Geosteering of horizontal well is a technique designed to maximize the well's contact with a preferred section of the reservoir. Operators often reduce drilling costs by reducing the number and quality of downhole tools while relying more heavily on the talents of their geosteerers. As a result, the stratigraphic based steering (SBS) has become ubiquitous for North American horizontal wells. SBS allows the projection of any/all horizontal MWD/LWD logs onto the true vertical thickness within an offset well logs domain. Using SBS geosteerers correlate this ‘verticalized’ log data to nearby exploration wells (typewells), and also to log data acquired uphole on the current well, to determine local apparent bed dip and wellbore location relative to the geologic target. Thus, SBS provides tools to generate and rapidly update a local geomodel which helps make informed well-placement decisions in real-time. At the same time, SBS techniques and workflows can vary dramatically among geosteerers and universal guidelines for interpretation and decision making do not currently exist.

In this paper, we analyze the SBS interpretations and decisions of 131 participants, representing six continents, of Geosteering World Cup (GWC) 2020. The GWC consisted of semifinal and final stages, each including one conventional and unconventional drilling simulation. All simulations were set-up based on real fields, from different regions, with varying formation dips and faults in the stratigraphy. Each simulation included a typical set of near-well logging tools, a pre-drill geological grid, and a designated geologic target zone (TZ) where the well was designed to be placed (every foot within the layer scored points). The geosteerers would target their wells and interpret the data generated for their unique well trajectory through a predefined model using a simulation function within a commercial SBS enabled geosteering software.

SBS geosteering is a reason-based choice process that answers the question "Where am I?" and uses the information available to decide "What to do next?". However, making these decisions is quite challenging because there is typically no direct measurement of what is ahead of the bit. We performed statistical analysis of GWC data to deduce how the geosteerers addressed these two questions. We highlight the differences in well positioning strategy with respect to the pre-drill grid, their own interpretation of the location of the TZ, and their responses when confronted with structural changes, such as faults bed dip inflections. Our results show that the quality of interpretation does not always correlate with the quality of the resulting well.

The studied empirical data produced in a controlled environment shines a light on better practices for SBS, which can be formulated as guidelines for geosteering training. At the same time, the gaps in data are identified, thus suggesting the changes to experimental set-up for further illuminate and unlock the geosteering human factor.

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