By Katie Mazerov, contributing editor
An index defining a broad range of surface and subsurface well complexity parameters has been developed and implemented by German operator Wintershall to assess risk, cost and contingency planning in drilling campaigns worldwide. “Our objective was to develop an easy-to-use scale, or well complexity index (WCI), that would immediately show us the complexity of the projects across our entire portfolio,” Blaise Nzeda, Senior Drilling Engineer for Wintershall Holding, said in a presentation at the 2014 IADC/SPE Drilling Conference & Exhibition on 4 March in Fort Worth, Texas.
“Complexity is basically a characterization of something with many parts in an intricate arrangement,” Mr Nzeda said. “By measuring the many and varied characteristics of a well, we can improve the planning process, compare projects for risk management and put the right experts in the parts of the operation that we believe are particularly complicated or may require special knowledge.”
The Wintershall WCI is made up of two categories of activities or conditions that make well design and drilling operations complex:
- A Drilling Complexity Index (DCI) that takes into account things such as well type, the well program, rig specifications, well trajectory, pressure and temperature and casing program. A myriad challenges are also measured, including formation issues that can be troublesome; operational issues concerning logging, coring, sidetracking and hole-cleaning; utilization of new technology; and HSE considerations regarding environmental and mud discharge requirements in a particular country;
- A Planning Complexity Index (PCI) that identifies surface challenges, including geopolitical issues, regulatory environment, security, planning time, weather and logistics, such as the ability to safely move equipment and personnel to the well site.
Factors influencing drilling and planning complexities are weighted on a scale of 0 to 10 on a Drill-O-Meter and Plan-O-Meter using a traffic light model: green for low complexity, yellow for medium complexity and red for high complexity. For a given well, the weighted parameters from each category are plotted on a well complexity graph to determine the overall well complexity.
In assessing the complexity of different well types, Wintershall determined that although offshore wells are generally more difficult than onshore wells, the latter can still be highly complex, he said. For example, the WCI determined that an onshore pilot drilling program for a sub-horizontal well with formation and stability issues and requiring a high mud weight in Russia was more complex than a 4,500-ft deep well with a normal pressure window in the North Sea.
“It is important to understand that we did not develop this tool to determine which wells we will drill, because we will drill all of them,” Mr Nzeda emphasized. “The objective is to determine the planning, resources and contingencies we need to assign to the project. The WCI allows us to look at each well to determine what is causing complexity at a certain level and address those issues methodically, in ways that make sense.”
As an example, he said that for a well drilled last year in Qatar with a value of more than 9 on the WCI, Wintershall immediately increased the contingency level. For exploration wells with little or no formation data, then the known parameters, such as water depth, are weighted, and a worst-case scenario is applied for the unknowns, he said. Wintershall also has used the WCI to verify a strong correlation between drilling complexity and nonproductive time in order to determine potential time and cost overruns on a well.
For more information, see IADC/SPE 167932, “Development of Well Complexity Index to Improve Risk and Cost Assessments of Oil and Gas Wells.”