Abstract

This paper presents a review and analysis of four Fry Sand waterflood projects in Runnels County, Tex. At the present time, two of these projects have been abandoned, one project has not shown any appreciable response to water injection, and one project has shown a response to water injection and is approaching its economic limit. One of the plugged out floods was an economic failure, and the other made a comparatively small profit.

The actual waterflood recoveries of all the Fry Sand Projects reviewed were much less than the calculated recoveries. From an analysis of the behavior of the floods, it is evident that the injected water was lost to other formations. Flood response was best in areas where the injection wells were not fractured, and it is reasoned that the fracture treatment created channels behind the pipe to a theif zone.

From the study of these waterflood projects, it has been concluded that the major problem in flooding Fry Sand reservoirs is the restriction of the injected water to only the Fry Sand.

Introduction

There have been many successful waterfloods in Strawn Sand reservoirs of West Central Texas during the past few years, but most of the attempts to water flood the Upper and Lower Fry sand reservoirs have been only partially successful. Four of these Fry sand floods of Runnels County, Tex. have been reviewed to determine the probable cause of low waterflood efficiency. Two of these floods have been plugged and abandoned, one is nearing its economic limit, and one has never responded to injection. A study of conditions which affected ultimate waterflood efficiency in these reservoirs should be of value to those investigating the teachability of secondary recovery projects in the many Fry sand fields which are still producing by primary means. Fairly complete information has been made available by the operators of these projects, and their interpretations have been used wherever possible.

FIELD "A"

Field "A" was discovered in 1952 and contained 30 wells located as indicated in Fig. 1. The estimated productive limits of the field are also shown on this map. Production is from the Lower Fry sand at a depth of approximately 3,600 ft. Cumulated production at the initiation of water injection was 123 bbl/acre-ft, and primary reserves at that time were estimated to be approximately 22 bbl/acre-ft. The production history of this field is shown in Fig. 2 [beginning about one year before injection commenced]. The field was producing approximately 270 B/D at the time water injection was started. Cambrian water handled in a closed system was injected into a peripheral pattern.

Since injection began the field has produced 82 bbl/acre-ft of oil which amounts to about 60 bbl/acre-ft in addition to that which would have been produced under primary recovery. The field is now producing about 1,000 bbl per month with only produced water being returned to the reservoir.

Nine producing wells were converted to injection wells, and five of these have been fractured. As indicated in Table 1, this field recovered a total of 205 bbl/acre-ft of which 60 bbl/acre-ft would not have been recovered by primary operations. Calculated waterflood recovery has been 34.2 per cent of the amount theoretically recoverable from the floodable portion of the reservoir. Oil saturation at the beginning of injection was estimated to be 50.1 per cent of the pore space. Total secondary recovery amounted to 41.5 per cent of estimated total ultimate primary recovery. This flood was successful and was profitable.

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