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Stimulation treatments are commonly performed in wells where poor permeability limits production due to naturally


tight


formations or formation damage. A common stimulation technique involves the injection of acid into carbonate formations. Acid dissolves some of the formation matrix material and creates flow channels that increase the permeability of the matrix.


The efficiency of this process depends on the type of acid used, reaction rates, formation properties and injection conditions. While dissolution increases formation permeability, the relative increase in permeability for a given amount of acid is greatly influenced by injection


formation to facilitate the flow of oil. Better still, wormholes require only a small volume of acid to produce significant increases in permeability. Researchers are therefore investigating factors that influence production of wormholes. CT scanning has proved instrumental in determining the effects that injection rate and spatial distribution of porosity have on dissolution patterns formed during stimulation experiments (below). Because it is nondestructive, this technique allows for characterization of the core before and after the treatment experiment so the development and shape of the wormhole can be evaluated.


applications, it is easy to envision the potential spread of new applications for µ CT. The technology will no doubt prove instrumental in improving the interpretation and application of laboratory and log data. As an increasingly important tool in nondestructive testing, its application can be extended to laboratory testing of unconsolidated or friable formation samples. The combination of µ CT imaging with numerical calculations may lead to more accurate predictions of a wide range of rock properties crucial to exploration, reservoir characterization and recovery calculations. Further applications include development of cross-property


improved correlations and development of libraries of 3D images that will


> Visualizing w orm hole developm ent. A sam ple of Winterset lim estone w as scanned b y CT b efore ( b ottom ) and after ( top) acid inj ection. This data volum e is display ed using Inside Reality visualization technology , in w hich pore space is rendered opaq ue, w hile surrounding vox els are rendered transparent. Initial distrib ution of pores ( b ottom ) show s discrete clusters of pores ( b lue) along the long ax is of the core. After acidizing ( top) , the core show s increased porosity , w ith a dissolution pattern ex tending from right to left that further m ark s the  ow of acid during inj ection.


conditions. At extremely low injection rates, acid is spent soon after it contacts the formation, resulting in relatively shallow dissolution along the face of the injection zone. High flow rates produce a uniform dissolution pattern because the acid reacts over a large region. In either case, the resulting gains in permeability require relatively large expenditures of acid. However, at intermediate flow rates, long conductive channels known as wormholes are formed. These channels penetrate deep into the


19 . Ab ove its critical point at 3 1. 1° C and 7 3 . 8 b ar, CO2


b ecom es a supercritical  uid. In this com pressed state, its properties lie b etw een those of a gas and a liq uid. With a low er surface tension than its liq uid form , supercritical CO2 can easily penetrate crack s and crevices. Unlik e CO2 gas, how ever, it can dissolve sub stances that are solub le in liq uid CO2.


Peering into the Future Tomography is not new to the oil industry. At the upstream end of the tomography spectrum lies crosswell seismic tomography; at the downstream end is industrial process tomography for refineries. As a research tool, µ CT is used across a broad suite of industrial applications to monitor performance of polymer-enhanced foams and polyethylene resins or to view phase separation and pore-space characterization in formation samples. Across this range of tomographic


20. Barlet-Goué dard V, Rim m elé G, Goffé B and Porcherie O:


“ Mitigation Strategies for the Risk of CO2 Migration Through Wellb ores, ” paper IADC/ SPE 9 89 24 , presented at the IADC/ SPE Drilling Conference, Miam i, Florida, USA, Feb ruary 21– 23 , 2006.


21. Neat cem ent has no additives that w ould alter its setting tim e or rheological properties.


allow a more rigorous and quantitative descrip- tion of rock type and texture. These quantitative descriptions can be integrated with classical sedimentological descriptions. The technology can also make a significant contribution to the study of elastic behavior, porosity-permeability trends and multiphase flow properties such as capillary pressure, relative permeability and residual saturations.


Future technological innovations will probably include higher resolution to overcome problems in predicting porosity when micropores fall below the detection capability of the present technique. With the improving resolution of their samples, µ CT technology is helping today’s geoscientists to better see their world in a grain of sand.


— MV Spring 2006 13


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