Drilling-Fluid Behavior During Reservoir-Formation Drilling and Completion

Abstract

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 The paper has not been peer reviewed. 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. Introduction 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. Do Filtrate Loss Volumes Tell Us How a Drilling Fluid Is Performing? In terms of aiding operational decisions, the remaining mudcake attachment after a period of production or injection is most relevant in maximizing hydrocarbon recovery. The cleanup of drilling mudcakes will be influenced by a range of factors. An approach that allows a holistic view of the changes related to drilling fluid, taking into account as many relevant factors as possible, is therefore desirable.