Locating enhanced-permeability streaks in a carbonate reservoir

Overview

The carbonate reservoir of an onshore producing field owned and operated by ADNOC in Abu Dhabi exhibits large lateral and vertical property variations in rock properties. These high-permeability streaks strongly influence flow patterns in the reservoir and could not be resolved using seismic. Any infill draining well that perforates one or more streaks will rapidly result in water breakthrough while leaving oil in place in the less permeable facies.

Solution

To locate and characterize the high-permeability streaks, an ensemble of 100 realizations was created in ResX representing the reservoir variability and uncertainties for both the geology and the reservoir’s static and dynamic characteristics. An iterative workflow history-matched the ensemble, resulting each time in small changes to equiprobable models to minimize the specified objective function (e.g., production rate, water cut and bottom hole pressure). The resulting P50 across 100 realizations shows clear high-permeability streaks (Figure 1b).

Figure: 1a (upper) and 1b (lower) We see the exposure of an enhanced permeability streak on the aggregated permeability P50 property for the ensemble after the history matching workflow (lower:), whereas it is not visible on initial ensemble (upper)

The cumulative oil production forecast from the history-matched ensemble captures the main uncertainties, notably FWL and structure. Seeing the spread of possible forecasts 5, 10, and 20 years ahead gives decision-makers a better grasp of business risks than the usual one or two deterministic scenarios yielded using conventional methods.

Figure: 2 The spread of possible cumulative production curves from the ensemble of 100 models, all fully matching all current data. The dotted red line represents the prior two base cases that fail to inform on the uncertainty of possible production outcomes as well as probably underestimating the field’s production potential.

Solution highlights include:

• Workflows honor the recommended static and dynamic modeling processes suited to the field’s complexity.
• Realistic sedimentological, structural and dynamic reservoir parameter uncertainties were identified and propagated to obtain a complete view of variability in the reservoir simulator response.
• Integrated workflows were used to generate an ensemble of equiprobable reservoir models.
• All realizations in the ensemble were history-matched simultaneously, delivering improved matches for the defined objective functions. Well-by-well matching achieves a good level match at the field level.
• Verification: PLT data was used as a blind test to validate the result. Trends of fluid flow along horizontal wells captured the characteristics of high permeability streaks.
• Analysis of forecasted ensemble dynamic responses helps evaluate the performance of existing infill targets and delineate new infill targets while understanding the associated risks under both static and dynamic uncertainty.

Result

The probable locations and likely properties of the high permeability streaks were established. This enabled the planning of infill drainage wells while avoiding a bypass of reserves or an acceleration of a water breakthrough.

References: Alqallabi, S., et al., 2021. An Integrated Ensemble-Based Uncertainty Centric Approach to Address Multi-Disciplinary Reservoir Challenges While Accelerating Subsurface Modeling Process in an Onshore Field, Abu Dhabi, UAE. Paper presented at the SPE Annual Technical Conference and Exhibition, Dubai, UAE, September 2021. Paper Number: SPE-205854-MS