Techniques For Accurate Geological Correlations in Highly Deviated and Horizontal Wells When Drilling Through Dipping Beds
Authors: Rasmus J.C. , Kashikar S. , Liu A. 
Published: December 2007

Abstract
Our ability to drill highly deviated and turned wells (sometimes exceeding both 90 degrees of build and turn) has increased significantly. This has necessitated the need for better tools for making accurate log correlations while drilling to keep the well in the pay zone.
A new technique is presented which allows for the changing wellbore trajectory geometry, horizontal displacement, and the dip of the bed to be accounted for simultaneously. This allows the beds to be represented accurately in space so that meaningful correlations can be made while drilling. This impacts steering decisions which aim to keep the wellbore in the pay zone.

Introduction
The ability to target smaller reservoirs with highly deviated and curved wellbores has been greatly supported by the development of new Measurements and Logging While Drilling (LWD) tools. Many of the directional drilling parameters and much of the geological data are being acquired real time, or shortly thereafter, and at the bit or closer to the bit then in the past. These parameters include continuous bit-tool-face and inclination readings, resistivity, and gamma ray data. With this data in hand the geologist has the ability to optimize the position of the well path while drilling, and make recommendations for setting casing, completing, or plugging and abandoning.
In areas with dipping geologic beds, the ability to correlate logs often increases in difficulty as the hole angle increases and/or the well path is turned. The standard procedure for correlating well logs involves transforming the logs from a measured depth index to a true vertical depth (TVD) index preferably referenced to sub-sea depths. Accurate correlations using TVD can only be obtained in vertical wells and/or wells drilled through beds with no structural dip. TVD correlations become meaningless when deviated, turning wells penetrate dipping beds. Another industry method is to use true vertical thickness (TVT) logs. By using a thickness rather than an absolute depth reference, deviated wells drilled through dipping parallel beds can be correlated. However, this technique fails 1.) when the wellbore inclination and/or azimuth change significantly, 2.) the wellbore no longer drills down section, or 3.) when non-parallel beds are drilled with deviated wellbores.
A new technique is presented which integrates changes in borehole azimuth, inclination, and beds dips simultaneously at the sample rate of the measured depth log. This technique calculates a true bed depth (TBD) for a given surface reference point allowing the beds to be more accurately represented in space so that meaningful correlations can be made.
An additional limit with using conventional TVD correlations occurs when a deviated well surpasses 90 degrees inclination. The TVD log will usually no longer display the data as TVD decreases. Even with programs that do display the data, the relative horizontal displacement associated with the data points are not known. TBD logs will not experience this problem as long as the wellbore continues to drill down section through the beds. When drilling up through the geological section, correlations and bedding interpretations can only be done with the logs plotted along the trajectory axis (measured depth) and the surfaces displayed in 3D.

Theory and Definitions

TVD
Fig. 1 illustrates a wellbore trajectory penetrating a bed and the definition of TVD. TVD is the horizontal projection of the wellbore depth back to a vertical line placed directly below the surface location of the wellbore. Conventional MWD or wireline survey programs compute this transformation using the measured depth along the wellbore inclination of the wellbore from the vertical, and azimuth of the wellbore all measured at given intervals along the trajectory. The most common technique is the minimum curvature method where a spherical arc is fit onto adjacent survey points. P. 907