Abstract

More than 17,000 gas storage wells in the U.S. annually lose about 5% of their deliverability (i.e., injectivity and productivity). A common problem is believed to be water blockage around the wellbore. For strongly water-wet gas reservoirs, capillary forces promote the retention of water in pore spaces, which in turn curtails the flow of gas in and out of the storage wells. This paper reports the use of surfactants to change the water-wetness of a gas reservoir in order to increase gas deliverability. Surfactant treatment can alter the strong water-wetness of the rock around wellbore to a neutral-wet condition, thus reducing water retention and improving gas flow. The objective of this laboratory study was to demonstrate improved gas deliverability by altering the wettability of core samples with proper chemicals from strongly water-wet to neutral-wet. Seven chemicals were screened for efficacy of changing the wettability of strongly water-wet glass capillary tubes. Following the capillary rise tests two surfactants were selected for the core sample experiments. Rock samples and water from the St. Peter formation, a sandstone aquifer gas storage reservoir, have been tested. Spontaneous imbibition tests of reservoir cores were used to evaluate the wettability before and after surfactant treatment. The cores were saturated with either initial formation water or surfactant solutions were displaced with wet nitrogen gas. Gas relative permeability was measured. A decrease in the trapped water saturation caused an increase in gas permeability. This increase indicated improved gas deliverability that resulted from wettability alteration away from very strongly water-wet conditions. Laboratory results demonstrated that reducing the water-wetness increased permeability and improved the deliverability of gas from the cores. This technique provides a cost-effective method to increase gas deliverability in fields where water blocking is a major factor in decreased gas deliverability.

Introduction

Approximately 400 underground natural gas storage facilities are currently operated in the U.S. They are mainly depleted natural gas and oil fields, aquifers, and salt caverns. Most gas storage operators experience an average loss in deliverability of approximately 5% per year, translating into about 3 Bcf/d for the entire industry (Yeager et al., 1997). Gas storage well deliverability can decrease progressively due to various factors. A principal cause of the loss of deliverability for aquifer gas storage wells is the capillary retention of water in the reservoir matrix, especially around the wellbores, due to the strong water wetness of the rock.

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