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.
A number of geophysical surveys in horizontal boreholes has already approached to that of conventional wireline logging. To date Logging While Drilling (LWD) systems and memory tools, pulling by drill pipes into a borehole, are provided with probes (sondes) for electrical, induction, neutron porosity, acoustic velocity, and other geophysical methods. Sondes of logging tools for horizontal wells, as a rule, are modernized versions of corresponding wireline tools. Unfortunately, a mechanically driven sonde such as used in a wireline caliper tool is not available because of the danger to the device of being crashed. As a result, caliper measurements have a limited implementation in horizontal boreholes, despite the great demand for information on a borehole size and shape. This information is necessary in correcting well logs and in making engineering decisions, for example, in determining the place for mounting a casing pipe seal.
A more direct measurement of a borehole size may be derived from ultrasonic caliper operating under reflected wave method known as a pulse-echo principle. The principle is applied in wireline logging devices used for borehole wall imaging and cement bond evaluation.(1),(2),(3),(4) These tools are able to measure the well bore geometry but only in consolidated formations and smooth borehole wall intervals. Deep caves (breakouts) are not measured but imaged like dark spots.(5) Some LWD systems include similar tools for providing borehole image and caliper measurements.(6),(7) Since walls of deviated and horizontal boreholes are usually highly rugous due to drill pipe rotation, the task of conversion pulse-echo signals into traveled distances is typically not quite simple. So, resistivity, neutron, density logging data are used together with an ultrasonic caliper in LWD system to get approximate evaluations of the borehole geometry.(7)