Reserves grading classification and development countermeasures for low-permeability tight gas reservoirs in the Ordos Basin

Low-permeability tight gas reservoirs in the Ordos Basin are characterized by poor reservoir physical properties, strong heterogeneity, low reserves producing degree and large difference in reserves producing degree, so even though their reserves scale is large, it is still a big challenge to keep long-term stable production and efficient development. To this end, this paper took five main gas fields in the Ordos Basin as the research objects to carry out reserves evaluation unit classification, reserves grading classification and evaluation and reserves developmental sequence establishment on low-permeability tight gas reservoirs by taking the benefit development as the orientation and the internal rate of return (IRR) as the key evaluation index, combined with dynamic and static characteristics. Furthermore, development technical countermeasures were put forward correspondingly for each type of reserves. And the following research results were obtained. First, the Ordos Basin is low in single-well dynamic reserves and gas production rate, and its gas producing patterns can be classified into two types, i.e., multiple-layer cooperative gas supply and single-layer main gas supply. Second, the five main gas fields in the Ordos Basin are divided into 11 reserves evaluation units, based on the similarity principle of geological conditions and single-well dynamic characteristics, combined with the distribution situations of development management blocks. And then by taking IRR of 30%, 8% and 5% as the boundaries, the 11 reserves evaluation units are classified into four reserves types, including high-efficiency, efficiency, low-efficiency and difficult-to-produce. Third, with IRR 8% as the threshold of effective development, the corresponding estimated ultimate reserve (EUR) per well is compared with the actual EUR per well of each reserves evaluation unit. And according to the benefit order from the top to the bottom, the cost-effective developmental sequence of the reserves evaluation units is built up. Fourth, it is suitable to apply pressurized exploitation and local well pattern adjustment to high-efficiency reserves, well pattern infilling to middle-efficiency reserves to increase the reserve producing degree, enrichment region selection and progressive development to low-efficiency reserves, and intensive technological research to difficult-to-produce reserves to realize benefit development. In conclusion, the research results are conducive to improving the producing degree of natural gas reserves in the Ordos Basin and provide technical support for the preparation of the Ordos Basin's long-term natural gas development strategy. © 2020, Natural Gas Industry Journal Agency. All right reserved.