随钻电磁波传播方位电阻率仪地质导向关键技术

随钻电磁波传播方位电阻率仪器在钻井过程中可以提供地层边界的方位及距离信息,因此在地质导向应用中发挥着重要作用,是提高油气资源开采率的重要手段.该技术是国外近十年来发展起来的前沿技术,在国内尚属空白.本文首次构思并实现了一种应用"交联天线"的随钻电磁波传播方位电阻率仪器,并从天线结构,测量原理,数据处理以及地层电阻率成像及解释等几个方面详细阐述.交联天线由极化方向正交的线圈串绕组成,同时具有方位探测和地层背景电阻率测量的功能.通过分析交联天线的电压在仪器旋转过程中的变化规律,可以计算出仪器所在地层的电阻率,判断地层边界的方位以及估算地层边界相距仪器的距离.该随钻电磁波传播方位电阻率仪器还包括用于常规电磁波传播电阻率测试的多频多测距补偿天线结构.结合地层电阻率测量及其在方位上的相对变化,可以实现对地层电阻率的全方位成像.该仪器在国内油田进行了多次实井测试,测试结果证明仪器能够在水平井地质导向中准确提供仪器所在地层的电阻率,以及地层边界方位和距离信息.已有实井测试结果表明仪器在油层(100Ωm)和泥岩(5Ωm)中的地层边界探测深度分别为2.2m和1.6m,平均误差在0.2m以内.

Theories and key techniques of directional electromagnetic propagation resistivity tool for geosteering applications while drilling

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 of a newly developed directional resistivity LWD tool equipped with a joint-coil antenna which fulfills all features mentioned above.#br#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 joint-coil 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.#br#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 oil-bearing zone and water-bearing zone as well as the well trajectory with respect to the formation boundary.#br#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 2.2 m when the tool is within oil layer (100 Ωm) and greater than 1.6 m when the tool is within shale (5 Ωm). The error of the estimated boundary distance is less than 0.2 m compared with previously known logging data.