How Well Logs Were Used To Improve Evaluation of a Gas Storage Project

Abstract

This paper describes how well logs were used to improve evaluation of a gas storage project in New Mexico. Log analysis and reservoir modeling can be used together to develop a composite picture of the reservoir and location of the gas bubble. The combination of modeling and logging allowed for better interpretation of reservoir structure and verification of gas inventory. Introduction This paper describes the application and interpretation of well logs in gas storage projects. A case history of a storage project in New Mexico is presented as this example had a full suite of base and monitor logs run during the life of the project. A combined application of surface geology, seismic interpretation, reservoir modeling, log evaluation, and field observations was used in developing the field and analyzing the storage operations to date. Time-lapse logging was used to monitor gas bubble growth and to help predict future gas movement. The combination of modeling and logging allowed for better interpretation of reservoir structure and faults (location and sealing efficiency), and verification of gas inventory. Description of Storage Project Location The Las Milpas gas storage project is owned and operated by Gas Co. of New Mexico, a division of Southern Union Co. It is located near the town of San Ysidro in Sandoval County, N.M., as shown in Fig. 1. Gas is being stored in the Aqua Zarca water sand that lies about 2,000 ft below the surface. The project is located on the Rio Grande watershed near the south end of the Nacimiento Mountains. Surface elevations range from approximately 5,000 to 6,000 ft above sea level. Geology The storage area is located in the extreme eastern portion of the San Juan basin, just west of the Nacimiento thrust fault zone that is considered the eastern limit of the basin. The Aqua Zarca water sand, of basal Triassic age, is a north-south oriented, doubly-plunging, faulted anticline with the major Las Milpas fault on the east side. A number of cross-faults lie normal to the structural axis. Fig. 2 is an early structure map of the top of the Aqua Zarca. (Note that the contours are elevations above sea level.) This map was based on surface geology and early well logs. The Aqua Zarca formation is overlain by the Chinle shale (known as the Chinle Redbeds) that serves as a caprock. Core data from the shale formation showed essentially no vertical permeability to water and a threshold pressure (gas displacing water) in excess of 1,000 psi. Reservoir properties were determined from well logs and cores. The Aqua Zarca sandstone is fractured uniformly in the area of interest, giving an average permeability of 5,000 md for reservoir engineering purposes. Net sand thickness (porosity higher than 10 percent) varies from 117 to 175 ft, with an average value of 140 ft. Average porosity is 16.7 percent. porosity is 16.7 percent. From cores, the Aqua Zarca formation can be described as a white, moderately hard, well-consolidated, clean quartzose sandstone with probable significant vertical fracturing. Microscopic examination shows the sandstone to be fine to coarse and moderately sorted with minor feldspar and mica. The grain size is essentially bimodal with fine sand forming the matrix of the sandstone. JPT P. 1550