Deformation Mechanism Of Reservoir Medium In Abnormally Pressured Gas Reservoirs

Abnormally pressured gas reservoirs are mostly in Midwest China, such as eastern Dachigan gas reservoir, Yanggaoshi gas reservoir in Luzhou, Sichuan, and Kela2gas reservoir in Talimu, all of which are symbolic abnormally pressured gas reservoirs. Abnormally pressured gas reservoir is high pressured, lowly compacted and fast permeated. When developing abnormally pressured gas reservoirs, the nearby reservoir rock could easily be deformed elastically or fractured plastically with the formation pore pressure going down, leading to the rapid reduction of porosity and permeability. As a result, production will decreased rapidly at the early stage of the abnormally pressured gas wells, and water will discharge seriously. These are critical disadvantage of production rate and stability maintenance for abnormally pressured gas reservoirs. The reason of those problems in this paper is considered to be deformation of reservoir rocks. To handle these problems, the first thing to do is to understand the cause of the deformation of abnormally pressured gas reservoirs.Based on the core experiments of stress sensitivity for abnormally pressured gas reservoirs, under the theories of large strain, dislocation, micromechanics basis, microscopic damage mechanics and coupling analysis, several models of abnormally pressured gas reservoirs deformation media are built:a model to describe large strain spatially and materially, a model of strain and fracture under the micro-, meso-, and macro-levels, a model of the coupling analysis of permeability, stress and damage for damaged rocks. These models are solved through numerical methods, and the variation characteristics of the parameters of the medium deformation and fracture for abnormally pressured gas reservoirs are obtained. Main topics involved in the thesis are as follows:(1) Abnormally pressured gas reservoirs can be classified as cracked and non-cracked reservoirs. This paper studies their reservoir medium deformation characteristics by rock stress sensitivity experiments, such as change rule for rock pore volume, porosity, compressibility coefficient, permeability along with the net confining pressuur.(2) With the knowledge of continuum mechanics, stress tensor under limited strain, and constitutive relation of large strain, spatial and physical large strain equations of abnormally pressured gas reservoirs are built in spatial and physical coordinate system; finite discreteness method and isoparametric element interpolation method are used to do the discreteness for the continuum equation and the balance equation, evolving to an increment finite element method for large strain consolidation of abnormally pressured gas reservoirs.(3) Based on the characteristics and the analysis of the influence factors of abnormally pressured gas reservoirs, the mathematical deformation and fracture model are built for abnormally pressured gas reservoirs, by the help of dislocation theories, meso-mechanics and continuum mechanics and developed from micro-, meso-, and macro-views. (4) By the definition of basic element and the Weibull statistic distribution function, this paper describes the heterogeneity of abnormally pressured gas reservoirs rock; then, based on the stress-strain relation of abnormally pressured gas reservoirs and gas permeability equation, the coupling model of permeability-stress-damage for abnormally pressured gas reservoirs are built.(5) Based on the annlyse for the deformation mechanism of abnormally pressured gas reservoirs, large strain consolidation module of abnormally pressured gas reservoirs, the module of the micro-meso-macro deformation and fracture, and the module of the coupling analysis of permeability-stress-damage are built by numerical program.(6) A physical model and the grid for abnormally pressured gas reservoirs are built. With the results obtained by the numerical program of this paper and the common computation (which doesn’t take into account the large strain of gas reservoir, fracture and stress-damage), pore index of the reservoir, porosity, horizontal displacement, vertical displacement and dynamical production index acquire why water encroachment is serious in the early stage and rapid production decrease of the gas wells.