Wireless Downhole Gauges Help Maximize Value of Appraisal Test in Abandoned Well
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This paper describes the acquisition and interpretation of long-term pressure-buildup data in a plugged and abandoned deepwater appraisal well. To accomplish the test objectives at an acceptable cost, a novel combination of well testing, wireless-gauge technology, and material-balance techniques was used to allow the collection and interpretation of reservoir-pressure data over a planned period of 6 to 15 months following the well test. The final buildup duration was 428 days (14 months).
Three interpretation methods of increasing complexity were used to provide insights into the reservoir. First, material balance was used to produce an estimate of the minimum connected reservoir volume. The advantage of material balance is that it requires very few input assumptions and produces a high-confidence result. Second, analytical models in commercial pressure-transient-analysis software were used to investigate near-wellbore properties and distances to boundaries. Finally, finite-difference-simulation models were used to investigate reservoir properties and heterogeneity throughout the entire tested volume. With increasing model complexity came additional insights into the reservoir properties and architecture but reduced solution uniqueness.
A key complication for the interpretation of the recorded pressure data was the potential for gauge drift. This was incorporated into the uncertainty range used in all three interpretation methods.
Analysis of conventional well-test designs (with varying flow rates and buildup periods) showed that the cost of resolving the key uncertainties exceeded the value of information significantly. To justify the appraisal, a way was needed to extend either the flow period or the buildup period without a rig on station and with the well left in a permanently abandoned state. To meet this objective, the potential of wireless-gauge technology to extend the buildup length was evaluated. Two competing wireless technologies were available, acoustic and electromagnetic transmission, both occurring up the tubing/casing. The key differentiator was that acoustic transmission required that cables be run through any cement plugs, which violated the barrier standards for abandoned wells. Accordingly, electromagnetic transmission was selected for the final system. The post-abandonment well concept is shown in Fig. 1. Of note is that the wellhead was not recovered and the top of the 20- and 36-in. casings have not been severed.
One critical design feature was the use of redundant gauges (four), repeaters (four), and subsea modems (four) to ensure no single point of failure existed within the wireless system. This also resulted in a narrowing of the gauge-drift and accuracy-uncertainty range as the response of individual gauges was thought to be independently and identically distributed.
Data were recovered from the wireless gauges in three tranches (corresponding to visits by a supply vessel to the well vicinity), with the subsea modems recovered to surface as part of the final tranche.
The well-test design called for an initial cleanup flow, initial buildup, multirate test, extended high-rate-flow period, and a final buildup. For operational reasons, an additional shut-in during the extended flow period was necessary to restock methanol for hydrate inhibition on the rig. The final buildup was split into three components: The first 24 hours were planned to be recorded with conventional drillstem-test (DST) gauges run on the completion to provide high-frequency data that could be interpreted for kh (the product of formation permeability and thickness) and any near-wellbore features. This was to be followed by a break in the recorded data while the long-term wireless-gauge system was installed and commissioned. The final stage was the long-term buildup recorded by the wireless gauges, which had a much lower acquisition frequency.
Interpretation: Initial Reservoir Pressure
The original intention was to use the interpreted reservoir pressure (P*) from the initial buildup following completion of the cleanup flow as the basis for the initial reservoir pressure used in the material-balance analysis; however, after calculating P* for the post-cleanup buildup, the same was performed for the buildup following emergency shutdown, which produced a higher P*. To investigate this further, P* was determined for each buildup and each gauge and then plotted against cumulative production, which produced a clear linear relationship between the interpreted P* and cumulative production. This is particularly notable because the duration of the buildups differed significantly, with the first buildup being 1.3 hours long, the second 9 hours, the third 2.3 hours, and the last 30.8 hours.
Interpretation: Material Balance
The initial reservoir pressure interpreted from the P* vs. production plot was combined with the pressure/volume/temperature data from fluid samples collected during the DST to allow a minimum connected volume to be calculated. The uncertainty range in the interpreted volume is based on the uncertainty in the reservoir-pressure measurement because of the potential for gauge drift, with the drift for each of the three gauges that survived for more than 6 months being treated as an independent and identically distributed random variable. This allows the uncertainty range to be reduced by a factor of the square root of the number of surviving gauges and illustrates an additional benefit of equipment redundancy. The initial pressure perturbation on the interpretation of the pressure data given the assumed drift range from the gauge specification is also of interest: The low-case value shown uses the pressure data from Day 205 of the buildup because this was the point at which the rate of pressure change recorded by the gauges matched the rate of pressure change corresponding to the gauge drift. Beyond this date, the adjusted low-case pressure has stopped increasing and begins to fall, indicating the adjusted data are no longer suitable for interpretation.
Interpretation: Pressure Transient Analysis
The final data set used for the pressure transient analysis included the effect of the gauge corrections described previously. The effect of the gauge corrections is to smooth out the derivative curve by eliminating discontinuities when switching between gauges. The data over the period from 0 to 40 hours are from the high-frequency gauges, and the remaining data are from the wireless gauges. Surprisingly, the derivative data were smooth enough (even without smoothing) to be interpretable all the way to the end of the recorded data.
The well test included four buildups: two with downhole shut-ins and two with surface shut-ins. The two buildups with downhole shut-ins result in smooth derivatives free of wellbore-storage effects from 0.003 hours, while the two buildups with surface shut-ins are dominated by wellbore-storage effects until 0.4 hours. The other major effect of downhole shut-in was the reduction in noise in the pressure measurements after the wellbore-storage region, likely because of reduced fluid redistribution from temperature effects.
Interpretation of the high-frequency-gauge data provided information on the skin and non-Darcy skin, formation permeability, heterogeneity (through indications of two-layer behavior with crossflow away from the wellbore), and the two closest boundaries.
Inclusion of the wireless-gauge data also allowed for resolution of the third boundary and provided an indication of the location of the fourth boundary, but the derivative response at the time of the last data point was still not indicative of a fully bounded system. Inclusion of the wireless-gauge data in the interpretation increased the distance investigated by the well test by a factor of ten, with the final path length to the outermost boundary estimated to be 15 km. The full test pressure match, however, indicated that the gas volume within the analytical model was a significant overestimate of the actual gas volume, which is consistent with the authors’ understanding of this environment of deposition.
Interpretation: Finite-Difference Simulation
The analytical model used for the pressure transient analysis was unable to reproduce both the derivative response and the pressure response simultaneously, so a more-detailed finite-difference simulation model was constructed to allow the modeling of more-complex reservoir heterogeneity. This model was able to identify the key factors affecting each section of the derivative response in this depositional environment. The learnings from this sector model were then used to guide population of the reservoir model, resulting in an excellent overall match with the only adjustment being a global permeability multiplier of 1.25. This model allowed testing of various features of the depositional environment, including channel relative orientation (varying orientation vs. parallel), overall net to gross (NTG), proportion of NTG in higher-quality channels, and permeability of the channel and nonchannel elements. The tradeoff for the improved match and additional insights is significantly increased model complexity and a higher dependence on external data sources.
Use of wireless gauges allowed the buildup duration to be extended from the 3 days used on previous DSTs in the region to 428 days, with the available rig day allocation used to produce a larger-pressure perturbance. This allowed the well test to investigate a reservoir volume similar to planned development wells (with the outermost boundary being 15 km from the well).
This paper shows how a progression of interpretation techniques of increasing complexity allowed additional insights to be drawn from the collected data but at the expense of increasing complexity, more degrees of freedom, and additional external constraints. While the more-complex techniques provide additional insights, the simplest technique provides a higher degree of confidence in its (more-limited) conclusions.
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Wireless Downhole Gauges Help Maximize Value of Appraisal Test in Abandoned Well
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