How does b-factor change with completion and well spacing?

Categories Reservoir Analysis


Many reservoir engineers are asking me what b-factor we should use to forecast wells?!

Well, the short answer is “it depends”!

b-factor is the derivative of 1/D (D=Nominal Decline) with respect to time and has no dimension. In early times, one could fit a set of production data with almost any value of b, and every one seemed to be a good fit. However, the value of b significantly affects the long-term forecast profile and the estimation of ultimate recovery.

b-factor changes with flow regimes and responds with cluster to cluster and well to well boundary interactions.

The best strategy to finding b is “analogy”. Find wells in close by area with same production zone and similar completion / well spacing design, and with a clean and sufficiently long production history (I would say at least 18 months of data), try to forecast those, and see what range of b-factor you get, then apply it to wells with minimum or no data.

It is, sometimes, hard to find wells with those exact similarities. In that case, you can simulate a nearby well and perform sensitivity on completion design and well spacing to estimate the b-factor. Also, always apply these b-factor general rules of thumb:

  • When adding more clusters, the b-factor should theoretically decrease due to faster fracture interaction.
  • Infill wells have lower b-factor than parent wells. Since the reservoir is partially drained by offset wells, the well has seen the well-to-well interaction.
  • The tighter the reservoir, the higher the b-factor, since these have longer transient flow.
  • Thicker reservoirs have higher b-factor.
  • The larger well spacing, the higher the b-factor.
  • Vertical wells with one frac normally have b-factors close to 2.0.
  • b-factor higher than 2.0 is possible! (don’t recommend using it, though).
  • Higher hydrocarbon in place (per section) means higher b-factor.
  • Closely staggering/ stacking wells will decrease the b-factor.

In shale gas and oil reservoirs, choosing a b-factor of less than 0.5 is normally considered conservative, and higher than 1.5 is aggressive.

Always remember that sensitivity on b-factor is crucial on reserve calculation. In modified hyperbolic DCA, EUR increases linearly with increasing b-factor (assuming a mechanical well life limit, e.g., 60 years).

At the end, always calculate Recovery Factor (RF) to make sure it’s in the practical range. You do not want to choose a b-factor that gives you a RF>1!!

By the way, the answer to the question in the headline image is well-A