Possible Misinterpretations in Well Test Analysis Due to Unfiltered Tidal Signal

Abstract This paper presents the most common misinterpretations in pressure transient analysis owing to unfiltered tidal signal in well test data. Pressure transient analysis gives reservoir engineers an idea about permeability, different components of skin, reservoir model, as well as reservoir geometry. While collecting downhole pressure data, the reservoir engineer is mainly concerned about pressure changes due to fluid flow in porous media. However, the downhole pressure data is affected by a few unavoidable natural phenomena. One of such natural phenomena is the tidal effect in offshore wells and on-land wells near to the shore. Most of the times tidal signals are filtered with the help of pressure recorded by sub-sea pressure gauges and maritime tide tables. However, when those data are not available, or in case of SRO (surface read-out) without provision for filtering tidal signal, build-up data is analyzed without filtering tidal signals. This paper discusses how tides with different time and amplitudes effect the interpretation of build-up tests having various QBμkh values. [GR1] Tidal signals that have different amplitudes and time periods were superimposed on pure build-up pressure of infinite acting radial model data, and an interpretation of results were compared. Our results suggest that the early time region of the pressure build-up is not significantly affected by the tidal signal because of the steep increase in pressure. If the build-up test is continued for a significantly long time, even after pressure stabilization (not practical), tidal signals could be identified on a log-log plot by inverted U shape patterns. However, in the case of build-up studies, which are not sufficiently continued, it is likely that geneous infinite acting reservoirs are interpteted as parallel fault, U shaped fault or closed boundary models. Several possible misinterpretations owing to the presence of diurnal and semi diurnal tidal signals in build-up data are listed in this paper. It has to be ensured that pressure transient data is free of tidal signal before concluding any boundary presence. Unfilterd tidal signals in the pressure transient data may lead to interpretation of unrealastic boundaries. The outcome of this research will help reservoir engineers avoid misinterpreting pressure transient data.