压裂停泵水击压力波信号滤波方法与特征分析

Filtering methods and characteristic analysis of water hammer pressure—wave signals from fracturing stop pumps

水击压力诊断是近年来兴起的压裂诊断技术,具有实时性强、成本低等优点,潜力巨大。该技术通过对停泵形成的压力波进行频谱分析,确定裂缝处压力波动传至井口的响应时间,然后乘以波速求得裂缝位置,实现裂缝识别。在压裂停泵过程中,由于管线震动、裂缝开启等各种井底波动事件,实际采集的水击压力波信号会包含有大量的随机噪声、固定频率噪声,大量噪声的存在会对压裂停泵频谱分析、确定响应时间产生强烈的干扰,影响分析的准确度。通过合理的滤波方式去掉停泵过程中存在的各种随机噪声、固定频率噪声,提高信噪比,是水击压力波诊断技术的关键问题。本文分析了压裂停泵水击压力波信号的特征,分别研究了在纯净水击信号中添加随机噪声、与模拟信号频率不相近的固定频率噪声以及频率相近的固定频率噪声后信号的特点,并以滤波信噪比的提高和倒谱效果为指标,进行了叠加平均法、FIR低通滤波法、自适应滤波法以及自适应噪声对消方法的原理分析及滤波效果的比较。在本文中,经理论分析和仿真验证,叠加平均法可以滤除随机噪声,且随着叠加次数的增大,叠加后信号的信噪比提高程度会逐渐变缓;自适应噪声对消方法对于固定频率噪声滤波效果最好,其信噪比提高最大且倒谱识别效果最好。该研究可以为压裂作业中信号的滤波提供指导。

Water hammer pressure diagnostics is a novel technique for fracture diagnosis developed in recent years.This advanced technique,with advantages of real-time monitoring and low cost,has potential for future applications.By analyzing the spectrogram of the pressure signal generated by pump shut-down in the fracturing process,this technique determines the response time corresponding to the pressure wave propagating from the fracture to the wellhead.Computing the product of the tube wave velocity and reflection time,estimates the depth of the fracture.In the process of the fracturing pump shutdown,because of the fluid compressibility,the water hammer pressure wave signal can be obtained at the wellhead.The pressure wave signal contains a large amount of random noise and fixed frequency noise.This noise is generated by the pipeline vibration,hydraulic fractures opening and other bottom wave events.The existence of a lot of noise affects the accuracy of spectrum analysis,which leads to the challenge of determining the response time.The key problem of water hammer pressure wave diagnosis technology is to remove all kinds of random noise and fixed frequency noise.It is necessary to improve the signal-to-noise ratio(SNR)through reasonable filtering.In this paper,we analyzed the characteristics of the water hammer pressure wave signal and the effect of filtering.Firstly,noise is added to the purified water pressure wave signal,including random noise,fixed-frequency noise consisting of frequency close to the purified water pressure wave signal and frequency greatly different from the purified water pressure wave signal.The characteristics of the mixed signal after adding various noise is analyzed.After that,the superposition average filtering method,the finite impulse response(FIR)low-pass filtering method,adaptive filtering method and adaptive noise cancellation method are used to filter out the noise.The mechanism of these filtering methods and the effect of these filtering methods on SNR are studied.The SNR gain and the cepstrum effect are selected to evaluate the effectiveness of these filtering methods.In this paper,through theoretical analysis and simulation verification,the superposition average method can filter the random noise.Besides,with an increase of stacking times,the degree of increase in SNR will gradually slow down.The adaptive noise cancellation method is the best method for fixed frequency noise filtering in this study.The adaptive noise cancellation method has the largest SNR gain and the best cepstrum recognition effect.This study can provide some technical guidance for signal filtering in a fracturing operation.