Delegates at the Offshore Europe conference in Aberdeen, Sept. 7-10, were treated to updates on the development of new technologies in sectors ranging from offshore exploration to deepwater pipeline installation.
Among the highlights, T.W.R. Harris and P.J.J. Vullinghs of Shell U.K. Exploration & Production reviewed pro- gress on the NeuRobot joint industry project, which is developing an advanced subsea field development system. And T.M. Gaynor, G.T. Irvine, and R. Boulton of Sperry Sun Drilling Services, plus D.A. Gilchrist of Shell Expro, told delegates of a step-change improvement in drilling efficiency with a new motor and bit system.
Among other emerging technologies discussed at the meeting:
- A design model to predict dynamic underbalanced drilling effects was unveiled by: R. Rommetveit and O. S?vareid of RF-Rogaland Research, Stavanger; A.C.V.M. Lage and E. Nakagawa of Petroleo Brasileiro SA; A. Guarneri of Agip SpA; C. Georges of Elf Aquitaine SA; and A. Bijleveld of Shell.
- A technique for incorporating seismic data of intermediate vertical resolution into 3D reservoir models was presented by Ronald A. Behrens and Thomas T. Tran of Chevron Petroleum Technology Co.
- Hurdles to developing new technology for the installation of pipelines in ultradeepwater areas were detailed by Colin McKinnon of J.P. Kenny Ltd., Staines, U.K.
Harris et al. told delegates that coiled tubing drilling holds promises of significant reductions in well costs, particularly with three technologies being developed under the NeuRobot program.
These are: advanced well bore construction technology comprising expandable solid tubulars; intelligent reeled drilling incorporating advanced geosteering; and small, highly automated, dynamically positioned reeled-system drilling vessels.
Shell Expro's NeuRobot project began in 1996 with the goal of building on the company's vision of slender reeled monodiameter wells to be drilled using reeled composite coil and an intelligent coiled tubing string.
"The ultimate NeuRobot vision," said the authors, "consists of a reeled-systems drilling vessel, termed NB-2, which constructs new reeled monodiameter wells.
"A more tangible shorter-term stepping stone towards this vision was identified-namely NB-1, a light subsea well intervention andellipsecoiled tubing drilling vessel. NB-1 functionality does not include the ability to pull completions, as to do so would be costly in terms of the vessel, blowout preventer, and riser equipment required.
"NB-1 will include three key features, which produce significantly faster subsea well intervention than traditional anchored semisubmersibles: dynamic positioning, which negates time consuming anchor handling; fast transit speeds of about 12 knots to and from port and between well locations; and innovative wire line and coiled tubing intervention systems, integrated and maintained as part of the vessel."
Last year, Shell Expro set about procuring an NB-1 subsea well intervention vessel. "Unfortunately, the request came in the middle of the oil price slump and the project has been put on hold," said the authors. "NB-1 is still being worked and may be realized in some form by 2001."
Gaynor et al. showed how drilling efficiency in five mature North Sea fields was improved by using a systems approach in a drilling program managed by a team comprising representatives from a services company, a drilling company, and an operator.
The authors said that all the fields' well sections were geometrically complex and that data were gathered with downhole pressure and vibration sensors and acoustic calipers.
"Measurement while drilling (MWD) annular pressure measurements," said the authors, "had shown that conventional steerable motor-bit combinations required extensive circulation and occasional back-reaming to ensure adequate hole cleaning.
"MWD vibration measurements and bottom hole assembly modeling had proved that particularly high levels of vibration were caused by both circulating and back reaming. This was directly associated with MWD, motor, and other downhole tool failures.
"It was concluded that the basic problem was hole quality, and attention was directed to eliminating hole spiraling deduced from MWD logs and calipers."
After studies and trials, the team found that the use of a novel system comprising a pin-down positive displacement motor (PDM) and long-gauge box bit improved the overall drilling performance.
Previously, it was though impossible to use long-gauge bits on steerable PDMs, because the combination usually gave a poor deviation performance, notably in initiating and sustaining a deviation from an existing well path.
"The problem therefore," said the authors, "was to find a way to retain the attributes of the steerable motor while retaining the benefits of long-gauge bits. The solution was discovered by treating bit and motor as a system, and fundamental changes were made to the design of the motor and bit to make them compatible. The most obvious was the use of a box-connection bit and a pin-connection motor."
No motor or MWD failures were reported with the new system because of less-severe drilling conditions. Also, the time required to drill each section decreased significantly, thus improving overall efficiency.
"This owes much to root cause analysis," said the authors. "Much of what has been achieved is counter to conventional wisdom but is based on knowledge of downhole processes based upon direct measurements."
Rommetveit et al. told delegates of a design model they have created to help prevent excessive pressure transients during underbalanced drilling (UBD) operations.
"Successful UBD," said the authors, "depends on a complete understanding of the reservoir and its geology as well as proper design and computer modeling of the complex multiphase flow system."
The authors created the DynaFloDrill model to incorporate multiphase hydraulics in a realistic well geometry and well-reservoir interaction. The heart of the model is a set of equations describing the way fluids flow in the well bore.
"We assumed," said the authors, "that all variables depend on only one spatial coordinate: length along the flow line. The temperature in the well is assumed to be known. The governing equations are those expressing conservation of mass and momentum for the system of fluid components present in a drilling situation."
To prove their model, the authors performed measurements in a vertical, extensively instrumented test well. These were focused on dynamic operations such as kick-off or unloading, changes in liquids and gas injection rates and pipe connections, as well as steady-state operations.
"The tests were performed with either 'parasite string' or drillstring gas injection," they reported. "Simulations with the dynamic model were performed using experimental quantities as input; for example, geometries and liquids rates.
"Results show that the model performs very well for parasite string gas injection. For drilling gas-injection systems, the model performs well for low and medium gas rates. For very high gas rates, there is still room for improvement."
Behrens et al. explained that 3D earth models are best created with a combination of well logs and seismic data, but that seismic data have poor vertical resolution compared with wells.
Normally, the vertical resolution defines how the data are handled, with different approaches assigning different "weights" to the vertical data to create reservoir models of various degrees of accuracy.
"We introduced a new multiple-map Bayesian technique," said the authors, "with variable weights for the important middle ground where a single seismic map cannot effectively represent the entire reservoir. This new technique extends a previous Bayesian technique by incorporating multiple seismic property maps and also allowing vertically varying weighting functions for each map.
"This vertical weighting flexibility is physically important because the seismic maps represent reflected wave averages from rock property contrasts such as the top and the base of the reservoir. Depending on the seismic acquisition and reservoir properties, the seismic maps are physically represented by simple but non-constant weights in the new 3D earth modeling technique."
This new technique was tried out on two seismic attribute maps from existing reservoirs. The authors concluded that, "Coupled with 3D visualization, it offers a powerful tool for quantifying uncertainty and hence facilitates risk-based decision making."
McKinnon told delegates that the development of deepwater fields is contingent on the ability to construct pipelines and flow lines in ever increasing water depths.
To date, in-field flow lines less than 14-in. in diameter have been installed in more than 1,500 m of water, he said, but the installation of larger-diameter pipelines has been limited to about 600 m.
Pipelines installed so far in deep water have been put in place mainly with techniques based on technology that first appeared 20 years ago. But now McKinnon feels new techniques may be required to match operators' deepwater aspirations.
"Extending existing technology may increase these (current) limits still further," said McKinnon. "However, if the step to ultradeep water is to be realized-that is, installation in water depths of 3,000-4,000 m-new technology has to be developed in terms of design approach, materials capabilities, and installation capability.
"With respect to design, methods are sufficiently advanced to enable accurate prediction of pipeline behavior, allowing realistic designs to be performed.
"The capability to manufacture large-diameter line pipe for ultradeep water application is currently not available. However, it is considered technically feasible to manufacture the necessary pipe.
"With respect to installation, J-lay is considered to be the most feasible method of installing pipelines in ultradeep water; however, further developments and significant increase in equipment capability would be necessary."