802 Subsurface Geologic Methods 



at shallower depths and with under-saturated crudes, it is not too large — 

 at great depth with large amounts of dissolved gas, the neglect of this 

 factor gives greatly exaggerated reserves. 



The last item in the formula is F for recovery factor, which is the 

 percentage of the inplace oil that the estimator believes is recoverable. It 

 is always an estimate, and all too often just an off-hand guess. It is one 

 of the greatest sources of errors in estimates. Many, many things go into 

 a recovery factor, and lots of them are not found in the science books. 



Chief among the scientific factors affecting recovery is the type of 

 explusive energy that is bringing the oil to the bore hole, and perhaps 

 removing the oil from other leases. If there is a fully effective water 

 drive, the lease high on the structure will have a very high recovery 

 factor — usually recovering more oil than could be calculated to be under 

 it. This fact is due to the Law of Capture ; the oil belongs not to the 

 lease owner under whose property it is discovered, but to the well owner 

 who produces it (reduces it to capture) . Courts are making some feeble 

 efforts to get away from the Rule of Capture, but it is still very strong in 

 most jurisdictions and must always be considered in every estimate of 

 recovery. The workings of the Rule of Capture are intimately related to 

 the subsurface geology of the property — and afford the geologically 

 minded operator many opportunities to take advantage of, or to protect 

 himself from, natural drainage. 



Just as the top lease in a water-drive field produces all of its own 

 oil and some of all the oil in the down-dip lease — conversely, the edge 

 lease produces only part of its own oil and none of anyone else's oil. 

 How much oil an edge-lease owner will produce depends on the allowable 

 production compared with those of up-dip leases. The down-dip lease 

 always shares its oil with the up-dip leases, but a larger allowable some- 

 limes results in the down-dip lease getting a fairer share. Water encroach- 

 ment is not always regular; frequently water will intrude through the 

 more porous permeable zones to reach the higher wells — ^this is particularly 

 true if the up-dip wells are pulled hard. In such cases, the edge lease is 

 left to produce its oil long beyond its computed time. Many edge leases 

 which should have gone to water years ago (if the water level evenly 

 moved up proportionately to the oil withdrawn from the reservoir) are 

 yet producing, and still make their allowable every day. 



Big recoveries from the leases high on structure are matters of 

 common knowledge. But enormous recoveries from a lease low on the 

 structure are just as likely if drive is from gas and the dip is steep. Gas 

 drive is only gravity drainage under high pressure — and the edge lease 

 will produce all of its oil and all of the oil that runs down into it, whereas 

 the top lease will produce only part of its oil and then go to gas. 



The study of "who gets the oil" is a fascinating combination of sub- 

 surface geology, some engineering, and an analysis of various statutes 

 and regulations of oil and gas boards. A change in field rules or in the 



