Layered Reservoir Testing

Key Takeaways

• Production logging (PLT) is the first step in characterizing a layered reservoir system before undertaking the more complex multilayer well test analysis, because PLT provides a direct measurement of the fraction of total well production contributed by each perforation cluster or zone under the actual flowing conditions of the well: a PLT run with spinner flowmeter, temperature, and gamma-ray tools records the cumulative flow profile from bottom to top of the perforated interval, and the step changes in spinner response at each perforation cluster quantify the flow contribution of each zone as a percentage of total production; zones with high PLT flow contributions have high flow capacity (kh/skin) and are the primary contributors to the composite well test response, while zones with low PLT flow contributions either have low permeability-thickness product, high skin damage, or have been pressure-depleted by prior production to a lower reservoir pressure than adjacent layers; the PLT results guide the layered well test design by identifying which zones need separate characterization and which are sufficiently similar that they can be grouped together in the multilayer model without significantly affecting the interpretation accuracy; PLT in gas wells uses a higher spinner sensitivity than in oil wells because the lower gas density requires faster spinner rotation to detect the same volumetric flow rate, and temperature surveys in gas wells can identify producing and injecting perforations through the Joule-Thomson cooling effect of gas expansion at producing perforations.
• Selective zone testing using mechanical isolation tools (bridge plugs, straddle packers, or selective completion hardware) enables direct individual zone testing by temporarily isolating a single layer from all others and conducting a conventional drawdown-buildup test on that layer alone: a straddle packer tool run on wireline or tubing spans the perforations of the zone to be tested and inflates above and below to isolate the perforated interval from the rest of the wellbore, with the test interval flowing and building up while the isolated zone is the only contributor; for wells with multiple perforation clusters over a long interval, straddle testing each zone independently generates a complete dataset of individual zone kh, skin, and average pressure, but requires a separate wireline or tubing run for each zone and the testing time accumulates proportionally with the number of zones; the individual zone test results from straddle packer testing are the most direct and accurate measurements of individual layer properties, but they are also the most expensive and time-consuming to obtain, making selective testing most justified in high-value wells with many production years ahead where accurate zone characterization will significantly improve production optimization; straddle packer test data also provide the initial pressure in each zone, which is essential for understanding the pressure depletion state of each layer and designing the production strategy to equalize drawdown and minimize cross-flow between layers at different pressures.
• Composite well test analysis for layered systems using the crossflow model addresses the situation where there is hydraulic communication between layers in the formation (interlayer crossflow) in addition to the commingled production through the wellbore: a layered no-crossflow system behaves during a buildup test like n individual wells in parallel, with the pressure buildup curve showing distinct slope changes as each layer equilibrates to a different pressure; a layered crossflow system (where permeable interbeds or vertical fractures connect the layers) behaves differently, with the pressure equilibration between layers occurring both through the wellbore (during production) and through the formation itself during buildup, producing a characteristic pressure derivative response that can be identified and used to determine the interlayer transmissivity (the property that controls how easily fluid moves between layers through the formation); the distinction between no-crossflow and crossflow behavior has major implications for production strategy because a crossflow system naturally pressure-equalizes between layers during shut-in periods, preventing the more severe depletion imbalance that develops in no-crossflow systems where high-permeability layers are depleted while low-permeability layers retain their initial pressure; identifying whether a layered reservoir has crossflow requires a sufficiently long buildup test to observe the characteristic crossflow derivative signature, and the test duration needed to observe crossflow response scales inversely with the interlayer transmissivity, which can be very small in systems with tight interbeds separating the production layers.
• Unsteady-state multilayer model inversion uses the pressure and rate data from a composite well test on a layered system to simultaneously estimate the individual zone permeability, skin, and storage for each layer, working backward from the observed composite pressure transient through a numerical model of the multilayer system: the inversion process minimizes the difference between the pressure predicted by the multilayer model (computed by solving the diffusivity equation in each layer simultaneously, with the wellbore connecting all layers) and the measured well test pressure, adjusting the individual layer properties until the model matches the data; the practical limitation of multilayer model inversion is non-uniqueness — multiple combinations of individual layer properties can produce very similar composite pressure transients, making it impossible to determine the individual layer properties uniquely from the composite well test alone without additional constraints; these constraints may come from PLT data (constraining the relative flow contributions of each zone), core permeability data (providing prior estimates of individual layer kh), or RFT/MDT pressure data (constraining the initial pressure of each zone); the combination of composite well test data with PLT flow profiles and MDT pressure measurements in each zone provides a substantially better-constrained multilayer inversion than the well test data alone, and this integrated approach is the standard methodology for layered reservoir characterization in complex stratified reservoirs.
• Production optimization decisions enabled by layered reservoir testing results include the recompletion of underperforming zones with stimulation, the mechanical shutoff of depleted or watered-out zones, the selective injection of water or gas into specific layers for pressure maintenance, and the optimal design of production rates to minimize interlayer depletion imbalance: a well test that shows one zone contributing 80% of production with a skin factor of zero while an adjacent zone contributes only 20% with a skin factor of +15 (indicating severe formation damage) immediately identifies the damaged zone as a candidate for stimulation (acid wash or fracture treatment) that could more than double total well production; a zone with substantially lower initial pressure than adjacent zones has been depleted by prior production and may be a candidate for water injection to restore pressure and production rate; a zone that is producing at high water cut while adjacent zones remain dry has watered out (breakthrough has occurred from the aquifer or an injection well), and the PLT-guided mechanical shutoff of the watered-out perforations can restore the water-free production rate of the remaining layers; all of these optimization decisions require knowledge of the individual zone properties that only layered reservoir testing and selective PLT can provide, making the investment in proper layered reservoir characterization directly translatable into production and revenue improvement over the life of the well.