First Break - Volume 41, Issue 11, 2023

The Kharita reservoir in Karam field is a prolific Early Cretaceous reservoir located in Badr El-Din concessions, Abu Gharadig Basin, Western Desert, Egypt. This reservoir is characterised by lateral and vertical variations in facies type, reservoir quality, connectivity and low gas recovery which in turn affect the efficiency of further exploitation in the Karam field. Herein, a 3D static reservoir model was conducted using multidisciplinary datasets to provide a comprehensive understanding of reservoir characteristics helping to optimise the strategies of hydrocarbon field development. The conducted well correlation reveals potentially multiple gas accumulations separated by a very low net-to-gross sequence. Two types of reservoir quality were interpreted: good-quality active distributary channels and poor tidal flat deposits. The different types of genetic facies have different property distribution and connectivity behaviours. Active distributary channels have higher connectivity than the tidally influenced channels and the tidal flat sands. The estimated initial gas in-place demonstrated that half of the gas volume is attributed to poor-quality sands which show very low productivity. This work procedure will lead to a more precise prediction of reservoir performance, and select the optimum subsurface development plan, including the location and number of infills required to increase the ultimate gas recovery.

This article discusses the development of a novel marine seismic acquisition system that utilises a fleet of fully autonomous underwater vehicles (AUVs) as ocean-bottom nodes (OBNs). The primary objective is to eliminate remotely operated vehicles (ROVs) or use of ropes for the deployment and recovery of ocean-bottom seismic sensors, thereby enabling ocean-bottom seismic surveys that are significantly more efficient, cost-effective with improved environmental impact. This is achieved by developing buoyancy driven AUVs, which offer long endurance and are capable of self-locating to pre-determined positions without the need for any direct surface control. Additionally, the system proposed is designed for scalability, allowing for large inventories of AUVs to be deployed in parallel to satisfy the requirements of large-scale seismic surveys. This system is expected to service sub-surface imaging needs of both hydrocarbon business and emerging low carbon businesses such as CCS. This system will also permit a shift from dense source side to dense receiver side sampling, with a clear benefit to limit sound emission levels even as sampling density (and so data quality) increases.

Despite a recovery in the number of towed-streamer surveys being conducted, OBN (Ocean Bottom Node) seismic projects continue to take an increasing market share over towed-streamer surveys. In OBN acquisition, each node is equipped with a pressure sensor (hydrophone) and three motion sensors (typically, geophones). The nearly-a-century-old geophone technology has, however, certain inherent shortcomings that degrade the recorded signal. Geophone performance deviates from reference specifications due to manufacturing tolerances, ageing and changes in temperature. As an example, for 15-Hz omnitilt geophones, as commonly used in OBN acquisitions, the variation in response reaches 3 dB in amplitude and 10 degrees in phase within their range of manufacturing tolerances. These uncertainties in sensor response prove particularly difficult to model and correct for in practice and result in final data sensor artefacts. The insensitivity of geophones to the gravity field also requires the use of additional tilt meters for the verticalisation of the 3C with resulting issues related to the relative orientations of these two pieces of equipment.

Today, MEMS (Micro-Electromechanical Systems)-based digital seismic accelerometers have proved to be the high-fidelity alternative to geophones. Their specifications are not affected by temperature, ageing or manufacturing tolerances, making the recorded signal accurate in phase and amplitude with the seismic signal over the entire seismic bandwidth. As MEMS can detect the gravity vector, the integration of this sensing technology into OBN has demonstrated that 3C MEMS provide, without pre-processing, seismic signal with true verticality, and a vector fidelity error (error in orthogonality between the three sensors) that is an order of magnitude lower than for 3C geophones. The excellent low-frequency performance of the latest, third-generation MEMS is also ideal for reaping the full benefit of novel low-frequency sources (Ronen 2017), and in this way pushing back further the limits of FWI.

This, along with other MEMS properties, makes this sensor a strong driver for the growth of OBN acquisition – especially for sparse or blended acquisition, where sensor fidelity matters more than ever. At the time of writing, the world’s largest OBN survey is continuing in the Middle East and is starting to deliver a promising dataset from the 23,000 MEMS-based OBNs deployed. Observations from this mega-survey, as well as from a previous experimental survey that includes direct comparisons with geophone-based OBN, are presented and discussed in this article.

In marine seismic surveys, data acquisition typically involves an end-on shooting geometry. While the Reverse Time Migration (RTM) method applied to end-on shot gathers often yields high-quality images, there are instances where certain dips may be inadequately represented or even missing in the migrated section. This paper introduces an efficient utilisation of the well-established principle of reciprocity to demonstrate the creation of a Pseudo Split-Spread (PSS) shot gather from existing one-sided offset shot gathers. Additionally, we illustrate that employing generated PSS shot gathers for RTM imaging of dipping events results in significant improvements compared to using recorded end-on shot gathers for both isotropic and anisotropic Vertically Transverse Isotropic (VTI) media. The proposed approach has been investigated in depth for its underlying methodology and additional advantages. The efficacy of this proposed RTM approach is substantiated through successful testing with synthetic and field data examples using isotropic and anisotropic VTI media.

The resistance of low-frequency seismic energy to scattering and absorption, and its resulting ability to be recorded over long ray paths through attenuating media is well established. This relative resistance to scattering and attenuation makes it vital for capturing information with depth and offset, both for model-building and illumination. There is likely to be no better example of this than modern long-offset OBN (ocean bottom node) surveys of the Gulf of Mexico – however the importance of high-quality low frequencies in unravelling imaging challenges below complex overburden such as shallow gas, carbonates, salt, and volcanics is well documented. Consequently, interest in recording low frequencies in the field extends beyond OBN surveys to include single-vessel and multi-vessel towed-streamer designs. We have seen strong focus on designing sources that produce rich low frequencies over the last decade as a result.