DRC Salvages Well Test

The role of a DRC technical sales engineer is not simply renting a gauge.  He acts as a well testing knowledge base and consultant to his clients.  The DRC well testing system, in addition to the SPIDR itself, includes the test design, conversion of wellhead pressure to bottomhole conditions and the interpretation of the data.  The system is dependant on both the technology and the experienced personnel who analyze the data.  Data Retrieval Corporation conducts over 700 well tests a year from a wide array of well types, locations and conditions.  A DRC sales engineer brings this extensive well testing experience to every sales call.

In a recent sales meeting, a DRC technical sales engineer sat down with a GOM engineer to discuss future testing opportunities.  The engineer had been testing his new gas wells with another service company.  They seemed to be happy with the results but had observed what they thought, were a few operational issues during a couple of their tests.  Due to these “operational issues” the service company concluded that the build-up test data could not be analyzed.  Since the build-up test could not provide the quantitative numbers he desired, the operator could not predict well performance and could only work with rough estimates based on production.

The operator mentioned that a “hiccup” occurred during the initial stage of the build-up followed by a “slow leak” during the remainder of the test. The DRC sales engineer recognized these key phrases, which prompted him to ask a few more questions.  As the conversation about the data set and well continued, the DRC engineer realized that the anomalies, the “slow leak” and “hiccup”, were actually wellbore effects that can be explained and corrected if one has the well testing experience and software to do so.  The “slow leak” at surface, which appeared to make the well test worthless for performing a transient analysis, was actually a phenomenon know as thermal decay.  Thermal decay is the cooling of wellbore fluids (gas/condensate) after production stops carrying reservoir heat to the surface.  This cooling of the wellbore increases the density of the fluid column, resulting in an increase in the hydrostatic pressure.  When the rate of hydrostatic pressure increase dominates the reservoir pressure response upon shut-in the wellhead pressure displays a downward trend.  This is mainly the case in wells with high bottomhole temperatures and moderate to high permeability.  Also note that not all cases of thermal decay will have a downward trend.  In some cases, the density increase of the fluid column does not dominate the reservoir pressure and the wellhead pressure simply trends upward less rapidly.  This will give erroneous reservoir property values from pressure transient analysis.  If not recognized by a skilled well tester, thermal decay can be mistaken as a “slow leak” or not noticed at all.

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The “hiccup” the operator spoke of was another phenomenon in well testing, which is encountered by both downhole and surface data acquisition devices.  The DRC sales engineer recognized this “hiccup” as liquid fallback and re-injection.  Liquid re-injection occurs when a well that produces some amount of liquid, whether condensate and/or water, is shut-in.  The liquid can fall back and create a column at the bottom of the well.  The gas from the reservoir displaces the liquid column from the well bore back into the formation.  This allows single-phase continuity to be re-established between the top perforation and the gauge and can be seen by a slope change (hiccup) on the pressure plot.  This is the case whether a downhole gauge or surface gauge is being used.  The extent of masking is the only distinguishing factor between the two.  The presence of liquid re-injection too can be misinterpreted as leaks or mechanical events if not looked at by an experienced eye.

Data Retrieval Corporation, with their expertise and experience in well testing, took this once non-analyzable data set and showed the operator that they had analyzable well test.  DRC’s reservoir engineers used their proprietary software to accurately convert the surface data to bottomhole conditions taking into account thermal decay and calculate values for permeability, skin and P*.  The quality of results provided by DRC could have been even better if a SPIDR gauge had been used.  The resolution and frequency of data would have been much greater and if a DRC engineer assisted in the well test design.  The first step in a quality test is the test design. 

Data Retrieval Corporation not only rents gauges, but is an authority on well testing and well test design which enables clients to achieve their test objectives at minimum expense.  The SPIDR well testing system provides operators with the most reliable and accurate pressure gauge on the market, but also a team of engineers who specialize in pressure transient testing.  Their experience and knowledge allows them to continually serve their clients better and stay the front-runner of surface well testing in the oilfield.