Horizontal wells dominate the development of unconventional shale reservoirs. Using real time drilling data to steer in a target zone is the key to economic success. Today structural interpretation in unconventional horizontal wells is a manual process that is time consuming, tedious, and error-prone, especially because gamma-ray (GR) logs are commonly the only available logging-while-drilling (LWD) data. For the first time, we have developed a method to automate interpretation of subsurface structure.
TST3D (true stratigraphic thickness in 3D) automates structural interpretation using pattern recognition. Given an initial structural model, TST3D automatically computes TST as the shortest distance from each wellbore survey location to the initial surface and then matches GR patterns in the horizontal well to those seen in a vertical pilot well in TST domain. TST3D inserts fold hinges, bends the structure, then recomputes the modeled GR response, progressively matching the pilot well log signature, from heel to toe in the horizontal well. We make three assumptions in the current TST3D: constant layer thickness across the drilled interval, GR variation follows structural layering and no faults. Those assumptions are reasonable in most shale plays.
TST3D can be applied in either a post-drill mode for structural interpretation or real-time mode for automated geosteering. Field tests in different shale plays and complex well trajectories demonstrate that TST3D runs quickly: a structural model of a 10,000-ft horizontal section can be computed in minutes, and a real-time update of 100 ft of new data takes less than a minute. Automating the geosteering correlation process would allow well placement engineers to cover multiple wells simultaneously, increasing the efficiency of the team while potentially improving service quality. Beyond structural interpretation, we believe that TST3D has great potential to contribute to the digital transformation of formation evaluation and drilling automation.
Many oil- and gas-bearing reservoirs have been drilled using highly deviated and horizontal wells. Such wells are essential to the successful development of unconventional reservoirs, especially when combined with advanced hydraulic fracturing techniques. Even though horizontal wells are more expensive to drill, they are still economically viable. Such wells are preferred by operators because they enhance productivity due to increased exposure of reservoir rock to the wellbore. However, structural interpretation of horizontal wells remains challenging because a horizontal wellbore may penetrate the same stratigraphic layer many times at varying angles, which makes it difficult to align and correlate measurement signatures along highly deviated or horizontal well trajectories.