The Spraberry Trend Field of West Texas was discovered in January, 1949.Drilling of 2,234 wells and production of some 45 million bbl of oil byJanuary, 1953, indicated this to be an important field which will ultimatelycover more than 400,000 acres. In addition to being the world's largest fieldin areal extent, the Spraberry has presented many problems in well completionand operation and has demonstrated unique reservoir performancecharacteristics.
The pay section consists primarily of a few fine grained sandstone orsiltstone members in a thousand-ft thick section of shale, limestone, andsiltstone. Since porosity averages only 10 per cent and nearly allpermeabilities are less than 1 md, conventional core analysis does notdelineate the "pay" section. Mercury injection was used as a capillarypressure test adaptable to rapid routine use to select those intervals havinglow enough connate water saturation to contain commercially significant oilsaturation. In the central area of the field this "pay" amounts to 16ft of Upper Spraberry and 15 ft of Lower Spraberry sands.
An interconnected system of vertical fractures, observed in cores, providesthe flow channels for oil to drain into the wells but most of the oil is storedin the matrix since the void volume of fractures is estimated to be less than 1per cent of that in the sand. Initial potentials of wells range up to 1,000 BIDafter fracture treatment which should be compared with estimated capacity of 5to 10 BID if oil had to flow into the wells through the sand itself.
Without exception initial pressures of later drilled wells weresignificantly lower than initial pressures of earlier drilled nearby wells in alarge area some 6 miles long. This means the earlier drilled wells had drainedfluids from areas much greater than their 40-acre proration units.