An evaluation of porosity and permeability based on an unsteady seepage model in a reservoir while underbalanced drilling

摘要

During underbalanced drilling, real-time monitoring of parameters such as the porosity and permeability of the gas reservoir is the fundamental basis for researching the formation properties and drilling risk prevention. As the drilled layer thickness increases, the exposed area of the gas reservoir also gradually increases, and the amount of gas that flows into the wellbore varies, which causes changes to the wellbore gas-liquid distribution and its control parameters. Such changes pose challenges to the precise interpretation of the gas reservoir in underbalanced drilling, and the difficulty of controlling the underbalanced well also increases. Therefore, a series of studies on computing formation gas output in underbalanced conditions was performed via real-time monitoring of the total hydrocarbon value using ground-monitoring equipment, and an unsteady seepage model of the reservoir gas was developed by considering the flow characteristics of the gas while drilling. We analyzed the time-varying gas output and seepage boundary characteristics of the gas reservoir that is opened gradually during the underbalanced drilling. Based on the superposition principle, a computational model of gas reservoir's permeability was derived based on the computational results of the unsteady seepage flow model. Moreover, the porosity distribution in the working area was obtained according to the relationship between porosity and permeability, and a highly efficient and accurate evaluation of the gas reservoir while underbalanced drilling was achieved. Based on a field example it was demonstrated that these methods can meet the demands of engineering projects. (C) 2015 Published by Elsevier B.V.