in sonic and density logs, and replaces them with realistic computed values, bringing them into harmony with velocities measured by check-shot surveys. Here we show that logs edited in the same manner produce synthetic seismograms which match the seismic data, allowing us to analyze the true phase and frequency content of the seismic data. This paper compares a South Louisiana well seismically analyzed in its recorded form to the same well analyzed in its edited form. The recorded logs contain enough “noise” to blur the wavelet extraction process, making it impossible to conclude much about the seismic data processing. One could incorrectly conclude from the results that the seismic polarity is reversed. The edited logs contain very little noise, making it easy to create robust wavelet extractions and excellent synthetic seismograms, yielding clear, valuable insight about the wavelet processing of the seismic data. The well used in this study is the Texaco 1 SL 9595 OKC. It is located east of New Orleans, Louisiana, on the southeast shoreline of Lake Borgne in St. Bernard Parish …

Test measurements with a new thermal conductivity borehole tool

A thermal conductivity tool for the pilot hole of the Continental Deep Drilling Program in the Federal Republic of Germany (KTB) was developed and constructed for depths up to 4000 m. The concept is based on the theory of a cylindrical heat source. First measurements in a shallow borehole show good agreement with laboratory measurements on cores.