基于成像矿物谱的页岩气储层脆性指数计算方法——以四川盆地南部下志留统龙马溪组为例

A calculation method for brittleness index of shale gas reservoirs based on the imaging spectroscopy mineral maps: A case study of the Lower Silurian Longmaxi shale gas reservoir in the southern Sichuan Basin

页岩气储层脆性指数是页岩气藏压裂方案设计所需的核心参数之一,但目前获取该指数的方法成本较高、耗时耗力且过分依赖于取心资料。为了解决上述问题,以四川盆地南部下志留统龙马溪组页岩气储层为研究对象,首先对该区的成像测井数据进行分析和校正、进行图像的直方图加强,将成像测井图像刻度为成像矿物谱;然后基于正态分布理论,并结合实验分析数据,通过解析成像矿物谱,获得了页岩气储层矿物含量;进而结合裂缝特征等建立了脆性指数的计算模型,形成了矿物含量及脆性指数的计算方法(以下简称新方法),并在该区的多口页岩气井进行了验证。研究结果表明:①新方法具有成本较低、适用性强、计算精度和实用价值较高等优点;②较之于目前常用的方法,新方法数据资料具有普遍性、脆性指数计算结果具有连续性;③新方法能够精确地反映层理、薄层及局部岩性变化等对脆性指数的影响,对于页岩气储层脆性及可压裂性评价有着重要的参考作用;④现场应用效果表明,新方法计算结果与实钻结果吻合程度高,该区多口井测试均获得了页岩气工业产能。结论认为,新方法为海相页岩气储层压裂优选评价提供了技术支撑。

The brittleness index of shale gas reservoirs is one of the key parameters for the fracturing scheme design.At present,however,the current method to calculate the brittleness index is expensive,time and labor consuming,and overly dependent on coring data.In view of this,the Lower Silurian Longmaxi shale gas reservoir in the southern Sichuan Basin was taken as the research object in this paper.Firstly,the imaging log data of this area was analyzed and corrected,the images were enhanced in the form of histogram and the imaging log image was calibrated as the imaging spectroscopy mineral map.Then,based on the theory of normal distribution,combined with experimental analysis data,the mineral content of shale gas reservoirs was estimated by analyzing the mineral map.Finally,based on fracture characteristics,the calculation model for brittleness index was established.In this way,a new method for calculating mineral content and brittleness index was developed and has been verified in many shale gas wells in this area.And the following research results were obtained.First,this new method has the advantages of low cost,strong applicability,high calculation accuracy and practical value.Second,compared with the brittleness index calculation methods that are currently used,the data obtained by the new method are universal and its brittleness index calculation results are continuous.Third,this new method can accurately reflect the effects of bedding,thin layer and local lithology change on the brittleness index,playing an important role in evaluating the brittleness and fracturing performance of shale gas reservoirs.Fourth,field application reveals that the calculation results of this new method are in a better accordance with the actual drilling results.Industrial productivity of sh ale gas is achieved in many wells of this area during the testi ng.In conclusion,this new method provides a technical support for fracturing opt imization and evaluation of marine shale gas reservoirs.