Accurate placement of a horizontal well within a reservoir can be complicated and with uncertainties (McLennan, 2006). That was the case for the 8.5-in. horizontal well in the study being reported. Uncertainty in the structural geology existed due to distance from the closest well (~500 m) and also the vast number of faults identified on the seismic data.With the well supposedly landed in the reservoir, the expectation on start drilling sand was not met upon drilling out the casing shoe. Approximately 180 m MD of shale was encountered before making a decision to use the well for appraising the upper seismic reflector. The section was subsequently abandoned for a sidetrack that aimed at producing the upper sand lobe.From the original casing shoe of the landing point, to access the upper sand lobe with the shortest shale section possible, a strong build in inclination to >90˚ would be required upon exiting the shoe. Once the wellbore entered the reservoir sand package through the base, a change in trajectory was immediately required to avoid exiting through the top of the thin sand. The sooner the well entered the sand, the greater the success of the well because drilling more than 410 m MD would intersect the drainage radius of another producing well; hence, creating undesired production interference.A new model was developed and the well plan was executed. Based on the model, approximately 140 m MD of shale was expected before intersecting the base of the reservoir; however, in actuality, 167 m MD of shale was drilled prior to intersecting the reservoir entrance. Within 20 m MD inside of the reservoir, an indication of the top of the reservoir was observed on the distance-to-boundary inversion. As a result, the trajectory was adjusted accordingly to prevent exiting the reservoir that resulted in achieving 60% of reservoir sand.This case study will highlight how the combination of real-time distance-to-boundary mapping technology and proactive steering decisions aided in eliminating a second consecutive sidetrack of the horizontal section.

The relatively recent development of azimuthal resistivity measurements enables proactive geosteering within complex reservoirs. These successful tools are the major contributor to the substantial expansion of horizontal drilling. The tools enable determining the distance (up to 5 m in ideal conditions) and the azimuthal direction to a resistivity boundary. In ideal conditions, the well is inside a high resistivity layer and the shoulder bed is low resistivity, giving geologists warning of approaching adjacent conductive beds. When the tool is in a low resistivity layer, the depth of detection of an adjacent high resistivity layer is much smaller. In these situations, it is often not possible to use the tool for effective geosteering. An extra-deep resistivity tool has been used for several years in Norway and has been introduced in the Peregrino Field in Brazil. It operates at lower frequencies, has large transmitter-receiver spacings and a depth of detection up to 25 m. This tool was deployed in addition to the conventional directional resistivity instrument. The new application in Brazil was supported by inversion software (still in development) to enable possible interpretation of the geology within the tool range. The inversion results provide information that can help identify adjacent reservoir layers while in the target zone and measure the thickness of the reservoir layer being drilled. Examples are presented from one well where the extra-deep resistivity provided early warnings and additional information that helped to steer the well successfully and maximize reservoir coverage. The extra-deep measurements from the tool also provide valuable reservoir understanding and knowledge for future well planning purposes.

Extra-deep reading azimuthal resistivity tools have been deployed in various reservoir settings around the world in recent years in an effort to further improve efficiencies in reservoir development. For many years field development relied on standard and azimuthal propagation resistivity tools with depths of investigation up to approximately 5m, contributing to optimized and pro-active geosteering. While effective at geosteering against adjacent boundaries to maintain position in oil bearing formation, more complex reservoir architectures require data sensing further into the formation to allow a closer correlation with seismic models and provide more complete reservoir mapping.The first extra-deep propagation resistivity tools were developed by employing lower frequency waves, increasing antenna spacing and eventually adding lower frequency azimuthal signals. The new designs greatly increased the depth of detection and also added directional components. However, due to the greater volume of formation being investigated, the deeper readings bring extra complexity and uncertainty to the interpretation process so that innovative inversion software is required to support the tools and produce results that can be used in real-time.The inversion method described in this paper for the interpretation of extra-deep azimuthal resistivities employs a-priori constraints and is user-controlled in order to accurately monitor laterally and vertically changing geology. The examples shown here will demonstrate how inversion results based on a full suite of resistivity measurements have brought benefits to reservoir understanding by deriving sandstone thickness, detecting multiple bed boundaries, locating remote sandstones and remote resistivity plus the relative dip between the tool and the formation. The integration of this data results in better constrained reservoir models and an improved field development strategy. This paper will present the results of wells drilled using extra-deep azimuthal resistivity tools on the Peregrino Field in Brazil. The reservoir comprises complex high energy gravity flows consisting of reservoir units difficult to map due to being below seismic resolution. The sandstones have limited lateral extent and thicknesses ranging from 2m to 25m. Originally developed to improve net sandstone drilled in the Peregrino heavy oil reservoir by allowing a more strategic approach to geosteering, the tool deployment has brought additional benefits in reservoir understanding which impact seismic model interpretation, future well planning, completion strategies and reduce the need of pilot holes.

This paper describes how petrophysics and geomodeling, integrated with advanced well placement technology, can be used to optimize well trajectory in horizontal well drilling operations. We have simulated a 3D facies model of the Mulichinco Formation within a hydrocarbon field located in the Neuquén Basin (Argentina) by taking into account petrophysical, geological and geophysical data. The integrated approach allows to analysis of possible drilling scenarios and to prepare contingency plans in time for maximizing hydrocarbon production. We have also compared our simulated 3D models with the LWD data of an existing horizontal well in order to optimize well placement in real time and to reduce risks associated with drilling in non-pay zones. The results presented in this paper are complementary to the prior information available in the studied field from other conventional techniques.Initially we tested the new approach on four existing horizontal wells, targeting shallow marine sandstones within the upper Mulichinco Formation (Valanginian), where an excellent match was observed between the predicted and actual facies encountered. We subsequently applied the technique by taking into account the data from 31 wells in the field. This new approach can be used to optimize well placement in any depositional environment (both onshore and offshore) by highlighting the geosteering hazards and ensuring that the wellbore is drilled inside the "sweet spot" and away from non-productive layers. The new approach and the associated horizontal well data also improve understanding of the distribution of facies and their geometrical configuration.

The relatively recent development of azimuthal-resistivity measurements enables proactive geosteering within complex reservoirs. The tools enable determining the distance (up to 5 m in ideal conditions) and the azimuthal direction to a resistivity boundary. In ideal conditions, the well is inside a high-resistivity layer and the shoulder bed is low resistivity, giving geologists warning of approaching adjacent conductive beds. When the tool is in a low-resistivity layer, the depth of detection of an adjacent high-resistivity layer is much smaller. In these situations, it is often not possible to use the tool for effective geosteering.An extradeep-resistivity tool has been used for several years in Norway and has been introduced in the Peregrino Field in Brazil. It operates at lower frequencies than the shallower reading tools, has large transmitter/receiver spacings, and a depth of detection up to 25 m. This tool was deployed in addition to the conventional directional-resistivity instrument.The new application in Brazil was supported by inversion software (still in development) to enable possible interpretation of the geology within the tool range. The inversion results provide information that can help identify adjacent reservoir layers while in the target zone and measure the thickness of the reservoir layer being drilled.Examples are presented from one well where the extradeep resistivity provided early warnings and additional information that helped to steer the well successfully and maximize reservoir coverage. The extradeep measurements from the tool also provide valuable reservoir understanding and knowledge for future well-planning purposes.Extradeep-Resistivity Measurements. To illustrate how the extradeep-resistivity measurements complicate visual interpretation, consider a simple model in Fig. 1.