Marginal oilfields normally refer to the oilfields that are less economical or less profitable when the conventional technology and management are applied in the process of oilfield's development and construction. They primarily show the characteristics of low permeability, remote, and high viscosity. There are abundant oil and gas resource in the Bohai Bay of China, and however, a large portion of them are marginal oilfields, which restrict high-efficient development and construction of the Bohai oilfields. Due to limited crude oil reserves and short development cycle, equipment and facilities should be simplified to reduce the development cost of platforms and subsea pipelines to achieve a higher IRR, especially in the current era of persistently low oil prices. So, the production platform is normally designed as unmanned wellhead platform. Based on the considerations above and a marginal oilfield NB located in Bohai bay, a new-style pipeline system, primary-secondary pipeline system, is proposed for delivering oil-gas-water mixtures from unmanned platform WHP to central processing platform CEP. The system has a great advantage over the conventional practice is that in the primary pipeline's displacement procedure when shutdown. Then the design principle, design method, and flow assurance schemes of the primary-secondary pipeline system are mainly studied and analyzed. The system is of great significance for marginal oilfields’ development and other situations.
There are a large percentage of marginal oilfields, which normally show the characteristics of low permeability, remote and high viscosity, in Bohai Bay of China. In general, the overall cost to produce one barrel of oil tends to be higher and internal rate of return (IRR) tends to be lower when the routine technology and management are used to develop the marginal oilfields. Because of pockets of oil reserves and short development period, and in order to gain an effective development of marginal oilfields, equipment and facilities simplification must be made to achieve the goal of reducing investment in platforms and subsea pipelines. Among them, the investment of the subsea pipeline, especially the fee of laying the pipeline, accounts for a large proportion in the entire project investment, even reaching 50%. Therefore, reduction the investment of subsea pipelines will have a greatly positive effect on lowing the cost and increasing the benefits of marginal oilfields, particularly in the era of current low oil price. Therefore, some simplifications and new technologies should be adopted to achieve an effective development.
A multi-piled oil pier is planned to be built in Bohai Sea. For such piled foundation structures in cold seas, ice loads on the piles are often considered as the dominant loads in design. A series of model tests were performed to evaluate the ice loads on the structure and the pile-group effect. Considering the ice conditions of the study sea area, in total, 6 tests were performed and the ice forces on each pile were measured under different test conditions. The test results and discussions are presented in this paper.
With the rapid development of marine transporting and oil exploring activities, large number of oil wharfs have been built up on the shorelines of cold region seas in recent decades. For example, from 2005 to 2015 more than 10 large ports with big oil or gas wharfs were built along the shorelines of Bohai Sea, which is the northernmost sea of China. To achieve large water depth for giant oil transporting vessels, the oil loading terminal needs to be designed to extend to deep water area through multi-piled oil pier. This kind of structure consists of a series of piles and its upper structure (Fig. 1). Furthermore, these structures usually have incorporated batter piles to resist lateral loading, such as ship impact or environmental loads (i.e. wind load, wave load, seismic load or ice load).
Ice loads on the piles are often considered as the dominant loads in design when such structures are located in cold seas. Mainly two types of models for estimating the ice load on single pile have been developed, i.e. analytical models and semi-empirical models. Various aspects of ice actions on multi-piled structures were prior investigated analytically and experimentally (Kato, 1983; Toyama, 1994). These experimental or theoretic studies have given some insightful investigations on the ice loads on multi-piled structures. It has been proven by many researchers that the key in determining ice loads on multi-piled structure is the evaluation of pile-group effect by which the ice load on single pile is reduced.
Piling-up of ice rubble in conductor array has been a significant threat to the jacket production platform in cold regions. This study investigates the ice pile-up process in the conductor array of a jacket platform by conducting a series of model tests in the ice tank at Tianjin University. The model of the jacket platform consisted of four legs with upward-downward ice-breaking cones and a 4 × 5 conductor array, and the ice pile-up process was achieved by towing the model jacket through the ice sheet under different speeds, directions and water levels. During the tests, various ice failure modes and pile-up patterns were observed, and the dimensions of the ice rubble piles under different conditions, as well as the horizontal ice forces acting on the conductor array, were measured. Based on the measured data, the distribution of the ice rubble pile in the conductor array was found to be non-homogeneous, both laterally and longitudinally, and the rubble pile was found “grounded” on the bottom of the model conductor array in some cases, which might bring about additional load to the jacket structure. Furthermore, comparisons of the ice forces based on the present tests with the calculated results from ISO algorithm were also made. A modified R value of 4.2 was recommended for the present jacket structure under the present ice conditions.
Explorations for offshore oil and gas reserves in cold regions have increased rapidly over the past decades. As one of the largest offshore oil production bases in China, the Bohai Sea contains significant hydrocarbon deposits, and jacket platform has been the predominant offshore structure for the oil exploration in this region. Being the northernmost sea of China, the Bohai Sea is always covered by ice in winter, which has caused many problems to the engineers. Among the ice-induced problems, the piling-up of ice within the jacket structure has frequently occurred in recent years.
Full waveform inversion (FWI) usually gives a poor update of the sediment velocity when in the close vicinity of the top of salt (TOS) reflection. This phenomenon is a common practical challenge and is due to the strong velocity contrast between the sediment and salt—although its exact cause is not yet well understood. We investigated the relationship between FWI’s sediment velocity update and the role of salt insertion, namely the accuracy of the salt interface used in the FWI input velocity model. The results indicated that the initial model with salt inserted helps update the sediment model, and furthermore, using a more accurate TOS (i.e., the TOS interpretation used to insert the salt is closer to the true TOS) provides a better sediment velocity update. However, an accurate TOS is not available unless the sediment velocity, particularly directly above the TOS, has been correctly updated. Therefore, we propose a workflow to obtain a more accurate TOS for the FWI sediment velocity update using iterative FWI and salt interpretation. Using 2D synthetic data and 3D real data, we demonstrate that our workflow yields a better FWI update above the salt compared to using the sediment model as the initial model.
FWI aims to minimize the misfit of phase and amplitude between real shot gathers and synthetic shot gathers (Lailly, 1983; Tarantola, 1984; Sirgue and Pratt, 2004; Virieux and Operto, 2009). Updating the velocity in areas of high impedence and/or velocity contrast, such as the sedimentsalt boundary, is challenging for FWI. Two approaches are commonly used to address this overburden velocity update problem. Kapoor et al. (2012) used sediment models created from ray-tracing tomography as FWI input. Chen et al. (2014) used a salt model approach that ran an initial pass of FWI with the sediment model to update the sediment velocity before creating a salt model for the second pass of FWI to update the sediment velocity again. However, few studies have been performed to understand the impact of the TOS interpretation on the sediment velocity update using FWI. By using band limited data and conventional FWI, the TOS singularities are not automatically updated at each iteration.
The presence of shallow gas anomalies continues to be a challenge in seismic imaging in the Gulf of Mexico (GOM). Due to the strong absorption property and irregular shape of a gas cloud, the seismic image below it could suffer from frequency, amplitude, and phase distortion. Q tomography and pre-stack depth Q migration (Q-PSDM) are efficient tools for compensating these distortion effects caused by absorptive heterogeneities. However, deriving Q models is a challenging task, especially when the shape of gas anomalies are complex and the velocity inside or close to the gas cloud area is not correct. Standard ray-based tomography fails to capture the detailed rapid velocity variation for the areas with or beneath a high lateral or vertical contrast (e.g., gas pocket, shale, salt, and carbonate). Full waveform inversion (FWI) provides high-resolution velocity details but is still an expensive process with its success highly dependent on the quality of the initial model. We present a case study at East Breaks, GOM, that combines adaptive data-selection FWI and Q tomography to invert the velocity and absorption model around a gas cloud area to improve the seismic image.
Elebiju, Bunmi (BP America) | Ariston, Pierre-Olivier (BP America) | van Gestel, Jean-Paul (BP America) | Murphy, Rachel (BP America) | Chakraborty, Samarjit (BP America) | Jansen, Kjetil (BP America) | Rodenberger, Douglas (Shell America) | White, Roy C. (Shell America) | Chen, Yongping (CGG) | Hren, David (CGG) | Hu, Lingli (CGG) | Huang, Yan (CGG)
Using the Kepler and Ariel Fields as a case study, this paper discusses the processing challenges and solutions applied to a 4D co-processing of Wide Azimuth Towed Streamer (WATS) on Narrow Azimuth Towed Streamer (NATS) data. Unlike a dedicated 4D acquisition, WATS on NATS 4D has relatively low repeatability in terms of acquisition geometry and bandwidth differences. All these factors can negatively impact the extraction of a meaningful 4D signal. In this paper, we demonstrate how processing techniques can help to increase repeatability and enhance 4D signal. We focus on the following 4D processing procedures: 4D co-binning, data matching, and post-migration co-denoise. Due largely to these techniques, the final co-processed volumes show an optimized 4D seismic signal with a median Normalized Root Mean Square (NRMS, which measures the repeatability between base and monitor. Details refer to Kragh and Christie, 2002) of 0.10 along the water bottom and 0.28 above the reservoir.
Pre-salt targets in Brazil’s Santos Basin have become the focus of much exploration in recent years, creating a need for higher resolution images over these complex structures. Variable-depth streamer acquisition is an emerging technology in the Santos Basin that can increase the usable bandwidth in both the low and high frequency ranges, gain stronger low frequency penetration, and reduce acquisition related noise. We examine the benefits of variable-depth streamer data for producing high-resolution pre-salt images and the potential for higher resolution velocity models in the Santos Basin. We utilize a real field data set to demonstrate that variable-depth acquisition, combined with advanced processing techniques, provides improvements to the pre-salt imaging in this region.
The impedance contrast of a salt/sediment interface can generate significant wave mode conversions in seismic reflection data. The salt-related converted waves (C-waves) travel at different velocities than pure-mode compressional (P) waves and thus generate artifacts if migrated with P-wave velocities. These artifacts can often interfere with salt and subsalt interpretation. However, C-waves can actually provide additional information about a structure, especially in regions of low illumination; if migrated with the correct velocity, they may actually aid imaging and interpretation. We explore the possibility of correctly positioning high-amplitude C-waves from the base of salt reflection with a dual-leg 3D acoustic modeling method. Additionally, we study the artifact removal through pre-migration adaptive subtraction and post-migration Vector Offset Output (VOO) RTM stacking using Gulf of Mexico (GOM) data. To aid base of salt interpretation, we run dual-flood RTM using C-wave energies as supplements for base of salt imaging.