Jurassic reservoirs in Kuwait fields are deep (upto 17,500 ft), high-pressure (>10,000 psi), high temperature (300°F) and having high H2S concentration upto 35%. Additionally, some reservoirs are having super saturated saline water. Exploratory wells drilled in these reservoirs are tested with Drill Stem Test (DST) tools. High quality reservoir fluid sample is necessary for meaningful PVT studies, which is very important for assessment of reserves and for future development. Many wells were tested in deep reservoirs but BottomHole Samples (BHS) could not be collected in most of the wells due to extreme well conditions namely well deviation, HPHT, H2S upto 35% and salt deposition inside tubing. Generally, Well Head Sample (WHS) and/or separator samples are collected for PVT studies. Downhole samples are preferred for carrying out PVT studies. However, using slick line/wire line for collecting downhole samples was considered unsafe and having potential risk due to extreme well conditions. Keeping in view of well profile and hostile environments, provision was made to collect representative reservoir fluid sample using downhole sampler in DST assembly. The DST sampler is rated to 20,000psi and 400°F, can collect upto nine 400cc samples, and are activated in sets of three by annulus pressure. It was very challenging to operate the downhole DST sampler, as the mud used in the annulus was as high as 20.8ppg OBM.
A pilot study on this DST sampler was implemented in two different exploratory wells. Reservoir fluid samples of sufficient quantity were collected at downhole condition in hostile environment. Quality of samples collected during DST is good and these samples were used to characterize the reservoir fluid. Based on the successful results, it has been planned to use DST sampler in these hostile environment in future.
This paper describes successful downhole sampling DST design/tools, together with the techniques implemented to obtain representative reservoir fluid samples at downhole.