Adding wells makes total rate rise. No decline curve can fit that, so the regression gives up and reports a straight exponential with a recovery estimate to match.
An Arps decline curve describes one well depleting one drainage volume. It assumes rate only ever falls. Field rate is not that. It is the sum of wells at different ages, and it steps upward every time a new well is turned to sales.
Point a least-squares fit at that series and it has no good options. It cannot bend a decline curve upward, so it flattens the curve until the residuals on either side of the step roughly cancel. The hyperbolic exponent is driven to zero, which is a straight exponential, and the initial decline comes back far shallower than any individual well actually exhibits. The fitted curve is not a bad description of the reservoir. It is not a description of the reservoir at all.
The reason it survives review is that the output still looks like a decline curve. It has an initial rate, a decline rate and a recovery number, all in the units everyone expects. Nothing about the printed answer says the regression failed.
Three checks, in order of how quickly they settle the question.
Look for the step. Plot rate against time for the whole history and mark every date a well was turned to sales. If the rate rises after any of those dates, a single curve cannot fit the series and whatever was fitted to it is meaningless.
Ask for the quality measure. A decline fit reported without its coefficient of determination is a decline fit somebody chose not to look at. Field-rate fits on a growing pad routinely land below 0.3. A clean per-well fit on the same data lands above 0.95.
Check the exponent. If the hyperbolic exponent came back at exactly zero, or pinned to whichever bound the solver was given, the solver hit a wall rather than found an answer. Real tight oil wells sit near one, and often above it early in life.
Normalise to rate per producing well. Divide by the count of wells actually online in each month, not the count drilled. Adding wells then stops looking like a reservoir doing something impossible.
Segment at each step change. Wells brought online two years apart are different vintages with different completions. Fit them separately and let the model add them, rather than asking one curve to describe both.
Drop the flowback month. The first partial month is choked, cleaning up, and not on the depletion trend that governs the rest of the life.
Fit on the logarithm of rate. Production spans two orders of magnitude across a well's life. A fit on raw rate is dominated by the first six months and effectively ignores the tail, which is exactly where the reserve lives.
Validate against something independent. Build a type curve from offset wells in the same interval and compare recoveries. Agreement inside a few percent means the fit is describing rock rather than arithmetic.
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