Improved Petrophysical Analysis in Horizontal Wells: From Log Modeling Through Formation Evaluation to Reducing Model Uncertainty - A Case Study
Authors: Valdisturlo A. , Mele M. , Maggs D. , Lattuada S. , Griffiths R. 
Published: June 2013

This paper presents the results of using a new workflow to correct and validate logging-while-drilling measurements (LWD) from horizontal wells, and the impact of the results in the petrophysical answers derived from the measurements. The workflow involves building a layered geological model and modeling the log responses in a model-compare-update loop to obtain the log properties of each layer. While similar methodology has been available in the past, the process was laborious and time consuming, and therefore it was not well applied in the industry. The paper demonstrates how the new process addresses the most common effects in horizontal wells in a timely and efficient manner, allowing it to form a part of petrophysical analysis in high angle and horizontal wells.

In high angle and horizontal wells it is often difficult to apply the traditional petrophysical interpretation techniques normally used in vertical wells, due to geometric effects on the data in particular the resistivity logs. These effects include local layering or resistivity anisotropy, and boundary effects such as proximity and polarization horns on the resistivity measurements. Other effects complicating the borehole environment include asymmetric invasion profiles, the presence of cuttings beds and drilling mud segregation. This means that the data is challenging to interpret and the petrophysical answers from horizontal wells are not always fully used in static reservoir models. The inclusion of the corrected petrophysical properties from this processing into the static reservoir model reduces uncertainty and improves model accuracy.

The workflow was applied on wells in a development field in North America. The reservoir consists of a few tens of feet thick silty sand and siltstone layers deposited in a shelfal environment. The extended reach wells used in the development of the field have long lateral sections (from 5,000 to 10,000 ft). Due to the geological complexity of the area, the wells often cross multiple layers and faults and are actively steered to optimize reservoir contact. The geological environment from static reservoir model was efficiently confirmed and refined, log responses corrected and verified before being used in the petrophysical analysis. The comparison of log responses between vertical and deviated wells was helpful both for quality control and in the well log modeling phase to assess the correct record of petrophysical properties for the input logs and for the modeling results. The update of the log measurements and resulting improvement in petrophysical answers is presented. The workflow requires strong integration between the Reservoir Geology, Drilling and Petrophysics teams.

The paper presents a case study of the application of a new workflow to improve petrophysical answers from logging while drilling measurements in high angle and horizontal wells. The study demonstrates how log modeling in high angle and horizontal wells can be used to improve formation evaluation. The improved formation evaluation and updated geological model reduces uncertainty and adds detail.