Significant savings in drilling costs are claimed for DROPS - the Drilling Optimization Simulator. DROPS uses information from the first or previous wells drilled in a reservoir to optimize the drilling of subsequent wells, offering substantial potential savings in both the appraisal and development drilling phases of deepwater fields.
The simulator is equally applicable in conventional water depths, and has proved its worth in drilling development wells for Saga Petroleum and Statoil, which is currently using it in the Yme, Gullfaks Satellites, and Gullfaks C drilling programs.
The program uses the information from previous wells to calculate a meter-by-meter rock strength log or geological drilling log (GDL). In Norway, this information is required by the Norwegian Petroleum Directorate, so no extra cost is involved in making it available for the simulator, says Geir Hareland, advisor to DROPS Offshore which owns the simulator.
The GDL is created using rate of penetration models inverted to calculate rock compressive strength. It then forms the basis for simulating any combination of operating conditions, bit designs, pull depths, weight on bit, and revolutions per minute. With knowledge of all relevant costs - rig day-rate, bit cost, rentals, trip times, mud costs, logistics costs, and so on - the simulator can then identify the well plan giving the optimal cost level.
"DROPS uses the learning curve as an illustrative tool," says Hareland. "When drilling a sequence of wells, the costs normally drop with each subsequent well. But by simulating all the wells, you can steepen the learning curve, and in theory make the second well as optimal as the last. In practice we have achieved cost reductions of 30-50%."
The company is now working to provide a real-time model for evaluating the main criteria in the course of drilling, so that the well can be optimized as it is drilled. This is mainly a question of the operator making the information available in real-time, as the simulator is already capable of rapidly providing the optimization, Hareland says.
The firm also is developing a suite of modules to be used in conjunction with the simulator to optimize other parameters, such as casing design and well path. It is furthermore working on the technology to create the rock strength log directly from four-component seismic. "We're looking at using the earth model to create a drillability log, which will make it possible to start estimating the drilling cost before the first well," says Hareland. Current development work is receiving funding from the Research Council of Norway.
In the case of deepwater wells, the need to maintain the riser margin makes it necessary to drill in an over-balanced fashion, which means there is greater scope for optimizing and hence for cost-saving, Hareland says.
