Does formation-damage coreflooding give a good representation of damage that occurs downhole? For those of us who are actively involved in coreflooding, this is a common question to be asked. In addition, it is central with regard to the design of corefloods that will provide information enabling the qualification of specific drilling and completion fluids or identification of damage mechanisms for wells in production. Key to all of this is the selection of representative core material from the main production or injection intervals.
Formation damage represents a near-wellbore reduction in permeability during drilling, completion, or production, and the plugs selected for testing represent a point around the wellbore in which we attempt to identify either the damage mechanisms that can be expected for new wells or that may have occurred for wells in production. In order to achieve this, return-permeability tests are performed that are designed to replicate drilling and completion of wells, cleanup, and production. Each company has its own coreflood procedure, but they follow a basic pattern of a baseline permeability followed by application of mud and completion fluid. A cleanup sequence is performed, increasing either drawdown or flowrate, after which the plug can be spun down in an ultracentrifuge to irreducible brine saturation and a final permeability is measured that can be compared to the baseline permeability. The percentage difference between these permeabilities gives the return permeability.
At a recent meeting of those interested in formation damage from North Sea operators, I asked the question, “What is return permeability?” Is the most representative return permeability that identified after the cleanup sequence of the coreflood or that identified after spin down? With regard to the latter, one of my formation-damage colleagues often says that we cannot spin down the reservoir. Therefore, how representative is this permeability?
Another question asked at the same meeting was, “What are the pass/fail criteria for a formation-damage test?” Each company tends to have its own specific criteria, but one criterion commonly used is that a 60% return permeability is acceptable. However, this in itself is an oversimplification because a high-permeability reservoir (e.g., 10 darcy) could experience significant formation/completion damage without affecting well productivity. Furthermore, it does not take into account that openhole completions tend to be more resilient to damage because of the high surface area available for inflow.
After we have performed our coreflooding, obtained return permeabilities, and identified the formation-damage mechanisms for the different tests performed, a common question asked is, “How do these values relate to potential well productivity?” This is a fundamental question and one for which we have not had an adequate answer. Recently, however, computational fluid dynamics has been used with some success to relate coreflood data to provide an indication of production rates. Predicted values have been shown to be similar to actual production rates achieved.
I hope you enjoy the papers.
Recommended Additional Reading
SPE 174174 Integrated Approach To Managing Formation Damage in Waterflooding by Sergey Aristov, Shell, et al.
SPE 174188 Bursting the Skin Bubble—Decoupling Formation Damage From Skin in Complex Well Geometries by Michael Byrne, LR Senergy, et al.
SPE 174199 New Laboratory Method To Assess Formation Damage in Geothermal Wells by Zhenjiang You, The University of Adelaide, et al.
Niall Fleming, SPE, is the leading adviser for well productivity and stimulation with Statoil in Bergen, Norway. He has worked previously as a production geologist, chemist, and engineer. Fleming’s main interest is within the area of formation damage from drilling and completion fluids and in wells under production. He holds a PhD degree in geology from Imperial College London. Fleming has authored several SPE papers, is an associate editor for SPE Production & Operations, serves on the JPT Editorial Committee, and has been a member of the organizing committees for several SPE conferences and workshops.
Niall Fleming, SPE, Leading Adviser, Statoil
05 January 2016
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