基于地层旋转的地层视倾角计算新方法——应用于水平井地质导向

随着页岩气勘探开发工作的逐步深入和开发规模的不断加大,如何更加精确地进行地质导向、指导钻井轨迹的调整、使水平轨迹在优质储层段穿行,对水平井地质导向技术提出了新的、更高的要求。为此,提出了一种在三维空间对地层进行旋转,以测井曲线上特征点匹配度最高为原则的地层视倾角计算新方法。新方法以地层等厚对比原则为基础,将地层视倾角的计算问题转化为全局寻优的过程并提出了适用于该方法的二阶段法。较之于常规地层视倾角计算方法,新方法的优势主要表现在:①在靶前大角度扭方位且地层倾向未知的情况下,同样可以准确计算水平段地层视倾角,同时计算出地层倾向和真倾角;②所选取标准井与水平井钻遇地层的铅垂厚度差别大且地层真厚度未知的情况下,也可以稳定有效地计算地层视倾角。基于实际资料的测试结果表明,利用该二阶段法可以在大角度扭方位情况下准确计算地层综合倾向,继而计算水平段地层视倾角,其结果与实际资料相吻合。结论认为,针对标准井因地层真倾角差异而导致地层铅垂厚度差别大的情况,新方法具有更强的适应性。

A new method for calculating apparent formation dips based on strata rotation

With the gradual deepening of shale gas exploration and development and increasing development scale, horizontal well geosteering technologies will be highly required to achieve more accurate geological guidance with the adjustment of drilling trajectory so as to make the horizontal track pass through high-quality reservoir sections. In this regard, a new method for calculating apparent formation dips was proposed, which revolves the strata in a three-dimensional space and takes the highest matching degree of feature points on the logging curves as the principle, and it was applied to the geosteering of horizontal wells. Based on the principle of equal thickness correlation, by this new method the calculation of apparent formation dips is transformed into a global optimization process, and a two-stage method suitable for this new method was also introduced. Compared with those conventional methods, this new method has the following advantages. First, in the case of large-angle torsional orientation and unknown formation dips before the target enters, the apparent formation dips of horizontal sections can be calculated accurately, so can be both stratigraphic tendencies and true dips at the same time. Second, if the vertical thickness of a horizontal well is quite different from that of a selected standard well, and the true thickness of the strata is unknown, the apparent formation dips can be calculated stably and effectively. The tested results from field practices demonstrate that the introduced two-stage method can accurately calculate the comprehensive stratigraphic tendency under the condition of large angle torsion orientation, and the apparent formation dips in the horizontal sections, which is consistent with the actual data. In conclusion, this new method is more suitable especially for the case that the difference of vertical formation thickness is great due to the variation of true formation dips.