Cleanup operations are often challenging to predict. The review of the major physical phenomena governing the behavior of a well cleanup sheds light on some important considerations to be taken to design and realize such operations. An optimal cleanup program will depend on the well construction processes, the lithological factors and the interaction between the drilling fluids and the formation, active sequencing of chokes. The coupling of these complex physical operations can be non-intuitive. A modeling approach is proposed and validated through comparison with field data.

The design of an optimal cleanup program is hampered coupling of two issues: the existence of formation damage due to the invasion of mud in the near well-bore area and the transient well bore phenomena associated with the replacement of drilling or completion fluid with lighter hydrocarbons. This paper investigates the integration of transient simulation of near wellbore multiphase phenomena with complex wellbore dynamics and provides recommendation on cleanup designs. The success of a wellbore cleanup is gauged in different ways, depending on the lithological, drilling and operational environments. Metrics of performance such as duration of the operation, productivity, recovery of loss fluids are commonly used.

We tackle the global issue with a predictive model specifically tailored to cleanup operations in a layered system that considers:

  • An internal mud cake (which is formed by mud solids intrusion into the formation)

  • An external mud cake (formed at the interface well / formation)

  • A mud filtrate invaded zone

  • Potential perforations

  • Dynamics of the multiphase (and multi-component) wellbore flow

  • Flow control devices

The paper discusses the laboratory validation of the near well bore model against dynamic core flooding and transient return permeability experiments. Comparisons against field data obtained with high speed multiphase flowmeter or dynamic production profiles further enhance confidence in the simulations.

A number of recommendations for cleanup designs are provided considering some of the challenging constraints such as:

  • Operational constraints: limited storage volume, rig time, pressure drawdown limits (collapse), noise, rates

  • Fluids limitations: avoiding drawing pressure below bubble / dew points

  • Geomechanics limitations: max drawdown or avoiding tubing collapse or protecting other completion elements such as screens

  • Lithological challenges: multilayer reservoirs and horizontal wells where it is necessary to clean all layers / drain.

  • Large drilling losses resulting in perforation channels not bypassing totally the mud filtrate invasion zone (and sometimes the internal mud cake area)

The analysis of the sensitivity of various model parameters confirms the need for robust cleanup designs that takes into account the actual uncertainties of the well construction process and of the formation heterogeneities and near wellbore characteristics. This study demonstrates that the principal cleanup characteristics are essentially dependent on properties of the drilling and completion fluids.

It is possible to give some practical operational recommendations for improved cleanup such as zone selectivity, choke sequencing and pressure controls.

The utilization of temperature variations at the on-set of the cleanup also provides important knowledge to the interaction of the drilling fluids and completion fluids with the formation prior to the test. This information can be used to optimize the next well. The monitoring in real-time (or in-time) of the downhole parameters such as pressure, temperature can significantly help to reduce the uncertainty of the cleanup operation and decrease substantially the rig time.

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