井下钻柱信道的声传播特性

Acoustical propagation characteristics of the communication channel of downhole drill string

基于钻柱信道的随钻数据声波传输方式能够满足现代钻井速度快、传输数据量大的要求。在声传播特性分析模型建立基础上,采用传递矩阵计算方法,对不同钻柱组合的声传播特性进行了研究。结果表明,钻柱信道的一个最明显特征是窄频带多波段通信。钻柱信道的几何结构、物理参数和信号频率对信道传输性能有显著影响,随着信号频率和传输距离的增大,接收信号衰减程度增大,同时信道的声传输性能还受钻柱材料的弹性模量和密度的联合作用影响。对于周期性钻柱,当钻杆长度增大时,通带中心向低频偏移,通带和阻带变窄;当接头横截面积减小时,通带变宽,阻带变窄,一个频带周期内通带中心向频带中心频率偏移,距离频带中心越近,其偏移量越大。对于非周期性钻柱,结构和物理参数差异的增大,会引起钻柱信道频域特性的显著变化,声传输性能显著下降。信噪比是影响井下信息声传输系统设计的关键因素,同时为了实现声波遥测,声传输方式的通信工作频率应尽量选择低频。

Using acoustic waves to transmit downhole information through the drill-string transmission channel is a promising ap- proach to meet the modern drilling requirements for the high-speed and large-volume transmission of data. Herein, we established a model to analyze acoustic transmission characteristics and examined acoustic characteristics of different drill-string assemblies by means of transmission matrix calculations. The results showed that a distinctive feature of the drill-string transmission channel lies in the existence of passbands and stopbands, in other words, the drill string, serving as a linear comb filter, is a narrow-frequency multi-band communication channel. The transmission performance of a drill string depends considerably on structure dimensions, physical parameters and signal frequencies. Along with the increase of signal frequency and transmission distance, the attenuation rate received increases obviously. In addition, the transmission performance is also co-influenced by the elastic modulus and density of drill-string materials. As for a periodic drill string, when the length of drill pipes increases, the passband center tends to shift to low frequency and both the passband and the stopband become narrower. When the sectional area of joints decreases, the passband widens while the stopband narrows, and the passband center within a band cycle tends to shift to frequencies of the band center. The closer to the band center the passband center is, the greater the offset is. As for a non-periodic drill string, the increase of differences in structure dimensions and physical parameters will change characteristics of the channel frequency domain of the drill string remark- ably and reduce the acoustic transmission performance considerably. The signal to noise ratio is the key factor in designing an acous tic signal transmission system within the drill string. In order to realize the acoustic telemetry, the communication frequency of a- coustic transmission should be chosen, to the greatest extent, within the low frequency range.