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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.[1] 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. The advent of nuclear tools for porosity and azimuthally sensitive gamma and resistivity tools improved the ability to infer whether the wellbore should be steered up or down. The development of the Troll Oilfield by Norsk Hydro (Later Statoil and Equinor) would not have been possible without the ability to precisely geosteer within a 4-meter thick horizon to avoid gas above and water below.
From 2D and 3D models of underground substructures, deviated wells (2D and 3D) are planned in advance to achieve specific goals: exploration, fluids production, fluids injection or technical.
坑井計画図とは、想定される坑井経路の幾何学的形状を表す、直線と曲線が連続して描かれた図のことです。坑井計画図は常に垂直および水平の地図上に投影されます。
坑井計画に従って掘削が進められる間、泥水検層、掘削中計測（MWD）、掘削中検層（LWD）から新たな地質情報が収集されます。これらの情報は通常、モデルから予測されるものとは若干異なる結果を示します。モデルは新たな地質情報（地層評価）と坑井位置（坑井偏向調査）によって継続的に更新されるため、地質構造に変化が現れ始め、修正された地質目標に到達するために坑井計画が更新される可能性があります。[ 2 ]
地質操舵には、MWD、LWD、イメージログ、2Dおよび3D地震データ、地質モデルなどのデータを利用できます。