Borehole Seismic: Essential Contributions Over the Oilfield Lifecycle

During the evolution of the petroleum industry, surface seismic imaging has played a critical role in reservoir characterization. In the early days, borehole seismic (BHS) was developed to complement surface seismic. However, in the last few decades, a wide range of BHS surveys has been introduced to cater to new and unique objectives over the oilfield lifecycle. In the exploration phase, vertical seismic profiling (VSP) provides critical time-depth information to bridge time indexed subsurface images to log/reservoir properties in depth. This information can be obtained using several methods like conventional wireline checkshot or zero-offset vertical seismic profiling (ZVSP), seismic while drilling (SWD) or distributed acoustic sensing (DAS) techniques. SWD is a relatively new technique to record real-time data using tool deployed in the bottomhole assembly without disturbing the drilling. It helps to improve decision making for safer drilling especially in new areas in a cost-effective manner. Recently, a breakthrough technology, distributed acoustic sensing (DAS), has been introduced, where data are recorded using a fiber-optic cable with lots of saving. ZVSP also provides several parameters like, attenuation coefficient (Q), multiples prediction, impedance, reflectivity etc., which helps with characterizing the subsurface and seismic reprocessing. In the appraisal phase, BHS applications vary from velocity model update, anisotropy estimation, well- tie to imaging VSPs. The three-component VSP data is best suited for imaging and amplitude variation with offset (AVO) due to several factors like less noise interference due to quiet downhole environment, higher frequency bandwidth, proximity to the reflector, etc. Different type of VSP surveys (offset, walkaway, walkaround etc.) were designed to fulfill objectives like imaging, AVO, Q, anisotropy, and fracture mapping. In the development phase, high-resolution images (3D VSP, walkaway, or crosswell) from BHS surveys can assist with optimizing the drilling of new wells and, hence reduce costs. it can help with landing point selection, horizontal section placement, and refining interpretation for reserve calculation. BHS offers a wide range of surveys to assist the oilfield lifecycle during the production phase. Microseismic monitoring is an industry-known service to optimize hydraulic fracturing and is the only technique that captures the induced seismicity generated by hydraulic fracturing and estimate the fracture geometry (height, width, and azimuth) and in real time. During enhanced oil recovery (EOR) projects, BHS can be useful to optimize the hydrocarbon drainage strategies by mapping the fluid movement (CO2, water, steam) using time-lapse surveys like walkaway, 3D VSP and/or crosswell. DAS has brought a new dimension to provide vital information on injection or production evaluation, leak detection, flow behind tubing, crossflow diagnosis, and cement evaluation during production phase. This paper highlights the usage of BHS over the lifecycle of the oilfield.

Application of Full Waveform Inversion to Time-Lapse Walkaway Vertical Seismic Profile Data for CO2-EOR Monitoring in Abu Dhabi: A Feasibility Study on Field and Synthetic Data

This study aims to investigate the feasibility of CO2-EOR monitoring by full waveform inversion (FWI) of time-lapse VSP data in an onshore CO2-EOR site in Abu Dhabi. CO2-EOR monitoring using conventional time-lapse surface seismic in onshore oil fields in Abu Dhabi is often technically challenging for two main reasons. The first is that elastic property change in response to pore fluid substitution is relatively small because the elastic modulus of the reservoir rock frame is far larger than that of the pore fluids. The second is the low repeatability of time-lapse survey data due to high amplitude surface-related noise which varies temporally. However, seismic monitoring with FWI of time-lapse borehole seismic data may offer a solution for these issues. FWI is capable of detecting small velocity changes such as those associated with pore fluid substitution. Furthermore, borehole seismic surveys may provide more highly repeatable, higher quality data compared to surface seismic surveys because borehole seismic data is less affected by surface-related noise. This study consists of two parts, a field data analysis and a synthetic study. In the field data analysis, we studied the resolution and repeatability of FWI results at field-data quality, including the presence of actual noise using time-lapse VSP data. VSP data was acquired at the very early stage of EOR and there was no CO2 injection in the time between the two time-lapse VSP surveys. As a result, a high-resolution P-wave velocity model, consistent with a sonic log, was obtained. The P-wave velocity model also revealed excellent repeatability between the two survey data sets. In the synthetic study, time-lapse FWI was performed using synthetic VSP data representing pre- and post- CO2 injection periods. The results of the synthetic study showed that even in the presence of realistic 4D noise, which was estimated in the field data analysis, FWI successfully delineated the distribution of velocity changes caused by CO2 injection when the cross-sectional area of the injection-induced velocity changes were larger than the resolution of the FWI results. With these results, we demonstrated that FWI using time-lapse VSP data was applicable for CO2-EOR monitoring in the field as long as the criteria were met. This conclusion encourages the application of FWI using time-lapse VSP data for CO2-EOR monitoring in onshore Abu Dhabi.

Earthquake monitoring of the Baribis Fault near Jakarta, Indonesia, using borehole seismometers

The geological setting of Jakarta and its immediate surroundings are poorly understood, yet it is one of the few places in Indonesia that is impacted by earthquakes from both the Java subduction zone and active faults on land. In this study, a borehole seismic experiment with low noise characteristics was deployed to record seismic activity on the ~ E-W oriented Baribis Fault, which is ~ 130 km long, passes to the south of Jakarta, and is only ~ 20 km away at its nearest point. A primary objective of this study is to determine whether this fault is seismically active, and therefore, whether it might pose a threat to nearby population centers, including Jakarta in particular. A total of seven broadband instruments that spanned Jakarta and the surrounding region were installed between the end of July 2019 and August 2020, during which time we were able to detect and locate 91 earthquakes. Two earthquakes were located close to the Baribis Fault line, one of which was felt in Bekasi (southeast of Jakarta) where it registered II-III on the Modified Mercalli Intensity (MMI) scale. The focal mechanism solutions of these events indicate the presence of a thrust fault, which is in good agreement with previous studies, and suggest that the Baribis Fault is active.

Borehole seismic methods for geologic CO2 storage monitoring

Borehole geophysical methods are a key component of subsurface monitoring of geologic CO2 storage sites because boreholes form a locus where geophysical measurements can be compared directly with the controlling geology. Borehole seismic methods, including intrawell, crosswell, and surface-to-borehole acquisition, are useful for site characterization, surface seismic calibration, 2D/3D time-lapse imaging, and microseismic monitoring. Here, we review the most common applications of borehole seismic methods in the context of storage monitoring and consider the role that detailed geophysical simulations can play in answering questions that arise when designing monitoring plans. Case study examples are included from the multitude of CO2 monitoring projects that have demonstrated the utility of borehole seismic methods for this purpose over the last 20 years.

A three-component optical sensor for borehole seismic applications

We describe a new optical three-component accelerometer for borehole applications. Field data acquired in early 2020 in a fiber-optic-instrumented well in Houston, Texas, show that the new optical accelerometer is a viable borehole seismic sensor, measuring signals at frequencies from subhertz to hundreds of hertz. It is argued that an array of these sensors could be used to complement distributed acoustic sensing (DAS) technology to compensate for the inability of DAS sensors to measure wavefield polarization. This hybrid fiber-optic receiver array would be a fully optical wide-bandwidth sensor array without any electronics in the well. With a maximum operational temperature expected to exceed 200°C, this array would not be affected significantly by possible high temperatures in the near-reservoir section of the well.

Spatial Aliasing and Three-Component Seismic Sensors

Temporal aliasing occurs when a waveform is sampled with less than two points per time period for a signal at a given frequency. This insufficiently sampled frequency will incorrectly be mapped into a lower (aliased) frequency. Analogous to this, spatial aliasing is said to occur when a propagating waveform is measured at spatial intervals larger than half the wavelength of any given signal in that waveform. Temporally aliased frequencies cannot be recovered with standard methods. On the other hand, we argue that "spatial aliasing" can be viewed as an expression of a non-uniqueness for estimating the direction of the propagation for signal at a given frequency, and that spatial aliasing may be overcome when three-component seismic sensors are used. Realizing this allows for using higher frequencies, and therefore enables the generation of higher resolution images from the data. This is particularly useful for borehole-seismic data which tend to contain higher frequencies than surface-seismic data, but does require that an array of three-component sensors is used, or that an array of less expensive single-component sensors is supplement by three-component sensors.