Practical Application of Interference Tests
Authors: Matthies E. Peter 
Published: December 2007

Abstract
This paper describes field application and calculation of an interference test in the North Anderson Ranch (Wolfcamp) field, Lea County, N. M.
With the proper application of core and fluid analysis information in connection with bottom-hole pressure measurements, the drainage area of producing wells and the spacing requirement for an entire field can be calculated. Published formulas are used in the calculation.
A modified 160 acre five-spot pattern was used in the field test whereby the center well as the observation well was surrounded by four producing wells. Bottom-hole pressures were measured continuously in all five wells during the first part of the test. The information was evaluated after the pre-determined amount of oil was produced. The test then was continued for the remainder of the month. Excellent agreement between calculated and measured drawdown was obtained, verifying the unsteady-state flow equations which have been developed and which are published in the literature.
Introduction
This paper describes the field application and calculation of an interference test in a reservoir in a Wolfcamp field in Lea County, N. M. It was the purpose of the test to determine the drainage area and spacing requirements for this reservoir. Well completion information indicated that each well location had experienced drainage prior to being drilled. When each well was completed it had a lower initial bottom-hole pressure than the preceding well, indicating some drainage, Furthermore, material balance calculations gave apparent drainage areas far in excess of the field-wide spacing rule of 40 acres/well. The test was undertaken to substantiate this behavior in order to avoid the drilling of unnecessary wells and to obtain 80-acre allowables for the field.
Primary Development and Production History
The North Anderson Ranch (Wolfcamp) field was discovered in May, 1958, with the completion of the well, Gulf Oil Corp. State 1-A. However, at that time it was not recognized as a new field. It was believed to be an extension to the Anderson Ranch Wolfcamp field, which was discovered in July, 1953. During 1958, two more wells were completed in the reservoir.
Then in 1960, Union Oil Co. successfully completed its State No. 1–33 in the center of the 360-acre exploratory unit. This touched off an active development program and by the end of 1961 there were 11 wells producing from the Wolfcamp in the Subject area. During the same time interval, the production rate more than tripled from approximately 450 BOPD during 1959 to over 1,500 BOPD, corresponding to the 40-acre allowables for the wells (see Fig. 2). All wells were completed as flowing producers. By now, the field is producing at a rate of 2,375 BOPD from 16 wells and has a cumulative production of 1.8 million bbl. Six of the wells producing from the Wolfcamp in the North Anderson Ranch area are completed in more than one zone, although they are all in the Wolfcamp. The zones cannot be correlated with 100 per cent certainty and only the five wells incorporated in the subject test are definitely producing from the same Source of supply. This has been called the C Zone. The other wells are producing from the A and B zones. No communication has been found between the zones.
JPT
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