Applications of Temperature Transient Analysis for Welltest Interpretation and Well Productivity Evaluation

Abstract

Abstract During transient tests, both pressure and temperature change depending on downhole flow rate. In gas producing wells, Joule-Thomson cooling and frictional heating effects are the main dynamic factors that cause flowing bottomhole temperature to differ from the static formation temperature at that depth. When a gas well is shut in, JT cooling effect is vanished and this causes a sudden sharp increase in sandface temperature. Then as the effect of wellbore storage ends, wellbore temperature gradually cools down due to heat conduction with near wellbore region. This paper demonstrates the applications of temperature transient data and proposes a new technique for using temperature transient data in gas wells in order to determine end of wellbore storage. Also, the effects of permeability and well productivity on temperature behavior are discussed. Three field examples are shown in which both temperature and pressure transient data were analyzed for more accurate welltest interpretation. This paper shows how knowledge of Joule-Thomson cooling effect and frictional heating effect can be applied for reservoir characterization. Introduction In pressure transient tests, the early portion of the well test data is usually affected by the wellbore storage effect and can be influenced by skin and reservoir permeability. Many analysts rely on pressure derivative curve to diagnose wellbore storage period and radial flow regime on pressure transient data. However, there are field examples that flow regimes can't be accurately determined. The wellbore storage effect delays the formation pressure response and distorts the early portion of pressure transient data. Diagnosing of the radial-flow regime is crucial to quantitative interpretation since it provides values for permeability and skin. Unit slope and the plateau on the pressure derivative curve as well as Horner plot are usually used to identify pure wellbore storage and radial flow regime as shown on Fig.1. The interpreter's first task always is to identify the unit slope line and derivative plateau to identify flow regimes. Since different factors including wellbore storage, skin and reservoir heterogeneity affect pressure response, detecting end of wellbore storage and flow regimes might have uncertainties. Horizontal wells, for example, pose two special problems for the reservoir engineer. The first is the unavoidably large wellbore storage effect as horizontal section may extend for thousands of feet and cannot be isolated from the transient. The second is the more complex nature of transient, which makes diagnosis more difficult. So wellbore storage may distort the early time flow regimes and cause uncertainty in detecting first plateau on derivative curve. Therefore, determining accurate time at which wellbore storage stabilizes, has a significant impact on the analysis and interpretation of pressure data.