Diagraphies de cimentation : vers une analyse de la qualité du contact ciment-formation Cement Logging: Toward an Analysis of the Quality of Cement-Formation Bonding

Les compagnies pétrolières ont un réel besoin d'évaluer correctement les cimentations de leurs puits : l'étanchéité entre les différentes zones est-elle assurée? Pour ce faire, les outils soniques et ultra-soniques ont été mis au point. Jusqu'à présent, la qualité du contactcasing-ciment était analysée quantitativement et celle du contactciment-formation était analysée qualitativement par les spécialistes (outil VDL). Le progrès le plus important que l'on pouvait apporter dans les logsde cimentation était de détecter les défauts à l'interface ciment-formation. C'est ce que nous avons fait dans le cadre d'un projet financé par l'ARTEP (Association de Recherche sur les Techniques d'Exploitation du Pétrole comprenant Total, Gaz de France GDF, Institut Français du Pétrole (IFP), et Elf Aquitaine Production (EAP)). Les expériences laboratoires effectuées au Service Analyse FLuides de Boussens ont été conçues en injectant du ciment entre un casing et une formation-simulée avec présence, ou non, de boue d'épaisseur variable. Des formations rapides ou lentes, ainsi que des ciments, rapides ou lents, ont été utilisés. Les échos ultrasoniques, obtenus à l'aide d'une sonde CET en céramique, ont été enregistrés et analysés. La théorie, comme les expériences, ont montré que les échos ultrasoniques sont modifiés en présence de boue et/ou de gaz. Les relations entre la forme de l'onde ultrasonique et la présence de boue et de gaz entre le ciment et la formation ont été établies. Une procédure de traitement est proposée avec ses limites. <br> Oil companies have a real need to make a correct assessment of cementing jobs in their wells. Is the seal ensured between different zones? To do this, sonic and ultrasonic logging tools have been developed. Up to now, the quality of the casing-cement contacthas been analyzed quantitatively, and that of the cement-formation contacthas been analyzed qualitatively by specialists (VDL tool). The greatest progress that could be made with cementing logs was to detect defects in the cement-formation interface. This is what we have done within the framework of a project financed by ARTEP (Association de Recherche sur les Techniques d'Exploitation du Pétrole, made up of Total, Gaz de France (GDF), Institut Français du Pétrole (IFP) and Elf Aquitaine Production (EAP)). Laboratory experiments performed in the Fluid Analysis Service at Boussens involved the injecting of cement between a casing and a simulated formation, with or without the presence of mud having varying thicknesses. Fast or slow formations as well as fast or slow cements were used. The ultrasonic echoes obtained by a ceramic CET sonde were recorded and analyzed. Both theory and the experiments showed that ultrasonic echoes are modified in the presence of mud and/or gas. The relations between the shape of the ultrasonic wave and presence of mud and gas between the cement and the formation were determined. A processing procedure is proposed with its limitations. Cementing Quality : If we had to set forth a quality criterion for cementing, we would say: For cementing to be considered as being of good quality, the cement sheath must be at least as leakproof as the formation it replaces. When a borehole is drilled, different zones are effectively brought into communication. The initial isolation will be established by the casing + cement sheath. This function will thus be correctly ensured if this combination lets pass only the start of a fluid leak (water, oil, gas) that is less than or equal to what is allowed by an equivalent section of formation drilled through. Naturally, this concept is mainly applied to sensitive zones, e. g. such as the overburden of a reservoir. However, it should be noted that this concept is also applied to the reservoir itself and not only to the overburden. Indeed, a leaky cement sheath at the level of a reservoir with relatively low permeability could, for example, promote the premature arrival of gas in the oil produced. The cementing defects that can disturb this seal are:(a) Cement-casing micro- or macro-annulus. (b) Mud channel at the level of the casing. (c) Cement-formation micro- or macro-annulus. (d) Mud channel at the level of the formation. (e) Gas percolation in the cement sheath. (f) Gas channel in the cement sheath. The aim of these logs is to determine whether one of these defect is present in the cement sheath. The causes of these defects may be numerous and varied:(a) Poor displacement of the drilling mud. (b) Improper spacer formulation. (c) Cement formulation not conforming to the requirements of the well. (d) Cracks in the cemented annulus as the result of thermal and hydraulic shocks and aging. (e) Variation of the permeability during aging. With the battery of logs now available, we know how to detect defects quite accurately at the level of the cement-casing contact as well as any gas percolation in the cement. On the contrary, local defects at the level of the cement-formation contact can be assessed only very qualitatively by a log of the VDL type. In addition, a VDL analysis requires serious expertise. The present challenge is thus to obtain a cementing log capable of making the same analysis at the level of the formation as what is now done at the level of the casing. This is all the more important as the percentage of risk in the presence of a defect is greater at the level of the cement-formation interface. We have thus worked on the possibility of making use of ultrasonic echo techniques for analyzing the quality of the cement-formation contact

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