A Practical Method for the Automatic Recognition of Rock Structures in Panoramic Borehole Image during Deep-Hole Drilling Engineering

Digital panoramic borehole imaging technology has been widely used in the practice of drilling engineering. Based on many high-definition panoramic borehole images obtained by the borehole imaging system, this paper puts forward an automatic recognition method based on clustering and characteristic functions to perform intelligent analysis and automatic interpretation researches, and successfully applied to the analysis of the borehole images obtained at the Wudongde Hydropower Station in the south-west of China. The results show that the automatic recognition method can fully and quickly automatically identify most of the important structural planes and their position, dip, dip angle and gap width and other characteristic parameter information in the entire borehole image. The recognition rate of the main structural plane is about 90%. The accuracy rate is about 85%, the total time cost is about 3 h, and the accuracy deviation is less than 4% among the 12 boreholes with a depth of about 50 m. The application of automatic recognition technology to the panoramic borehole image can greatly improve work efficiency, reduce the time cost, and avoid the interference caused by humans, making it possible to automatically recognize the structural plane parameters of the full-hole image.

Khuff and Pre-Khuff Reservoir Fracture Characterization using the Cross Dipole Shear Wave Imaging and Borehole Imaging Data Integration

Increasing demands for gas in UAE have led to increased focus on more tight gas reservoirs like Khuff and pre-Khuff formations, away from the conventional oil-bearing carbonate reservoirs. The case study presented is in an offshore field, Northwest of Abu Dhabi city. The structure, with an area of 50 Sq.km was first identified in 1966 and it is part of the regional N-S extending structural. The multi-discipline approach applied in this study required the integration of a suite of open-hole data over a variety of length scales. Combination of the Borehole Acoustic Reflection Imaging technique and borehole imaging logs (BHI) in 3D, provides a better understanding of the complex fracturing network and the associated formation stress orientation up to 100ft away from the wellbore. The ability to "see" away from wellbore what was previously hidden on seismic, allows unlocking further potential reserves or avoiding certain production hazards. The well has penetrated the highly economical tight clastic Pre-khuff formation and the carbonate Kuff formation, allowing the analysis over a large geological history of offshore Abu Dhabi. The coherency of all data has helped establish for the first time a baseline understanding of the role of the fractures and fault in the petrophysical properties distribution along the wellbore and the 3D structural characterization in an larger area around the wellbore (up to 100ft). The emphasize in this paper is on the Borehole Acoustic Reflection Imaging technique (DSWI), which allows the identification of both intersecting and non-intersecting of geological features with a depth of investigation up to 100 ft away from the borehole. Moreover, the combination of DSWI with BHI have been used for the anisotropy estimation away from wellbore especially in a very tight and fractured reservoir deciphering multiple fault orientation, which potentially, cancel the anisotropy estimation due to destructive interference. In addition to the presence of drilling induced fractures interfering in with the natural fracture as seen on the BHI. The detailed BHI interpretation and the petrophysical data revealed that the fracture densities and orientation vary from bottom to top interval indicating tectonic regimes affecting the field. The lithological variation due to the evolution of the depositional setting has significantly influenced the fracture distribution and their length. The presence of these induced fractures and how deep they propagate into the formation, dominates the behavior acoustic anisotropy by reaching the flexural (dipole shear) investigation zone (3 to 4ft deep). It is also interesting to see the behavior of both natural and induced fractures and their respective strike change over the different formations revealing a geomechanically complex structure.

The Synergy Between Borehole Imaging and Geosteering

The quest to gain more knowledge of the subsurface and to reduce uncertainty in the interpretation of subsurface data has been an age-long effort in the oil and gas industry. To achieve this, asset owners use tools with improved resolution, utilize different types of logging tools and integrate the interpretation from these logging tools. This paper will review some projects where data from borehole imaging tools were used to support geosteering decisions and to gain more knowledge of reservoir structure. Borehole images are logs based on the circumferential measurement of a petrophysical parameter along a borehole wall. Logging-while-drilling borehole images can be used for structural, sedimentological and petrophysical analysis. These near-wellbore analyses contribute greatly to the success of most geosteering jobs. Geosteering is a process used in placing high-angled and horizontal wells in subsurface intervals of interest. It involves the use and integration of data from varied sources. This paper will show different scenarios, in different depositional environments, where borehole imaging supported the geosteering process and how geological interpretations from geosteering brought more clarity to borehole imaging analysis. Examples of these will highlight the stratigraphic relationship between geological structures and wellbore trajectory, detection of subsurface structural discontinuities, primary sedimentary structures, and the interpretation of complex geological structures. This paper will broaden our understanding of the applications of borehole imaging and how it integrates with geosteering in achieving oil and gas well objectives.

Rock joint detection from borehole imaging logs based on gray-level co-occurrence matrix and Canny edge detector

Rock joints play an important role in characterizing the rock mass quality for geo-mechanical design and stability analysis. An approach was developed to detect and characterize the rock joints from images collected by a borehole imaging system. A gray-level co-occurrence matrix was employed to locate the joint regions, allowing more focused and effective detection processing, followed by extractions of the upper and lower edges of rock using the Canny algorithm. Four basic geometrical parameters of rock joints－orientation, depth, aperture, and core length－were determined based on the fitting of sinusoids to joints’ edges. Furthermore, the joint density was determined based on the geometric parameters. To calibrate the proposed approach, a borehole in the Rumei hydropower station engineering at Lantsang River was selected as a case study. Orientation of rock joints with gentle dip angles, which was determined from borehole imaging logs, corresponded to the measurement in three horizontal tunnels. Additionally, both joint density and pressure-wave velocity revealed that jointed rock mass was observed in the depth from 100 m to 120 m, and intact rock mass was presented in the depth of 150 m to 170 m, indicating the good performance of the proposed method.

Research on Rapid Evaluation of Rock Mass Quality Based on Ultrasonic Borehole Imaging Technology and Fractal Method

The distribution of discontinuities in the deep stratigraphic rock mass is random and irregular, and the spatial distribution has statistical self-similarity, which can be analyzed by fractal theory. The borehole wall image obtained based on the ultrasonic borehole imaging technology can clearly reflect the rich structural surface information such as the inclination, density, and crack width of the structural surface in the rock mass. Due to its inherent fractal characteristics, the fractal box dimension of the hole wall image can be used to describe the complexity of the structural surface. In the study, the fractal box dimension of the hole wall image and the RQD value of the rock mass are compared, and it is found that there is a linear correlation between the two. Therefore, the rock quality can be evaluated according to the fractal box dimension value of the hole wall image. Finally, a rapid quantitative evaluation method for rock mass quality classification is established based on ultrasonic borehole imaging technology and fractal method.

FIELD-TESTING A THROUGH-THE-BIT HIGH-DEFINITION ELECTRICAL BOREHOLE IMAGER FOR OIL-BASED MUD

A new, through-the-bit, ultra-slim wireline borehole-imaging tool for use in oil-based mud provides photorealistic images. The imager is designed to be conveyed through drill-pipe. At the desired well section, it exits the drill pipe through a portal drill bit and starts the logging. Field test measurements in several horizontal, unconventional wells in North America show images of fine detail with a large amount of geological information and high value for well development. A relatively new solution for conveying tools to the deepest point of a high angle or horizontal wells uses a drill bit with a portal hole at the bit face. As soon as the bit reaches the total depth, a string of logging tools is pumped down through the drill pipe. The tools exit the bit through the portal hole, arriving in the open hole and are ready for the up log. The tools operate on battery and store the log data in memory so that no cable is interfering as the drill pipe is tripped out of the well while the tools are acquiring data. The quality of wireline electrical borehole images in wells drilled with oil-based mud has significantly improved in recent years. Modern microresistivity imagers operate in the megahertz-frequency range, radiating the electromagnetic signal through the non-conductive mud column. A composite processing scheme produces high-resolution impedivity images. The new, ultra-slim borehole-imager tool uses these measurement principles and processing methods. Innovating beyond the existing tool designs the tool is now re-engineered to dimensions sufficiently slim to fit through drill pipes and to use through-the-bit logging techniques. The new, ultra-slim tool geometry proves highly reliable and, due to the deployment technique, highly effective in challenging hole conditions. The tool did not suffer any damage and showed only minute wear over more than twenty field test wells. The tool’s twelve-pad geometry provides 75% coverage in a six-inch diameter borehole and its image quality compares very well with existing larger tools. The field test of this borehole imaging tool covers all scenarios from vertical to deviated and to long-reach, horizontal wells. Geological structures, sedimentary heterogeneities, faults and fractures are imaged with detail matching benchmark wireline images. The interpretation answers allow operators of unconventional reservoirs to employ intelligent stimulation strategies based on geological reality and effective well development. A new high-frequency borehole imager for wells drilled with oil-based mud is introduced. Deployed through the drill pipe and its portal bit, the imager carries photorealistic microresistivity images into wells where conventional wireline conveyance techniques reach their limits in both practicality and viability.

ASSESSMENT OF NEW HIGH-DEFINITION BOREHOLE IMAGING-WHILE-DRILLING TECHNOLOGY: LEARNINGS FROM PRE-SALT CARBONATES OF BRAZIL

The pre-salt carbonates of Brazil pose drilling and characterization challenges associated with inherent reservoir heterogeneity; and borehole imaging while drilling often provides insights helpful for both, operational and subsequent decisions. The findings and learnings from a 3-well campaign, offshore Brazil are presented to assess and validate a recently deployed high-definition borehole imaging technology that provides industry’s first real-time ultrasonic amplitude images and time-to-depth corrections for best possible images maintaining the geological features integrity. High-definition ultrasonic measurements were acquired at two central frequencies with 0.2-in resolution and provided amplitude and transit time images for geological characterization and petrophysical evaluation in addition to azimuthal ultrasonic calipers. The lossy nature of amplitude data makes it difficult to transmit in real-time; therefore, a unique data compression technology was used to achieve industry’s first high quality amplitude images streaming while drilling. In deepwater operations acquisition of high-definition logging while drilling (LWD) images can be severely degraded if time-to-depth offset due to heave is not compensated. Recently developed heave-filtering workflows ensured the integrity of subsurface features. The time-indexed data was processed with this application in real-time, providing good results and confidence in the capability of this technology. Image-logs of the first well were helpful in interpretation and added value to the reservoir understanding; however, many intervals suffered from lack of confidence in image features. Simulations were performed to improve the images acquisition parameters based on learnings from this experience. New optimized operational parameters were applied in next two wells, resulting in image logs of excellent quality. Data from second well suffered from high heave while drilling, which required implementation of the heave-filtering memory data workflow. For the third well, an additional requirement for real-time image quality-control was defined, requiring data to be processed after every drill-stand. Real-time data quality provided confidence in optimal quality of memory data, thereby eliminating the need of post-drilling wireline operations in open-hole. The images acquired in memory helped characterize intervals of stromatolites with various morphology, and zones of vugs distribution, providing excellent alternative for wireline logging, de-risking the operations in pre-salt carbonate logging in Brazil offshore operations.