Simplifying Geosteering interpretation and decision making in complex environments using Deep Resistivity Images from Azimuthal and Multiple Propagation Resistivity

Abstract

Abstract Directional drilling, particularly in conjunction with reservoir navigation (i.e., geosteering), is used to land wells at preplanned points in a reservoir and to optimize the wellbore placement of production sections with respect to reservoir boundaries and fluid contacts, by adjustment of well trajectories in real-time. Geosteering is accomplished and facilitated by the recognition of approaching conductivity contrast boundaries not yet penetrated by the wellbore by the use of deep reading tools and geologic models used to predict the boundaries approach. The decision making process needs to be quick and efficient as drilling progresses whilst decisions and interpretations are being made. Using deep resistivity images, and example templates for interpretation, assists in timely decision making and aid in simplifying what is a complex problem. The efficacy of the deep resistivity image interpretation concepts are illustrated by comparison of motifs observed in field data in complex channel sand environments to synthetic models and numerically modeled images of observed instrument responses. Addition of receiver antennas transverse to the axis of the coaxial antenna array permits acquisition of information on the direction to the boundary or contact. The physics of the method dictate that there is little or no detectable signal from the formation except in the presence of a conductivity contrast boundary within the (relatively large) volume of investigation of the antenna array. Electromagnetic radiation propagates considerable distances into resistive reservoir rocks and fluids, enabling electromagnetic logging-while-drilling instruments to detect such boundaries and estimate their orientation and distances in space relative to the wellbore. Images constructed from a combination of the usual coaxial dipole electromagnetic signal augmented by signals from transversely mounted receiver coils offer a visual interpretation option for the responses of logging-while-drilling propagation instruments. However, methods for interpretation of standard borehole resistivity images cannot be applied directly to the interpretation of deep resistivity images. These deep resistivity images vary with hole inclination, conductivity contrasts, and geologic structure, but usually produce a recognizable pattern, called a motif, that can be diagnostic of the geological_structure in the vicinity of the wellpath. Moreover, the motifs can be organized into a manageable number of themes that aid in their interpretation. Proper interpretation of the themes and motifs not only aids in geosteering decisions, but also enhances conventional formation evaluation by bringing an element of directionality to resistivity measurements that has not been possible in the past. In this paper we present real life examples from a challenging environment in the middle east, where simple motifs can help identify common responses and in future speed and aid interpretation for both the contractor and the client. In addition, the naming of these assists in the training of new Navigators, and clients alike. As the industry expands and the real-time decisions are pushed onto often less experienced personnel, and with the rapid change in technology the industry experiences, making interpretations simpler and more memorable is of key import.