3D Finite Element Simulation of In-Situ Stress and Its Application in Coalbed Methane Exploration - A Case Study in Southern Qinshui Basin

Abstract

Permeability of coal reservoirs is very complex and influenced by a series of geological factors including tectonics, stress state, coal petrography etc. in different degree. While, since China underwent multi-stage tectonic evolution and ended up with complicated tectonic background in the present day, the influences of in-situ stress on coal permeability are of remarkable importance. The prediction of present day in-situ stress distribution based on finite element simulation is significant for permeability evaluation of coal reservoirs. On the basis of plenty of well drilling, logging and testing data collection and statistical analysis in Southern Qinshui Basin, the finite element simulation software was introduced to establish a 3D geological model and finite element meshes were divided using SOLID 186 element type. Combined with the tectonic evolution and fracture monitoring data of coalbed methane wells with hydraulic fracturing stimulation, the reasonable boundary constraints were loaded on the finite element model boundaries. According to the examination of limited calculated in-situ stress values from well data inside geological model, the loading forces were optimized. Finally, the in-situ stress distribution of the study area was simulated. The theoretical model indicates that the permeability of fractures is proportional to the third power of the fracture aperture while inversely proportional to the average fracture spacing. The geological controls of in-situ stress and effective stress on permeability are derived from their controlling on fracture aperture, that is, with the increasing of effective stress, fractures tend to be closed and then the permeability is reduced. The results of 3D finite element simulation of in-situ stress in Southern Qinshui Basin indicate that: (1) the vertical stress is controlled by buried depth showing a positive correlation which means permeability will decrease with the increasing of vertical stress induced by buried depth, (2) the value of horizontal stress comes both from the compressional pressure of various tectonic evolutions that the coal or strata underwent and from pressure of overlying strata, (3) low values of maximum horizontal stress occur in the vicinity of fault belts and on the top of fold axis, (4) the stress concentration during tectonic evolutions and stress release afterwards on fold axis could create a great quantity of structural fractures, which are beneficial to coalbed methane production. 3D finite element simulation of in-situ stress can not only be an effective method to predict coal permeability but also provide a significant parameter for trajectory design of horizontal wells and hydraulic fracturing design.

Copyright 2019, IFEDC Organizing Committee.

This paper was prepared for presentation at the 2019 International Field Exploration and Development Conference in Xi’an, China, 16–18 October, 2019.

This paper was selected for presentation by the IFEDC Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the IFEDC Technical Team and are subject to correction by the author(s). The material does not necessarily reflect any position of the IFEDC Technical Committee its members. Papers presented at the Conference are subject to publication review by Professional Team of IFEDC Technical Committee. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of IFEDC Organizing Committee is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of IFEDC. Contact email: paper@ifedc.org.