ABSTRACT:

Gas reservoirs of north-west China are characterized by deep burial, complex lithology, high-angle beds, low porosity, low permeability, and high water salinity. As such, reservoir evaluation with conventional logging tools like resistivity and neutron-density have a high degree of uncertainty. Incorporating more modern logging techniques like nuclear capture spectroscopy, epithermal neutron measurements, borehole imaging, and dipole sonic reduces uncertainty and helps identify gas-saturated reservoirs. Lithology evaluation of the complex sandstone reservoirs in north-west China is significantly improved with the addition of nuclear spectroscopy logs. Nuclear capture spectroscopy data indicate that the sandstone formations contain both calcite cement and disseminated anhydrite. An accurate understanding of lithology is critical for porosity estimation and is particularly important for the low-porosity formations encountered. The low-porosity reservoirs also contain dissolved pores, intergranular pores, and fractures. Reservoir water analysis indicated that formation water salinity is approximately 200,000 ppm. Compared to thermal neutron measurements, the epithermal neutron shows reduced environmental effects due to both high salinity and clay volume and thus provides more representative measurements of formation porosity. Computed formation dips from borehole image data ranged from 30° to 70°. The combination of a complicated pore structure and environmental effects makes traditional approaches of determining fluid saturations from resistivity logs problematic. The paper highlights a practical approach in these complicated reservoirs for fluid identification and saturation estimation with the addition of modern logging tools.

INTRODUCTION

Gas reservoirs of north-west China are characterized by deep burial, complex lithology, high-angle beds, low porosity, low permeability, and high water salinity. These complex reservoirs have traditionally been evaluated with conventional well logs and a relatively simple lithology model of sand/shale. Errors in lithology determination can lead to errors in the computation of porosity, a key formation evaluation parameter.

This content is only available via PDF.
You can access this article if you purchase or spend a download.