An Unusual Case of Salt Plugging in a High-Pressure Sour Gas Well

Abstract

Abstract During the course of development of the deep Mississippi Smackover trend Shell encountered a particularly troublesome production problem in one well, the Shell Ridgway 1-R well in the Southwest Pineywoods Field. This well plugs after producing approximately 1 BCF of gas and must be treated on a regular basis by injecting fresh water to restore the productivity of the well. Eight other Shell wells in three fields in the deep Mississippi Smackover trend have produced without encountering this problem. We have concluded that the plugging observed in the Shell Ridgway 1-R well is caused by the deposition of NaCl in the well bore. We believe that a trace of brine (±0.05 bbl/MMCF) is produced from an anomalously high water saturation interval in this well. This high water saturation interval is located ±300 ft. above the gas water contact in the field. Other zones having low water saturations were penetrated at greater depths in the well. We believe that this unexpected production of a small amount of brine from an isolated interval within the pay section possibly combined with the nature of the well completion created the plugging problem. Initially we did not consider that brine flow could occur in a well completed ±300 ft. above the gas water contact. Instead we thought that transport of NaCl in the high pressure sour gas might be responsible for the plugging. To evaluate this possibility we supported a program conducted by Dr. James B. Hyne of the University of Calgary to measure the solubility of salt at reservoir conditions in the sour gas. His measurements show that trace amounts of NaCl are carried in the high pressure sour gas. However, we were not able to use Hyne's results to explain the difference between the production characteristics of the Shell Ridgway #1-R well and the other wells in the trend. Hyne's results do raise a warning that under certain conditions plugging due to NaCl deposition in the reservoir may occur in high pressure sour gas wells. Tn this paper we shall develop our understanding of this plugging problem by presenting material on two wells: the Shell Ridgway 1-R well in the Southwest Pineywoods Field, and the Shell Burch #1 well in the Thomasville Field. These two wells have pay zones of similar quality and are located in similar positions relative to the gas water contacts in their respective fields. The following items will be discussed: Characteristics of the pay intervals, reservoir conditions and structural settings for the two wells. Completion and production histories for the two wells. The solubility of NaCl in sour gas based on the work of Dr. Hyne and the implications of these data for the plugging problems. Chemical evidence to support the hypothesis that NaCl deposition occurs from brine flowing into the Shell Ridgway 1-R well. A suggested prognosis of the sequence of events during a plugging and treatment cycle of the Shell Ridgway 1-R well.