< Formate Matters

CFD uncovers the truth behind erratic formation-damage testing

Wells drilled and completed with cesium formate fluids all enjoy high rates of production (SPE 165151-MS). Unfortunately, this excellent well performance is not always replicated in reservoir condition coreflood testing, especially for high-permeability reservoirs.

CFD model showing how heavy cesium formate filtrate has drained away from a deviated well in a 1 darcy, 200°C gas reservoir after only two days of shut-in followed by four days of production.

CFD model showing how heavy cesium formate brine has segregated to the bottom of a horizontal coreflood test cell, effectively preventing the gas from cleaning up the core plug during drawdown.

Reservoir-condition coreflood testing is a laboratory test method commonly used to evaluate a drilling or completion fluid’s formation damage potential. The typical formate 'damage' mechanism is reported as "filtrate retention." Using computational fluid dynamics (CFD), Lloyds Register has now confirmed this as a laboratory artifact. 

During well cleanup, there are two forces that help displace drilling fluid filtrate from the near wellbore. These are drawdown pressure and gravity. Independent of well configuration or test-cell configuration, the drawdown force will always try to clean up the rock matrix through low-permeability filtercake. This can be a very slow process.

In a reservoir, there will always be regions where filtrate is free to drain away from the wellbore with gravity – often to an extent where it never returns. This has been confirmed by comparing LWD logs with after-drilling wireline logs, which shows that cesium formate filtrates in gas reservoirs are mobile and are replaced by reservoir fluids within days or weeks.

Using CFD modeling, it is now possible to correlate the exact behavior of heavy cesium formate filtrate inside a typical coreflood test cell with its behavior in the reservoir. The CFD model demonstrates the key role gravity has in cleanup of wells exposed to heavy cesium formate filtrate. It also confirms how a lack of gravity drainage in a standard coreflood cell leads to the artifact of filtrate retention by trapping the fluid. The CFD study concludes that retention of heavy formate brine in a laboratory core is a laboratory artifact rather than a formation damage mechanism.

Further research will be conducted by Lloyds Register to investigate how artifacts in the laboratory test cells can be eliminated.