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Concept
Geosteering
Résumé
Geosteering is the optimal placement of a wellbore based on the results of realtime downhole geological and geophysical logging measurements rather than three-dimensional targets in space. The objective is usually to keep a directional wellbore within a hydrocarbon pay zone defined in terms of its resistivity, density or even biostratigraphy. In mature areas, geosteering may be used to keep a wellbore in a particular reservoir section to minimize gas or water breakthrough and maximize economic production from the well. In the process of drilling a borehole, geosteering is the act of adjusting the borehole position (inclination and azimuth angles) on the fly to reach one or more geological targets. These changes are based on geological information gathered while drilling.
Originally only a projected target would be aimed for with crude directional tools. Now the advent of rotary steerable tools and an ever-increasing arsenal of geophysical tools enables well placement with ever-increasing accuracy. Typically a basic tool configuration will have directional and inclination sensors, along with a gamma ray tool. Other options are neutron density, look ahead seismic, downhole pressure readings et al. Due to the vast volume of data generated, especially by imaging tools, the data transmitted to the surface is a carefully selected fraction of what is available. Data is collected in memory for a data dump when back on the surface with the tool.
Geosteering only practically became possible with the advent of deep reading 2 MHz resistivity LWD tools from the major LWD vendors (BakerHughes Reservoir Navigation Tool, SperrySun and Schlumberger) and other tools in the early 1990s, and the forward modeling software from a number of vendors capable of predicting resistivity tool responses for different relative angles and formation resistivities. Prior to this, Gamma ray gave some bed information, but was rarely used to dynamically adjust the well path to the best oil saturation and porosity.
Source officielle
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