Effects of Harsh Weather on Delayed Wireline Formation Fluid Sampling
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This paper discusses two cases where formation fluids were sampled after prolonged exposure to drilling fluid invasion. The long exposure occurred due to suspension of rig activities following the arrival of the Typhoon Meranti, in South China Sea. This comparison highlights the various factors influencing fluid sampling after unexpected delays. This paper attempts to answer the question, “In what ways does prolonged exposure to invasion affect formation testing and does sampling need to be cancelled due to such unforeseen delays?” Both of these wells were ready for the formation testing operation when the typhoon arrived and caused suspension of all rig activity indefinitely. The testing started 13–16 days after drilling, once the typhoon dissipated. Representative formation oil samples were collected in both wells, but the clean-up and sampling performances were dissimilar. The formation oil breakthrough times were very different, about 6 h with pumped volumes of 120–200 l for the medium-high viscosity oil (>200 cP) and about 3 h with pumped volume of 30–50 l for the light oil (<1 cP), respectively. These results provided a good opportunity to study the influence of unexpected delays on formation testing. Two 3-D near-wellbore reservoir models were set up using data from coring, open-hole log interpretation and laboratory PVT analysis results. Simulation runs were carried out to history match the pressure profile, pumped volume history and water cut (WBM contamination) performance. The invasion profile, which includes the flushed zone, the transition zone and the virgin zone, was studied to compare the influence of prolonged invasion on each of the cases. Results of the study show that the influence of extended time exposure to drilling fluid invasion is limited for a light oil reservoir for which there is a small viscosity difference between formation oil and the WBM filtrate. By contrast, medium-high viscosity oil with its deeper flushed zone required longer WBM filtrate clean-up. The observations provide a useful background for pre-job modelling and post-job analysis of formation testing jobs.