Title: Theories and key techniques of directional electromagnetic propagation resistivity tool for geosteering applications while drilling
Abstract: Geosteering plays a very important role in enhancing hydrocarbon recovery rates by keeping drill bit within hydrocarbon bearing zone to the maximum extent.Precise geosteering requires information of exact position of the down-hole assembly and geological characteristics of surrounding formations,which,e.g.includes if the surrounding formation is an oil-bearing zone and how far and in which direction the oil-bearing zone boundary is located with respect to the down-hole tool assembly.Directional resistivity logging while drilling(LWD)technique is one of the most effective means for real-time geosteering applications by providing information of formation resistivity as well as the relative location(distance and direction)of formation boundaries with respect to the down-hole tool assembly.This paper presents key techniques ofanewly developed directional resistivity LWD tool equipped with a joint-coil antenna which fulfills all features mentioned above.The directional resistivity LWD tool equipped with the joint-coil antenna has been developed to implement three functions:(1)providing azimuthal direction of a formation boundary;(2)estimating the distance to the boundary from the tool;(3)generating azimuthal resistivity image of surrounding formations.Two orthogonal coils denoted as Rz and Rx coils are connected in series to compose the joint-coil antenna.The Rz coil has the same polarization as the transmitter coil antenna whereas the Rx coil has an orthogonal polarization.The total response of the jointcoil antenna is the sum of the responses from both Rz and Rx coils.The Rz response only reflects background resistivity of formation whereas the Rx response is only related to formation boundaries.The azimuthal direction of a formation boundary can be derived by analyzing the Rx coil response,which varies sinusoidally during a rotation circle of the directional resistivity LWD tool.The vertex of the Rx response corresponds to either the exact direction of the boundary or the complete opposite direction.Combing the background resistivity of the formation obtained from Rz coil response can eliminate the ambiguity through knowing whether the tool is within an oil-bearing zone or within a water-bearing zone.Thus the azimuthal direction of the boundary can be uniquely determined.The amplitude of the Rx coil response is related to the distance to the boundary from the tool as well as resistivity contrast of the formation separated by the boundary.Thus the boundary distance can be estimated by quick inversion with knowledge of the amplitude of the Rx response and the formation resistivity derived from Rz coil response.Combining the Rx and Rz coil response,an azimuthal resistivity image of the formation can be obtained based on the fact that the Rz coil response represents background resistivity and the Rx coil represents variation in azimuthal direction.The azimuthal resistivity image illustrates the relative position of the oilbearing zone and water-bearing zone as well as the well trajectory with respect to the formation boundary.The directional resistivity LWD tool has been tested in several wells in Northeast China.Both azimuthal direction and distance of formation boundaries were estimated precisely during geosteering.The results show that the depth of investigation is greater than 2.2m when the tool is within oil layer(100Ωm)and greater than 1.6m when the tool is within shale(5Ωm).The error of the estimated boundary distance is less than 0.2m compared with previously known logging data.
Publication Year: 2015
Publication Date: 2015-01-01
Language: en
Type: article
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Cited By Count: 6
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