You don’t get the gas until the drilling is completed.
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but can’t they use that gas to power nearby engines that are drilling as well as all the vehicular traffic in the neighborhood? That would really cut initial costs.
Two items, it is often miles, so building pipelines for that temporary power is less cost effective than building pipelines for a permanent gas supply to the main pipelines.
Keep in mind, you are not going to be sure about how much gas is produced until after you drill.
Secondly, wellhead gas is not really clean enough for use. It can and has been used, as some location, but there is a gas processing plant between the wells and the consumers in natural gas systems.