深孔水压致裂地应力测量及应力场反演分析

GEOSTRESS MEASUREMENTS BY HYDRAULIC FRACTURING METHOD AT GREAT DEPTH OF BOREHOLES AND NUMERICAL MODELLING PREDICTIONS OF STRESS FIELD

万福煤矿是一座拟建特大型矿井,上覆冲积层厚度高达700m,主煤层平均埋深达1004.12m。选择7个勘探钻孔、37个试验段进行深孔水压致裂地应力测量。为保证超千米深孔地应力测量的成功,对传统水压致裂法的测试技术和设备进行改进,提高系统的耐压能力和测试精度。测孔最大水平主应力与垂直主应力的比值较大,平均为1.97,说明勘探区现今应力场以水平应力为主导。测得的21个印模中有18个点位于NEE-SWW向,3个点位于NW-SE向,符合本区震源机制解得出的挤压应力场方向,且与新构造活动所反映的区域构造应力场方向一致。结合实际的地质条件和地层参数,在实测地应力结果的基础上,提出基于有限元三维地应力拟合分析法反演勘探区现今地应力场,给出煤层顶板主应力分布曲线并分析井下可能出现的各类不良地质现象。

Hydraulic fracturing is an efficient method for determining the stress field and is suitable at the early stages of projects when no underground access exists.Wanfu coal mine is located in the southwest of Shandong Province,China,which is prepared to construct with great annual coal capacity.The overburden soil layer is the alluvium of the Yellow River with the thickness of over 700 m.The depth of coal deposit is about 1 004.12 m.Determination of both the orientation and magnitude of in-situ stresses at great depth is quite difficult,so some new stress measurement techniques are employed.In the exploration area,a total of 37 hydrofracturing tests in 7 vertical exploration boreholes drilled from the ground surface are carried out.The results suggest that the maximum horizontal stress is dominantly oriented NEE-SWW.This stress direction is also in agreement with the direction derived from the focal mechanism in the area.At the same time,the maximum horizontal stress orientations are closely consistent with those in the general contemporary stress field in southwest of Shandong Province.The ratio of maximum horizontal principal stress to vertical principal stress is 1.44-3.22 with the average of 1.97 in 37 measuring points.It is shown that the prevailing stress field in Wanfu coal mine is controlled by the tectonic horizontal stress.Based on the geological conditions and rock mass properties,a 3D mechanical model is established and a regressive method for obtaining regional stress from limited stress data by using a 3D finite element method is proposed.According to the spatial distribution of the stress filed,the curves of principal stress for coal roof are plotted;and some adverse geological phenomena are discussed.