Since Cullender and Smith developed their WHP to BHP conversion program in the early 1950’s, surface pressures have been used to calculate bottomhole pressures on shallow, dry gas wells. If the original Cullender and Smith equations are modified to account for produced liquids, and the higher pressures and temperatures encountered in deep wells, the correlation may be extended to gas/condensate wells that are single-phase in the well bore. This then means that single-phase liquid wells (water injectors and oil wells above the bubble point) can also yield accurate well test results from the surface. Testing from the surface reduces the cost and eliminates the risk of running tools into well bores. Surface testing also allows the testing of high-pressure/high-temperature wells that cannot be tested with a downhole gauge because of harsh conditions. Thus, to reduce the cost and risk (or when no other option is available), many operators have chosen to run their pressure transient tests from the surface on single-phase wells. In addition to the traditional pressure transient testing methods of build-up and drawdown, surface testing is an invaluable tool for other pressure transient tests. These include pre-fracture injection fall-off tests, waterflood optimization, CO2 injection and production testing and injectivity and withdrawal testing on storage reservoirs.
It is possible to test most naturally-unloading gas/condensate and oil wells from the surface. This is due to advances in multi-phase wellbore modeling which take into account the fluid’s behavior in the well bore. The purpose of this presentation is to show the validity of pressure transient testing via surface pressure measurements. First, the general framework of the surface-to-bottomhole pressure calculation will be presented. Next, multi-phase wells will be categorized based on the type of fluid and the behavior of the fluid both in the reservoir and in the well bore. This categorization will be the basis for both surface testing candidate selection and recommended test procedures. Afterwards, wellbore phase and temperature modeling will be discussed. Next, instrumentation requirements will be presented. Field data comparing calculated bottomhole pressures from surface gauges to measured bottomhole pressures from downhole gauges (and the subsequent analysis) will be presented. Finally, there will be a discussion of the data acquisition methods and requirements for performing unconventional pressure transient tests.