Buy the oil from foreign sources.
Many will go on about the natural resources of Texas, current budget, etc. However, it may be a fools proposition to think the other 49 states will take so kindly to succession and the additional burdens they would be subject to. There are tremendous federal dollars and benefits that the residents of a new independent state would no longer have access to, or would have to pay for very dearly. Not to mention new requirements for treaties, tariffs, and other infrastructure currently funded by or provided by a federal government.
Not saying it's a bad idea, but the devil is in the details and it does not appear to be well thought out prior to the push for execution.