Geological Steering of Horizontal Wells
Authors: Meehan D. Nathan 
Published: December 2007

Technology Today Series

Introduction
Horizontal well technology has evolved rapidly. Our most difficult concerns have changed from borehole stability and drilling techniques to completion, stimulation, and formation evaluation. Tomorrow's challenge lies in steering the well path precisely by use of formation geologic and geophysical information. This "geosteering" technique is credited with major improvements in drilling results.

Geological Control
Horizontal wells involve all the planning, safety, and environmental controls used for vertical wells as well as a variety of new challenges related to directional control. Vertical wells must be drilled deep enough to penetrate the horizons of interest. A small error in estimating the actual depth of a target horizon can be corrected by sample analysis or by a log run.
Horizontal wells approach the target formation nearly parallel to bedding planes. Medium- and long-radius wells have excellent geometric depth-control capabilities. However, a small error in estimating the true vertical depth(TVD) of a target horizon may cause significant problems in horizontal wells. For example, if a horizontal well is initially in the center of a 40-ft-thicktarget with trajectory parallel to the estimated dip, a 2 dip error will cause the well to be out of zone in less than 600 ft. This has a two-fold impact. First, the spatial geometry of the reservoir and identification of the actual target zone must be accurately defined. Small errors will result in costly failures. Second, TVD errors induced by inaccurate estimates or small faults will require significant additional drilling lengths (usually nonproductive) to get the well back "in zone." Many horizontal well failures are the result of these two problems that require the operator to be able to assess both well path and geologic variations rapidly and accurately.
One definition of geosteering is "the planned interactive navigation of a wellbore using geological criteria." Geosteering implies feedback, with all available data continuously entered into the model of the well path and reservoir. Potential gains in production must be balanced with additional drilling and formation evaluation costs. Proper characterization requires knowledge of where the well path is located, where the current trajectory will take the well path, and where the wellbore should go. Uncertainty in geological modeling and the need to maximize profitability require an interdisciplinary team approach.
Geosteering is probably unnecessary for wells that have target windows of100 ft. Conventional steering without geological feedback may suffice for such wells. Successful geosteering is more likely when lateral continuity of the target interval is good. Detection of lateral discontinuities (e.g., faults and pinchouts) is improved by incorporation of 3D seismic. When the goal of geosteering is to maintain an optimal separation from a fluid contact, successful geosteering requires a detectable variation in the formation evaluation response across the contact. The decision to use geosteering must consider the type and quality of data available and the ability to steer the wellbore accurately.

Evaluation of Horizontal Wells
Reservoir characterization now is recognized as a critical aspect of successful horizontal well implementation. Vertical well formation evaluation combines the analysis of such measurements as mud logs, cores, and open- and cased-hole logs. While many of the same technologies are applicable in horizontal wells, major differences exist and are important. These differences are often critical in evaluating the reservoir and in determining where and how the well is drilled. Offset well data are the primary source for selecting the best formation evaluation measurements to use for geosteering. Mud logging, Dexponent, and log analysis are the principal sources of formation evaluation used for geosteering.
Conventional vertical wells, especially exploration wells, routinely incorporate the "near real-time" services of a mud logger. Mud loggers interpret drilling data (rate of penetration, revolutions per minute, and weight on bit), analyze drill cuttings entrained in the mud, and analyze hydrocarbon or water influxes into the well during drilling. Detailed characterization and identification of lithologic variations often can be made as lithologic and paleontological markers are penetrated. Mud-log-derived" show" information is generally semiquantitative.
Drill cuttings are circulated to the surface with a time delay caused by the time to circulate "bottoms up" and by particle-settling velocities. Different formations generate varying amounts of cuttings that have different settling velocities. It is possible to recover rock from any exposed interval resulting from spalling (a shear-stress-induced wellbore failure) and clay swelling, for example. Also, cuttings are not recovered during intervals of lost circulation. For these reasons, correct identification of lithology with depth is not always possible, especially for long horizontal intervals.
Mud logging in horizontal wells is further complicated by cuttings transport issues in the horizontal and deviated intervals. Lost circulation is more common in certain horizontal wells. An influx of salt water or hydrocarbons in a vertical well is usually detected quickly at the surface compared with horizontal wells. Light fluids may accumulate in local "high points" of the well, masking the source of the influx. Some operators have had significant success using paleontology data to steer horizontal wells in layered reservoirs. This requires a depositional environment with distinct fossil markers in the intervals above and below the target.

Mechanical Aspects of Horizontal Well Logging.
Vertical well logs are lowered to total depth (TD) by the aid of gravity and conveyed by electrical cables. In steeply dipping and horizontal wells, the tools must be conveyed to TD without the aid of gravity. This requires a rigid means of conveyance capable of physically displacing the tools as well as communicating the information to surface. Tool diameters in vertical wells only need to be small enough to clear the minimum hole size. In deviated wells, the tool must be able to negotiate hole curvature, which limits tool diameter and length.
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