Shale gas horizontal well productions vary from hundreds of m3/day to 1 million m3/day, however, there are no significate variations in the reservoir petrophysical properties, such as TOC, porosity and gas content, etc. Natural fractures play an important role in the performance after hydraulic stimulation. The presence of natural fractures can sometimes optimize the stimulation by complexing the hydraulic fractures network, but they can also cause problems, such as sand plug, fluid leak-off, etc.
High-resolution wellbore measurements such as micro-resistivity images are used to find natural fractures along the borehole, however, the depth of investigation (DOI) is about 1-2 cm. Stoneley wave attenuation and reflection techniques have a DOI of about 0.7m. Dipole shear wave reflection imaging techniques can be further used to extend the volume of investigation to about 20-30 m into the reservoir. An integrated workflow which incorporates the micro-resistivity images and acoustic images compensates the vertical resolution and DOI of the measurements. It enables the prediction of the natural fractures developed in the reservoir.
A Case study is presented in the Changning shale gas field of PetroChina. The development magnitudes of the natural fractures in the vertical and horizontal wells are different. High angle natural fractures are seldomly seen on the micro-resistivity images and the drilling cores in the pay zone of the vertical wells. However, natural fractures can be found using measurement of stoneley wave attenuation and reflection, and dipole shear wave reflection imaging techniques in the reservoirs of the horizontal wells. The presence of nature fractures must be considered in the hydraulic fracturing of the horizontal wells. A completion design based on the reservoir quality, brittleness, stress magnitudes and natural fractures has improved perforation efficiency and production performance.
This paper discusses a novel application of multi-scale fracture detection from borehole micro-resistivity image and dipole shear wave reflection imaging in shale gas horizontal wells. The hydraulic stimulation based on the natural fractures proves to be successful, and the gas production is promising.