MWD - the basis of geosteering
The basis (the main input data in other words) for geosteering is MWD data. MWD, or measurement while drilling, is a tool that is used downhole as a part of drilling assembly next to the bit while drilling. MWD tool is capable of sending real-time data including vertical inclination and azimuth as magnetic direction from the north at specific measurement depth through acoustic signal (no wires required). The interpolation of this data results in the production of a three-dimensional path of the wellbore and provides the engineer with the most important knowledge so that the wellbore is effectively steered in preferred direction.
Another type of data that is transmitted to the surface at every depth point as a part of MWD dataset is a gamma ray signal. The patterns (signatures) of gamma ray signals represent the geology of the reservoir where the horizontal well is being drilling at the current moment. By comparing the gamma ray signatures of the well under drilling vs. the typical gamma ray signature in surrounding wells the geosteering specialist makes a decision about the geological horizons behavior and require a further direction of well steering.
Since its conception and introduction to the drilling industry in 1980s MWD has continued to evolve by building better and smarter features that make geosteering procedures far less complicated than ever before. A more advanced form of MWD is logging while drilling (LWD).
Unlike MWD systems, Logging while drilling (LWD) also carries the well logging tools as a part of bottom hole assembly (BHA). In other words in addition to the directional survey you are getting a number of useful logs in real time. They can provide the geosteering specialist with pertinent petrophysical data, including spectral gamma ray, neutron-density, resistivity, sonic, etc.
These log data offers significant advantages for geosteering comparing to regular MWD based geosteering by helping the geosteering engineer to perform detailed analysis of the reservoir while drilling for solid decision-making in drilling projects with the high structural and reservoir property uncertainties.
In the drilling industry, a more advanced and scientific tool such as LWD increases efficiency of the drilling process and minimizes risk associated with the activity but it is usually more expensive than regular MWD technologies. Therefore it is typical for LWD services to be ordered frequently by big oil&gas companies whose main goal is effectiveness and safety. There is a tendency that LWD service is getting cheaper every year. In ten years from now it may replace MWD by providing more information for the same price.
The need for technologically advanced solutions of LWD is becoming more evident. LWD systems are setting the trend for today’s drilling and geosteering, and there is no sign of going back.
Borehole Image Log Data
LWD tools are also capable of capturing high-quality images of the borehole called Image Logs (Image Log resolution depends on Image Log type: resistivity, density, gamma ray).
Borehole Image Logs is special type of logs that give one the most useful input to the process of well placement optimization (geosteering in other words). Those logs can be interpreted in real-time with estimation of true dips of the beddings that well has just penetrated, identifying faults and fractures crossed by well. Having these structural details of the reservoir in real-time the geosteerer can better plan his actions of “well driving”.
Seismic data interpretation is an important piece of data that can be used for well planning and geosteering. Having a seismic cross-section as a backdrop of your plan trajectory, geosteerer can predict the position of faults, dip changes and be ready to change the trajectory according to those geological features. This data is available before the well drilling, so the geosteerer can spend some time analyzing possible pitfalls and be ready for all major changes of reservoir geometry.