Drilling-Fluid Behavior During Reservoir-Formation Drilling and Completion

Topics: Drilling fluids
Fig. 1—Example of the change-mapping technique applied to a core sample. The after-test image (a) shows a drilling mudcake attached at the top, which is not present in the before-test image (b). Image (b) is subtracted from image (a), leaving behind the change (c).

A reservoir-conditions coreflood study was undertaken to assist with design of drilling and completion fluids for a Norwegian field. Multiple fluids were tested, and the lowest permeability alterations did not correlate with the lowest drilling-fluid-filtrate-loss volumes. This paper will examine the factors that contributed to alterations in the core samples.


A range of measurements are made during reservoir-condition studies, with typical metrics of the performance of a fluid or sequence including the following:

  • Permeability measurements are made at initial reservoir conditions and then again at various points throughout the study.
  • Filtrate-loss volumes are used to compare the performance of various fluid types, including bridging design (drilling fluids), activation of crosslinked gels (kill pills), and breakthrough time/rate (treatment fluids).
  • Production/injection rates or differential pressures can give some broad indications of cleanup but are generally prone to artifacts or misinterpretation caused by multiple mechanisms occurring simultaneously within samples.
  • Visual observations can give an excellent overview of external features of the samples such as drilling-mudcake cleanup and sanding or sample failure or fracturing. However, they do not show what changes have occurred within samples and cannot visualize changes at a microscopic level, and both are generally key to understanding results.

These metrics are unfortunately subject to a number of factors that make interpretation difficult and therefore add risks to the decision-making process. In order to reduce these risks, a number of interpretive techniques are used. These include scanning electron microscopy (SEM), thin sections, and computed-tomography (CT) scanning.

In order to overcome some of the limitations posed by existing techniques, a micro-CT change-mapping technique was developed to show the distribution of alterations within samples at selected points in a study.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 185889, “The Nature of Drilling-Fluid Invasion, Cleanup, and Retention During Reservoir-Formation Drilling and Completion,” by Justin Green, Ian Patey, and Leigh Wright, Corex; Luca Carazza, Aker BP; and Arild Saasen, University of Stavanger, prepared for the 2017 SPE Bergen One Day Seminar, Bergen, Norway, 5 April. The paper has not been peer reviewed.
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Drilling-Fluid Behavior During Reservoir-Formation Drilling and Completion

01 February 2018

Volume: 70 | Issue: 2