Geosteering is the interactive placement of a wellbore using real-time geological criteria; its use allows increasingly more complex geologic targets for horizontal producers and injectors. This paper presents a case study of Geosteering horizontal wells in an onshore field of the western Niger Delta. The results demonstrate how an integrated team uses modern LWD tools to make real-time well path corrections in a challenging environment. The objective of geosteering is to direct the well using real-time logging and interpretation to a specific target within the reservoir. Various best practices are discussed in which geologic and structural data are combined with LWD measurements to lower the risk of " landing" and subsequently maintain the horizontal portion of the well in the desired location. Real-time dip estimation is a tool in the geosteering arsenal that can assist with geosteering decisions both during landing and while maintaining the lateral portion of the wellbore in the target window. Field examples illustrate the use of azimuthal density measurements to enhance real-time evaluation and decision-making processes. Quadrant density measurements and density images are used to calculate dips and enhance steering decisions made by an asset team of geologists, petrophysicists, and geosteering engineers for a field development project in Nigeria. Present methods in geosteering for real- time dip estimation include interpretation of resistivity and density images, estimation from the quadrant density log, and detailed stratigraphic correlation during approach hole drilling. Associated with each estimation method is an inherent uncertainty with the dip calculation; these uncertainties can lead to geosteering decisions of low confidence. To reduce the total uncertainty, more than one method for calculating dip can be used to assist with well plan adjustments. The thinner the sand is, the tighter the steering tolerance becomes, requiring the well to be drilled closer to bed or target boundaries. The most accurate possible dip estimation is needed for these cases. Introduction The " Swamp" Field is located onshore in the western Niger Delta, and is operated by Chevron Nigeria Limited (CNL) in a joint venture with Nigerian National Petroleum Company (NNPC). There are presently 35 horizontal development wells and 6 horizontal water injectors in this development. Production in March 2003 was 39,800 BOPD, with 47,200 BWPD being injected. The reservoirs comprised 10 to 60 ft thick shoreface sands with occasional tidal channel facies. Lateral variability in reservoir thickness and quality demand that all horizontal wells in the " Swamp" Field be geosteered.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe horizontal well placement within a thin sand with structural dip and economically produce these reserves are a challenge for the oil industry. New Reservoir Management and Formation Evaluation techniques are making these very thin hydrocarbon sands accessible and economic for development. Upfront planning by multi functional team to define well placement, uncertainty management and tool selection are key. Next execution of landing and lateral require maximum flexibility and accuracy from tools and team.In this paper, two case studies are presented where Logging-While-Drilling (LWD) resistivity and Nuclear Magnetic Resonance (NMR) were utilized to geosteer a horizontal section through a thin sand and to geo-stop the horizontal well after drilling an optimal drain hole length.Resistivity and NMR-LWD provides critical formation evaluation information such as resistivity at bit, up and down resistivities, structural dip, formation porosity, bound-fluid volume, free-fluid volume and permeability.Obtaining this information while drilling has a significant impact on drilling and completion decisions in Niger Delta. In the first case study, based on the information of NMR LWD from a pilot hole, a horizontal sidetrack was optimised in the high permeable section of the thin reservoir. In addition, NMR logs demonstrate the horizontal section was placed generally within the sweet spot of high free fluid section.Drawing experience from the first case study, a more difficult situation of drilling a horizontal well with a six feet window was addressed. The objective was to place the well within the sand and drill an optimal length that will meet hydrocarbon deliverability. First, resistivity at bit was used to restrict the well within the target sand. Within this heterogeneous sand, the while-drilling resistivity image and up and down deep Resistivity measurements allowed the geosteering team to stay in the "sweet spot" of the reservoir.Concurrently, permeability was estimated from the NMR log. Based on this permeability, hydrocarbon producibility was computed and decision was reached to drill the optimal length. This novel approach combining geosteering techniques using flexibly LWD tools are key to future development of thin hydrocarbon reservoirs.
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