Improving LWD Image and Formation Evaluation by Using Dynamically Corrected Drilling-Derived LWD Depth and Continuous Inclination and Azimuth Measurements

Abstract

Summary The paper illustrates the improvements in logging while drilling (LWD) images and subsequent formation evaluation by using a new methodology for depth and survey measurements corrections. LWD depth measurements are often considered inaccurate and, therefore, not as reliable for well-to-well correlations, correlations with data acquired with wireline measurements and formation layer thickness determinations. The reasons for these inaccuracies generally originate from the traditional practice that LWD depth is purposely made equal to the driller's depth, which is a static pipe length measurement made by tape at the surface. There is almost always a difference between the actual measured depth (MD) of the LWD sensor downhole and this static pipe measurement, because downhole the drillpipe is subject to an environment that is not representative of the derrick (e.g., varying drilling mechanical conditions and temperature changes). Here, we demonstrate the applications of the method, which allows dynamic driller's depth correction for the effects of drillstring weight, downhole friction, weight on bit, thermal expansion, residual rig heave, and tide. Another significant inaccuracy source is a standard practice of calculating borehole position from stationary survey points typically taken every 90 feet (ft) using the minimum curvature method. Neglecting the complex borehole shape between survey stations can lead to a systematic error in determining the borehole position. We consider using continuous inclination and azimuth measurements along with stationary surveys to correct these errors. We provide comparisons of LWD images before and after the depth and survey corrections to illustrate how the measurement errors affect formation dips interpreted from the images. We demonstrate how improved accuracy allows filtering out the artifacts and provides more decisive and accurate identification of geologic features. We show how using the corrected 3D position improves accuracy of the formation thicknesses calculations and therefore improves the reservoir summation results. As a result, we propose a borehole 3D position measurement that is accurate, consistent between wells (regardless of rig type or bottomhole assembly [BHA] configuration), and independent of the drilling mode. Using this new measurement significantly improves the quality of the formation evaluation.