Ultra-Deep Directional Resistivity Measurements and The Way of Revealing Formation Tops Ahead of Time

Abstract

Abstract Petroleum Development of Oman (PDO) operates most of the fields of the Sultanate. In one of the developed fields, the formation has many internal subdivisions and discontinuous sublayers separated by dense argillaceous zones that make it difficult to land the well. Challenges to landing include depth, thickness, and dip uncertainties. Additionally, there are the errors that are inherent to correlating between the reservoir scale and the seismic scale. Finally, it is difficult to discern boundaries in this low-resistivity environment characterized by very low resistivity contrast. Resistivity contrast, in particular, is of major importance as it plays the main role in determining the operability of common distance-to-boundary tools. A reservoir mapping-while-drilling tool with a large depth of investigation was used to reveal details of subsurface bedding and fluid contacts at the reservoir scale. The data provided were integrated with seismic surveys to refine the reservoir structure and geometry interpretation. The improved understanding made it possible to optimize recovery and production through optimum landing of the well and to map the overlying and underlying reservoir parameters. By selecting the right tool configurations in the bottom hole assembly (BHA), formation tops were detected from a distance of up to 27-m true vertical depth (TVT), well before the drilling bit reached the formation. Resistivity contrast was as low as 1 ohm.m versus the more typical 1.4 ohm.m when detecting the limestone marker in the shale unit. In two wells, a formation petrophysical evaluation tool deployed in the BHA was used to calculate and confirm the reservoir formation tops while drilling to serve as a conventional means of confirming the tops that had been detected in advance of the bit by the new technology.