Wireline pressure testers and reservoir fluid sampling tools have, for quite some time now, been considered viable alternatives to well testing. These tools are widely used to identify reservoir fluids and obtain representative samples for laboratory analyses. In order to recover uncontaminated samples, fluid is first pumped out of the formation into the wellbore, until real-time downhole monitoring of the fluid in the tool flowline ensures it is clean. The reservoir fluid is then captured in sampling bottles or chambers.
Gas-condensate sampling has always been the trickiest, because even little traces of contamination may render the sample useless. Besides that, the tool pumps gas into the wellbore during the cleanup phase, raising issues of well control. Another important factor to consider if the dew point is close to the formation pressure, is that pressure drawdown has to be controlled while pumping - this is imperative in order to be able to sample the fluid at downhole reservoir conditions, and to minimize change in phase characteristics and composition of the fluid sample. Despite advancements in the field of Downhole Fluid Analysis (DFA*), there have been ample instances of recovering contaminated gas-condensate samples.
This paper discusses an innovative technique, successfully applied in the North Sea, to acquire clean gas-condensate samples.
Firstly, a well-kick and under-balance drilling simulator was used to calculate how much gas could be safely pumped into the wellbore. This, coupled with continuous downhole monitoring of well hydrostatic pressure, addressed well control concerns.
In the Wireline toolstring that was used for the operation, 450 c.c. sample bottles were used for collecting the fluid. These bottles were positioned on the top end of the toolstring and the pump module was at the bottom. The basic principle used was that after cleaning up, the pump was stopped for a couple of minutes allowing the fluid to be stationary in the tool flowline, thereby letting any contaminants present in the fluid to settle down. The pump was then restarted, to slowly push the clean fluid column into a sampling bottle. Downhole Fluid Analysis (DFA*) was used to constantly monitor the flowline. The fluid analyzer was positioned in between the pump and the bottle, and as soon as it detected traces of contaminants, the sampling bottle was shut off immediately, even if it was not completely full. For this set-up to work, the volume of flowline between the pump and the bottle had to be enough to fill at least one bottle. This was achieved by adding extra sample chambers in the string, which served the dual purpose of increasing the length of the flowline and taking additional big volume fluid samples. The fluid analyzer also served as a warning device to indicate if there was any undesirable gas-liquid segregation in the tool flowline, upstream of the sampling bottles. Determination of the drawdown threshold and controlling the pumping rate ensured that liquid drop-out did not occur in the flowline, during the critical phase of the operation.
This technique yielded very clean samples, validated by laboratory analyses - four out of six bottles were full and pressured-up; two were not completely full, but clean. This may be adopted as a standard best practice and is applicable to any Wireline gas-condensate sampling operation that applies a similar setup and principle.