This paper presents two case histories that demonstrate the effectiveness of an active geosteering approach using computer networking between rig site and operator office. A satellite link was used to update well trajectory and LWD data in StrataSteer™ geosteering software located within the operator's asset group. This link provided the key advantage of allowing subsurface staff to become an integral part of the geosteering process. The geosteering software enables a geological / petrophysical model to be created based on offset well log data and seismic profiles. The LWD log responses are modeled along the planned well trajectory and then compared to the actual LWD log responses (gamma, multi-depth resistivity, neutron, and density). The responses are then used to adjust the geological model and take geosteering decisions. The amended geological situation enabled immediate and enhanced decision-making regarding adjustments to the well path in order to remain within the optimum reservoir zone. Good communication between all parties involved, both onshore and offshore, undoubtedly contributed to the success of both operations. The two example wells, "A" and "B", present different challenges. In both cases, it was clear prior to drilling that active geosteering would be necessary. Well A was planned as a horizontal oil producer. The vertical thickness of the pay section was estimated at 13 ft, and the bed dip was expected to vary between 0 and 2 degrees in the direction of drilling. Following successful landing of the well, a total of 1300 ft of target reservoir was drilled using the geosteering software to actively guide the well path. Production rates from this well were significantly above pre-drill expectations. The objective in Well B, a deviated sidetrack, was to drill through four reservoir zones, two on either side of a major fault. Recognition of the fault in real-time was critical due to different reservoir thickness and bed dips across the fault and significant uncertainty regarding the position of the fault. The use of geosteering software enabled the fault to be quickly recognized when it appeared — some 450 ft along hole earlier than expected. The geological model was quickly revised and the well path adjusted to optimize placement in the final two targets. Introduction Geosteering is the process of actively adjusting a high angle well trajectory to remain within a target zone based on real-time formation evaluation and geological information. In practice, it is commonly a complex and difficult operation. Key factors, which contribute to the success of a geosteering operation, include:Planning, including design of a well path based on the existing geological interpretation and engineering constraints. If the understanding of the geological situation is accurate at this point, the placement of the well becomes a ‘geometric’ steering exercise. In practice there is commonly a degree of uncertainty and the geological situation must be re-evaluated as the well is being drilled. Modeling of the expected LWD log responses is often beneficial for interpretation during the drilling phase.Accurate steering decisions, which depends upon the ability to quickly visualize and interpret the geological situation as the well is being drilled.Communication, as geosteering involves several disciplines and their respective personnel. Geologists, geophysicists, petrophysicists, drilling engineers, directional drillers, and LWD specialists can all contribute to the success of a geosteering operation; and it is often impractical to have all this expertise available at the rigsite. Communication and transfer of information between rig and office therefore becomes an important issue.
Two examples are discussed of a vertical pilot hole and a horizontal side-track, in which the resistivities recorded in the vertical hole are significantly different from those recorded in the horizontal hole. It is concluded that the differences may be caused by (1) variations in reservoir quality, (2) nearby layers, (3) resistivity anisotropy or (4) borehole and invasion effects. Resistivity tool modelling is found to be an essential tool in the analysis. However, the presently available computer codes are capable only of handling the effect of nearby, parallel layers. Full 3D modelling will be required to account for borehole and invasion effects and more complex layering. In addition, core data or data from "directional-resistivity" logging tools will be required to quantify the effect of resistivity anisotropy. As long as it is impossible to fully quantify the above-mentioned effects, it is recommended to treat hydrocarbon saturations from horizontal well logging data with great care, especially when they are to be used in oil-in-place and/or reserves calculations. This applies in particular when no nearby vertical calibration well is available. Introduction With the maturing of horizontal well drilling technology, future oil-in-place and reserves estimates are likely to become more and more dependent on logging data from (near) horizontal wells. The prime means to determine hydrocarbon saturations is via resistivity logging, and one might expect that resistivity log interpretation in a horizontal well is similar to the interpretation in a vertical well. However, a comparison of resistivity log readings obtained in several horizontal wells with those from nearby vertical wells shows that the recorded resistivities can be substantially different. Two examples will be discussed in detail, involving both a vertical pilot hole and a horizontal side-track. In the analysis of this phenomenon, the resistivity tool responses in both the horizontal and vertical wells were simulated using forward modelling computer codes. The codes calculate tool response by solving Maxwell's equations for a simplified logging environment: borehole effects and invasion are neglected and the formation is described by a layered-earth model. The codes can, however, handle any dip between the borehole and the layers, which allows analysis of the above-described phenomenon insofar as dip is an issue. In order to take borehole effects into account, an approximate procedure was applied in which the modelled tool response was compared with the actually measured response, corrected for borehole effects according to the charts provided by the logging contractor. The error that is introduced by this procedure is probably quite small as the borehole corrections were found to be negligible for the two examples discussed in this paper. Unfortunately, it is not possible to apply a similar procedure to account for invasion. In a horizontal borehole, the invasion profile is expected to be asymmetric. Since the invasion correction charts provided by the logging contractor are based on symmetric (circular) invasion profiles, they are not applicable here. P. 151^
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