Numerical Test Theroy Research And Application Of Gas-Water Two-Phase Flow On Horizontal Well

At present, with the development level of low permeability gas reservoir for horizontal wells deepening, horizontal gas well testing technology faces challenge of new problems emerging. In particular, in the late and middle period of the gas reservoir and gas wells development, to see different levels of water flow to wellbore flow and the formation of multiphase flow present situation have become increasingly prominent for the results of well test analysis. Another problem for the single well testing, well test analysis of existing traditional methods often take only the flow of formation as the object of study, few consider the wellbore flow situation, and wellbore flow can’t be ignored an important factor for transient testing. Therefore, it need to find new test theory and methods considering generally above complex factors, make the results of well test interpretation closer to reservoir’s realities.This article is based on the above, new test theory and methods of gas and water two phase flow in the horizontal wells in low permeability gas reservoirs was regarded as the research object, and on the basis of research of a lot of literature, this paper established mathematical model and numerical well test fully implicit numerical model of gas-water two phase flow in reservoir coupled horizontal wellbore flow through the integrated use of reservoir engineering, reservoir mechanics, computational mathematics, computer technology and other disciplines and methods. The wellbore storage, skin effect, slippage effect, and threshold pressure gradient were also taken into account in the model. On this basis, a gas-water flow numerical well test simulator for horizontal wells was developed to provide a theoretical basis to low permeability gas reservoir development.Through this study, the main results obtained are as follows:1) Based on black oil model and generalized Darcy’s law, established a mathematical model and numerical well test fully implicit numerical model coupled horizontal wellbore flow and gas-water two-phase flow in reservoir. Model considered the wellbore storage, skin effect, slippage effect and pressure gradient, the whole system more accurately reflected the actual flow law of gas-water two-phase flow in gas reservoir.2) Using hybrid grid encryption technology to split gas reservoir, radial grid was applied near well region, and gas reservoir region was rectangular grid, which eliminated required number of multi-layer grid the conventional grid simulation of radial flow near wellbore region that reduced largely the necessary grid number of the accurate simulation of wellbore fluid flowing in the vicinity.3) For solving the Jacobi coefficient matrix formed according to fully implicit numerical model, the paper introduced the Newton-Raphson iterative method and the pretreatment orthogonal minimizing iterative method which improved largely stability, calculation speed and convergence rate of the full implicit numerical model compared to other traditional methods such as SOR, SIP and so on.4),On the basis of theory research for numerical well test on gas-water flow t in horizontal well, under the environment of Windows XP operating system, using VB, VC, and MATLAB language, the paper developed numerical well test simulator about gas-water flow in the horizontal well considering the multi-parameter and wellbore flow according to object-oriented programming design which further broadened and enriched the well test analysis measure.5) The paper calculated and analyzed a single well and single layer homogeneous gas reservoir model using separately developed well test simulator with the analytical and numerical solution, the result showed numerical well test curve shape consistent with of the analytical well test curve under the conditions of single-phase flow which proved accurate and reliable of the methodology of the model and the simulator.6) The calculation results of wellbore flow showed that the radial inflow of gas flow along the horizontal well bore length distribution is uneven, the whole shape show "U" shape, reflecting that the supply scope of different position of horizontal wellbore are different, while the influx in the horizontal wellbore is variable, wellbore flux appeared increasing trend from the heel to the toe of horizontal well. However, because the existence of pressure drop well increasing from the heel to the toe, thus wellbore flux would not w increase unlimitedly with the increase of the length of horizontal section.7) Through the analysis of sensitivity factors of the gas-water flow numerical test well curve in horizontal well, the results showed that large differences the between gas and water in physical properties in gas reservoir made the test curve shape was quite sensitive for changes of the physical parameters. One slip factor, pressure gradient and saturation affected the whole pressure derivative curve shape; the skin effect only affected the transition section shape of the pressure derivative curve; effect of wellbore storage and location of horizontal well was the location of the early segment of the pressure derivative curve; the effect of horizontal wellbore’length for the pressure derivative curve showed radial flow and the early stages of the border.Based on the theory research and application of gas-water two-phase flow numerical well test in horizontal well, it further enriched and broadened the application area of the horizontal well testing theory and methods, and provided a strong theory support for resolving the well testing problems of gas-water two-phase flow in horizontal wells in low permeability gas reservoirs and coupling with wellbore flow, while promoted the further development of numerical test theory.