Abstract

The acquisition of downhole pressure data representative of reservoir response enabling subsequent pressure transient analysis has been one of the primary drivers for running drill stem tests. However, many factors can influence the representativity and interpretability of the data acquired that are not related to reservoir properties.

To our knowledge, while many publications have presented challenges in acquiring representative pressure data those have not been compiled in a comprehensive revies, and there are no practical recommendations that would summarise causes and effects and offer procedures to eliminate or at least manage those effects and enable end-users to maximize the value of acquired data.

This paper describes in details today's challenges associated with the acquisition of high-quality, representative and undisturbed bottom hole pressure data during well test operations. Many different effects, including gauges’ deployment methods, wellbore effects and operational aspects of the test can compromise the quality of bottom hole data acquired while running a welltest.

Therefore, the origin and impact of each of these effects needs to be evaluated at the design stage of the test to develop appropriate mitigation actions. To address these issues, actual examples and methodologies derived from various locations are presented.

Over the years the metrological performances of downhole memory gauges such as resolution or drift have improved drastically, reaching a point where gauge specifications have become less influential on data quality than environmental effects. Many improvements have also been made in DST tools to increase the representativity and interpretability of acquired bottom hole pressure data such as the introduction of downhole shut-in valves or compensation for tubing contraction and expansion due to temperature change during the test. However, there remain several occurrences today where memory gauge data are affected by the various wellbore phenomena making interpretation of downhole pressure transient test data complicated. The selection of an appropriate location of pressure sensors in the DST string also remains a crucial task.

The paper provides analysis, explanations and practical recommendations allowing to mitigate the most common effects typically observed during welltest operations performed around the world, such as:

  • Tidal effect

  • Fluid segregation effect in the wellbore

  • Pressure noise propagation from the surface due to rig movement

  • The impact of application of electrical submersible pump (ESP) on the quality of pressure build-up data

  • "Hammer effects" during well shut-in

  • Impact of circulation above the test valve during PBU

  • Impact of pressure bleed off and top up in the annulus

  • Fluid cooling effect in the wellbore

  • Gauge movement due to string contraction and expansion

This paper will summarise the observation and lessons learned from hundreds of welltest operations performed around the globe with different reservoir fluids and environments through a few telling examples. Furthermore, the paper provides practically proven well-test techniques allowing to manage those adverse effects on bottom-hole pressure data. Recipes for success are provided to ensure that high-quality data can be acquired during welltest operations in a challenging environment while keeping the cost in line with the AFEs.

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