Geological Steering of Horizontal Wells
- D. Nathan Meehan (Union Pacific Resources Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- October 1994
- Document Type
- Journal Paper
- 848 - 852
- 1994. Society of Petroleum Engineers
- 5.8.6 Naturally Fractured Reservoir, 5.6.1 Open hole/cased hole log analysis, 4.1.2 Separation and Treating, 3 Production and Well Operations, 5.1.2 Faults and Fracture Characterisation, 4.1.5 Processing Equipment, 1.7.7 Cuttings Transport, 1.12.2 Logging While Drilling, 1.6.7 Geosteering / Reservoir Navigation, 5.1 Reservoir Characterisation, 2.4.3 Sand/Solids Control, 1.10 Drilling Equipment, 1.12.6 Drilling Data Management and Standards, 3.3.2 Borehole Imaging and Wellbore Seismic, 1.6 Drilling Operations, 1.1 Well Planning, 3.3.1 Production Logging, 1.12.3 Mud logging / Surface Measurements, 1.12.1 Measurement While Drilling, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation
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Technology Today Series
Horizontal well technology has evolved rapidly. Our most difficult concernshave changed from borehole stability and drilling techniques to completion,stimulation, and formation evaluation. Tomorrow's challenge lies in steeringthe well path precisely by use of formation geologic and geophysicalinformation. This "geosteering" technique is credited with majorimprovements in drilling results.
Horizontal wells involve all the planning, safety, and environmentalcontrols used for vertical wells as well as a variety of new challenges relatedto directional control. Vertical wells must be drilled deep enough to penetratethe horizons of interest. A small error in estimating the actual depth of atarget horizon can be corrected by sample analysis or by a log run.
Horizontal wells approach the target formation nearly parallel to beddingplanes. Medium- and long-radius wells have excellent geometric depth-controlcapabilities. 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 causethe 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 actualtarget zone must be accurately defined. Small errors will result in costlyfailures. Second, TVD errors induced by inaccurate estimates or small faultswill require significant additional drilling lengths (usually nonproductive) toget the well back "in zone." Many horizontal well failures are theresult of these two problems that require the operator to be able to assessboth well path and geologic variations rapidly and accurately.
One definition of geosteering is "the planned interactive navigation ofa wellbore using geological criteria." Geosteering implies feedback, withall available data continuously entered into the model of the well path andreservoir. Potential gains in production must be balanced with additionaldrilling and formation evaluation costs. Proper characterization requiresknowledge of where the well path is located, where the current trajectory willtake the well path, and where the wellbore should go. Uncertainty in geologicalmodeling and the need to maximize profitability require an interdisciplinaryteam approach.
Geosteering is probably unnecessary for wells that have target windows of100 ft. Conventional steering without geological feedback may suffice for suchwells. Successful geosteering is more likely when lateral continuity of thetarget interval is good. Detection of lateral discontinuities (e.g., faults andpinchouts) is improved by incorporation of 3D seismic. When the goal ofgeosteering is to maintain an optimal separation from a fluid contact,successful geosteering requires a detectable variation in the formationevaluation response across the contact. The decision to use geosteering mustconsider the type and quality of data available and the ability to steer thewellbore accurately.
Evaluation of Horizontal Wells
Reservoir characterization now is recognized as a critical aspect ofsuccessful horizontal well implementation. Vertical well formation evaluationcombines the analysis of such measurements as mud logs, cores, and open- andcased-hole logs. While many of the same technologies are applicable inhorizontal wells, major differences exist and are important. These differencesare often critical in evaluating the reservoir and in determining where and howthe well is drilled. Offset well data are the primary source for selecting thebest formation evaluation measurements to use for geosteering. Mud logging, Dexponent, and log analysis are the principal sources of formation evaluationused for geosteering.
Conventional vertical wells, especially exploration wells, routinelyincorporate the "near real-time" services of a mud logger. Mud loggersinterpret drilling data (rate of penetration, revolutions per minute, andweight on bit), analyze drill cuttings entrained in the mud, and analyzehydrocarbon or water influxes into the well during drilling. Detailedcharacterization and identification of lithologic variations often can be madeas 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 thetime to circulate "bottoms up" and by particle-settling velocities.Different formations generate varying amounts of cuttings that have differentsettling velocities. It is possible to recover rock from any exposed intervalresulting from spalling (a shear-stress-induced wellbore failure) and clayswelling, for example. Also, cuttings are not recovered during intervals oflost circulation. For these reasons, correct identification of lithology withdepth is not always possible, especially for long horizontal intervals.
Mud logging in horizontal wells is further complicated by cuttings transportissues in the horizontal and deviated intervals. Lost circulation is morecommon in certain horizontal wells. An influx of salt water or hydrocarbons ina vertical well is usually detected quickly at the surface compared withhorizontal wells. Light fluids may accumulate in local "high points" ofthe well, masking the source of the influx. Some operators have had significantsuccess using paleontology data to steer horizontal wells in layeredreservoirs. This requires a depositional environment with distinct fossilmarkers 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 andconveyed by electrical cables. In steeply dipping and horizontal wells, thetools must be conveyed to TD without the aid of gravity. This requires a rigidmeans of conveyance capable of physically displacing the tools as well ascommunicating the information to surface. Tool diameters in vertical wells onlyneed to be small enough to clear the minimum hole size. In deviated wells, thetool must be able to negotiate hole curvature, which limits tool diameter andlength.
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