MWD - The Basis of Geosteering
Geosteering focuses on the interpreting of Measurement While Drilling (MWD) data to determine the current wellbore position relative to stratigraphy. MWD data is acquired and transmitted to the surface by tools incorporated into the Bottom Hole Assembly (BHA). MWD data can be sent in real-time via acoustic or electromagnetic communication systems. One critical type of MWD data is the positional data which includes vertical wellbore inclination as well as a magnetic azimuth relative to north associated with a specified measure depth (MD). The interpolation of this data produces a three-dimensional wellbore path that is used by the drilling team in order to steer the well in the preferred direction.
Another type of data which is transmitted to the surface as part of the common MWD data set is the gamma ray signal. As different stratigraphic layers display different gamma ray magnitude signatures they can represent the wellbores current geologic position within the reservoir. By comparing the MWD gamma ray signature to the signature from surrounding wells the geosteerer determines geological horizon behavior and provides direction for future wellbore placement.
Since its introduction in the 1980s MWD has continued to evolve by building more accurate and higher resolution tools which make geosteering less complicated and more precise.
Logging While Drilling (LWD) solutions carry additional well logging tools as part of the BHA. In addition to Positional and Gamma Ray data you are receiving a number of additional logs in real-time. They can provide the geosteerer with pertinent petrophysical data, including spectral gamma ray, neutron-density, resistivity, sonic etc.
These log data offer significant advantages when compared to simple MWD based geosteering. They allow the geosteerer to perform detailed analysis of the reservoir while drilling which allows for solid decision-making on projects with high structural and reservoir property uncertainties.
In the drilling industry a more advanced tool, such as LWD, increases efficiency of the drilling process and minimizes risk associated with these wells. LWD is often significantly more expensive that standard MWD technologies so it is most often used by larger oil and gas operators whose primary goal is maximal efficiency and safety. LWD prices have been trending down for several years making what was price prohibitive for many projects now accessible. At some point LWD may completely displace MWD by providing more data at a similar cost.
Technologically advanced LWD solutions are setting the trend for today’s complicated drilling and geosteering projects and there is no sign of going back!
Borehole Image Log Data
LWD tools are also capable of capturing high-resolution images of the borehole called Image Logs (Image Log resolution depends on Image Log type: resistivity, density or gamma ray).
Borehole Image Logs are a special type of log which provide one of the most effective data streams for use in well placement optimization. Those logs can be interpreted in real-time to make estimations of apparent and true dips of penetrated bedding planes. It can assist in more accurately determining the position of faults and fractures encountered by the wellbore. Having these structural details of the reservoir in real-time is a powerful tool for the geosteerer in planning his optimal “well driving” path.
Seismic data interpretation is an important component in both the planning and the geosteering of a well. Having a seismic cross-section as a backdrop of your plan trajectory allows a geosteerer can accurately predict the position of upcoming faults and dip change. This allows adjustment of the wellbore trajectory in preparation for those geologic features. This data is available before drilling commences so the geosteerer can prepare for possible operational pitfalls and all major changes in reservoir geometry.