Waterflooding the Grayburg Formation on the J.L. Johnson AB Lease: Experience in the Johnson Field
Authors: Prentice R.S. 
Published: December 2007

Summary
The Grayburg formation in the Johnson field, Ector County, TX, is a more complex reservoir than originally believed. Poor response from water flooding the J.L. Johnson AB lease with 40-acre [161 875-m] five-spots led to development with 20-acre [80 937-m] line drive patterns. This caused a substantial production increase. patterns. This caused a substantial production increase. Infill drilling led to the discovery of random, anhydrite-filled sections which act as barriers to flow. They are probably interconnected and may be the cause of poor response to injection on wide spacing. Anhydrite barriers may exist both in other parts of the Johnson field and in surrounding fields. These barriers could play an important role in determining how other water floods play an important role in determining how other water floods are designed.

Introduction
Poor response to water flooding with 40-acre Poor response to water flooding with 40-acre [161 875-m] five-spot patterns prompted ARCO Oil and Gas Co. to undertake a major infill drilling program on the J.L. Johnson AB lease in the Johnson field of west Texas. As might be expected, closer spacing increased production significantly. It also led to the discovery of production significantly. It also led to the discovery of random, anhydrite-filled porosity areas in the entire zone under water flood. These areas have small lateral dimensions but probably wind through the unit in meandering channels created by leaching ground waters after the structure was formed. They act as barriers to flow and are the probable cause for the poor response on wide spacing.

Location
The Johnson field is on the eastern edge of the Central Basin Platform of the Permian Basin in Ector County, TX. As shown in Fig. 1, it encompasses parts of 26 sections in Blocks 42 and 43, Township I South, Texas and Pacific Railroad Co. Survey, and is bounded on the north Pacific Railroad Co. Survey, and is bounded on the north by the North Cowden field and on the south by the Foster field. All three fields produce from the Grayburg formation. The J.L. Johnson AB lease is an 840-acre [3 399 373-m] 100% working interest lease in Secs. 37 and 48, Block 43, and Sec. 43, Block 42, T-1-S, operated by ARCO Oil and Gas Co.
Pay Description Pay Description The Grayburg formation is the productive interval and is found at about 4,150 ft [1265 m] on the lease. It is a tight Permian Age dolomite that lies between the Queen and the Permian Age dolomite that lies between the Queen and the San Andres formations. Fig. 2 is a type log of the Grayburg pay found in the J.L. Johnson AB No. 70 water injector well pay found in the J.L. Johnson AB No. 70 water injector well (WIW). Table 1 describes each zone. The upper pay is a 10-to 15-ft [3- to 5-m] thick sandy dolomite. Porosity averages 10 to 15% and permeability is 1 to 2 md. Continuity is erratic, with porosity in the zone completely missing at times.
The main pay is a tight dolomite. Porosity ranges from 1 to 15%. The geometric average permeability to liquid is 0.6 md. Continuity is very unpredictable. The formation correlates between wells, but parts or all of the porosity development can disappear completely from one well to the next.
The lower pay is a porous, permeable, sandy dolomite that is found throughout the area. Porosity is typically 10 to 15% and permeability is 8 to 10 md. This zone was productive once, but it has been flooded out over the years and is now water bearing. The lower dolomite resembles the main pay except that it is wet. This zone appears never to have been productive on the AB lease.
Currently, only two zones, the upper pay and the main pay, are under flood. Although the gamma ray curve indicates shale streaks through the interval, cores contain only minute quantities of shale. Radioactive dolomites are causing the gamma ray kicks. Rock porosity and permeability may vary widely from those properties listed in Table 1, depending on location.

Field History
ARCO Oil and Gas Co. discovered the Johnson field when drilling the J.L. Johnson A No. 2 in the southwest corner of Sec. 48 in Feb. 1937. Development began on 40-acre [161 875-m] spacing and continued through the early 1940's. At the same time, the adjacent field to the south, the Foster field, was discovered. Drilling in the Johnson field began again in the mid-1950's. By 1960, drilling activity had slowed, but several secondary recovery units were formed. Flooding began in Cities Service Co.'s Johnson Grayburg-San Andres Unit in late 1960. In the mid-1970's, drilling on new leases in the field began again. Development was continuing as late as 1982. As of Jan. 1, 1984, six operators were using 155 producers and more than 100 injectors. Two operators, ARCO Oil and Gas Co. and Cities Service Oil and Gas Corp., conduct most of the water flooding. Current field production is approximately 1,786 B/D [284 m3/d] oil.
JPT
P. 1773