Infill Drilling for Incremental Recovery
Authors: Gould T.L. , Sarem A.M. Sam 
Published: December 2007

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.

Summary
The purpose of this paper is to summarize what has been learned over thelast several years about infill drilling and to recommend when it should andshould not be considered. In general. the more a reservoir deviates from idealbehavior, the greater the opportunity for incremental recovery by infilldrilling.

Introduction
Infill drilling of pattern waterfloods has received remarkably littleattention from both the public and professional communities. Given the rightcircumstances. this process can compete favorably with EOR processes on arecovery basis for much less investment and operating cost. And yet, to thebest of our knowledge, there are no research institutes, university projects, government programs, or tax incentives to encourage projects, governmentprograms, or tax incentives to encourage the development and application ofinfill drilling. There is virtually no technical literature on the subject, compared with EOR, and until recently very little field data supported ordenied any claim concerning the benefits of infill drilling. In 1980, vanEverdingen and Kriss provided our industry yet another service by raising acontroversy around infill drilling. Their statement that "infill drilling, ifdone properly, can be used to recover at least as much oil as the U.S. alreadyhas produced" (120 billion bbl [19 X 10(9) m3]) created a controversy thatstill goes on today. Although we believe this statement to be exaggerated forthe U.S. as a whole, it is interesting to note that there are no projects underway to determine just how much could be recovered. As a result of his originalposition papers, van Everdingen was asked by the U.S. DOE to investigate thesubject further. His resulting proof was inconclusive, primarily because of alack of good proof was inconclusive, primarily because of a lack of good fielddata and reservoir description. In 1980, Holm provided a thought-provokingdiscussion of van Everdingen and Kriss' work, but his analysis was limited bythe same lack of field data. At that time, very little technical analysisexisted of infill drilling as an incremental recovery process and themechanisms involved. Holm's estimate of the potential of infill drilling ismuch more conservative: "with our best efforts we could add to U.S. reservesabout 1 to 1.5 billion bbl/yr for about 10 years." In our current times ofreduced reserve additions from exploration. such additions would be mostwelcome. Holm's estimated national average was 34 × 10(3) to 47 × 10(3) bbl[5406 to 7472 m3] per infill well. In 1983. a very significant paper by Barberet al. confirmed Driscoll's 1974 observations of 2 to 8% incremental recoveryfrom infill drilling. Barber et al. analyzed nine sets of field data showingvery positive incremental recovery from infill drilling. Two of the reservoirs, Dorward and Sand Hills, were primary projects and cannot be used for comparisonwith secondary pattern floods. The Howard-Glassock reservoir is a peripheralflood and as such is also not directly comparable. Data from Barber et al., Driscoll, and Ghauri et al. are summarized in Table 1 for a total of 1,323wells with an average incremental recovery of 107.1 × 10(3) bbl [ 17 × 10(3)m3] per infill well. This average is greater than Holm's by a factor of two tothree but is weighted specifically for west Texas carbonates. If Holm's moreconservative estimate is correct, 10 to 15 billion bbl [1.6 × 10(9) to 2.4 x10(9) m3] still a very large number for a virtually unstudied incrementalrecovery process. This represents 2 to 3% of the national original oil in place(OOIP) of 460 billion bbl [73 × 10(9) m3]. Current field data for a limitednumber of projects show estimated infill recoveries of about 5% OOIP. Althoughthe true benefit of infill drilling is unknown, its potential is wellestablished. The Natl. Petroleum Council (NPC) published a study on EORpotential in the U.S. in 1984 that concluded that "as much as 14.5 billion bblof additional oil could ultimately be recovered with the successful applicationof existing EOR current technology, under current economic conditions."
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