Abstract
This paper discusses how the pressure measured by the downhole gauges during a drillstem test (DST) is affected not only by hydrostatic, friction or kinetic energy pressure losses, but also by changes in temperature. In oil wells, the bottomhole temperature is decreasing when the pressure is building up after a flow period. The change in temperature is inducing a change in the wellbore fluid density, itself affecting the bottomhole pressure. Most numerical simulators modeling flow in pipes do not take into account this effect, as it is not significant for low to moderate productivity wells. In high productivity wells however, the temperature change can affect drastically the bottomhole pressure measured above the reservoir and the pressure can even decrease at the end of a static period. A mathematical model is presented to compute the pressure drop caused by the nonisothermal effects and can be used to correct the measurements. Field examples are discussed to illustrate this effect and recommendations are made on how new technology can improve pressure measurements during well tests.
In order to measure the effects on the pressure measurements due to the position of the gauges in the DST string, two gauge carriers were run in some well tests in deepwater wells offshore Brazil, one at the level of the tester valve, and one below packer, as deep as possible in the test string. The gauges could transmit data with a wireless telemetry system that allowed the operator to monitor in real-time the pressure changes and the difference between the two sets of gauges.
In high permeability wells, using the uncorrected pressure measured with a gauge 50 to 100 m above the reservoir as is typically the case, can result in a wrong productivity index and at times, in uninterpretable results. Locating pressure gauges as close to the reservoir as possible is crucial in highpermeability reservoirs, to minimize the measurement errors, but also to avoid misinterpretations when an obstruction forms below the tester valve. With the new wireless telemetry systems, it is possible to monitor in realtime gauges mounted below the packer, much closer to the perforations than the standard gauges placed at the level of the tester valve, thus avoiding taking wrong decisions during the test.
A long pressure transient test could be uninterpretable because the gauges are located at the wrong depth. Ideally, the pressure gauges should be placed in front of the perforations but it is not always possible. Therefore, non-isothermal effects on pressure measurements during a well test needs to be clearly understood and taken into account when designing and interpreting well tests, particularly in high productivity reservoirs.
