Study On Fracture Evaluation Methods Based On Multi-Pole Array Acoustic Logging Data

Fractures in rocks are not only channels for pore fluid migration,but also important storage spaces.Fracture identification and evaluation has always been key and difficult points of petroleum exploration and development due to its distribution complexity.There are many methods for fracture identification,such as core observation,logging identification,and seismic identification.Multi-pole array acoustic logging is a relatively reliable fracture identification technology.This paper studies fracture identification methods of multi-pole array acoustic logging,and develops a set of computer programs to implement various fracture identification methods such as high-resolution acoustic attenuation processing method,acoustic attenuation extraction method,wave field separation technology,method of distinguishing natural fractures from induced fractures,etc.Four acoustic attenuation extraction methods are compared through example analysis,and the results show that the energy method,envelope amplitude method and area method on the accuracy of attenuation calculation are similar;the acoustic attenuation calculated by the main frequency amplitude method is smaller,and the peak and valley of the curve have the highest contrast.The research on the method of distinguishing natural fractures from induced fractures shows that the borehole mode acoustic attenuation spectrum and Stoneley wave reflection coefficient spectrum can distinguish natural fractures from induced fractures well;natural fractures could increase the acoustic attenuation and Stoneley wave reflection coefficient in the entire frequency band;induced fractures only increase the acoustic attenuation and Stoneley wave reflection coefficient at high frequencies,while don't change them at low frequencies.At the same time,the acoustic attenuation and Stoneley wave reflection coefficient caused by induced fractures are generally much smaller than natural fractures.The developed program is used to process oilfield multi-pole array acoustic waveform data.The ratio of compressional slowness to shear slowness is used to judge the fracture dip angle;the borehole mode acoustic wave attenuation is used to judge the fracture dip angle as well as fracture validity;the Stoneley wave reflection coefficient is used to characterize the fracture position and estimate the fracture aperture;the fracture development is estimated by shear wave anisotropy and the difference between fast and slow shear wave spectrum,and the fracture strike is assessed by the azimuth of the fast shear wave surface;the borehole mode acoustic attenuation spectrum and Stoneley wave reflection coefficient spectrum are used to distinguish natural fractures from induced fractures.At the same time,combined with the electrical imaging logging data,further comparison and verification are carried out.The result shows that the above methods can qualitatively or semi-quantitatively evaluate formation fractures.But a single method can only determine some fracture parameters,and there are many interference factors,which causes a certain degree of multiplicity in identifying fractures.Therefore,in the practical application,we should choose more proper methods to process logging data so as to get more fracture parameters and the multi-solvability of single method could overcome effectively as well,which make the fracture evaluation based on multi-pole acoustic waveform data more accurate and reasonable.