Geosteering Diagnosis: A New Approach to Monitor the Well Position within a 3D Geological Model

Abstract

With the increasing demand for more efficient drilling operations, the oil industry is searching for new ways of utilizing all data available while drilling. The integration of the realtime drilling information with data from offset wells and the reservoir model is considered of high value for accurate wellbore placement and geosteering. This paper introduces an original methodology to better understand and analyze the well position within the subsurface and to warn the end user when predefined geological and drilling events are appearing or prognosed ahead of the drill bit. The concept is called Geosteering Diagnosis. We identify a monitored trajectory as composed of a real-time and a project-ahead section. The bit and the MWD/LWD sensors are monitored objects attached to the trajectory. A number of targeted objects (i.e. horizons, contacts, faults, targets, planned and previous drilled wells trajectories, well and grid properties) are accessible from the 3D geological model. Rules can be defined based on the relationship between the monitored objects and any targeted objects. Various thresholds are used to identify events at the time of the drilling and alarms can be triggered on specific cases. The Geosteering Diagnosis is applied to a production well in the North Sea. The rules are used to monitor drilling against the predefined geological model. They provide quantitative information, which can be used to either update the geological model, or take actions regarding the course of the well. Introduction 3D visualization tools are today part of the drilling operation process, allowing a better communication and understanding between the experts of the asset team. Real-time data are integrated within the earth model and can be visualized in real-time operation centers. Measurement While Drilling data (MWD) and the Logging While Drilling data (LWD) are used to evaluate the position of the well, assess the physical properties of the rocks and fluids drilled, and analyze the behaviour of the drillstring in real-time. Positioning horizontal wells within few centimetres height of targeted areas is only possible because of the evolution of the combined drilling and logging technologies. From the kick-off point, through the landing area and within the reservoir it is critical to have the best possible understanding of the formations crossed by the drill bit. The LWD data provides that knowledge in real-time at a high resolution scale. An optimal decision making process will not only rely on a constant monitoring of the drilling data within the 3D geological environment, but also on an accurate understanding of where these measurements are performed along the drillstring. Some logging data can effectively be measured tens of meters behind the bit. Each of the physical property measured by the MWD/LWD tools can be defined onto the Bottom Hole Assembly by its distance from the bit. The experience shows that it is often difficult to measure the distances between a measurement point along the BHA and a particular object in the 3D geological model. But this knowledge is important for the asset team for taking actions regarding the update of the 3D earth model at the time of the drilling. A diagnosis approach was then proposed to constantly monitor and analyse the MWD/LWD information against the geological model and predict future trends of the data based on the project-ahead trajectory. The methodology relies on the definition of rules based on measured distances and property differences between tools on the BHA and objects of the geological model. By applying multiple thresholds on the rules it is possible to define different levels of warnings, which will help to identify and predict coming events. At the same time the project-ahead approach is used to predict when possible events will appear. The Geosteering Diagnosis can be an excellent tool to help validating the geological model update at the time of the drilling. A final objective, but somewhat far away at this preliminary stage, would be automated well follow-up and steering reducing on-site personal.