How to Obtain Quality PVT Samples for Heavy Oils
Authors: Platt Chris , Chevarunota Natasha , Taksaudom Pongpak , Daungkaew Saifon , Duangprasert Tanabordee , Khunaworawet Tanawut , Thiti Lerdsuwankij Thiti , Wattanapornmongkol Sawit , Thongpracharn Payap 
Published: November 2019

Exploration activity is always associated with many challenges such as uncertain pore pressure, and uncertain formation depths and characteristics. Unconsolidated formation could cause more serious troubles for drilling, formation evaluation, and production such as borehole washout, wellbore collapse, and sanding if proper planning is not in place. In addition, a viscous oil can add another complication for fluid sampling operations. An unsuccessful logging program could have a major impact on the field development plan (FDP) and further field investment decision (FID).

In the Gulf of Thailand (GoT), high temperature Pattani basin discovery wells, reservoir fluids are mainly gas and condensate. There are numbers of waxy oil reservoirs1–5 in certain area in the GoT, notably in the cooler peripheral Tertiary basins. However, the subject field is the first one that was identified as having productive heavy oil reservoirs. The viscosity variation ranges between 1 and 100 cp2–6. It was observed that there was a depth related variation with deeper reservoirs having higher viscosities, and therefore, reservoir fluid information is crucial for the FDP and FID resulting from a field extension drilling campaign in early 2018.

This paper will discuss step by step (1) reservoir characterization challenges (2) proposed methods to obtain reservoir and fluid information, as well as the interval pressure transient test, (3) the actual field results, (4) recommendations and way forward for similar reservoirs.

Different proposed options are also discussed with field examples to obtain high quality PVT samples. Pumping to clean up high viscous oil contaminated tends to attract finer particulates towards the probe and into the flowline, causing plugging issues in other probe types even though a modified sand filter was added. In the end, the 3D Radial probe was proven in making this exploration campaign a success story for acquiring the heaviest oil samples to date in the GoT. The 3D Radial probe equipped with mesh filter plays an important role to restrict ingress of small sand particles, thereby allowing both sustainable pumping speed and flowing pressure. The single packer design also helps to support the formation preventing drawdown collapse. Coupled with larger flow area of the probe itself, the 3D Radial Probe has ability to control flowing pressure to stay above the sand break-away pressure even as more viscous formation oil enters. However, job objectives were achieved, which were formation pressure acquisition, high-quality fluid sampling, and Interval Pressure Transient Testing (IPTT) as well as Vertical Interference Testing (VIT).

This paper also discusses the comparison between Downhole Fluid Analysis results and PVT lab analyses. Limitation and challenges for downhole measurements for this heavy oil environment. Advantages and disadvantages for different testing methods for this heavy oil reservoir will also be discussed.