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46375-G9
Productivity Loss in Unconventional Natural Gas Wells Due to Salt Crystallization

Jagannathan Mahadevan, University of Tulsa

Report of Progress:

I wish to sincerely thank the ACS PRF for supporting research project on the crystallization of salts and its impact on productivity of natural gas reservoirs. The award (ACS PRF# 46375-G9) has helped me support two undergraduate students, provided summer salary and purchase of computing equipment in the first year in addition to other equipment such as software and printers (see financial report for details). Research was conducted with the support of undergraduate students to investigate the salt crystallization in porous rocks using modeling and numerical solutions. The research resulted in greater understanding of the salt saturation evolution during evaporation. The results have been sent to Transport in Porous Media Journal for peer review and publication. The results of the work were also integrated with a student project in an undergraduate class taught in the spring 2008 called natural gas engineering. Two of the four main objectives were achieved during the first year of the two year award. It is hoped that in the remaining year the remaining two objectives will be achieved and also a proposal to continue the exciting investigation of crystallization in porous media will be prepared. It is expected that paper proposals will be made in future to present results in both chemistry related symposium such as the ACS national meeting and the Society of Petroleum Engineers conferences. Due acknowledgement to the award from ACS PRF will be made on all the publications and presentations. The achievements in the first year are described in detail in the following paragraphs.

Research Summary

Natural gas generally occurs along with connate brine or reservoir that is high in dissolved salt content. Production of natural gas is achieved by reducing the pressure in the bottom-hole of a well drilled into the reservoir that holds the natural gas in pore spaces. The expansion of gas during production itself can lead to an evaporative effect which can in turn lead to brine super saturation and hence salt crystallization. Halite precipitation, triggered by evaporation, is an important formation damage mechanism that affects productivity of a gas well. In year 2007 a proposal to investigate the productivity loss in unconventional natural gas reservoirs was funded by the American Chemical Society – Petroleum Research Fund. The proposal that was submitted originally introduces the reader to the current status of the understanding of gas flow in cylindrical laboratory samples of sandstones partially saturated with water. Additionally, a model for evaporation including transport of liquids by capillary wicking in a porous rock sample was presented. Following the model for evaporation, a mass balance for salt species was introduced along with the evaporative effects and multiphase gas and liquid flow. The main aims of the proposal were to

1. understand the impact of flow through drying on the salt concentration in porous medium.

2. investigate the effect of capillary films on salt transport in porous medium.

3. evaluate the rate of change of porosity and permeability due to salt crystallization during flow through drying process including transport through films.

4. predict the impact of following parameters on the gas flow rate by including salt crystallization with the above model: absolute initial permeability, temperature, and pressure gradient.

In the past year, since the award was made, considerable progress has been made to identify the model that describes evaporation based salt crystallization in porous samples. In particular, an evolution equation for liquid salt concentration that couples the capillary driven film flow along with the evaporative effects has been developed. Additionally an evolution equation for solid salt concentration has also been developed. All the details of the models and the resulting graphical representations are presented in a manuscript that is submitted for peer review to Transport in Porous Media Journal.
Objectives Covered:

1. One of the main objectives of the proposal was to obtain a better understanding of the impact of flow through drying on salt concentration. The model results and the solutions show the impact of flow through drying on the salt concentrations.
2. The second objective was to identify the impact of capillary films on the salt transport. We show that the capillary driven films transport salt from low drying rate regions to high drying regions. This leads to greater salt concentrations in both the liquid and solid phases.

Objectives Remaining:

3. evaluate the rate of change of porosity and permeability due to salt crystallization during flow through drying process including transport through films.
4. predict the impact of following parameters on the gas flow rate by including salt crystallization with the above model: absolute initial permeability, temperature, and pressure gradient.

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