Pressure Transient Analysis (PTA) of bottom-hole pressure data (BHP) is a well-established method for estimating reservoir flow parameters and identify well behaviour. Unfortunately, permanent recording of bottom-hole data is not always operationally possible, for example in the case of high pressure and high temperature (HP/HT) reservoirs. On the other hand, most wells are equipped with gauges at the well head which record well head pressure data (WHP) continuously. This paper investigates the feasibility of using WHP data for identifying well test behaviour. The objective is to assess the ability to derive key well and reservoir parameters from WHP data in the absence of BHP data, focusing primarily on the estimation of permeability and skin.
Three different actual reservoir fluid and wellbore conditions were studied: a water injector with a single phase fluid in the wellbore and a reasonably constant fluid density; a dry gas well, also with single phase in the wellbore but changing fluid density; and a gas condensate well with multiphase flow and varying density in the wellbore. In each case, both WHP and BHP data were available. These WHP and BHP data were analysed separately with conventional PTA methods in order to compare resulting permeabilities and skin factors. WHP data were then converted to BHP data, using methods available in the literature in the water and dry gas cases, and two different approaches developed in this work for the gas condensate case.
In the case of the water injector, analysis of both the original WHP data and the converted BHP data provides a good estimate of permeability while overestimating the skin factor. A correction can be applied to the WHP derived skin, to match the BHP skin value.
In the dry gas and gas condensate cases, WHP analysis overestimates both permeability and skin factor. In the dry gas case, the WHP derived permeability and skin can be corrected to yield the BHP values, whereas PTA on the converted BHP data does provide the correct permeability. Finally, in the gas condensate case, it is possible to obtain the correct permeability from converted BHP in the absence of phase redistribution, whereas the skin factor remains overestimated.