Downhole fluid sampling in shallow unconsolidated reservoir having high inter-granular porosity and filled with viscous oil is quite challenging. The dominant formation properties, relevant to fluid flow, like low pressure, rock’s mechanical weakness, drilling damage and fluids mobility ratios, despite very high permeabilities, have resulted in the failure or impracticality of the conventional and most obvious methods- for example the sampling with Dual/Straddle Packers.
The problems faced during sampling are: i) plugging of flowline, ii) emulsion generation iii) sand production- leading to caving around sampling tool and iv) marginal operating limit of pressure drawdown-dictated by extremely low formation pressure and little difference between hydrostatic and formation pressures. Mostly, the flow happens in surges and any increase in flow rate causes large drag on sand grains and excess pressure drawdown.
A thoughtful mix-up of technology, understanding of rock mechanics, innovative operating techniques and proper coordination by aligning all concerned has helped in meeting the challenge of sampling viscous oils in the unconsolidated sand.
A log-based Geo-Mechanical study suggested very low unconfined compressive strength for the sand, restricting the flow rate to as low as 2 cc/s. Dual/Straddle Packers, with its large volume at this low flow rate, would require very long hours of pumping. Typically, the volume between packers would take 5 to 6 hours of continuous pumping at the suggested maximum rate of 2cc/s; just sufficient to start pumping out from the formation. A normal probe would cause sanding at probe head due to the reduced cross section to flow. The most successful approach was the selection of: a) probes with enlarged cross section to flow, which reduce the drawdown by decreasing the flow velocity at the sand face, b) ultra slow pump out rates to negotiate drawdown, formation weakness and mobility ratios of water/filtrate and viscous oil.
Multiple sand filters in the flow line, segregation of emulsified filtrate through innovative techniques and state-of-the art fluid analysis methods helped to determine exactly when the viscous oil started to flow. However, the surge nature of flow still resulted in 20 to 30% contamination in the sampled oil.
The obtained PVT-quality downhole sample helped in determining the in-situ fluid properties of the viscous oil.