IOR potential for the Mogollón formation onshore northern Peru.

Current oil prices and the present economic climate in Peru have made the Talara Basin an interesting play, with plenty of opportunities for further investigation and exploration. Petrobras, which is active in Peru, engaged Knowledge Reservoir to perform a study of the complex dual porosity Mogollón formation in the Talara Basin in the northwestern part of the country.

The Mogollón formation is a thick section of sandstone, conglomerate, conglomeratic sandstone, and shale of Eocene age. Matrix permeability is low, and economic flow rates rely on a natural fracture system and massive hydraulic fracturing of production wells. Challenges presented by low permeability and fracture orientation led to the decision to carry out additional studies of the Mogollón to determine the best sectors and water injection patterns to enhance recovery.

Setting the scene

Production in the study area began in 1956. To date, approximately 300 wells have been drilled to produce the reservoir. The field has been divided into 12 segments to more efficiently evaluate the original oil in place (OOIP), oil recovery, fracture flow characteristics, and development alternatives for each area. The peak oil rate was nearly 9,000 stock tank barrels (STB) per day in 1987. The current Mogollón oil rate is just 1,000 STB/day from approximately 220 wells. The reservoir is under primary depletion. Oil recovery is forecast at between 2% and 10%, depending on the subarea and density of the natural fractures.

With the goal of determining the best production strategy from the naturally fractured dual porosity reservoir, the primary objective of the study was to evaluate alternative reservoir development strategies for improving oil recovery. The objectives also included estimating OOIP (calculated by a probabilistic analysis using a Monte Carlo approach), providing geologic and engineering interpretations (the structural interpretation was made quickly to calculate volumes in a timely fashion), and estimating the incremental oil potential of different field development alternatives.

Interpreting the data

The project presented a complex challenge given the dual porosity nature of the reservoir. The oil recovery factor was low as a result of poor reservoir properties — low permeability and solution gas drive in a naturally fractured reservoir.

Logs for 234 of the wells were analyzed in the Mogollón formation using PowerLog software. The petrophysical interpretation included estimation for porosity, volume of shale (Vsh), water saturation, and net pay. Results of the log analysis were then written in *.LAS format and imported to Petrel software for geologic modeling. Additional log interpretation was carried out in an effort to classify the fracture system and lithofacies. Also, logs were analyzed for 115 of the wells in the Echino and Ostrea formations to provide input for geologic modeling of OOIP.

The results of these completed objectives were used to build two static models using the Petrel software. The first model was made for an area encompassing the Mogollón, Echino, and Ostrea formations and was used for volumetric calculations of net pay and OOIP.

The location of this model was based on high cumulative oil and fracture production, older and more recent production wells, some infill drilling, good well coverage, and conventional core coverage.

The following areas were examined when constructing this sector model: location, Segment 2 structure and cellular model, facies, Vsh and net to gross, porosity and permeability, water saturation, fracture permeability, and upscaling, volume, and Eclipse export.

The second model was a sector model with dimensions of 1 mile by 0.5 mile (1.6 km by 0.8 km). This model was used to history match production from 10 wells and to forecast IOR through infill drilling, water injection, and non-miscible gas injection. The model was created in Eclipse and was based on the same criteria as the Mogollón model, which included high cumulative oil and fracture production, older and more recent production wells, some infill drilling, good well coverage, and conventional core coverage.

The simulation sector model was used over a small representative area of the Mogollón formation of the reservoir with the objective of understanding fluid flow and oil recovery dynamics.

The following areas were evaluated in the construction of this simulation sector model: Eclipse model initialization, porosity and permeability, PVT (pressure-volume-temperature) data, relative permeability, analytic aquifer, pressure history, well bore, history matching, and simulation forecasts. The resulting model was used to show the benefits of incremental field development projects. Despite the complex nature of the reservoir, good IOR and EOR potential were identified for both water and gas injection.

The Eclipse reservoir simulation was used to model a detailed area of the Mogollón in Segment 2 that contained 10 wells. Three history matches were achieved: one used a dual permeability model, and the others used dual porosity models that varied the amount of OOIP in the fractures. Forecasts from the three history matches provided
a range of potential upside reserves for infill drilling and gas and water injection cases. IOR ranged from 0.6% to 12.3% of OOIP, depending on the history match model and development case.

The value of refracturing

Another objective included selecting 30 wells as re-fracturing candidates based on formation specific to the technical and operating criteria. The first step was to re-fracture wells that fell into the top 10% in terms of oil rate.

Next, the wells were selected by an initial screening process, and recommendations were made for a more thorough review of their mechanical integrity. Additional strategies for identifying re-fracturing candidates were also provided for future production enhancement. Knowledge Reservoir used a workflow that provided a quick-look estimate of IOR potential for different development scenarios.

Fracture characterization was performed using a core description and analysis, micro-resistivity logging tools, and a review of historical well production to determine the relative fracture-matrix contribution. There were 12 subareas classified as flow, predominantly from dual porosity, dual permeability, and matrix with few fractures. The geostatistical static modeling included facies population and the population of fracture permeability that was soft-conditioned spatially to lithofacies and laterally located to tectonic faulting. Seismic data in this area were extremely poor and could not be used for spatially locating fracture swarms.

Finally, the sector simulation results were upscaled to the entire field using a quality index factor for each of the 12 segments and a definition of reservoir matrix-fracture flow characterization. The IOR was scaled to all areas to determine total field IOR potential and benefits.

Non-miscible gas injection provided the best opportunity to increase oil recovery with a target IOR of 5.2% to 12.3% depending on the area. Only minor upside reserves were identified for infill drilling due to the low reservoir pressure and energy. However, a combination of drilling new production wells and injecting in existing, high-water-cut producers was recognized as having significant economic and recovery advantages.

The results, further study

Petrobras Peru received recommendations for further developing the Mogollón formation. Knowledge Reservoir advised that a detailed structural reinterpretation be made. The results of this interpretation could be critical in helping locate areas with a greater abundance of natural fractures.

The company also suggested that a more detailed study of the Mogollón conglomerate (complied deposition of thicknesses and percentages) be conducted. It would be beneficial for additional sector models to be constructed for selected areas in the northern part of the basin where high production occurs in areas with lower net pay and OOIP. To achieve the best results, several areas should be selected for consideration for a field water or gas injection pilot. Segments 2 and 4 were recommended as good candidates for the IOR pilot projects. The company further suggested that a study be conducted on the economics of water injection verses gas injection. This study should include details of field equipment and the cost of water and gas supply. More in-depth studies are also required to evaluate well spacing, well locations, the amount of injected fluid required, and the optimum reservoir pressure for gas and water injection schemes.