Application Of Acoustic Logging Technique In Evaluating Hydraulic Fracture Height Of Reservoirs In Xihu Sag

Recent years,in order to increase the natural gas productivity of low-permeability and tight reservoirs in the Xihu sag,many exploration wells have been reformed by fracturing operations.However,because of the limitation of platform operation environment,it is impossible to employ the relatively mature land-use micro-seismic fracturing monitoring technology.For a long time,it has been lack of effective evaluation methods for fracturing effect,especially the height of fracturing induced fractures and as a result,it is impossible to know whether the water is from the layer or the water layer above or below the target layer connected by the induced fractures when the well produces water.In this paper,a novel technical method for evaluating the height of fracturing induced fractures of combined use of array acoustic wave anisotropy,P-wave travel time tomography and dipole shear wave remote sensing is proposed and its practical applications have achieved remarkable results.For reservoirs with significant stress differences,a single main fracture is often generated along the direction of the maximum principal stress after fracturing,so significant differences appear in slowness and energy anisotropy when comparing to the one before fracturing.Thus,the post-fracturing fracture height can be obtained from this anisotropy.When the rock around the borehole is broken,a large number of fractures will be induced,causing the rock expansion.The acoustic wave velocity change is proportional to the degree of fracture fragmentation which decreases with the increasing radial depth.Therefore,the radial extension and post-fracturing fracture height can be both determined by this method.After the reservoir is fractured,there are generally two types of fractures,one-way fracture and net fractures.Even if the micro fractures,induced by the one-way fracture around the main fracture,which is not intersected with the borehole,is actually developed reflection interfaces to the dipole shear wave remote sensing technology so it can be detected by this technology,moreover,radial extension of fractures can also be determined by this technology.Dipole shear wave remote sensing technology also extend the measure range from 1m to tens of meters,providing a great improvement in postfracturing effect evaluations.By using the methods above,the practical applications in a couple of exploration wells in the Xihu sag are good,also,it can accurately determine the vertical extension heights of fracturing induced fractures in vertical wells,which provides accurate data for the comprehensive evaluations of reservoir fracturing,and improves the exploration of lowpermeability reservoirs in Xihu sag.