Prediction of Reservoir Fluid Facies Through Collaborative Workflow of Advanced Surface and Downhole Methodology
Authors: Ali H. , Mavromatidis A. , Strapoc D. , Dmello A. M. , Khan S. , Al Blooshi A. , Lyonga S. M. 
Published: November 2017

Arab reservoirs of UAE are complex in terms of fluid properties (condensate window) and reservoir quality (multiple targets). To characterize complex reservoirs in a continuous fashion and to update reservoir model, there is not always sufficient data available in terms of core samples and reservoir fluid samples. Therefore, advanced surface and downhole fluid sampling compositions were integrated to determine reservoir fluid type and distribution across the field and hence, help with field development program (FDP).

In this study, measurements from two different techniques were used to develop integrated workflow for fluid distribution: advanced surface fluid logging (ASFL) and bottomhole sample (BHS) analysis. Thermodynamically controlled mud-gas extractors and acquisition system provided continuous and quantitative fluid composition between C1 and C5 at surface for each individual reservoir layer. Near-real time, advanced surface fluid-facies had good agreement with bottomhole sample (BHS) composition. Fluid typing logs were anchored by later BHS lab complete composition and fluid properties analysis, which helped more accurate and locally calibrate surface fluid logging in follow up wells.

Advanced surface and downhole fluid signatures showed good correlations with key fluids’ properties, such as GOR. In order to further understand reservoir fluid chemistry and relationships to GOR, alkanes and aromatic components were studied using published models based on worldwide BHS. Fluid from each individual reservoir layer appeared to vary across the field. Specifically, in places where excess C1 and C2 was found using the mentioned above models.

Additionally, the C1 and C2 concentrations were the main driver controlling fluid GOR. Specifically, the recharge of mature C1 and C2 was observed in Arab-A reservoir, which was characterized by composition deviated from typical condensates. Yet, despite relatively decreased C7+ fraction, the wet gas and liquid concentration slopes remained very similar throughout Arab-A. Integrated fluid-compositional correlation pointed to a similar fluid signature in Arab-B, C and upper-D but different from the Arab-A. In terms of C7+ modelling, Arab B and Arab C fluids appear to be the unaltered parent condensate. Compositional ratios with GOR showed good relationship from surface and downhole measurements. This relationship serves as calibration point, including non-target yet promising intervals from which bottomhole samples were not available.

Proactive assessment of expected reservoir fluid is the primary objective in well planning and FDP. First time in Abu Dhabi, improved and reliable surface technology with incorporated downhole measurements, provides an efficient and economical integrated workflow. This established workflow lead understanding of reservoir fluid types and alterations (here specifically secondary gas recharge) and their distribution across the field for quick decision making, including: i) optimized downhole sampling involving investigations around GOWC and fluid anomalies, ii) lateral-well placement, iii) completion and frack stage planning.