Abstract

Utilized for natural gas storage since 1953, the Six Lakes Field has experienced an annual deliverability decline of 5.6%. Peak days in February have decreased from a high of 568 MMcf/d in 1968 to a low of 182 MMcf/d in 1992. This decline occurred in spite of well stimulations in the early 1980s and infill drilling in the 1970s. Unabated, the decline would have resulted in a February 1997 peak day capability of 137 MMcf/d. Together with the drilling of six horizontal wells, stimulations increased the February 1997 peak day deliverability to 238 MMcf/d. This represents an increase of 31% since 1992.

Introduction

Six Lakes Storage Field is located in west-central Michigan and serves MichCon's 1.2 million customers with an annual working capacity of 40 Bcf. In addition, the field provides the transmission system with a high degree of flexibility. Only one hour is required between injection or withdrawal turnarounds.

The reduced performance during the 1970s and 1980s is well documented, and the 5.6% decline rate is at the national average. Five possible causes for the decline have been identified, four have been evaluated and three field tested. Mechanisms investigated include clay migration, salt deposition, compressor oil deposition, Fe scale and bacterial growth. The interpretations of flow tests before and after stimulation for individual wells and the total field show significant improvements. Downhole videos document the effective removal of the most significant deliverability inhibitor.

Field Background

Six Lakes is a sandstone reservoir of Late Mississippian age. The reservoir consists of a series of three parallel offshore barrier bars trending northwest-southeast. Structurally, the maximum closure is 80 feet. The thickest sand sections are across the peaks, thinning toward the edges and flattening to the northwest. The storage zone thickness (gas bearing sandstone) is a maximum of 60 feet across the top of the bars and tapers off to 10 – 20 feet on the edges.

Reservoir Characterization

The storage reservoir at Six Lakes is in the Stray "B" Sandstone, the lower most sand of the Michigan Formation. It overlays the Marshall Sandstone and may in fact be reworked Marshall sand. Within the reservoir itself, three distinct zones can be defined. The Tite Zone, which is typically 4 - 10 feet thick, underlies the National City Anhydrite caprock. The Pay Zone extends down to the top of the Water Zone which is the original gas-water contact of (-325) feet subsea. Years of storage operation have pushed the gas-water contact down to (-335) feet subsea when the field is at maximum pressure.

One of the first steps in this study was to compile and correlate all the existing petrophysical data. The data consisted of openhole log evaluations, core data and various special core tests. The data was also evaluated for validity and to determine if all areas and zones of the reservoir were represented. Considering x-ray diffraction evaluations from 40 samples, it was determined that the reservoir rock is an orthoquartizite, being almost 95% quartz. The average clay content is approximately 2%. There is little variation in mineralogy or grain size among the three zones. The zones are differentiated by changes in porosity and permeability. The average porosity and permeability values for the reservoir are shown in Table 1.

Historical Declines

The annual deliverability decline is illustrated in Figure 1. In 1993, a survey of storage operators in the United States indicated that the average decline was 5.2%. Figure 1 illustrates one way of depicting declining deliverability and its dramatic impact on peak-day capability. Data for the flowpoints was taken at the same conditions of 300 psig keywell pressure and a flowing inlet station pressure of 210 psig. In the late 1970s, thirty new vertical wells were drilled resulting in improved deliverability; however, the annual decline continued. The equation for either line is:

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