engleski

Multidisciplinary approach in core analyses of the Legrad-1JR reservoir

The wider area of the Legrad-1J (Leg-1J) exploration well has been reinterpreted and a prominent fan-shape feature has been recognized. The new deviated well Leg-1JR (reentry) has been drilled from the existing borehole to reach that more than 20 m thick sand body. The well was positioned in mid fan to gain better reservoir properties and thickness. Primary target was Upper Miocene reservoir of Okoli sandstone Member (B5 series) from Ivanić Grad Formation. The well Legrad-1JR has been drilled and completed in accordance with planned activities. Standard laboratory analyses and methods have been used on all samples from the borehole to complete well data interpretation and the Final well Report. Information from mud log and core data suggested the bottom sand formations have hydrocarbon shows. Therefore core that has been recovered from the interval of interest become the most valuable. The C#1 (interval 2286-2295 m) is made of light gray to brown gray mostly fine-grained, moderate to well lithified, cemented (dolomite – ankerite ; calcite ; quartz overgrowth), UV positive litharenite in type sandstone with good intergranular porosity (~ 15%) but low permeability (~ 1 mD). The analyses also included palynological Upper Pannonian age definition, Paratethys dinoflagellate biozonation affiliation to Spiniferites bentorii zone (Pontiadinium pecsvaradense subzone), palynofacies characterisation, as well as petrographic and sedimentological analyses and interpretation that enabled setting in brackish lake toe-of slope channel complex environment with subaquatic fans, channel and inter-channel deposits. Standard geochemical procedure was performed on cores and cuttings as well. Through the well profile the organic contaminations (graphite, lignite, polymers and D2) are documented and distinguished from extractable organic matter (EOM) in the core sandstone. The mature bitumen from the core originated from the mixed kerogen type II-III that reached the mid stage of oil window (0.8% Ro). Five intervals have been chosen for further reservoir evaluation, assessment of formation fluid presence, productivity performance and reservoir physical properties. Selected intervals have not behaved as expected during well testing. This initiated number of additional multidisciplinary analyses and measurements on core, rock cuttings and fluids. In order to resolve emerged problems, petrographic analyses of drill cutting sample from each available interval have been done. Also, detailed SEM and EDS analyses of the pore space, pore throats, cement and clay minerals in core samples have been conducted as well with special emphasis on diagenetic processes in rocks. Since the well is deviated many lubricants and additives have been used in drilling which might resulted in formation damage and occurrence of rigid skin and precipitation of sludge. Further organic and inorganic geochemical analyses of detected emulsion (mixture of water, D2 diesel fuel and submicron graphite and iron particles) and acid solubility tests were performed. It seems that interaction between acid used during acidizing operations and organic contaminant (D2) in the well probably produce the emulsion blockage phenomenon. Additional analytical and numerical simulation analysis has been done to validate initial production potential of the reservoir using relevant petrophysics and thermodynamics properties. Although only formation water presence was confirmed, sensitivity analysis with different fluid saturations and level of formation damage (skin) was performed. The overall conclusion indicated a poor quality reservoir with low productivity performance and possible high skin present due to mud not completely removed from the wellbore.

Special (geological, geochemical, petrophysics, termodynamics) core analyses, low perm sediments, Upper Miocene, Croatia

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