Abstract
The idea of using wellhead data for pressure transient analysis is very attractive, as if it is successful it spares us from the need to deploy any type of downhole tools, which are costly and wireline conveyed tools pose various mechanical risks including planting fish in the wellbore. The economics is overwhelmingly in favour for deep and hot wells where wireline intervention costs are high and workovers are expensive. Wellbore storage, presence of a second phase, cooling of the fluid column are some of the major obstacles in making use of wellhead data for pressure transient analysis.
In this paper we describe our experience of Pressure Transient Analysis using wellhead data for some dry gas fields in Saudi Arabia. For most of the cases wellbore storage is not an issue, as we can design the test for a period long enough to get out of that region. We have conducted some pressure build-up tests with gauges at both downhole and wellhead. Then we have converted wellhead pressure (WHP) data to bottomhole pressures (BHP) using the popular Cullender-Smith method and its various modifications. Two phase region and wellbore storage effect have been overcome. However, temperature transient has been found to be the biggest challenge which affects not only the absolute BHP, but also the bottomhole pressure trend which forms the very basis of transient pressure analysis. A thermal decay function was used to model the temperature transient using the initial and final temperature profiles. The resulting calculated BHP trend gets closer to the actual measured data, however differences still remain. Then we spell out a detailed plan to mitigate the remaining uncertainties to make the best use of these wellhead pressure data and obviate the need for running wireline gauges.
