A low frequency forward looking antenna array for LWD and MWD

Abstract

A new forward looking antenna array applied in logging while drilling (LWD) or measure while drilling (MWD) is presented in this paper. The so called forward looking antenna array corresponds to an ability to probe the formation in front of the bit, and it is based on the character occurs in the near field while the working frequency of solenoid coil is relatively low. Regardless the complexity of the formation such as the contrast of the electrical parameters in different formation layers, and the influence resulted from multipath that electromagnetic wave propagates from radiator to aim area, the antenna array is able to realize in-phase superposition of the field radiated by different array elements in front of the bit. The difficulty of designing such forward looking antenna is the main lobe of the radiation pattern always remove, because of a homogeneous boundary condition locate at the mandrel. And what's more, the rapid attenuation for radio frequency electronic wave propagates in formation make it hard to obtain a relative strong radiation field. Facing this, the method mentioned in this paper use a low frequency radiation source (solenoid coil), and what we concern is no longer its radiation field but a near field instead. Furthermore, compared with conventional design of the forward looking antenna used in LWD and MWD, the proposed method first introduces the antenna array theory to enhance the amplitude of the field in thus near field case. At last, some numerical examples are presented to validate the effectiveness of the proposed method in LWD and MWD working environment.