Field classification of fractured reservoirs of crystalline basement

Abstract

Development of oil and gas deposits in fractured reservoirs entails certain risks due to peculiarities of geological structure. Classification and identification of fractures in reservoirs is of high-priority importance and makes it possible to assess the impact of both fractured systems and matrix blocks on field development parameters. This article presents the results of statistical and qualitative analysis of the influence of fracture systems and fracture heterogeneity to classify reservoirs in crystalline basement granitoids using the example of the White Tiger (Bach Ho) and Dragon (Rong) fields located on the southern shelf of the South China Sea (Viet Nam). Field classification of fractured reservoirs is based on a well-marked difference in parameters between wells within a field, due to fracture heterogeneity. In order to solve the tasks set, construction and analysis of graphs of well performance parameters distribution (productivity, flow rates, accumulated indicators, etc.) as well as Lorenz curves were carried out. According to the results, all the objects under study are characterized by asymmetrical shape of distribution curves, which indicates a significant influence of fracturing. Based on the calculated values of the fracture influence coefficient, it is found that fractured reservoirs in crystalline basement, as a first approximation, belong to type 2. This fact is inconsistent with the earlier works on crystalline basement, in which rocks are classified as reservoirs of type 1. Such contradiction is explained by the fact that the microfracture systems and the blocky low-permeability part exhibit matrix properties, but are not fully matrix. This part of the reservoir is proposed to be called a “pseudomatrix”. If macrocracks dominate in the section, the basement rocks are identified as type 1 fractured reservoirs, but if microfracture systems (“pseudo-matrix”) dominate in some parts of the void space, they may show the properties of type 2 reservoirs forming a mixed type of fractured reservoirs.