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31.1° C and 73.8 bar [ 87.9° F and 1,070 psi] — conditions


medium to deep wells.1 9


that are easily exceeded in most Therefore, an important


aspect of any CO2 sequestration project is to know how downhole materials will react to supercritical CO2 (scCO2).


Long used in oil and gas wells to hydraulically isolate pay zones from the surface and other permeable zones, portland-based cements play a critical role in wellbore integrity. This study focused on a sample of neat The cylindrical cement sample was


cement.2 1


cured for three days at 90° C and 280 bar [ 194° F and 4,061 psi] . Scientists obtained CT scans of the cement cylinder before exposing it to scCO2. The cement was then subjected to a wet scCO2 environment and kept at 90° C and 280 bar for 30 days. Two sample plugs were cut from the original cylinder and then scanned. Using Inside Reality software, researchers were able to manipulate the data volume to visualize porosity and microfractures and arbi- trarily slice through zones of interest. By comparing scans acquired before and after treatment, researchers noted significant changes to the cement plug, resulting from scCO2 attack. Of particular interest were the formation and distribution of microfractures, along with a zone of aragonite replacement and a zone of mineral alteration characterized secondary porosity.


by high


The reaction between scCO2 and cement produced an irregular carbonation front, extending 4 mm [ 0.16 in.] from the outer edge of the core toward its center. This lighter colored carbonation front was readily apparent in the gray-scale 3D volume, and in a color-coded slice (above right). Subsequent X-ray diffraction analysis determined that the alteration front had a different composition than the original cement, which had been replaced by aragonite. Porosity was clearly enhanced in the regions around the microfractures and the aragonite front (right). The tests suggested that exposure to scCO2 may cause conventional cement to lose more than 65 % of its strength after only six weeks. These important observations provided an impetus for creating new blends of cement. Schlumberger researchers have developed new scCO2-resistant cementing materials that display


D issol ution f ront Z one of very high porosity F il l e d m ic rof ra c ture 0 cm1 2


> A sam ple plug of neat cem ent. Only a few centim eters in length, this sam ple revealed im portant inform ation concerning the b ehavior of supercritical CO2 on portland cem ent. The tom ographic gray - scale im age of the cem ent sam ple ( right) , scanned w ith a resolution of 18. 3 3 µ m , show s a high concentration of aragonite along the edge of a carb onation front, accom panied b y an alteration front. An additional dissolution front of high porosity ex tends farther into the core. Circular holes w ith a diam eter of 5 00 µ m m ay represent air b ub b les. Microfractures are  lled w ith aragonite cry stals. Lighter features represent higher CT values, signify ing different m ineralogy in the case of the  lled m icrofracture, or different am ounts of m icroporosity , in the case of the alteration front.


Scientists at Schlumberger Cambridge Research in England have collaborated with their counterparts at Schlumberger Riboud Product Center in Clamart, France, to investi- gate long-term effects of CO2 storage on wellbore integrity. One such experiment sought to determine how scCO2 would react with casing cement.2 0


good mechanical behavior after exposure to scCO2 gas. Laboratory tests on these new materials show only a slight decrease in compressive strength during the first two days, and essentially no loss for the subsequent three months.


Examining Wormholes Caused by Stimulation Treatments Researchers have also used CT imaging to study the effects of heterogeneity on carbonate matrix stimulation. In one experiment,


it was instru-


mental in visualizing the effects of the original porosity distribution on acid-dissolution patterns.


S a m p l e P l ug


CT I m a g e


A l te ra tion f ront Ca rbona tion f ront


Z one of very low porosity A ir bubbl e


( Diameter 0 .5 mm)


Tool s System Menu


Colormap Fault Fence


Growing


Reservoir Ruler Sketch


Slice Surface Stereo AUTOSAVE SCR_040917_1736_1 Inside Reality N eat cement


Restore Scene Save Scene


Snapshot Aragonite front


> Highlighting the ex tent of supercritical CO2 alteration. Color-coding enhances features that m ay not


b e readily apparent in gray -scale im aging. Microfractures form ed during the supercritical CO2 attack served as conduits for further aragonite alteration. The concentration of aragonite along the fractures and the edge of the alteration front can b e visually distinguished using color-coding provided b y Inside Reality softw are. Materials im aged are unaltered neat cem ent ( green) , an alteration front


( y ellow ) , and m ineral- lled m icrofractures or carb onation front ( red) . Increased porosity ( b lue) m ark s the ex tent of various dissolution patterns.


12


Oilfield Review


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