Complex Resistivity Responses Explained Using LWD Laterolog Curves and Images, LWD Propagation Resistivity and Wireline Dielectric Measurements

Abstract

During field testing of a logging-while-drilling (LWD) laterolog resistivity and imaging tool, formation resistivity differences were observed between the new laterolog and standard propagation resistivity. This paper compares the resistivity measurement acquired in the same borehole using different tools in both sand and shale formations. In addition, the high-resolution images acquired by the new tool are used for a detailed geolgical analysis of the sequence. The high-resolution images acquired by the tool are used to determine the sedimentary environments in this complex fan delta sequence. A wide range of facies types can be identified on the images and correlated to available core with detailed examples shown of the key reservoir facies (distibutary channels and mouth bars). The images also provide valuable structural, depositional trend and insitu stress information for this well. The laterolog resistivities were higher in the shales and lower in the sands than the propagation resistivity values. The data was acquired while drilling in a water-based mud, sub-vertical exploration well in the South China Sea. While the main objective of the data acquisition in the siliciclastic formations was high-definition resistivity borehole images for detailed geological description, the radial laterolog resistivity response was also of interest. An advanced wireline multi-frequency dielectric measurement was also acquired, and its response was used for comparison and validation. In this paper, we associate the differences in resistivity response for varying formation properties to the tool physics, vertical resolution, depth of investigation, and time after bit between the measurements. In the sands, a resistivity inversion was applied to correct the logs for invasion effects and forward modeling used to resolve the resolution differences. The inverted formation resistivity from the LWD laterolog matches the deeper reading LWD propagation resistivity. The shale response was initially found to be more difficult to explain. It is commonly and historically accepted that due to resistivity anisotropy laterolog reads higher than propagation resistivity in low angle wells with laminated formations. Advanced forward modeling was used to investigate the laminations observed on the high-definition images and high-resolution laterolog resistivity curves. Although a model could be created to match both sets of resistivity measurements, the level of anisotropy required was considerably higher than expected, and supplementary information was required to validate the model. The wireline multi-frequency dielectric measurements provided the additional information required to confirm the anisotropy contrast observed by the resistivity modeling and confirm the LWD tool responses. This paper will compare the tool responses, and to determine the correct sand and shale resistivity. It will show how by combining different measurements, additional insight can be obtained into the nature of the formation and its properties.