Abstract
The South Fuwaris Field is located in the Partitioned Zone of Kuwait and Saudi Arabia. The main reservoir is the Lower Cretaceous Ratawi, which commenced production in 1963. The reservoir is an elongated anticlinal structure, with good porosity (15-20%) but low permeability (1-10 md). The Ratawi Oolite was historically ranked as a lower priority compared to Ratawi Limestone due to poor productivity from the vertical wells. In recent years, the well performance has improved significantly through the use of open hole horizontal wells and Electric Submersible Pumps (ESPs). 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, as no representative PVT samples had been available.
Multiple attempts to collect downhole oil samples using Wireline Formation Tester (WFT) in this reservoir had failed due to low formation permeability, low fracture density and high pressure drawdown. A new technique was developed from comprehensive review of log and core data including:
Nuclear Magnetic resonance (NMR) for identification of mobile hydrocarbons, presence or absence of bitumen, estimation of oil viscosity, pore size distribution and permeability with core data integration.
Review of sonic and core data to optimise perforation design.
Open hole perforation in the zone of interest to bypass the damaged zone, increase the area open to flow and intersect natural fractures.
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 downhole 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.
