Maintaining wellbore stable is one of the key tasks in the oil and gas industry, in order to reduce nonproductive time during drilling, because wellbore instability problems would lead to higher than necessary drilling costs and have a severe impact on drilling schedule. Wellbore stability is controlled by two type of factors, one type is the factors which are completely out of our control, such as in-situ stresses, pore pressure, and rock strength, the other type is the factors which we can optimize and design to minimize geomechanical related stability problems, such as well trajectory, casing seats, mud system, mud weight, and proper drilling practice including minimizing swab and surge while running pipe and reducing the stationary time during connection.In this paper, only the mud weight optimization and casing seats design will be focused on, in order to avoid geomechanical related instability issues for drilling a recently planned appraisal well in an offshore field.Mechanical properties including Young's modulus, Poisson's ratio, USC, friction angle, tensile strength, bulk density and pore pressure gradient obtained from two offset wells A1 and A2 were projected to the planned appraisal well A3 with tops were used as guidance. The same parameters optimized from those two offset wells were used to estimate horizontal stresses for the planned well, and the same failure criterion was used to carry out wellbore stability for the planned well. Meanwhile the mud weight corresponding to kick, breakout, loses and breakdown were obtained. Based on those results, the safe mud weight window was established and the casing seats were placed to prevent and reduce the geomechanics related instability problems.This methodology allows us to predict and prevent the geomechanics related instability issues in advance before drilling starts, thereby to reduce the non-productive time and drilling costs. Continuous updating of the geomechanical model is necessary during drilling if the geological environment is complicated.

A field test was conducted utilizing autonomous marine vehicles (AMVs) and 3D sensor arrays (3DSAs) to record and compare seismic data generated during an ocean-bottom cable (OBC) survey. The test was a field verification to check that the AMV platform and the sensor array can deliver high-quality seismic data in a form that can be successfully processed and compared to ocean-bottom fixed-receiver data. Three AMVs, each towing a 3D sensor array, were deployed during the acquisition of an OBC 3D survey in shallow water, offshore Abu Dhabi. The test was conducted to (a) assess the feasibility of seismic acquisition using AMVs and 3D sensor arrays including safe deployment and retrieval, (b) evaluate the performance of the 3D sensor arrays based on holding station capability, maintenance of desired depth, and accuracy of measurements of pitch and orientation, and (c) compare the quality of the acquired seismic data with the pressure data recorded in the OBC survey. The water depths across the acquired survey area average approximately 20 m. Due to the very shallow water depths, towed-streamer acquisition is seldom used offshore Abu Dhabi, limiting the marine seismic acquisition methods to either OBC or ocean-bottom nodes (OBN). The AMV and 3D sensor array are ideally suited to operate and record seismic data in this shallow-water environment, providing a potentially viable alternative for off-bottom recording or as a method to supplement OBC or OBN data acquisition. The feasibility test conducted in offshore Abu Dhabi demonstrated the successful and safe deployment, seismic data acquisition, and retrieval of the AMV and 3D sensor array. The evaluation criteria indicate the consistent performance of the AMV and 3D sensor array, and that the recorded data are comparable to the OBC component data in terms of signal-to-noise ratio and frequency bandwidth. The processing flow that may typically be applied on the OBC data for signal processing, velocity model building, and imaging may also be applied to the 3D sensor array data. Marine seismic data can be acquired using towed-streamer arrays, ocean-bottom cables, or nodes in which the receivers are placed in fixed positions on the seafloor. Limitations to acquisition in each case may be determined by water depth and seafloor topography, or operational constraints due to in-situ infrastructure and obstructions. In the cases where these sorts of limitations arise, a loss in operational efficiency may easily result, which will ultimately drive the acquisition cost up, or may even result in poor or no data coverage. Using the AMV offers the potential to acquire seismic data in these cases where adverse existing factors may hamper standard acquisition methods.

An Ocean Bottom Cable (OBC) 3D/4C seismic survey covering 2730 square kilometers and spanning 23 months has been undertaken by ADMA-OPCO. This survey covers 2 offshore congested producing fields, plus several exploration areas currently planned for development in the near future. This large acquisition when coupled with two previously acquired 3D OBC surveys (in 2000 and 2007) offers 4580 square kilometers of continuous 3D OBC converage. Coordination and intensive communications between 6 shareholders, 14 ADMA-OPCO divisions and 4 governmental agencies were required to facilitate this survey. The initial survey design (used for 2 months) used Distance Separated Simultaneous Source (DS3) acquisition but was revised to Managed Source and Spread (MSS) acquisition for improved fold, offset and acquisition efficiency. During the course of survey acquisition, 3 different test data sets were also acquired for technical analysis. A short history on the feasibility and complicated process leading up to the start of the survey will be offered, as well as samples of the onboard processing from the various areas will be presented. This survey was acquired in a little over half the time of previous OBC surveys in offshore Abu Dhabi. The data, based on limited onboard fast processing, is of high quality and illuminates deeper gas bearing structure and stratigraphy. Formal onshore processing has begun and promises excellent results. This innovative acquisition has provided more efficient data acquisition saving time and money, reduced HSE exposure in busy fields and will in conjunction with previous surveys provided one of the largest continuous offshore 3D datasets in the Arabian Gulf.

The 3D ocean bottom cable technique allows for acquiring long offset and wide azimuth seismic data. The use of simultaneous sources reduces the acquisition turn-around and HSE exposure. In shallow water environments, simultaneous source data are highly contaminated by surface waves and interference noise. Poor signal to noise ratio (S/N) affects velocity estimation, wavelet stability and overall image quality. This paper demonstrates the successful implementation of different processing and interpretation tools to deal with these challenges. The initial velocity model was built by extrapolating checkshot corrected sonic velocities along the interpreted key horizons and was subsequently updated to achieve final PSTM velocity. Several passes of noise attenuation were applied. Volumetric curvature analysis was used to monitor and protect fault planes from smearing during the denoising process. Seismic to well ties were continuously monitored to quantify the improvement after each key process was applied and to QC the seismic wavelet through different processing steps. A key factor to achieve a stable wavelet, at the end of the processing in the shallow water environment offshore Abu Dhabi, was the well driven horizon consistent velocity modeling. High seismic to well synthetic cross-correlation was observed on the final processed data due to the high S/N achieved by several passes of denoising, plus attenuation of strong multiple energy by velocity discrimination. High S/N, pickable geological events, and high resolution fault images are some of the key features of the final stacked image. In pre-stack data, long offset information is available to facilitate AVO and AVAz studies. Incorporating geological knowledge in the interpretation of horizons and faults and using well data during the course of seismic processing proved to be effective in obtaining a high quality seismic dataset.

Faults and fractures play an important role in reservoir production, since they can act either as barriers or conduits for fluid migration. They are also one of the key factors to be considered for drilling trajectory design. However, it is very challenging to delineate fractures due to the limitation of seismic resolution. The objective of this study was to conduct fracture cluster characterization for one of the carbonate reservoirs in Abu Dhabi, using a recently acquired 3D seismic survey. In this study, the similar workflow used by Singh et al. (2009) was followed. First of all, structural evolution and regional paleo including current principal stress orientations were investigated. Three major tectonic events were identified, and they are the main contributors to the development of faults and related fractures. Secondly, seismic data was processed based on an objective-driven processing sequence. Three fracture volumes were generated and five sets of fractures were interpreted. They were validated against the regional principal stress orientations, BHI interpretation, mud losses and well test data. In addition, previously the discontinuities along NNE-SSW direction in the area of interest were considered as seismic acquisition footprints. Through this study, it was proved that those discontinuities are small scale faults, hereby enhanced the existing reservoir characterization. The findings on faults and fractures characterization in the study area are critical in field development plan and drilling efficiency. This study showed that integrating data from different disciplines is a reliable and effective way to delineate fracture clusters.