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Permalink Reply by dbob on August 17, 2012 at 14:45

Martin - it depends on the field rules - I know i'm going to butcher this, but here goes.  For conventional gas formations, a spacing of 40 acres per well or 80 acres per well is pretty common.  therefore, that particular formation could have between 8 and 16 wells per 640 acre unit.  Of course, many areas have multiple pay zones in different formations - each of those formations could likely have a theoretical maximum of a similar number of wells.  

In certain situations, even more limited spacing may be allowed - such as 20 acre spacing, or possibly even less in certain salt dome situations.  

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Permalink Reply by Martin Wampler on August 17, 2012 at 16:19

Thanks dbob,

Does that apply to horizontal wells also? Thanks again for answering.

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Permalink Reply by Andrew on August 17, 2012 at 21:13

Are there different spacing rules for oil vs gas?

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Permalink Reply by dbob on August 18, 2012 at 3:04

Andrew, in Texas, the typical maximum size for an oil 'proration" unit is 160 acres +10% overage, while for gas its 640 acres +10% over.  The spacing within those proration units is established by the field rules, which, in theory, should avoid the "waste' of resources which could mean either leaving some in the ground, or having wells to dense or producing at an excessive rate, which could lead to formation damage or excess water intrusion.  

Martin Many times the filed rules are expressed in terms of distance between wells, and distance from well to the unit line.  In the haynesville, 330' from the unit line is the minimum.  In theory, if there wasn't a practical limit to drilling length, one could have a unit 660' wide, and about 42000 feet long, and produce it all from a single well.  If you take the maximum practical drilling distance (about 7500' in a haynesville lateral) and divided it in to 42000' you get a maximum of about 5-6 wells per unit.  

The best way to figure out for your area of interest is to look at the field rules and do a bit of geometry.  

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Permalink Reply by Martin Wampler on August 18, 2012 at 9:17

dbob, Thanks once more for the explanation. Can't begin to express my thanks to you and the other folks that take the time and energy to help our understanding of what the details are. Geometry was not my strong suit! Thanks again!!

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Permalink Reply by Joe Aldridge on August 21, 2012 at 2:33

Dbob,

I'm sorry I don't understand the 42000'. Could you explain that number? Also, I always thought that the size of the production unit had something to do with the porosity and permeability. In the case of the Tuscaloosa Sands it was determined that a 640 acre unit was capable of draining that size area and that has been proven to be correct. There would be little to no porosity or permeability  beyond the frack in a shale well. So In that case I'm assuming that the area that the well can drain is the area that was fracked. How would you figure the number of wells that would be in the unit given the fact that you are only producing the frack area? Can an operator tell what the real area of frack covers?

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Permalink Reply by Les Bamburg on August 21, 2012 at 2:48

Joe, for shale gas formations a well would recover gas from a limited area beyond the fractures.  Operators have estimates of the area that is being drained by a single well and use that as the basis for well spacing or number of wells in a unit.  Well flow testing helps verify if the spacing should be increased or decreased.  For the Haynesville Shale the standard was a drainage area of 660 ft X 5280 ft (80 acres) or 8 wells per 640 acre unit but some units have been developed with 6 or 7 wells.  The well spacing in other shale gas plays (Barnett, Fayetteville, Eagle Ford) has been more variable. 

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Permalink Reply by dbob on August 21, 2012 at 3:32

A 640 acre unit contains roughly 27878400 square feet.  27878400 square feet/660 feet wide = 42240 feet long - take off 330' from each end, and you get roughly 41580 feet that would be the hypothetical maximum length of production in a 640 acre Hanyesville unit.  Of course, that length is technically impossible to drill or compelte at the present time.  If units are sized appropriately, you could get between 5-6 wells of 7500' in length - the longer laterals are more efficient than an 8 well configuration. 

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