Reservoir Pressure Data Used To Justify Infill Drilling in a Low Permeability Reservoir
Authors: Davis Elbert F. , Shepler James C. 
Published: December 2007

The initial development spacing for a field may or may not be the optimum spacing, depending upon the reservoir and producing characteristics and upon the development economics of that particular field. Reservoir pressure data can be helpful in determining the best pattern. pressure data can be helpful in determining the best pattern. Introduction
Engineers and conservation officials in the various oil and gas producing states have been concerned for some time with the efficient development of our oil and gas resources through optimum well spacing. Usually this concern has centered on justifying wider spacing than is authorized by the various state oil and gas regulatory agencies. Surely each reader of this paper knows of a field, or fields, where engineers, paper knows of a field, or fields, where engineers, through the presentation of sound engineering principles and data, have succeeded in obtaining wider principles and data, have succeeded in obtaining wider spacing rules to develop oil and gas reservoirs more efficiently and economically and to make maximum profits. In recent years the case for wider spacing has profits. In recent years the case for wider spacing has been recognized by most regulatory agencies; for example, in Texas in 1965, the Statewide Spacing Rule for oil wells was changed from 20 acres to 40 acres. In general, this has been a wise decision; however, it is still the responsibility of the engineer to evaluate each developing field to determine the optimum spacing for that field.
The Sacatosa (San Miguel-1 sand) field, which initially was developed on 40-acre spacing, represents a case in which infill drilling on 20-acre spacing was justified by the use of reservoir pressure data. Development of the Sacatosa field has generated considerable interest, particularly in Southwest Texas, not only because of its size, but also because of the challenge it has presented to the engineering and production people to extract efficiently and economically production people to extract efficiently and economically the petroleum hydrocarbons from this shallow, low-permeability oil reservoir.

Location and Geology
The Sacatosa (San Miguel-1 sand) field, henceforth referred to as the Sacatosa field, is in Maverick County in Southwest Texas and is approximately 20 miles from the Rio Grande River or Mexican border (Fig. 1). Locally the field is more generally known as the Chittim field, because most of the San Miguel-1 sand, development is on Continental Oil Co.'s lease on the N.J. Chittim ranch.
Fig. 2 is a structure map of the San Miguel-1 sand, which shows that the formation dip is to the southeast and varies from 140 to 200 ft/mile. Minor faulting occurs in the field with the fault traversing Sec. 42, providing closure for a small, isolated gas cap around providing closure for a small, isolated gas cap around Well 42-7.
Although not shown in Fig. 2, the San Miguel-1 sand pinches out updip against the Chittim arch, thereby forming a stratigraphic trap for the accumulation of oil. The present productive limits of the Sacatosa field are defined by an oil-water contact on the east and southeast, or downdip, side of the field and by permeability and sand thickness that decrease on all other sides of the field.
Geologically, the San Miguel-1 sand is generally considered a member of the Upper Taylor formation in the Gulf Series of the Cretaceous system of Mesozoic Age.
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