A New Method for the Detection of Wellbore Phase Redistribution Effects During Pressure Transient Analysis
Authors: Qasem F.H. , Nashawi I.S. , Mir M.I. 
Published: December 2007

Pressure - Time data obtained during a pressure transient test include the effect of non-reservoir factors besides the reservoir response. Improper diagnosis to differentiate between reservoir and non-reservoir effects causes misinterpretation of well test data. Phase redistribution is one of the major non-reservoir factors. Even the best available diagnostic techniques fail to detect the phase redistribution effects when they are not obvious from the analysis plots and the data do not match the pressure derivative type curves over the complete time range. One can only suspect the phase redistribution in this case. Verification of anomalous effect due to wellbore phase redistribution is required. In the field of exploration geophysics, there have been established methods for the detection of anomalies in the gravity, magnetic or seismic data. In this paper new analysis techniques besides some of the major techniques from exploration geophysics are applied to transient well test data. And their relative success is discussed for the detection of wellbore phase redistribution effects.

Introduction

Pressure transient well testing is one of the major tools for formation evaluation. These days highly sophisticated electronic gauges are used for pressure measurement. A significant amount of these very accurate pressure data that are measured during a well test, do not reflect reservoir phenomena at all but are dominated by wellbore related effects or pressure gauge related effects. If not properly identified these effects could easily be misinterpreted as reservoir characteristics. In that case it will lead to erroneous answers in both the identification of the reservoir model and the calculation of parameters values. Therefore the measured data must be critically evaluated and processed prior to the pressure transient analysis.

Once the non-reservoir effects have been identified, they can either be filtered out, corrected or ignored but must not be interpreted as reservoir effects. Mattar1–2, has discussed in detail the importance of pre-processing of well test data before its analysis or interpretation.

Wellbore effects have been recognized since pressure transient testing was first established as a viable method for evaluating well and reservoir performance. In an effort to quantify and evaluate these effects the concept of wellbore storage was introduced3–6 followed by the concept of phase redistribution phenomena7–8. The phenomena of wellbore phase redistribution occur in a well, which is shut in with gas and liquid flowing simultaneously in the tubing. As shown by Stegemeier and Matthews7, when such a well is shut in at the surface, gravity effects cause the liquid to fall and the gas to rise to the surface. Because of the relative incompressibility of the liquid and the inability of the gas to expand in a closed system, this redistribution of phases causes a net increase in the wellbore pressure. When this phenomenon occurs in a pressure build up test, the increased pressure in the wellbore is relieved through the formation, and the equilibrium between the wellbore and the adjacent formation will be attained eventually. However, at earlier times the pressure may increase above the formation pressure, causing an anomalous hump in the buildup pressure, which cannot be analyzed with the conventional techniques. In less severe cases, the wellbore pressure may not rise sufficiently to attain a maximum buildup pressure.