A large amount of published literature exists detailing the effects of HF on sandstone cores.1  The bulk of this work concerns flowing large numbers of acid pore volumes through relatively small cores having pore volumes of about 5 to 15 mL. Some HF acidizing studies have analyzed the effluent from these tests to determine ionic composition in an effort to study the chemistry of HF acidizing. Chemical and engineering simulators use the results of such work in an attempt to predict the performance of HF in sandstone formations.2-6  Returns analyses7-9  from many wells treated with HF do not support the results of these multiple-pore volume flow tests and associated simulators. Some researchers now use long-core permeameters to improve the prediction of the laboratory tests and to answer key questions concerning well response to HF acidizing treatments.10-13 

It has been recognized that inadequate laboratory experimental models have caused the discrepancies between the experiments and the well returns. To solve this problem, a laboratory acidization apparatus has been designed, constructed, and tested. The apparatus has a pore volume of about 300 mL, which allows fractional pore volume tests to be conducted and effluent to be analyzed on the basis of fractional pore volume. For such a test to be analyzed, the effects from mixing and ion exchange must be considered. A computer code written explicitly for the apparatus evaluates the effluent ionic data.

The apparatus is essentially a 3-ft Hassler sleeve with multiple pressure taps. The sleeve can be packed either with a blend of sand and clay or loaded with a solid core. Two computers and two microprocessors networked together control temperature, valve switching, pressure measurements, flow measurement, pump control, and data collection. A typical two-to four-pore volume experiment requires about 4 to 24 hours to complete, depending on pump rate.

The new apparatus successfully has modeled (1) mixing between two separate fluid stages, (2) ion exchange on bentonite with various brines and acids, and (3) leaching of aluminum from clays by HCl. Results indicate that the new apparatus and analysis tools are particularly suited for the study of HF acidizing chemistry.

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