Borehole Televiewer Logs Research Articles

Metre-scale damage zone characterization usingS-coda waves from active ultrasonic transmission measurements in the STIMTEC project, URL Reiche Zeche, Germany

SUMMARYStudies of controlled hydraulic stimulation experiments with active and passive seismic monitoring conducted in Underground Research Laboratories (URLs) benefit from specific knowledge of hydraulic parameters, close by microseismic monitoring revealing structural details of the rock mass, and detailed evolution of seismicity in response to injection operations. Microseismic monitoring is commonly used to characterize a stimulated reservoir volume, for example, in terms of damage evolution of the rock mass. Since seismic attenuation is affected by damage of the rock volume, active seismic sources covering sizes from the centimetre to decimetre scale may help us to investigate space–time varying attenuation properties in a reservoir. This may allow us to monitor damage evolution of the stimulated rock volume in more detail, also since active seismic sources produce stronger signals leading to a broader frequency range that can be analysed compared to passive seismic signals. Within the STIMTEC project in the URL Reiche Zeche (URL-RZ) in Freiberg (Germany), more than 300 active Ultrasonic Transmission (UT) measurements were performed before and after hydraulic stimulations in two boreholes in the targeted rock volume, an anisotropic metamorphic gneiss. The signal-frequency content ranges between 1 and 60 kHz. Assuming scattering attenuation to dominate over intrinsic attenuation, we here apply the single isotropic scattering model. S-coda waves of 88 spatially representative UT measurements are used to estimate the coda quality factor (QC). We obtain stable QC estimates for centre frequencies of octave-width frequency bands between 3 and 21 kHz. We group neighbouring UT measurements to stabilize the observations and form eight UT groups in total, covering different depth intervals in three boreholes and four different time periods to investigate scattering attenuation changes in a spatiotemporal manner. Our final mean QC ($\overline {{Q}_C} $) estimates show characteristic frequency-dependence as observed at the field scale in geological reservoirs. We find temporal variations of QC are strongly connected to hydraulic stimulation, and these variations are more significant than those resolved from velocity changes. $\overline {{Q}_C} $ estimates at frequencies above 15 kHz indicate healing of injection-induced small-scale fractures during a two-months post-stimulation phase. Larger fractures, mostly sampled by lower frequencies (<15 kHz), seem to be more persistent with time (over 15 months). We observe spatial differences of $\overline {{Q}_C} $ values near the mine galleries (driftway and vein drift) and relate these observations to different extents and characteristics of the galleries’ excavation damage zones. Our results further support previous assumptions based on borehole televiewer logs and mapped structures of an existing fault with larger damage zone that crosses the stimulated rock volume NW-SE between the galleries. We conclude that the coda analysis of active UT measurements complements established imaging methods used during experiments in URLs. In particular, coda analysis is a powerful tool for the detection of damage zones and for monitoring local fracture networks with immediate application for imaging georeservoirs considered for exploitation or underground storage of gases and liquids.

Investigating fault zone hydraulic properties and groundwater potential in a fault-dominated aquifer system: A case study of the Dili fault in Central Taiwan

Discovering groundwater resources in a fault-controlled aquifer system appears to be a viable solution to water scarcity in the mountainous areas of Taiwan. Regoliths, fractured bedrock, and faults commonly exist in such aquifer systems. To investigate the groundwater behavior in the three subsurface media, a 3-D hybrid model (equivalent porous media/discrete fracture network) involving various in situ investigation techniques (borehole drilling, outcrop investigation, borehole televiewer logging, sonic logging, and downhole hydraulic tests) was proposed to assess the hydraulic properties and groundwater storage of the Dili fault zone of Central Taiwan and its surrounding formations. Subsequently, the model was verified through a case study of a simple fracture network. Based on the validated model, flow path analysis was successfully performed to reveal the hydrogeological role of the fault zone, which acts as a conduit for groundwater flow. To ensure the simulated result regarding the hydraulic properties of the fault zone, this study proposes three additional cross-checking techniques, namely stable water isotopes, derivative plots of pumping test data, and identification of hydraulic gradients. The results of the investigation show that the hydraulic properties of the fault zone obtained by the three approaches are consistent with those determined by the numerical method, and the use of the three cross-checking techniques has been confirmed to be suitable for effectively exploring the hydraulic properties of a fault because the site characterization data are insufficient. Finally, the developed hybrid model was used to calculate the groundwater storage of each geological zone, indicating its great potential for the development of groundwater resources in the study area. Therefore, this study integrates in situ investigation techniques, numerical models, and three evaluation approaches of fault zone hydrogeology to construct a systematic, feasible, and applicable exploration methodology that can be employed for groundwater resource exploration in a fault-dominated aquifer system.

Statistical methods of fracture characterization using acoustic borehole televiewer log interpretation

AbstractAcoustic borehole televiewer (BHTV) logs provide measurements of fracture attributes (orientations, thickness, and spacing) at depth. Orientation, censoring, and truncation sampling biases similar to those described for one‐dimensional outcrop scanlines, and other logging or drilling artifacts specific to BHTV logs, can affect the interpretation of fracture attributes from BHTV logs. K‐means, fuzzy K‐means, and agglomerative clustering methods provide transparent means of separating fracture groups on the basis of their orientation. Fracture spacing is calculated for each of these fracture sets. Maximum likelihood estimation using truncated distributions permits the fitting of several probability distributions to the fracture attribute data sets within truncation limits, which can then be extrapolated over the entire range where they naturally occur. Akaike Information Criterion (AIC) and Schwartz Bayesian Criterion (SBC) statistical information criteria rank the distributions by how well they fit the data. We demonstrate these attribute analysis methods with a data set derived from three BHTV logs acquired from the high‐temperature Rotokawa geothermal field, New Zealand. Varying BHTV log quality reduces the number of input data points, but careful selection of the quality levels where fractures are deemed fully sampled increases the reliability of the analysis. Spacing data analysis comprising up to 300 data points and spanning three orders of magnitude can be approximated similarly well (similar AIC rankings) with several distributions. Several clustering configurations and probability distributions can often characterize the data at similar levels of statistical criteria. Thus, several scenarios should be considered when using BHTV log data to constrain numerical fracture models.

Evidence for tectonic, lithologic, and thermal controls on fracture system geometries in an andesitic high‐temperature geothermal field

AbstractAnalysis of fracture orientation, spacing, and thickness from acoustic borehole televiewer (BHTV) logs and cores in the andesite‐hosted Rotokawa geothermal reservoir (New Zealand) highlights potential controls on the geometry of the fracture system. Cluster analysis of fracture orientations indicates four fracture sets. Probability distributions of fracture spacing and thickness measured on BHTV logs are estimated for each fracture set, using maximum likelihood estimations applied to truncated size distributions to account for sampling bias. Fracture spacing is dominantly lognormal, though two subordinate fracture sets have a power law spacing. This difference in spacing distributions may reflect the influence of the andesitic sequence stratification (lognormal) and tectonic faults (power law). Fracture thicknesses of 9–30 mm observed in BHTV logs, and 1–3 mm in cores, are interpreted to follow a power law. Fractures in thin sections (∼5 μm thick) do not fit this power law distribution, which, together with their orientation, reflect a change of controls on fracture thickness from uniform (such as thermal) controls at thin section scale to anisotropic (tectonic) at core and BHTV scales of observation. However, the ∼5% volumetric percentage of fractures within the rock at all three scales suggests a self‐similar behavior in 3‐D. Power law thickness distributions potentially associated with power law fluid flow rates, and increased connectivity where fracture sets intersect, may cause the large permeability variations that occur at hundred meter scales in the reservoir. The described fracture geometries can be incorporated into fracture and flow models to explore the roles of fracture connectivity, stress, and mineral precipitation/dissolution on permeability in such andesite‐hosted geothermal systems.

Heterogeneity of structure and stress in the Rotokawa Geothermal Field, New Zealand

AbstractGeometric characterization of a geothermal reservoir's structures, and their relation to stress field orientation, is vital for resource development. Subsurface structure and stress field orientations of the Rotokawa Geothermal Field, New Zealand, have been studied, for the first time, using observations obtained from analysis of three acoustic borehole televiewer logs. While an overall NE‐SW fracture strike exists, heterogeneity in fracture dip orientation is evident. Dominant dip direction changes from well to well due to proximity to variously oriented, graben‐bounding faults. Fracture orientation heterogeneity also occurs within individual wells, where fractures clusters within certain depth intervals have antithetic dip directions to the well's dominant fracture dip direction. These patterns are consistent with expected antithetic faulting in extensional environments. A general SHmax orientation of NE‐SW is determined from induced features on borehole walls. However, numerous localized azimuthal variations from this trend are evident, constituting stress field orientation heterogeneity. These variations are attributed to slip on fracture planes evidenced by changes in the azimuth of drilling‐induced tensile fractures either side of a natural fracture. Correlation of observed fracture properties and patterns to well permeability indicators reveal that fractures play a role in fluid flow in the Rotokawa geothermal reservoir. Permeable zones commonly contain wide aperture fractures and high fracture densities which have a dominant NE‐SW strike orientation and NW dip direction. Studies of this kind, which show strong interdependency of structure and stress field properties, are essential to understand fluid flow in geothermal reservoirs where structural permeability dominates.

Processing and analysis of high temperature geothermal acoustic borehole image logs in the Taupo Volcanic Zone, New Zealand

Acoustic borehole televiewer (BHTV) logs provide direct observations of lithology, structure and in situ stress in reservoirs, essential for successful well targeting and field management. Analyses of BHTV logs acquired in twenty-three high temperature (≤288°C) geothermal wells in the Taupo Volcanic Zone, New Zealand, resulted in the modification of BHTV processing techniques and the creation of a descriptive feature classification for hydrothermally altered, volcano-sedimentary-basement type reservoirs lacking other complementary information common in hydrocarbons or lower temperature geothermal systems. Lithological characteristics observed on these BHTV logs are presented, alongside an assessment of the reliability of the structural measurements and image interpretation.

Structural controls on groundwater flow in a fractured bedrock aquifer underlying an agricultural region of northwestern New Brunswick, Canada

A hydrogeological study was conducted in northwestern New Brunswick, Canada, to improve the predictability of fracture-dominated groundwater flow within folded bedrock composed of fine-grained turbidites. Borehole televiewer logging and outcrop mapping, integrated with hydraulic packer tests revealed enhanced hydraulic conductivity associated with northeasterly striking bedding-plane fractures formed during folding and flexural slip. These fractures impart azimuthal anisotropy to the aquifer because of moderately dipping fold limbs. High-angle fractures form a well-developed non-stratabound network, comprising two open fracture sets striking NNE parallel to the current direction of principal stress, and WNW parallel to the direction of principal stress that dominated during the Acadian orogeny. The subset of fractures showing significant oxidation, deemed most important to the groundwater flow system, is dominated by bedding-plane and high-angle fractures striking near-parallel to the maximum principal stress direction, resulting in extensional opening and enhanced hydraulic conductivities. An equivalent porous media model, incorporating anisotropy and varying hydraulic conductivity with depth, indicates that horizontal flow dominates the aquifer with relatively minor exchange between different model layers. These findings have implications for understanding flow directions in the Black Brook Watershed and elsewhere in the Matapedia Basin where fractures formed under similar stress conditions.

The combined use of heat-pulse flowmeter logging and packer testing for transmissive fracture recognition

This paper presents an improved borehole prospecting methodology based on a combination of techniques in the hydrogeological characterization of fractured rock aquifers. The approach is demonstrated by on-site tests carried out in the Hoshe Experimental Forest site and the Tailuge National Park, Taiwan. Borehole televiewer logs are used to obtain fracture location and distribution along boreholes. The heat-pulse flow meter log is used to measure vertical velocity flow profiles which can be analyzed to estimate fracture transmissivity and to indicate hydraulic connectivity between fractures. Double-packer hydraulic tests are performed to determine the rock mass transmissivity. The computer program FLASH is used to analyze the data from the flowmeter logs. The FLASH program is confirmed as a useful tool which quantitatively predicts the fracture transmissivity in comparison to the hydraulic properties obtained from packer tests. The location of conductive fractures and their transmissivity is identified, after which the preferential flow paths through the fracture network are precisely delineated from a cross-borehole test. The results provide robust confirmation of the use of combined flowmeter and packer methods in the characterization of fractured-rock aquifers, particularly in reference to the investigation of groundwater resource and contaminant transport dynamics.

Borehole and Ice Feature Annotation Tool (BIFAT): A program for the automatic and manual annotation of glacier borehole images

We present a stand-alone software platform, Borehole and Ice Feature Annotation Tool (BIFAT), for the examination, annotation and analysis of glacier borehole images. This tool aids in the lengthy and often subjective process of annotating layers and other features in optical and acoustic glacier borehole televiewer logs. Since these view 360° around the full circumference of an exploratory borehole, intersecting planes are reconstructed on the televiewer image as sinusoids, the amplitude and phase of which can be used to calculate, respectively, the dip and direction of dip of each of these planes. The program aids in the annotation and examination of such planes, as well as a number of other features, including clusters and inclusions. BIFAT also provides an automatic layer detection option to aid and speed up the often lengthy process of identifying planar features in glacier borehole images. The software also shows promising results in rock borehole images and in the detection of planar layers in line-scan ice core images. We describe the capability and operation of BIFAT, and illustrate its application with reference to the automatic identification and annotation of sections from an optical televiewer (OPTV) borehole log from Roi Baudouin, Dronning Maud Land, Antarctica. The software is freely available for download from http://users.aber.ac.uk/byh/iceoptv.

Using Borehole Prospecting Technologies to Determine the Correlation between Fracture Properties and Hydraulic Conductivity: A Case Study in Taiwan

This study conducted different borehole prospecting techniques for hydrogeological investigations of fractured rock at three active landslide sites in southern Taiwan. Borehole televiewer logging, flowmeter logging, and packer hydraulic tests were performed to quantify various hydrogeological parameters including fracture width, fracture angle, flow velocity and hydraulic conductivity. The dependence of hydraulic conductivity on fracture properties was evaluated by correlation analysis. While a high positive correlation was found between hydraulic conductivity and fracture width ([Formula: see text]), and flow velocity ([Formula: see text]), no correlation with fracture angle was observed. In addition, it is worthwhile to note that the product of fracture width and flow velocity showed a strong correlation with hydraulic conductivity ([Formula: see text]). The regression analysis also revealed that a power law relationship, with a coefficient of determination of 0.83, exists between the hydraulic conductivity and the product of fracture width and flow velocity. The rationality of this relationship was carefully verified by using another group of geophysical borehole measurements. The results demonstrated that it is capable of predicting the hydraulic conductivity of fractured rock based on borehole televiewer and flowmeter logging results.

In Situ Stresses in Borehole Blanche-1/South Australia Derived from Breakouts, Core Discing and Hydraulic Fracturing to 2 km Depth

The development of Hot-Dry Rock (HDR) geothermal energy in Australia with drillings to some kilometres depth yields an impetus for deep stress logging. For the Olympic Dam HDR-project, borehole Blanche-1 was drilled to almost 2 km depth and provided the possibility to estimate the in situ stresses within the granitic borehole section by the analysis of borehole breakouts and core discing, as well as by hydraulic fracturing combined with acoustic borehole televiewer logging for fracture orientation determination. Although the stress magnitudes derived by the different methods deviate significantly, they clearly indicate for the depth range between 800 and 1,740 m a compressional stress regime of Sv ≤ Sh < SH and a consistent East–West orientation of maximum horizontal compression in agreement with existing stress data for Australia. The minor horizontal stress Sh derived from the hydraulic fracturing closure pressure values is about equal to the overburden stress and may be regarded as most reliable.

Analysis of fractures intersecting Kahi Puka Well 1 and its relation to the growth of the island of Hawaii

As part of the Hawaii Scientific Drilling Project, Kahi Puka Well 1 penetrated about 275 m of Mauna Loa basalts overlying a sequence of Mauna Kea flow units as it was drilled and cored to a total depth of 1053 m below land surface. A borehole televiewer (BHTV) was run in most of the well in successive stages prior to casing in order to obtain magnetically oriented acoustic images of the borehole wall. A total of 283 individual fractures were identified from this log and characterized in terms of strike and dip. These data are divided into three vertical sections based upon age and volcanic source, and lower hemisphere stereographic plots identify two predominant, subparallel fracture subsets common to each section. Assuming that most of the steeply dipping fractures observed in the BHTV log are tensile features generated within basalt flows during deposition and cooling, this fracture information can be combined with models of the evolution of the island of Hawaii to investigate the depositional history of these Mauna Loa and Mauna Kea basalts over the past 400 kyr. The directions of high‐angle fractures appear to be generally parallel to topography or to the coastline at the time of deposition, as is supported by surface mapping of modern flows. Consequently, an overall counterclockwise rotation of about 75° in the strike of these fractures from the bottom to the top of the well represents a systematic change in depositional slope direction over time. We attribute the observed rotation in the orientations of the two predominant fracture subsets over the past 400 kyr to changes in the configurations of volcanic sources during shield building and to the structural interference of adjacent volcanoes that produces shifts in topographic patterns.

Fracture Statistics Derived From Digital Ultrasonic Televiewer Logging

Abstract A 300 m section of core from a highly fractured igneous formation is directly compared to digital ultrasonic televiewer images of the borehole wall. The two different images consist of a mapping of the amplitudes travel-times of ultrasonic pulses reflected from the wall rock with respect to depth and magnetic north. These data were acquired during logging using a specially designed high-speed digitization system. The number of planar features seen in the amplitude and travel-lime images were 867 and 322, respectively, whereas 567 fractures and veins were observed in the core. The depth distributions of planar features seen in the core and in the travel time image appear to correlate. This correlation suggests that the travel-time log features in this paper are indicative of those in situ fractures which are open and permeable as determined by other means. Further, the travel-time planar features dip in the same direction as that expected for the greatest compressive stress. Conversely, this observation implies that the travel-time feature orientations possibly reveal in situ stress directions. Core examination indicates that the observed fractures are predominantly shear and, for a given state of stress, the orientations of these type of fractures differ from the orientations of extensional joints which open perpendicular to the direction of the least compressive stress. Extensional joints have been reported in televiewer logs by other workers. Hence, to delineate in situ stress directions from fractures observed in televiewer images, it must be distinguished whether a fracture results from either shear or extensional failure. Introduction Both natural and artificial fractures enhance fluid flow in the crust. In dense limestone, tight gas sand, coal and most igneous and metamorphic rock formations where the intrinsic permeability is low, economic extraction of hydrocarbon and geothermal resources can only originate from natural or artificial joints and fractures. Further, natural joints also impede and complicate underground construction. Hence, some knowledge of the density and orientation of in situ fractures is desirable for the efficient completion of many different engineering endeavours and both fracture detection and evaluation is important. A short listing of fracture detection methods includes borehole and surface seismics(1, 2, 3), radar(4), temperature gradient perturbations(5), sonic logging(6), resistivity scanning(7), and ultrasonic borehole televiewer analysis(5, 8, 9, 10). Each of these methods provides a piece of relevant information on fracture density, aperture, orientation, permeability, or location. Differentiating which fractures are open and permeable from those which arc closed and impermeable is especially crucial. Because the resolution of the fracture sensing techniques varies from a few metres for seismic measurements to centimetres for some wireline logging procedures, accurate characterization of in situ joints and fractures requires application or a number of methods. Of the above techniques mentioned, high resolution ultrasonic borehole televiewer logging(8) has been commercially exploited for nearly two decades. In conventional analog usage the televiewer provides a two-dimensional linear grey scale image of the borehole wall by mapping me strength, i.e. amplitude, of ultrasonic pulses reflected from rock from the interface between the wellbore fluid and the wall rock.

Introduction to the Dianaut Program: A scientific wireline re‐entry in deep ocean boreholes

Downhole measurements were successfully made, during the DIANAUT program, in three Deep Sea Drilling Project (DSDP) holes in the North Atlantic, utilizing the wireline re‐entry capability of the French logging shuttle Nadia which, seated in the drill re‐entry cone, provides a completely stable platform with precise control of depth and logging speed. Logging activities included temperature profiles, borehole televiewer logs, flowmeter measurements, fluid sampling, and a three‐component magnetometer profile. The different parameters were measured to study the heat transfer regime and hydrogeological processes within the oceanic crust as a function of age and distance from the ridge axis. Interpretation of the data obtained from holes 333A, 395A, and 534A, drilled in rocks ranging in age from 3.5 to 154 m.y., provide evidence of cold seawater circulation in the upper permeable layer of the young crust (sites 333A and 395A), thus maintaining very low temperatures and heat flow; higher temperatures were obtained from the older crust at site 534A.

Analysis of the borehole televiewer log from DSDP hole 395A: Results from the Dianaut Program

The French wireline re‐entry expedition known as DIANAUT culminated in the successful completion of a series of downhole measurements in three Deep Sea Drilling Project (DSDP) holes in the north‐central Atlantic Ocean. Among these measurements was a borehole televiewer (BHTV) log obtained in Hole 395A from bottom of casing at 112 meters below seafloor (mbsf) to a depth of 605 mbsf. In contrast with a BHTV survey previously conducted in this well during DSDP Leg 78B in 1981, televiewer data were not degraded by random cable oscillations due to ship heave and the quality of this log is excellent. These latest data were digitized and processed in terms of acoustic travel time and amplitude to compute hole size and shape, evaluate the structural integrity of the surrounding rock, and characterize intersecting fractures. The acoustic caliper and reflectivity logs correlate well with the lithologic column, particularly at the boundaries between major units which often are marked by breccias. Fractures in the lower part of the hole appear to be effectively sealed, in contrast with the open fractures identified in the upper sections. This supports a systematic pattern derived from complementary geophysical logs of increasing density, electrical resistivity, and elastic‐wave velocity, and decreasing in situ permeability as a function of depth. Fractures intersecting the well exhibit a wide range of orientations marked by some clustering of dip azimuths approximately east‐west. The presence of moderately dipping fractures striking subparallel to the ridge axis is consistent with both ridge‐parallel topographic elongations observed in this area and with azimuthal seismic anisotropy reported elsewhere, suggesting that fractures such as those identified from the BHTV record contribute to these effects.

State of lithospheric stress and borehole stability at Deep Sea Drilling Project Site 504B, eastern equatorial Pacific

Hole 504B in the eastern equatorial Pacific is the deepest hole to penetrate oceanic basement, extending more than 1500 m beneath the seafloor. Two borehole televiewer (BHTV) logs were obtained in this hole, one through the upper 1000 m of basement during Deep Sea Drilling Project leg 83 and the other extending an additional 260 m deeper into the crust during Ocean Drilling Program leg 111. The two BHTV records have been combined and processed in terms of both acoustic amplitude and travel time in order to evaluate the extent and distribution of rock failure along the borehole wall. A histogram of borehole enlargements versus azimuth depicts a dominant breakout azimuth of N122.5°E which corresponds to the direction of minimum principal stress Sh and correlates well with stress orientations inferred from solutions to regional earthquake focal mechanisms. Furthermore, the bimodality of this histogram, with a secondary mode orthogonal to Sh, indicates that a significant number of enlargements are coalesced tensile fractures occurring along the orientation of SH, the maximum principal stress. The appearance of this orthogonal, bimodal distribution suggests that the regional horizontal stress field is highly anisotropic, a condition supported by seismic data which show all of the earthquakes in the area to be strike‐slip. The frequency of borehole enlargements increases with increasing depth and depicts a systematic structural deterioration of the well bore. The tensile fractures along the SH azimuth contribute to this degradation and appear to be induced by thermal stresses due to the injection of cold water into hot rock. The frequency of these extensional features does not increase with depth. Rather, their appearances can be directly correlated with shipboard efforts at deliberately cooling the well and/or with the sudden resumption of drilling after the hole had been allowed to reequilibrate thermally for several days. These latter borehole enlargements are more pronounced then those commonly associated with hydraulic fracturing. The mechanisms for fracture initiation and growth, based upon temperature contrasts between the well bore fluid and the adjacent rock may enhance rock failure.

Analysis of fractures from borehole televiewer logs in a 500m deep hole at Xiaguan, Yunnan province, Southwest China

Fractures from a 500 m deep hole in the Red River fault zone were analyzed using an ultrasonic borehole televiewer. Four hundred and eighty individual fractures were identified between 19 m and 465 m depth. Fracture frequency had no apparent relation to the major stratigraphic units and did not change systematically with depth. Fracture orientation, however, did change with stratigraphic position. The borehole intersected 14 m of Cenozoic deposits, 363 m of lower Ordovician clastic sediments, and 106 m of older ultramafic intrusions. The clastic sequence was encountered again at a depth of 484 m, suggesting a large fault displacement. Fractures in the top 162 m of the sedimentary section appear randomly distributed. Below that depth, they are steeply dipping with northerly and north-westerly strikes, parallel to the major active faults in the region. Fractures in the ultramafic section strike roughly eastwest and are steeply dipping. These orientations are confined to the ultramafic section and are parallel to an older, inactive regional fault set.

Fracture Detection in Low-Permeability Reservoir Sandstone: Comparison of BHTV and FMS Logs to Core: ABSTRACT

Detection and characterization of fractures in low-permeability reservoirs is an important goal of reservoir analysis. Natural fractures may increase porosity in low-permeability rock and may enhance or limit the success of hydraulic fracture treatment. Two logging tools used for fracture detection are the borehole televiewer (BHTV), an acoustic device that maps the smoothness of the borehole wall, and Schlumberger's Formation Microscanner (FMS), a resistivity tool that produces a conductivity map of two 2.8-in. wide strips of the borehole wall. Their analysis compares BHTV and FMS logs to 1028 ft of core from three wells in the 2000-ft thick Lower Cretaceous Travis Peak Formation, a sandstone and shale unit that produces gas from low-permeability sandstone in east Texas. Core was obtained from depths of 5900 to 9900 ft. The Travis Peak provides a good test of fracture-imaging logs because natural fractures have complex geometry and variable mineral fill and because there are borehole breakouts, drilling-induced fractures, and vertical sedimentary structures that must be distinguished from natural fractures for successful fracture analysis.

Borehole Televiewer Improves Completion Results in a Permian Basin San Andres Reservoir

Summary The borehole televiewer (BHTV) is an acoustic logging tool that has a fine vertical resolution (⅓ in. [8.5 mm]) that is extremely useful for detecting bedding orientation, thin high-permeability zones, lithology changes, and zones of secondary or vuggy porosity. BHTV logs were acquired from 10 wells during the recent infill drilling of the North Hobbs (Grayburg/San Andres) Unit. These logs, when correlated with conventional logs and core data, have improved completion strategies, resulting in higher oil and lower water production in new infill wells.

A New Method for Fracture Identification Using Array Sonic Tools

Summary. The development of array sonic tools has enabled the recording of an array of sonic waveforms at each depth location with several receivers. In this paper, we develop a new method to study the sonic amplitude/energy by taking advantage of the redundant information in these waveforms. First, we identify the wave components of interest. Second, we apply windowing procedures to extract the desired wave components. Third, we calculate the energy of the wave component at each receiver depth location. Finally, the calculated energies are displayed as an energy log or a variable-density image as a function of depth. Open fractures are detected when the method is applied to the low-frequency Stoneley wave. Reductions of the Stoneley wave energy caused by open fractures appear as a characteristic pattern on the output. The vertical extent of the pattern provides an estimate of the fracture dip angle, and the magnitude of the energy reduction is related to the hydraulic conductivity of the fractures. We show examples of fracture identification using data from the Conoco Test Well 33–1 located in Oklahoma. Complementary information regarding the location and nature of the fractures is available from full-core and borehole televiewer (BHTV) logs. The Stoneley wave response to different fractures is discussed. Combined with borehole images, this method provides a valuable way for distinguishing between open and closed provides a valuable way for distinguishing between open and closed fractures. Introduction Modern digital sonic tools are capable of recording down-hole sonic waveforms with an array of receivers. The recorded waveforms contain information about the in-situ rock properties, such as compressional and shear slownesses, and attenuation. In this paper, we propose a new method to estimate wave amplitude/energy from recorded array sonic waveforms and to display the information as a well log. Three important contributions are emphasized. First, we discuss ways to extract the sonic amplitude/energy information from array sonic waveforms. Second, we display the information in a meaningful way, taking full advantage of the data redundancy in array sonic waveforms. Third, and most important, we apply the method to formation evaluation, where we have found that the method has great potential for describing the heterogeneous properties of fractured rocks. This is important because the location and evaluation of fractures is critical for oil and gas production in low-permeability formations. Method The acoustic wavefield recorded in the borehole is extremely complex because of the presence of the overlapping wave components: compressional and shear headwaves, trapped-fluid modes, and surface waves. One way to separate the components in the recorded waveforms is to use a sonic tool with a long transmitter-to-receiver (TR) spacing. In this way, wave components that propagate with different velocities can be isolated with a time window. Besides exhibiting a time separation, the wave components may also have harmonic separation in frequency. The frequency content of compressional and shear waves, for example, is generally higher than that of the Stoneley wave. This allows us to apply low-pass filtering to extract the Stoneley wave from the waveforms. Fig. 1 shows an array of sonic waveforms recorded in hard rock with a digital array sonic tool and shows that the different arrivals can be isolated from one another by application of a time-windowing technique. Fig. 2 shows the amplitude spectra of the time-windowed compressional, shear, and Stoneley waves. It is clear that the low-frequency Stoneley wave (2,000 to 5,000 cycles/sec [2 to 5 kHz]) is the strongest event and is separated from the headwaves (approximately 6,000 to 15,000 cycles/sec [6 to 15 kHz]). The filtered output (2,000 to 4,000 cycles/sec [2 to 4 kHz]) of the waveforms is shown in Fig. 3. Note that the filtered waveforms contain only the low-frequency Stoneley wave and not the headwaves. Wave components of interest may be extracted with time windowing or frequency windowing, or with a combination of the procedures. The choice of the windowing procedure and its implementation will depend on the procedure and its implementation will depend on the application. For example, one might be interested in the compressional headwave in the 10,000- to 15.000- cycle/sec [10- to 15-kHz] band. JPT P. 677