ABSTRACT

A portion of the Countess B petroleum reservoir in the Lower Cretaceous Glauconitic Sandstone located near Brooks, Alberta, which had initially been waterflooded, was converted to wet in-situ combustion. A mineralogical examination of a post-combustion core from this pilot was used to determine the water-rock reactions that occurred in the reservoir during combustion. At this location, the reservoir can be split into three zones on the basis of colour; a top (A) and bottom (C) brown reduced zone, and a middle (B) red oxidized zone. Before combustion, the unaltered reservoir sandstone consisted of framework grains that are composed of 75-90% quartz, 5-15% chert, 5-15% other rock fragments and trace to minor amounts of feldspar, carbonaceous material, pyrite and grain-coating siderite crystals. The clay mineral fraction of the unaltered rock consists of kaolinite and illite, with small amounts of chlorite and smectite. Smectite growth was common to the zones of the post-combustion core analyzed. The reaction was of the form

7 kaolinite + Na⁺ + K⁺ + 8 H₄SiO₄ = Na/K-smectite + 2H⁺ + 23 H₂O

In addition, hematite and alunite grew in the middle oxidized zone. The alunite formed from the breakdown of illite and kaolinite by reactions of the form

illite + 4H⁺ + 2SO⁼₄ + 6H₂O = alunite + 3H₄SiO₄

The interstitial waters were high in sulfate and low in chloride compared with the water compositions recorded at the producing wells. Low pH waters exist in the oxidized zone. Such high sulfate and low pH waters favour the stability of alunite. Temperatures did not exceed 400°C for extended periods of time, based on the presence of alunite and kaolinite. A possible scenario for reaction events in the reservoir during combustion is as follows: combustion of oil produced sulfate and low pH conditions; illite and kaolinite became unstable and alunite began to precipitate in zone B. After the combustion front passed through, conditions changed to a lower temperature but higher pH; waters moved into the stability field for smectite, and smectite began to form from illite and kaolinite. Oxidation was recorded by the formation of hematite. Zone C escaped the combustion front because of lack of fuel, Zone A, because of its relatively low permeability. Only steam conditions prevailed in these two zones, which caused smectite to form.

COMPOSITION MINERALOGIQUE ET COMPOSITION CHIMIQUE DE L'EAU DE LA ZONE BURNT D'UN PROJET PILOTE A COMBUSTION HUMIDE EN ALBERTA

RESUME

Une partie du reservoir petrolifere Countess B dans le gres Glauconitic du Cretace inferieur situe pres de Brooks, Alberta, qui avait d'abord ete soumi a l'injection d'eau a ete converti a la methode de combustion humide sur place ("wet in-situ combustion"). Un examen de la composition mineralogique d'une carotte post-combustion de ce projet pilote a ete utilisee afin de determiner les reactions eau-roche qui se produisent lors de la combustion. A cet endroit, le reservoir peut-etre divise en trois zones, en se basant sur la couleur; une zone superieure (A) et une zone inferieure (C), reduites et de couleur brune, et une zone moyenne (B), rouge et oxidee. Avant la combustion, le gres reservoir consistait en grains composes de quartz (75-90%), de chert (5-15%), d'autres fragments de roche (5-15%), et des traces et quantites mineures de feldspath, de materiel carbone, de pyrite et de crystaux de siderite recouvrant des grains. La portion de mineraux argileux de la roche non-alteree est composee de kaolinite et d'illite, ainsi que de petites quantites de chlorite et de smectite. La croissance de smectite etait commune a toutes les zones de la carotte post-combustion analysee. La reaction etait de la forme suivante:

7 kaolinite + Na⁺ + K⁺ + 8 H₄SiO₄ = Na/K-smectite + 2H⁺ + 23 H₂O

En plus, l'hematite et l'alunite crurent dans le centre de la zone oxidee. L'alunite est formee a partir de la decomposition de l'illite et de la kaolinite par des reactions de la forme suivante:

illite + 4H⁺ + 2SO⁼₄ + 6H₂O = alunite + 3H₄SiO₄

Les eaux interstitielles avaient un pourcentage eleve de sulfate et un bas pourcentage de chlorures, a comparer aux compositions de l'eau enregistrees dans les puits producteurs. Des eaux a pH bas existent dans la zone oxidee. De telles valeurs de sulfate elevees et de pH bas favorisent la stabilite de l'alunite. La temperature n'exceda pas 400°C pendant de longues periodes de temps, si l'on se fie sur la presence d'alunite et de kaolinite. Un scenario possible pour la serie de reactions dans le reservoir durant la combustion est le suivant: la combustion du petrole produisit les sulfates et le bas pH; l'illite et la kaolinite devinrent instable et l'alunite commenca a se precipiter dans la zone B. Apres le passage du front de combustion, la temperature diminua, mais le pH augmenta; des eaux passerent dans le champ de stabilite de la smectite, et la smectite commenca a se former a partir de l'illite et de la kaolinite. L'oxidation est representee par la formation d'hematite. La zone C echappa a la combustion, a cause du manque de carburant; la zone A elle aussi, a cause de sa permeabilite relativement peu elevee. Seulement les conditions de vapeur prevalurent dans ces deux zones, ce qui causa la formation de smectite.

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