The era of easy oil is gradually coming to an end. Over the last 5 years, the big discoveries around the globe have been in areas with complex geology that are not easy to access and, in most cases, present logistical difficulties.
One of the most promising of these regions is the so-called Golden Triangle, which includes offshore US Gulf of Mexico, offshore Brazil, and offshore western Africa. These areas lie in deep or ultradeep waters, ranging from 2000 to 3000 m, and are below a considerable layer of salt rock—up to 2000 m thick in the Brazilian case. The reservoir geology is also complex, ranging from microbial carbonates to dolomitized limestones to intercalated sandstones. Exploiting these resources in a cost-effective manner presents a number of challenges for the industry, which I shall discuss as they relate to offshore Brazil.
Reservoir Imaging
Presalt reservoir imaging and characterization are the starting points for achieving a successful development campaign. Although it has improved during the past few years, reservoir imaging still poses big challenges to operators and service companies offshore Brazil. Room for further improvement certainly exists. The high reflectiveness of the postsalt/salt interface leads to major attenuations of the seismic signal, jeopardizing vertical resolution. Additionally, the salt layer heterogeneity, composed of halite intercalated with layers of anhydrite, carnalite, and tachyhydrite, brings additional hurdles to the modeling of the velocity cube, which is used in the time-depth conversion. The reservoir carbonate rock also presents a high degree of heterogeneity, with important insertions of silica nodes and, at least until now, an unknown fracture pattern, in addition to the need for better understanding of pore size distribution and connection.
However, technologies are emerging in Brazil that enhance both vertical seismic resolution and rock characterization. Wide-azimuthal seismic acquisition, using a single- or dual-coil survey, is a technique being introduced into the presalt with encouraging results for both the enhancement of vertical resolution as well as improvement of the velocity cube accuracy. Additionally, the use of variable-depth streamers in a slanted cable configuration in conventional seismic acquisition also improves the seismic illumination of the reservoir rock.
Areas of research seeking similar results are related to the seismic processing workflow. New ways of processing seismic data, introducing new steps or shuffling existing steps, can also improve reservoir characterization. These include using new special core analysis techniques, such as high-field nuclear magnetic resonance (NMR) imaging and determining accurate rock fabric and flow units, which seek to better characterize the pore size distribution and connection of the reservoir rock. Additionally, these techniques will improve the upscale process workflow envisaging geologic and reservoir models. Other processes that will help illuminate faulting and fracturing patterns existing in the reservoir rock are “walk away” and “walk around” 3D vertical seismic profile (VSP) techniques; however, the processing workflow of these techniques needs improvement to enhance the accuracy of the seismic calibration.
Because of the low compressibility of the carbonate rock, 4D seismic may not be an effective tool for reservoir monitoring. The transient electromagnetic method is a technique under discussion that may help shed some light on the problem when it comes to detecting the injected water front away from the producing wells. Use of this technique might result in better management of the reservoir sweep efficiency and, as a consequence, maximize recovery factors.
Well Construction
Drilling costs represent one of the biggest hurdles associated with presalt field development. Nonproductive time (NPT) due to blowout preventer (BOP) stack failure is costly and needs to be addressed. Excessively slow penetration rate while drilling the target reservoir is another hindrance. Other areas of research interest are lost circulation prevention, drillstring vibration, and stuck pipe. To address these challenges, a number of efforts are under way, such as influencing BOP manufacturers to rethink current BOP stack design; improving bit design with emerging new designs, such as the Kymera hybrid drillbit; optimizing the bottomhole assembly to reduce drillstring vibration; and developing microemulsions that can provide proper lubrication to the drillstring in order to avoid stuck pipe.
Another major problem associated with presalt development is well stability during the production lifespan and the corresponding uncertainty of how the salt layer will react to potential subsidence. In order to minimize possible impacts, geomechanical reservoir flow simulation two-way coupling will help predict stresses and strains acting upon well structures and will provide safer well trajectories as field development advances. Seismic/geomechanical inversion may provide the means to update and calibrate the flux-deformation model.
Flow Assurance
Potential flow assurance issues in presalt fields are being treated through the development of a combination of products that minimize environmental footprint. Nanotechnology can be applied to develop nanomembranes to be used in production facilities, reducing the size of the processing equipment, which helps reduce the corresponding environmental impact of such installations. Intelligent production equipment, coupled with supervisory algorithms, helps automate well operations and optimize reservoir management and productivity.
Conclusion
The deepwater Brazilian presalt represents a huge opportunity for operators, service companies, and academia to develop leading-edge technologies that will help the industry access these hard-to-reach resources, as well as help the country improve its technology base. TWA
