页岩气水平井钻井的随钻地质导向方法

四川盆地川南地区页岩气开发的主力目标层位为厚约6 m的下志留统龙马溪组海相页岩,具有靶窗较小、构造复杂、地层倾角变化大等特点,实钻井眼轨迹存在脱靶和出层的风险。为此,以该盆地威远区块页岩气水平井为例,通过分析该区水平井地质导向过程中出现的技术难点及其原因,从入靶导向和水平段地质导向等关键环节入手寻找出解决上述难点的办法:1应用随钻自然伽马依次测量地层剖面上的高伽马页岩标志层,配合地质录井确定轨迹精确入靶点,保证井眼轨道按设计井斜角姿势入靶,避免了提前入靶和推迟入靶;2应用随钻自然伽马测量优质页岩储层低值层段特征,配合地震深度剖面判断地层大致视倾角以及微构造的发育情况,实现实钻轨迹在钻进箱体中位置的正确确认,确保了储层的钻遇率。2年的现场应用效果表明,该区块已完钻27口水平井,平均深度为5 043 m,平均水平段长度为1 428.96 m,优质页岩箱体钻遇率达95.75%。结论认为,优质页岩箱体地质导向方法配合地质、三维地震、气测全烃含量等多种方式,可实现箱体的准确着陆及水平段的精准穿行。

Geosteering technology in the drilling of shale gas horizontal wells

In the southern Sichuan Basin, the major target horizon of shale gas development is the 6 m-thick Lower Silurian Longmaxi Fm marine shale. It is characterized by smaller target window, complex structures and greatly varied stratigraphic dips, so it is possible for the actual drilling trajectory to miss the targets and run out of the target layers. In this paper, the shale gas horizontal wells in Weiyuan Block in the Sichuan Basin were taken as examples in analyzing the technical difficulties that occurred during the geosteering of horizontal wells in this block and the related reasons. Then, the solutions were worked out starting from the key points of target orientation and horizontal section geosteering. First, the high-GR shale markers on the stratigraphic section were measured one by one by applying GR detection while drilling, and combined with the geological logging, the accurate targeting point of hole trajectory was determined, so that the well track can run into the target accurately in accordance with the designed deviation angle. Second, the characteristics of low-value intervals in quality shale reservoirs were identified by applying GR detection in drilling, and combined with the seismic depth profile, the approximate apparent stratigraphic dips and the development situations of micro-structures were discriminated, so that the position of actual trajectory in drilling box can be confirmed accurately to guarantee the drilling ratio of reservoirs. Based on the field application within 2 years, 27 horizontal wells have been drilled in this block with an average depth of 5 043 m, average length of horizontal sections 1 428.96 m and drilling ratio of quality box shale 95.75%. It is concluded that the quality box shale geosteering method, together with geologic, 3D seismic and gas logging total hydrocarbon content technologies, can guarantee the accurate landing of boxes and the precise drilling of horizontal sections.