Determination of Horizontal Borehole Geometry by Ultrasonic Pulse-Echo Method

Abstract

Abstract The information on the horizontal borehole geometry (size and shape) is needed in well logging interpretation and solving engineering tasks, for example, choosing a place for mounting a casing pipe seal. Direct application of mechanical driven logging devices such as used in a wire line caliper tool is impractical in horizontal boreholes. The borehole geometry can be determined by ultrasonic caliper operating in a pulse-echo pattern. Such techniques have been successfully used in wireline logging for borehole imaging and cement bond evaluation, but meet some obstacles in horizontal wells. A rugous borehole wall and low difference in acoustic impedances of formation and drill mud are the reasons of acoustic pulse high attenuation. In horizontal boreholes the logging tool is always decentralized and consequently the pulse travel time may be less than transducer ringing duration. These concerns with ultrasonic measurements lead to a task of finding a reflected signal amid overwhelming noise. Using transducers similar to those of wireline scanners and applying robust algorithm to data processing can solve the task. The principal procedure of processing includes regular noise extraction by compiling waveforms of the neighboring depth steps in time domain. The search of reflected pulses in waveforms is provided by time windowing and finding the first reflections in frequency domain. Application of described techniques in memory logging tool and data processing discovered a durable method of caliper measurements in horizontal boreholes. Field tests have demonstrated the capabilities of proposed logging method in borehole shape determination. Actually in more than a hundred boreholes average diameters were measured without discontinuities. The error of diameter determination was in the range of ±2 mm. The results of field tests gave a hope for the ultrasonic caliper application to both borehole shape measurements and stress direction determination.