Case Study Shows Applications of Pay Zone Steering in Directional Wells
Authors: Jackson Charles , Ridley Robin , Worthen John 
Published: December 2007

A case study is presented for a horizontal well drilled in the Ship Shoal 207 field. This Gulf Coast well illustrates several advantages of the Pay Zone Steering method. Although there was active faulting in the area and some subseismic faulting was encountered in the new well, standard LWD 2-MHz resistivity and gamma ray tools were run during drilling. The data from these tools were used with PZS modeling studies to determine the well's location relative to the reservoir and help land the well in the reservoir and make critical decisions about the well's final completion strategy.

Pay Zone Steering is an approach to geological steering that integrates the logging and drilling disciplines to optimize wellbore exposure in the desired portion of the reservoir. The Pay Zone Steering method relies heavily on forward modeling of LWD data, particularly from standard LWD resistivity tools, to correlate and steer the well to the desired portion of the reservoir. The build and reach sections of directional wells each have distinctly different steering requirements, and the Pay Zone Steering process helps facilitate these requirements to first land and then stay in the targeted portion of the reservoir.

During the planning of a well in which Pay Zone Steering is to be used, the overall drilling objectives are addressed and then incorporated into a basic earth model for the subject well. The expected log responses for the well are computed with forward modeling software. Once drilling commences, these expected log responses are continuously compared with the actual log response to determine if the well is on track. The comparison can reveal that the target is at an unexpected position, or that unexpected variations exist in the reservoir itself. This information is used during the build section to adjust the true vertical depth (TVD) of the landing. During the drilling of the reach section of the well, the resistivity profile can indicate approaching beds or fluid contacts and can be used to adjust the well path to help maintain the well at an optimum position in the reservoir.

Introduction

The Ship Shoal 207 field has been extensively produced from five different platforms since 1968. The objective of this drilling project was to obtain production from a small, structural trap in the area illustrated in Fig. 1. In this trap, there is only a single offset well for geological control. The size of the target was a major factor in the decision to drill the well horizontally. A single horizontal well could replace two or three wells at substantial savings, and water production could be minimized by the lower drawdown pressures. Prior to drilling this well, the operator, Norcen Explorer, Inc., had had experience with several horizontal wells; however, this well would be the first that they would drill in this field. Because of the lack of geological control, the plan was to drill the well in two stages: a pilot hole and, after a kick-off, a medium-radius hole into the target sand.

Because knowledge of the local geology was limited, the Pay Zone Steering technique was offered as a means of assisting the drilling operations. Predicted logs from forward modeling with the technique were incorporated into the drilling plan and were used for reference during the sidetrack drilling operations. For maximum effectiveness of the technique, close communication was maintained throughout the project between the Pay Zone Steering service provider, and the operator's field management and office personnel. Also, to help reduce costs associated with potential tool loss in the horizontal section of the well and yet still provide a geological reference, the operator and service provider decided to use a standard 2-Mhz LWD resistivity tool instead of a more expensive geological steering tool.

The ensuing discussion gives an overview of the Ship Shoal Block 207 field in which the subject well was drilled as part of a major development project. This discussion is followed by a basic description of the Pay Zone Steering method and how it was implemented into the planning, drilling, and completion of the subject well.

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