Using Pay Zone Steering in High-Angle and Horizontal Wells
Authors: Jackson Charles E. 
Published: December 2007

This paper examines the benefits of Pay Zone Steering, an integrated approach to drilling high-angle and horizontal wells incorporating advanced resistivity forward modeling into the overall planning and drilling phases. Pay Zone Steering provides quick detection of geological changes and allows for subsequent adjustments to the well plan. Key to this method are (1) the forward modeling of the responses of logging-while-drilling (LWD) resistivity tools, as well as other LWD devices, for different scenarios and the provision of complete and clear communications among all members of the drilling team throughout all phases of well construction.

This paper addresses the necessary steps required for a successful steering operation. The theory of the forward modeling technique and the way it identifies a well's critical features are discussed, as well as applications for post-well analysis. The Pay Zone Steering process, from establishing objectives to final completion, is illustrated with case studies.

The paper also demonstrates the benefits of this geological steering method, which include recognition of true-vertical-depth shifts in the well's trajectory, recognition of unexpected lithology, and improved capability to keep the well within the desired reservoir.

An appendix is included that discusses a means of resolving any discrepancies between the model and the actual resistivity measurement.

Introduction

When geological steering was introduced for directional drilling operations, it was intended as a means for proactively drilling a well to maximize reservoir exposure. Unfortunately, geological steering often focuses on the process from the point of view of logging-tool response and, consequently, it tends to reflect the description of the formation only from a geological or log-analysis standpoint. The Pay Zone Steering (PZS) technique integrates all available information into the overall drilling plan to help guide the bottomhole assembly during steering operations (Fig. 1). The technique is designed to accommodate changes in the drilling plan at any point in the drilling operation. During development of the PZS process, it became apparent that elimination of any single element of the process could compromise the chances of a successful operation. Also, it was evident that the implementation of the PZS service must begin at the earliest possible opportunity, preferably immediately after acceptance of the well proposal and at the beginning of the well planning operations.

Common directional-drilling goals include drilling high-angle or horizontal wells along thin beds and, in thicker beds, maintaining the well at a specified distance from some interface (for example, the top of the reservoir or an oil-water interface). These goals present a challenge, especially in complicated reservoirs where formation parameters, such as dip and water saturation, change rapidly.

During the drilling operations, the primary focus is on answering the following questions in a timely fashion:–Where is the bit?–Where is the well heading?–Where should the well be heading?–If necessary, how can the well path be corrected?

The questions are usually addressed in two ways:by direct use of at- or near-bit sensors, such as resistivity, inclination, and gamma ray, orby modeling the response of standard LWD tools that may be located some distance behind the bit.

Each technique has been successfully applied numerous times, and the choice of which technique to use may often be decided on tool availability or service costs. However, these techniques complement each other and in many cases, should both be run where possible.

The direct use of at- or near-bit sensors is most popular because these sensors provide early feedback about the formations encountered during drilling. Such early feedback helps the driller recognize changing conditions that may affect the well trajectory and allows more effective steering of the well to and through the target. P. 487^