Downhole Tests Research Articles

Advances in Study of Mechanical Properties of Gas Hydrate-Bearing Sediments

Gas hydrate (GH) is defined as the crystalline solid, or clathrate hydrate, which are formed by some kinds of low mass molecular gases, such as methane, carbon dioxide, and hydronitrogen, with water at relatively high pressure and low temperature conditions. Gas hydrate-bearing sediments (HBS) are some sand, clay and mixed sediment containing gas hydrates. Advances in the study on the mechanical properties of HBS are summarized mainly in aspects of the laboratory test, in-situ investigation, and theoretical model. Firstly, the main factors are discussed including the structure of GH, formation method and matrix characteristics of HBS; Secondly, progress on the laboratory tests and results are discussed, which mainly includes the tri-axial tests with the natural and synthesized HBS samples, the acoustic tests for measuring the elastic coefficients, the tests for investigating the effects of main factors such as the gas and water contents and soil types on the strength of HBS. Thirdly, for in-situ investigations, including the geophysical surveying, in-situ tests (such as downhole tests) and results are summarized; Fourthly, several theoretical models for estimating the mechanical properties of HBS are introduced; At last, the emphases and the tendency in the future study on the mechanical properties of HBS are discussed.

Spatial Distribution of Shear Wave Velocity for Late Quaternary Alluvial Soil of Kanpur City, Northern India

Empirical correlation between standard penetration resistance (SPT-N) and shear wave velocity measured by seismic downhole techniques are prepared of the alluvial soil of quaternary age for the Kanpur city. The Kanpur city is having seismic threat from Himalaya and it falls in seismic zone III according to seismic zones of India. Standard penetration test as well seismic downhole test has been carried out up to 30 m at twelve different locations of Kanpur city. The measured SPT-N values and shear wave velocity values are used to develop empirical correlation between SPT-N and shear wave velocity. The proposed correlations have been compared with the existing regression equations by various other investigators. It is found that the proposed correlation exhibit good performance (10 % error bar). Also the measured shear wave velocity has been used to prepare spatially distributed contour map of 50, 75 and 100 m/s using ArcGIS-9 software. It is observed that the shear wave velocity values for the northern part of Kanpur city vary from 125 to 825 m/s. In southern part, it is varying from 125 to 500 m/s where as in the central part of the city the shear wave velocity varies from 125 to 375 m/s. The eastern part of the city also shows some variation in shear wave velocity which ranges from 250 to 625 m/s. The western part of the city shows the variation of shear wave velocity from ≤125 to 500 m/s. The soil type of the study area are classified as per NEHRP and new Italian O.P.M.C classification system as B, C and D type soil with having site period of 0.1–0.9 s and Poisson’s ratio varying from 0.1 to 0.4.

Two-dimensional imaging of soil–bedrock interface by short-array beamforming technique

The depth to soil–bedrock interface, which is one of the major parameters in the site response analysis, has been often investigated by surface-wave tests. The round-robin tests for a surface-wave method in Korea revealed that a long measurement array in surface-wave tests is not appropriate in locating soil–bedrock interface. In this paper, for the improved profiling of depth to soil–bedrock interface in 2-D image, short measurement array was introduced for the beamforming technique, which is a robust array processing technique adopted in a long-array format for stiffness profiling. Numerical simulation and field applications of the short-array beamforming technique indicate that the method is valid even for surface wave propagation with mode-related complexity. Depth to soil–bedrock interface and shear-wave velocity profiles determined by the short-array beamforming technique were in good agreement with layer stratifications of boring logs, resistivity map, shear-wave velocity profiles of downhole tests and CAP-SASW tests.

Post-Processing Continuous Shear Wave Signals Taken During Cone Penetrometer Testing

Abstract Continuous seismic velocity measurements use a special automated wave source and advanced post-processing analyses to provide fast, detailed, and reliable profiles of shear wave velocity (Vs) with depth. Conventional geophysical techniques such as crosshole tests (CHTs) and downhole tests (DHTs) in boreholes are slow because they have several required steps: (1) rotary drilling, (2) installation of casing and grouting, (3) inclinometer measurements (for CHTs), and (4) deployment of geophones for seismic readings. Direct-push technologies include the use of seismic cones and seismic dilatometers that offer DHT-type Vs data at intervals of 1 m or less without the need for drilling, casing, grouting, or separate field events. The recent development of a new portable autoseis source allows the generation of reliable and consistent shear waves either intermittently or as frequently as every 1 to 10 s. Continuous shear wave measurements can provide improved detailing of the small-strain stiffness (G0) at frequent depth intervals and fast field production times. Appreciable sensitivity errors in Vs calculations can be experienced because of the extremely small time shifts between adjacent shear wave records, as well as significant signal noise due to vibration, external sources, and refracted waves. This paper details continuous-interval seismic piezocone testing and explains how to handle signal post-processing in both the time domain and the spectral frequency domain in order to obtain a reliable in situ Vs profile.

Reservoir Testing System Evolves to Meet New Frontier Challenges

Technology Update As today’s deep wells challenge the bounds of pressure and temperature, accurate reservoir testing has become essential to mitigate production risk. The information contained in a series of downhole pressure transient measurements and representative reservoir fluid samples can make or break a prospect’s potential. The consistent and accurate acquisition of this information requires the precision instrumentation and functionality of a drillstem testing system. Whereas excellent downhole logging instruments can describe the near-borehole regions, only a drillstem test can reach deeply into the reservoir to discover boundaries, faults, and fractures; confirm volumetric reserves estimations; and acquire the cleanest, most representative fluid samples to provide accurate fluid phase behavior, physical properties, and com-positional analyses. Safety, Reliability, and Efficiency The Schlumberger Quartet high-performance downhole reservoir testing system combines four proven downhole technologies into a package that can isolate the zone, control flow, measure pressure and temperature, and acquire multiple single-phase reservoir fluid samples on a single trip into the well. The system provides enhanced safety, increased efficiency, and improved reliability to testing operations. Compared with conventional downhole test strings, the system functions with a 35% lower tool operating pressure, 50% fewer seals, and 60% fewer connections, while requiring 90% less nitrogen. The system is four times shorter than a conventional tool string because it eliminates the requirement for a safety joint, jar, slip joints, and drill collars. Despite its smaller size, the system is sufficiently durable to permit perforating guns to be run below the packer and fired without incurring damage to the tools or impairing their accuracy or integrity. The system also includes a below-packer circulating valve to allow a more efficient well kill following the test, which is especially important when testing gas wells. Each of the four tools that make up the downhole reservoir testing system operates individually and collectively to perform the test. The CERTIS high-integrity reservoir test isolation system delivers production-quality well isolation. The isolation system combines the features of a retrievable drillstem test packer with a hydraulically set permanent packer, making the safety joint, jar, slip joints, and drill collars unnecessary. Because the system is actuated by annulus pressure, no rotational or set down forces are required.

대형 사력댐에서의 전단파속도 평가를 위한 단측선 빔형성기법

대형 사력댐의 내진성능 평가에서 필히 요구되는 입력상수는 사력재료, 코아매질의 전단파 속도이다. 이를 표면파 시험으로 평가하기 위해서는 사력골재의 불연속, 매질의 비균질, 사면 경계면 등 표면파 시험결과의 신뢰도를 떨어뜨리는 조건을 극복해야 한다. 본 연구에서는 이러한 표면파시험의 한계를 극복하기 위하여 기존 빔형성기법의 원리를 응용한 SBF (Short-Array Beamforming) 기법을 제안하였다. SBF 기법은 3~9 m의 짧은 측선과 원거리 발진원을 이용함으로써, 빔형성기법 고유의 장점인 측정자료의 자동화분석뿐만 아니라 근접장 문제의 해결, 국부적 이상대의 발견 등의 기능을 가지도록 개발되었다. 본 연구에서는 이러한 SBF 기법과 IRF(Impulse-Response Filtration) 기법을 활용하여 대형 사력댐의 전단파속도를 신뢰성 있게 평가하는 방법을 정립하였다. 정립된 기법은 사력댐의 사력재료와 유사한 암버럭으로 매립 성토된 철도 노반에서 다운홀 시험, CapSASW (Common-Array-Profiling SASW) 시험과의 비교를 통하여 그 신뢰성과 실용성을 검증하였다. One of the input parameters in the evaluation of seismic performance of rockfill dams is shear-wave velocity of rock debris and clay core. Reliable evaluation of shear-wave velocity by surface-wave methods requires overcoming the problems of rock-debris discontinuity, material inhomogeneity and sloping boundary. In this paper, for the shear-wave velocity investigation of rockfill dams, SBF (Short-Array Beamforming) technique was proposed using the principles of conventional beamforming technique and adopted to solve limitations of the conventional surface-wave techniques. SBF technique utilizes a 3- to 9-m long measurement array and a far-field source, which allowed the technique to eliminate problems of near-field effects and investigate local anomalies. This paper describes the procedure to investigate shear-wave velocity profile of rockfill dams by SBF technique and IRF (Impulse-response filtration) technique with accuracy and reliability. Validity of the proposed SBF technique was verified by comparisons with downhole tests and CapSASW (Common-Array-Profiling Spectral-Analysis-of-Surface-Waves) tests at a railroad embankment compacted with rock debris.

Wellbore Permeability Estimates from Vertical Interference Testing of Existing Wells

Wellbore integrity is considered an important risk factor for leakage of CO2 and formation fluids out of geological CO2 storage sites. Quantifying the effective hydraulic parameters that control vertical migration of fluids along the wellbore involves data collection through numerous field and laboratory experiments. The vertical interference test (VIT) is a downhole test designed to measure hydraulic communication of the outside-of-casing wellbore barrier system over a selected well section. Results from these tests can be analyzed numerically to determine the average permeability of the section. Several field surveys of existing wells have resulted in 9 VIT datasets, of which three are presented here. The effective permeability estimates for the three tests span two orders of magnitude, from approximately 1 mD to more than 100 mD. When compared with companion sidewall core analyses of the cement matrix that have permeabilities in the microD range, the VIT data suggest that interfaces or defects in the cement sheath are responsible for flow. Initial analysis of the remaining 6 datasets suggests an even larger range in effective permeability values, as low as microD to more than 1 D, indicating that well permeability can be highly variable from well to well and that high values of permeability are possible. These data provide important insights into realistic wellbore integrity of typical wells in N. America, and help us constrain models for understanding and mitigating risk of leakage during CO2 storage operations.

A Comparative Study on Borehole Seismic Test Methods for Site Classification

In this study, crosshole seismic test, donwhole seismic test, SPT uphole test, and suspension PS logging (SPS logging) were conducted and the shear wave velocities of these tests were compared. The test demonstrated the following result: Downhole tests showed similar results compared to those of crosshole tests, which is known to be relatively accurate. SPS logging showed reliable results in the case of no casing, i.e. in the rock mass, while, in the case of soil ground, its values were lower or higher than those of other tests. SPT-uphole tests showed similar results in the soil ground and upper area of rock mass compared to other methods. However, reliable results could not be obtained from these tests because SPT sampler could not penetrate into the rock mass for the tests.

Topographic effects on the Monte Po Hill in Catania (Italy)

The study regards an evaluation of site effects on and near the Monte Po hill, located in the north-eastern part of the city of Catania (Italy), an area at high seismic risk. At the beginning of 2007 a seismic station was located in a school building, situated at the slope toe, but no seismic events have been recorded as yet. Therefore, synthetic seismograms have been used to evaluate the ground response analysis at the surface. Because the average slope is moderate (less than 15°), 1-D computer codes have been used to model the equivalent-linear earthquake site response analyses of layered hill deposits, as generally performed by professionals. However, the slope response has also been analysed in greater detail, using a 2-D computer code and the soil characterisation has been evaluated accurately by means of borings, Down-Hole tests, SDMT tests and laboratory tests. Comparing 1-D with 2-D results the stratigraphic site amplification and the Topographic Aggravation Factor (TAF) have also been computed. The aim of the study is that it will form a basis for the design of works to remediate the damage caused by a landslide reactivated by the earthquake in Eastern Sicily on December 13, 1990 (ML=5.4).

Evaluating the In Situ Lateral Stress Coefficient (K0) of Soils via Paired Shear Wave Velocity Modes

AbstractThe utilization of shear wave velocities toward the evaluation of the in situ geostatic horizontal stress state in soils is validated, specifically the lateral stress coefficient K0. Field shear wave velocities from paired sets of different directional and polarization modes are compiled from 16 well-documented test sites involving a variety of geomaterials. Focus is particularly placed on shear wave velocities measured by downhole tests (VsVH), crosshole tests (VsHV), and special rotary-type crosshole tests (VsHH). At these sites, field K0 stress states have been quantified using one or more direct assessment techniques, including self-boring pressuremeter, total stress cells, and hydrofracture in field testing, as well as suction measurements, special consolidometers, and/or triaxial arrangements on undisturbed samples in the laboratory. Although the specific delineation of stress-induced versus inherent or fabric anisotropy may be difficult, it is shown that the ratio of horizontally polarized ...

Developing a System for Down-Hole Seismic Testing Together with the CPTU

. This paper presents a system for performing down-hole seismic test together with the piezocone test in order to determine the shear wave velocity (Vs) and for calculating the maximum shear modulus (Go); a basic parameter for analyzing the dynamic soil behavior and a reference value of the soil stiffness. The system components are described and tests results for checking the geophone response are also presented, both before and after installation into the probe. The system was used in down-hole tests carried out at three experimental research sites located in the interior of Sao Paulo State, Brazil, where in situ seismic test results are available. The Vs values measured in down-hole tests carried out with this system were consistent with those determined in cross-hole tests and with a commercial seismic piezocone, which enabled to validate the developed system.

Focused inversion of vertical radar profile (VRP) traveltime data

The reconstruction of the GPR velocity vertical profile from vertical radar profile (VRP) traveltime data is a problem with a finite number of measurements and imprecise data, analogous to similar seismic techniques, such as the shallow down-hole test used for S-wave velocity profiling or the vertical seismic profiling (VSP) commonly used in deeper exploration. The uncertainty in data accuracy and the error amplification inherent in deriving velocity estimates from gradients of arrival times make this an example of an ill-posed inverse problem. In the framework of Tikhonov regularization theory, ill-posedness can be tackled by introducing a regularizing functional (stabilizer). The role of this functional is to stabilize the numerical solution by incorporating the appropriate a priori assumptions about the geometrical and/or physical properties of the solution. One of these assumptions could be the existence of sharp boundaries separating rocks with different physical properties. We apply a method based on the minimum support stabilizer to the VRP traveltime inverse problem. This stabilizer makes it possible to produce more accurate profiles of geological targets with compact structure. We compare more traditional inversion results with our proposed compact reconstructions. Using synthetic examples, we demonstrate that the minimum support stabilizer allows an improved recovery of the profile shape and velocity values of blocky targets. We also study the stabilizer behavior with respect to different noise levels and different choices of the reference model. The proposed approach is then applied to real cases where VPRs have been used to derive moisture content profiles as a function of depth. In these real cases, the derived sharper profiles are consistent with other evidence, such as GPR zero-offset profiles, GPR reflections and known locations of the water table.

The Measurement and Analysis of Depth in Archaeological Geophysics: Tests at the Biesterfeldt Site, USA

ABSTRACTInformation on depth may be gained through the analysis of geophysical maps, by utilizing soundings, pseudosections, or time‐slices, or via the application of downhole measurements. Various methods and techniques were compared at the Biesterfeldt site, a proto‐historic earth‐lodge village in the Northern Plains, USA. Both traditional and more experimental approaches were used, including ground‐penetrating radar time slices and overlay analyses, a resistivity pseudosection, two‐ and three‐dimensional magnetic and resistivity modelling, and downhole tests of magnetic susceptibility, magnetic viscosity, total magnetic field, capacitance and resistivity. A direct‐push colour system, measuring the reflection of light over the red–green–blue (RGB) and near‐infrared spectrum, provided another useful tool for mapping the depths of archaeological features and stratigraphic layers.Early historic maps of the site show more than 60 earth lodges and an encircling ditch. Current management efforts aim not only to georeference and evaluate the accuracy of these maps, but to determine the state of preservation of the archaeological features. These efforts are complicated by historic cultivation of the southern half of the site. Geophysical tests focused on depth yielded locations and depths for several lodges and the filled ditch as well as small and thin features and layers within them. A buried soil was also identified. Down‐hole geophysical and direct‐push color techniques were particularly helpful for mapping lodges in the plowed portion of the site. These results were confirmed through excavation. Adding geophysical tests focused on depth at Biesterfeldt provided information useful for understanding feature formation, context, and post‐depositional processes, and for designing effective at‐surface geophysical surveys, planning excavations and site management. These tests highlight the advantages of integrating techniques focused on depth with geophysical area surveys. Copyright © 2011 John Wiley & Sons, Ltd.

Shear wave Velocities for Sample Quality Assessment on a Residual Soil

For the assessment of the quality of laboratory samples, a number of methods are available, though not universally applicable to any soils. This paper examines the issue of sampling quality and its assessment using comparisons between shear wave velocity measurements in situ and in recovered samples as the base, which is very useful in naturally structured granular soils, like residual soils. For this purpose, cross-hole and down-hole tests were performed in the field and bender elements measurements were made on triaxial samples collected from two experimental sites on residual soil from Porto granite. Various sampling techniques and tools were used, including block sampling and different tube samplers. The analysis of the results has led to a new classification of sampling quality and sample condition based on the comparison of normalised shear wave velocities in the field and in the laboratory.

Peculiar earthquake damage on a reinforced concrete building in San Gregorio (L’Aquila, Italy): site effects or building defects?

In San Gregorio (L’Aquila, Italy) a three-story, reinforced concrete (RC) building had the first floor collapsed following the earthquake of April 6, 2009. The remaining two stories fell with a displacement in the horizontal projection of about 70 cm. This unusual behaviour is made more puzzling by the fact that buildings located at a short distance and with similar features had little or no damage reported. To understand the causes of the collapse we performed strong motion recordings, microtremor measurements, a detailed geological survey, a high-resolution geo-electrical tomography, a borehole with a down-hole test. On the building we performed a geometrical survey and laboratory tests on concrete cores. The acceleration and noise recordings have shown a high amplification with uphill-downhill direction. The geological survey has revealed the presence of co-seismic fractures on stiff soils. Geo-electrical tomography has shown an unexpected, strong discontinuity just below the building. Taking advantage of excavations in adjacent lots, we have highlighted rare cataclastic decimetric bands with a very low resistance material incorporated in well-stratified calcarenites. The same soft material has been founded in the borehole down to 17 m from ground level, showing a shear wave velocity that starts at 250 m/s, increases with depth and has an abrupt transition in calcarenites at 1,150 m/s. The surface geophysical measurements in the vicinity of the site have not shown similar situations, with flat HVSR curves as expected for a rock outcrop, except for a lateral extension of the soft zone. The analysis on the quality of the building materials has yielded values higher than average for the age and type of construction, and no special design or construction deficiencies have been observed. A strong, peculiar site effect thus appears to be the most likely cause of the damage observed.

Prediction of Shear Wave Velocity in Underground Layers Using SASW and Artificial Neural Networks

This research aims at improving the methods of prediction of shear wave velocity in underground layers. We propose and showcase our methodology using a case study on the Mashhad plain in north eastern part of Iran. Geotechnical investigations had previously reported nine measurements of the SASW (Spectral Analysis of Surface Waves) method over this field and above wells which have DHT (Down Hole Test) result. Since SASW utilizes an analytical formula (which suffers from some simplicities and noise) for evaluating shear wave velocity, we use the results of SASW in a trained artificial neural network (ANN) to estimate the un- known nonlinear relationships between SASW results and those obtained by the method of DHT (treated here as real values). Our results show that an appropriately trained neural network can reliably predict the shear wave velocity between wells accurately.

Automated instrumentation to measure rock mass stresses in parallel-drilled holes

The prototype of a new-developed program-instrumentation system provides automated measurement and processing of data on stresses in a rock mass by employing the parallel-hole method. The respective instrumentation and the procedure for execution of tests on stress evaluation within a rock mass are also briefed on. Recorder software provides the prompt control of the entire process with visualized parameters, thus contributing to the proper decision-making with account for experimental conditions and expediency to go on with downhole tests.

Shallow subsurface geology and Vs characteristics of sedimentary units throughout Rasht City, Iran

The Manjil-Rudbar earthquake of June 1990 caused widespread damage to buildings in the city of Rasht located 60 km from the epicenter. Seismic surveys, including refraction P-wave, S-wave and downhole tests, were carried out to study subsurface geology and classify materials in the city of Rasht. Rasht is built on Quaternary sediments consisting of old marine (Q1m), deltaic (Q2d), undivided deltaic sediments with gravel (Qdg) and young marine (Q2m) deposits. We used the variations of Vp in different materials to separate sedimentary boundaries. The National Earthquake Hazard Reduction Program (NEHRP) scheme was used for site classification. Average S-wave velocity to a depth of 30 m was used to develop site categories, based on measured Vs values in 35 refraction seismic profiles and 4 downhole tests. For each geological unit histograms of S-wave velocity were calculated. This study reveals that the Vs(30 of most of the city falls into categories D and C of NEHRP site classification. Average horizontal spectral amplification (AHSA) in Rasht was calculated using Vs(30 . The AHSA map clearly indicates that the amplification factor east and north of the city are higher than those of south and central parts. The results show that the lateral changes and heterogeneities in Q1m sediments are significant and most damaged buildings in 1990 Manjil earthquake were located in this unit.

Seismic characterization of an Alpine site

ABSTRACTSite amplification of earthquake ground motion can arise from local stratigraphic and morphological conditions. In this respect, a detailed geo‐mechanical 2D model is needed, in particular for peculiar situations such as the ones in Alpine valleys. This paper reports a site characterization case history for seismic response analysis, where a combination of both body and surface wave seismic techniques has been used. In particular, it is shown that synergies and mutual cross‐checking of information lead to a reliable 2D model for site amplification studies.The evaluation of the deep structure and the location of the seismic bedrock have been obtained through seismic reflection and combined active‐passive surface wave analysis, while the shear‐wave velocity profile of the sediments has been obtained by downhole measurements and surface wave tests. Furthermore, a detailed compressional wave velocity model has been retrieved by tomographic inversion of the first arrivals picked on seismic reflection records. This model is useful for geological interpretation as well as for quality control of the other methods. Surface wave analysis has also been performed on the ground roll of the seismic reflection records. The large amount of profile oriented data has made it possible to reconstruct lateral variations of the shear‐wave velocity in the soil deposit. The final pseudo‐2D shear‐wave velocity model is the product of an innovative inversion approach, based on the integration of the Monte Carlo and laterally constrained least‐squares techniques. In the laterally constrained inversion the shear‐wave velocity profiles obtained from downhole tests and data from passive surface wave measurements have been used to constrain the inversion. This has greatly improved the final solution.This case history underlines the necessity of combining different seismic techniques to derive an accurate and reliable 2D model for site amplification studies. Through several different and independent analyses of the same seismic records, the cost effectiveness of the whole survey is optimized and a full exploitation of the information contained in the reflection seismic data set is achieved.

Development of an apparatus for down-hole triaxial tests in a rock mass

Development of an apparatus for down-hole triaxial tests in a rock mass