Technical Paper: Improved Technique for Wireline Oil Sampling of a Tight Carbonate Reservoir in the Partitioned Neutral Zone (Kuwait/Saudi Arabia): A Case Study

Society: SPE
Paper Number: 132867
Presentation Date: 2010
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The main reservoirs of South Fuwaris field are the Lower Cretaceous Ratawi Limestone and Ratawi Oolite which commenced production in 1963. Ratawi reservoir is an elongated anticlinal structure with good porosity (15-20%) but low permeability (1- 5 mD). The Ratawi Oolite was historically ranked as a lower priority due to the poor productivity from vertical wells. The use of open hole horizontal wells and ESPs lifting has therefore become the favoured development option in recent years. The reservoir is currently undergoing further development with critical reservoir inputs being reviewed to enhance the reservoir model.  One of the key model uncertainties is related to oil properties where no representative PVT samples were available.

Multiple attempts to collect downhole oil samples using Wireline Formation Tester (WFT) in this reservoir had failed due to low formation mobility (<0.5 mD/cp) low fracture density and high pressure drawdown during sampling. A new technique was developed from comprehensive review of log and core data that includes:

i.   NMR for estimation of the extent of the damaged zone and identification of mobile hydrocarbons.

ii.   Review of sonic and core data for: estimation of rock strength; wellbore integrity; and perforation design.

iii.   Open hole perforation in the zone of interest to bypass the damaged zone increase the area open to flow and intersect natural fractures.

iv.   Reduction of perforation damage due to mud reinvasion through reduction of hydrostatic overbalance (from WFT pressure data) and the replacement of the wellbore mud system with clean completion fluid during perforation and sampling operations.

This paper presents a case study of successful oil sampling in a very tight carbonate reservoir through the use of innovative techniques and detailed job planning. The potential of applying this technique to heavy oil reservoirs within our asset portfolio is under evaluation.

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