Research On The Method Of Well Test Interpretation In Shale-gas Reservoirs

With the growth of world energy consumption and the increasingly outstanding imbalance between supply and demand, shale gas resources have aroused widespread attention of the US, Canada, Russia and have already entered the development stage. At present, the exploration and development of shale gas resources is put on the important agenda by China, through geological survey and exploration, an abundant shale gas resources are found in China, according to preliminary estimate, about (15～30)×1012m3of resources reserves may be existed, thereby indicating wide development prospects. Shale gas, an unconventional gas resource, mainly stores in high carbon shale and dark mud shale of very low porosity and permeability in adsorbed and free states, showing adsorption, desorption and diffusion characteristics. Shale gas is formed by gathering nearby in hydrocarbon source rock after natural gas generation, showing typical in situ accumulation model.Based on a lot of investigation, clear of shale gas in the process of developing the seepage mechanism and gas adsorption and desorption characteristics of.In shale gas reservoirs, a considerable part of shale gas in the adsorbed gas state Shale gas exploitation without drainage, fracture of gas and free water exploitation, the formation pressure decreased, as the gas reservoir pressure decreases, gas adsorption desorption diffusion occurs, this phenomenon will process the effect of pressure change in the accumulation of shale gas.On the basis of previous studies, according to the Langmuir adsorption isotherm and the Fick’s law of diffusion, a single medium with shale gas wells, gas water two phase flow in porous media, established a mathematical model of seepage theory, and the difference of the model was solved using numerical, the double logarithm curve drawing the bottom hole pressure difference and pressure difference derivative, analysis of adsorbed gas desorption and diffusion, skin factor, Langmuir pressure, Langmuir volume, start-up pressure gradient, geometrical factor, storativity ratio, interporosity flow coefficient, influence on the bottom hole pressure. Analysis results show that, in a single medium characteristics of well test curves, compared with conventional gas reservoirs, considering adsorption gas desorption and diffusion, the downhole pressure decrease slows down; Langmuir volume increases, gas desorption quantity increase adsorption formation, bottom hole pressure decreased slowly. Langmuir pressure close to the original formation pressure, bottom-hole pressure decreased most slowly; in the dual-porosity characteristics of well test curves, considering the pressure derivative curve appeared in the late period of upswept starting pressure gradient; consider the storativity ratio, transition matrix system and fracture system of the curve deepened; in well testing curve of gas-water two phases, gas reservoir pressure extraction, amatrix surface desorbs and diffuses, accompanied by the matrix shrinkage and porosity increases and so on. Desorption gas complement fluid supply, improve the formation energy, the bottom hole pressure reduces slowly, pressure difference curve downward; matrix shrinkage and porosity increased improve physical properties of reservoir and flow coefficient, the pressure derivative curve early depression deepened. At the same time, based on the material balance principle, the use of Bangham’s solid deformation theory, the Langmuir isotherm model, get the rock compressibility, and deduces the shale gas material balance equation, the original geological reserves of shale gas reservoir. Analysis results show that:Considering adsorption gas desorption and the matrix shrinkage effect, in shale gas reservoir rock compressibility is reduced, the porosity decreases slowly, properties compared with conventional gas reservoirs become good; increase the content of available reserves of gas adsorption, increase the content of free gas, make for the shale gas geological reserves increase.On the basis of bottom hole pressure research of shale gas reservoir, a shale gas well testing interpretation software is developed, the virtual base class (interface) of air reservoir, shale gas derived class and the realization of class function are written and the confidentiality and inheritance of reservoir property and behavior are realized through the application of computer professional knowledge such as object-oriented development language, data structure, database and the three layer architecture model. What’s more, a friendly interface is created to ensure better application of research results.The following results and understanding were obtained in this paper:(1) Analysis of the absorbed gas desorption and diffusion characteristics, and the percolation mechanism in the process of mining of shale gas.(2) Established the single medium seepage mathematical model and numerical model of shale gas reservoir with solution of the model, analyzed the effect of formation parameters such as the skin factor, wellbore storage coefficient, Langmuir pressure, Langmuir volume(3) Established the double medium seepage mathematical model and numerical model of shale gas reservoir considering start-up pressure gradient, and the solution of the model, analyzed the effect of formation parameters such as Interporosity flow coefficient, storativity ratio, start-up pressure gradient, wellbore storage coefficient, Langmuir pressure, Langmuir volume.(4) Established the single medium seepage mathematical model and numerical model of gas, water two-phase shale gas reservoir with solution of the model, analyzed the effect of formation parameters such as gas adsorption, skin factor, saturation.(5) From material balance principle, using Bangham solid deformation theory, the Langmuir isotherm adsorption model, deduced the shale gas reservoir material balance equation and analyzed porosity variation characteristics considering of the adsorbed gas, and obtained original geological reserves of shale gas reservoirs.(6) According to horizontal three-layer architecture thought (user interface layer, business logic layer, data access layer) and longitudinal three-tier architecture thoughts (interface layer, implementation layer, logical objects) application of object-oriented programming ideas, to develop shale gas reservoir well test interpretation software.