Abstract

The St. Peter aquifer in northern Illinois is used to store natural gas during the summer months for the winter heating season. Since the fresh water aquifer is strongly water-wet, the capillary force promotes the retention of water in the pores to curtail the deliverability of gas to the wellbore during withdrawal. Similarly, the injection of gas during the filling cycle is also hampered. Laboratory work performed earlier demonstrated that the surfactant Tomadry N-4 could alter the wettability of the St. Peter core samples from strongly water-wet to neutral-wet, which in turn increased the gas deliverability. A field test was performed based on the laboratory results. The Waverly Storage Facility operated by Southern Union/Panhandle Energy was selected as the site for the field test of surfactant treatment. The storage reservoir is an 1800-ft sandstone aquifer located near Springfield, Illinois. Expansion and contraction of a spherical gas bubble provided pressure support as gas was injected and withdrawn from 37 individual wells. Three wells were selected for treatment with 1000 bbl of ~4% Tomadry N-4 solution. An additional three wells nearby were selected as control wells. The wells were treated at the end of the 2007 injection cycle, and the rate-pressure performance of the three treated wells and three control wells through the first month of 2009 was analyzed. Use of the productivity index technique was not as insightful as the conventional rate vs. cumulative production analysis technique. The Waverly test is believed to be the first field experiment of altering wettability to improve gas flow rates. Graphical presentation of the data suggests that at the beginning of the withdrawal cycle and injection period, the performance of the surfactant-treated wells was better than the control wells. The test results are encouraging, and further field tests should be developed. It appears that the gas withdrawal rate from two of the three treated wells was about 33% greater than prior to treatment.

Introduction

In strongly water-wet gas storage reservoirs, the capillary force, EQUATION, promotes the retention of water in the pore space, which curtails the deliverability of gas to and from the wellbore. To improve the gas deliverability, chemicals, including crude oil, can be used to alter the strong water wetness in order to decrease capillary pressure and release the retentive water. Wardlaw and McKellar (1998) observed that gas reservoirs with residual oil drained water more efficiently than oil-free reservoirs; the presence of crude oil altered the reservoir rock wettability to less water-wet. Sharma, Firoozabadi, and their group members have extensively studied alteration of rock wettability with chemicals in the laboratory (Kumar et al., 2006; Mahadevan and Sharma, 2005; Li and Firoozabadi, 2000; Tang and Firoozabadi, 2002, 2003; Fahes and Firoozabadi, 2004). By using a surfactant, Xie et al. (2009) altered the strongly water wet sandstone from the St. Peter.

The St. Peter Sandstone is a multi-state fresh water aquifer in Illinois. The aquifer is sporadically used to store natural gas. Expansion and contraction of a spherical gas bubble around the wells provided pressure support as gas was injected and withdrawn from the individual wells. One such storage facility located near Waverly, IL (near Springfield in central Illinois), operated by Southern Union/Panhandle Energy, was selected as the site for a field test of the surfactant treatment. The field test was performed to investigate whether the wettability alteration technology developed in the laboratory could improve the gas deliverability, or productivity index, defined as

EQUATION (1)

Where q is the wellhead flow rate; is the wellhead flow pressure; and is the reservoir pressure measured at an observation well. A successful test would improve operational flexibility leading to better economics.

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