A key element in formation damage assessment of drilling and completion fluids is return permeability tests on representative core samples. One of the most important test phases is the simulation of production or injection. In a well, this natural clean-up is crucial to the well performance but it is often disappointingly replicated in laboratory tests. Traditionally, drawdown in the laboratory has been simulated by rate control or pressure control. We demonstrate that neither of these techniques adequately capture the transient nature of pressure drop and fluid flow in the near wellbore area, leading to artifacts that produce unrepresentative and questionable return permeability results.
The proposed method allows both simulation of pressure controlled drawdown and a flow rate limit so that only realistic velocities are simulated. The well drawdown during start-up is determined and is scaled to reflect the likely initial drawdown across the very near wellbore. During the laboratory test, the wellbore pressure is gradually reduced simulating "bean-up" and flow rate monitored. Independently, an overriding maximum potential flow rate for the candidate lithology is determined using core plug end face surface area and assumptions on the surface area of exposed wellbore and likely well rates. This maximum flow rate constraint guarantees that unrealistic production or injection rates are never achieved and thus enhanced clean-up or unrepresentative fines movement are never experienced.
The new laboratory return permeability test protocols has resulted in much more realistic laboratory core test data. These data can therefore be used with more confidence to predict the relative performance of different drilling and completion fluids and the absolute impact of formation damage on well productivity and injectivity. It is recommended that this more credible and rigorous methodology should be considered for all return permeability tests.