Analysis of factors affecting the coal seams distribution permeability in the southern part of the Tutuyasskaya Area of the Kuznetsk Basin, Russia

Abstract

Introduction. The gradual depletion of hydrocarbon condensate fields is already forcing the industry to pay attention to those sources, which are included to the off-spec and difficult to recover categories. Therefore, the study of reservoir permeability, associated with the pore-fractured structure of coal, is the most important parameter in the unconventional gas resources development, which determines both economic efficiency and approaches to the development of such deposits. In this regard, the investigation on permeability distribution of coal seams over an area is critical to fairway selection and coal bed methane resource assessment. The research objective. The factors analysis that affect the permeability value change of the Tutuyasskaya area, situated in the southern part of the Kuznetsk Basin in Russian Federation. Materials and methods. Within the framework of the research, target coal seams were selected that have sufficient data for the permeability primary assessment. As a result of wells exploration drilling in the productive coal seams intervals, core sampling was carried out by removable core and gas sampler to investigate their methane content, as well as sampling for technological, sorption and petro graphic analysis. Results. The primary visual assessment allows to conclude that a high (for unconventional reservoir) permeability is predicted almost equally in all the investigated coal seams. For a more specific assessment of the predicted endogenous fracturing and permeability of the target coal seams, the results of petro graphic analysis were used. Therefore, high permeability values were almost equally expected in all the investigated coal seams because of the dominated vitrinite group minerals, a medium coal rank and a developed network of endogenous fractures. Dynamic well tests were used to quantify the filtration parameters of coal seams and permeability. The results obtained showed a different amount of coal seams permeability despite the same tendency to high permeability. Moreover, the permeability of coal seams in the southern part of the Tutuyasskaya area of Kuzbass does not depend on the depth of the seam, but on the main stresses acting in the rock mass, the influence of exogenous fracturing and the presence of a faults near the penetrated coal seam. The results of 1D geo-mechanical modeling allowed to conclude that higher permeability values of the investigated coal seams are associated with a minimal difference between geostatic and horizontal stresses with conditions of almost equilateral volumetric compression. Furthermore, lower permeability values can be explained by the influence of exogenous fracturing, which reduces filtration characteristics. Conclusions. The analysis of cores from investigated wells in the southern part of the Tutuyasskaya area of Kuzbass showed the tendency to high permeability of the coal seams as for unconventional reservoirs. Dynamic well tests of coal seams have shown that the primary assessment of permeability does not agree with its quantitative determination and depends not on the depth of the coal seams, but on the main stresses acting in the rock mass, the influence of exogenous fracturing and the presence of a faults near the penetrated coal seam. Moreover, the one-dimensional geo-mechanical modeling allowed to conclude that high permeability values of the investigated coal seams are associated with a minimal difference between geostatic and horizontal stresses. Suggestions for the future research. The results of this research can be used in the design of coal seam degassing works for productive coal mines in order to extract coal bed methane in advance and also can be used in the future methodological frameworks development for choosing locations of wells extracting coal bed methane. Furthermore, the advanced further research direction is the evaluation of the efficiency of the coal bed methane usage in an effort to solve the urgent environmental and economic problems in the Kuznetsk Basin.