Producing Formation

Key Takeaways

• Multi-zone completions in a single wellbore may produce from multiple producing formations simultaneously (commingled production through a single tubing string without isolation between zones) or separately (with mechanical isolation using packers and bridge plugs between producing intervals to allow each zone to be managed independently for pressure and rate control) — commingled production from multiple formations simplifies the completion architecture and reduces the number of tubulars required but creates formation management challenges: if the zones have significantly different pressures, the higher-pressure zone may cross-flow into the lower-pressure zone through the wellbore when the well is shut in; if the zones have significantly different quality or drive mechanisms, the better zone may deplete faster than the weaker zone and require different production strategies; regulatory requirements in many jurisdictions prohibit commingling across formations that cross state or provincial ownership boundaries (different royalty owners may hold mineral rights in different formations), requiring separate completions and production records for each forming even when they occupy the same wellbore; separate production records for each producing formation allow the operator to accurately account for royalty obligations, track formation-specific production trends, and identify which formations are underperforming relative to expectations for individual stimulation or workovers.
• Producing formation changes in a wellbore (where the well is re-completed from one formation to another by setting a bridge plug below the original perforations and perforating a new interval above) are called zone changes or re-completions and require filing amended completion reports with the regulatory agency, updating the production allocation among the formations in the commingled production accounting, and revising the well's reserve assignment to reflect the new producing formation's properties and expected recovery — re-completions are common in mature wells where the original producing formation has depleted to uneconomic rates but an overlying or underlying formation with remaining hydrocarbon potential can be accessed with a relatively simple wireline operation (setting a plug below the depleted zone and perforating the new zone); the decision to re-complete is an economic analysis comparing the cost of the wireline operation, the formation damage risk (perforation and stimulation costs), and the incremental production expected from the new formation against the value of the original zone's remaining slow decline production; formations that are bypassed during re-completion are tracked in the well file as potential future workover targets in case the new formation also depletes before economic abandonment.
• Producing formation pressure management is the core activity of reservoir engineering in a mature field, and the per-formation production records that identify which formation each well is producing from form the database that drives pressure maintenance decisions — average reservoir pressure decline in the producing formation is tracked by periodic shut-in wellbore surveys or by static pressure measurements during workovers, and the rate of pressure decline relative to cumulative production from the formation determines the formation's drive mechanism and recovery efficiency; formations under active water drive show slower pressure decline per unit of production than formations under solution gas drive, and formations with gas cap expansion show intermediate behavior; mixing production data from different formations in a single average would obscure the individual formation pressure trends that are needed to identify which formations need pressure maintenance (water injection or gas injection) before the pressure drops below the efficient recovery range; the formation-level production and pressure database is also the input to the volumetric material balance calculation that provides an independent estimate of the original oil in place for each producing formation, which is compared with the geological reserve estimate to validate the reservoir model.
• Regulatory reporting of producing formation designation is mandatory in all oil and gas jurisdictions and links the production records to the geological framework used for resource estimation and environmental impact assessment — in the United States, state oil and gas commissions require that each completion report, monthly production report, and well history report identify the producing formation by its designated name in the state's geological nomenclature (the Wolfcamp A, the Codell Sandstone, the Haynesville Shale, the Permian Bone Spring); these designations are entered into the state's well database and used to compile formation-specific production statistics that track basin-wide production from each formation over time, identify which formations are being developed and at what pace, and provide the production history used to calibrate basin-wide geological models; federal reporting requirements (for wells on federal lands) additionally require formation-level production records that link to the mineral lease records for royalty calculation, with the royalty rate, decimal interest, and ownership applicable to each formation specified in the lease documents; incorrect formation designations in regulatory filings can result in under-reported or over-reported royalties that require retroactive adjustments.
• Water production from the producing formation versus from a non-producing formation behind casing is a critical operational distinction in wells with elevated water cut — if the water is being produced through the perforations from the target hydrocarbon-bearing formation itself (as formation water from the aquifer below the oil-water contact or from water breakthrough in a waterflood), the water is an expected consequence of the reservoir drive mechanism and the producing formation designation is unchanged; if the water is entering the wellbore through a casing leak, a failed packer, or an uncemented annular channel from an adjacent water-bearing formation (a non-producing formation that happens to be wet and pressure-connected to the wellbore through a mechanical integrity failure), the water is a wellbore integrity problem rather than a formation production issue; distinguishing these two sources of water requires production logging (spinner and temperature logs to identify the depth of water entry), noise logs or tracer surveys to locate casing leaks, and pressure testing of the casing and packer to confirm mechanical integrity; misidentifying casing leak water as formation water leads to an incorrect water-oil ratio in the producing formation's material balance and may incorrectly advance the timing of the well's economic limit calculation.