Time Is the Budget: What an Exact Reconciliation of Lost Time and Well-Cost Variance Can and Cannot Identify

Drilling performance is usually analysed in hours and well cost in money. We give an exact reconciliation of the two on a declared operating window. A benchmark splits productive time into a technical limit and invisible lost time (ILT) and the programme's allowance into the same limit and planned slack; plan and actual window prices turn the hours into a bridge from the authorisation for expenditure (AFE) to the outturn, with explicit terms for the programme clock, prices and costs outside the window. The Best Composite Well is a limiting case of the ILT estimator. The central result is an identity: for every benchmark, slack minus ILT equals allowance minus productive time, so a change of benchmark changes both by the same amount. An exactly closing bridge therefore cannot say how a variance divides between an accommodating plan and slow execution; the benchmark decides. The identity bounds the magnitude of the slack term from below, an accounting allocation rather than avoidable expenditure, and shows that lost time against the plan is the least ILT a benchmark must assign if its limit fits within the allowances. On the look-ahead the accounting supports conditional scenario reporting line by line, and a realised risk needs no new bridge term: coding its hours as NPT or as productive trouble decides which term carries them, without changing the total. Constructed examples demonstrate the accounting results. We specify a held-out calibration test for the benchmark, show that a paired forecast from one productive-time model cannot rank the split, and describe the interventional evidence that could.Supplementary file: supplementary_scripts.zip contains two Python scripts that recompute the constructed examples and redraw the figures (run: python3 example.py --check).