Mangala is the largest discovered oil field in the Barmer Basin of, Rajasthan, India having a STOIIP of over 1 billion barrels in multiple stacked fluvial clastic reservoirs. It contains medium gravity (20–28oAPI), waxy, viscous crude (9–17 cp) in high permeability (1–25 Darcy) clean sandstone reservoirs. Initial development plans for the field are based on waterflooding, with at least the initial volumes of water heated to minimize any issues with wax dropout in the reservoir. Owing to the relatively high oil viscosity and adverse mobility displacement for waterflooding, the desirability of implementing an appropriate EOR process was identified shortly after the field discovery. Screening studies identified aqueous-based chemical flooding processes as the most favorable for Mangala. Detailed laboratory studies have now identified the potential of alkaline-surfactant-polymer (ASP) processes in significantly improving the sweep and displacement efficiency. The laboratory studies involved screening and optimization of the ASP slug formulation based on IFT measurements, adsorption measurements, polymer rheological and thermal stability studies; followed by a series of linear and radial corefloods. Experiments indicate the ASP process is the most effective chemical EOR process for Mangala, with improvement in recovery efficiency above the waterflood recovery of over 30% STOIIP. Chemical EOR simulations with using STARSTM have been used extensively to understand the process mechanisms via matching of the coreflood experiments. Simulation parameters tuned to the laboratory data were used to evaluate the process performance under field conditions. A closely spaced five-spot pilot with 100m well spacing has been designed to evaluate the process. The pilot consists of four injectors, the central producer, three saturation monitoring wells and two post-pilot core holes. An ASP pilot will be implemented in the field shortly after startup of the waterflood. Introduction Production startup of Mangala under waterflood is planned for 2009. Given its adverse waterflood mobility ratio, the importance of EOR for Mangala was realised soon after its discovery in 2004. The bulk of the Mangala reservoir contains a medium- to light-gravity oil of moderate viscosity, with a minor biodegraded zone of higher viscosity above the OWC. The oil also has a reasonably high petroleum acid content, which will react with an injected alkaline solution to form natural "soaps" that can help to increase oil recovery. Reservoir geology varies from massive and well-connected stacked channel sands to relatively discrete single-storey channels with somewhat poorer (but still good) lateral communication; all with high porosity and permeability. Further, the injection water available has low salinity and is reasonably soft. Mangala oil, rock and water attributes are all positive for the application of mobility-controlled chemical EOR methods. The base waterflood is expected to recover more than 35% of STOIIP, with an oil plateau of 4–5 years duration before the field goes on steep decline. Significant fluid handling and water injection will be required to achieve the projected recovery. Thus, huge potential exists for mobility control chemical flood processes, not only in terms of extending the plateau and increasing reserves but also in terms of significantly reducing total fluid production and water injection requirements. This is important for the project economics as it implies that the oil production facilities will work longer at designed capacity longer, and total fluid handling requirements may go down if an EOR process is implemented sufficiently early, before the field goes on waterflood decline. This will result in significant CAPEX and OPEX savings as well. Early implementation of the process will also ensure a more better efficiency of the process, as the in situ oil saturations will be high. Otherwise late in the field's life, the higher water saturations that will exist later in the field's life can partially shield the remaining oil from the action of the injected chemicals.

The paper discusses the full-field polymer flood implementation plan post successful pilot result in one of the largest onshore fields in India. Mangala field contains multi-Darcy sandstones that have high porosity and very low connate water saturation and low temperature (65 degC). The connate water salinity is low (~6500 total dissolved solids), The crude is waxy and viscous, which results in unfavorable mobility for water-flooding, Chemical flood was identified as a potential EOR method during a screening study. A normal 5 spot polymer flooding pilot has been conducted and results are encouraging in terms of additional oil recovery and reduction of water cut. Additionally much has been learnt about how to optimize the surface facility, polymer preparation, and polymer quality monitoring. Following the successful completion of the pilot, it is planned to implement a polymer flood for the fullfield. All the lab results and pilot learnings have been incorporated to minimize the uncertainty and risks. Different subsurface injection pattern concepts have been evaluated based on the reservoir characteristics. Various parameters like pattern and number of wells, polymer viscosity, tapered vs. fixed concentration, polymer slug sizes and other variables have been optimized and are discussed in the paper. Sensitivities to capture the impact of polymer adsorption, implementation schedule, conformance in injectors, visco-elastic effect of polymer, polymer shear thinning characteristics etc. on recovery have been studied. Learning from pilot has been used in selecting the surface facility, well design, pumping facility, selective completion for conformance control, surface pressure requirement, and designing to minimize the viscosity loss across the completion and sand face. Surface facilities concepts have been finalized to prepare concentrated polymer at a single location and pumping it to each well for injection. Polymer flooding has the potential to significantly improve the sweep efficiency in the field and to increase expected ultimate recovery (EUR). The project is one of the largest in the world in terms of scale, polymer usage, and related facility and logistics.

Detailed laboratory experiments were carried out to evaluate the potential for various chemical flood processes (polymer, alkali-polymer, and alkali-surfactant-polymer) in Mangala, a large oilfield in India containing waxy crude with viscosity of 7–20cp. Experiments included fluid-fluid and fluid-rock interaction studies, followed by a series of linear and radial corefloods. Simulation of the corefloods was carried out using STARS, the advanced compositional simulator available from CMG. The main coreflood simulation objective was to shed light on the various process mechanisms and to generate chemical flood parameters for field-scale simulation forecasts. Primary matching parameters included performance of the base waterflood, injection pressures to define shear-thinning polymer behavior, chemical flood performance to define interpolations between appropriate relative permeability curves based on capillary numbers, and produced chemical concentrations to define adsorption parameters. Sensitivity studies on the matching parameters examined the impacts of heterogeneity within the large-diameter radial cores, chemical adsorption, and capillary pressure and gravity effects. The coreflood simulations provided significant insights into the chemical flood process mechanisms and eliminated uncertainties such as the effect of gravity in radial coreflood experiments; although some issues including the role of capillary end-effects have not as yet been completely resolved. The modeling showed that in situ saturation monitoring (scanning) of the cores is critical for future planned corefloods. The study also showed some of the current limitations in modeling these complex processes. Chemical flood parameters fine-tuned during the coreflood simulations have been used in field-scale simulations to evaluate expected performance and to aid in design of a field-scale pilot project. Extensive field-scale simulations have been used to design an appropriate chemical flood implementation strategy. These works indicate potential field-scale incremental recoveries over waterflooding of ~7% STOIIP for a polymer flood and ~15% STOIIP for an alkaline-surfactant-polymer flood.

The Mangala Field is the largest discovered oil field in Barmer Basin, Rajasthan, India. The field contains medium gravity viscous crude (10-20 cp) in high permeability sands. The field is being developed under pattern and peripheral water injection. Due to the adverse mobility ratio, waterflooding is expected to result in lesser sweep and ultimately affect oil recovery. The potential for chemical enhanced oil recovery (EOR) has been recognized from an early stage in the field development, with polymer flooding identified for early implementation, followed by staged-wise implementation of Alkaline-Surfactant-Polymer (ASP) injection. To understand and evaluate the efficiency of chemical flooding, a closely spaced normal five spot EOR pilot is being conducted in the fluvial FM1 reservoir units of the Mangala Field. Polymer injection in the pilot commenced in August 2011, with a series of short term polymer injectivity tests including multi rate test in one of the injectors. These tests provided useful insight about polymer rheological characteristics under reservoir conditions. Non-Newtonian behaviour of the polymer helped in injecting the ~30 cp solution within the designed surface pressure. Radial well modeling confirmed the shear thinning characteristics of the polymer which were in agreement with laboratory data, and that there has been no significant mechanical degradation of the polymer inside the well. Production logging conducted before, during and after the injectivity tests also show no significant change in injection profile as a result of the polymer injection. The injectivity test establishes that polymer injection is viable in Mangala reservoirs. The pilot results are important for an early implementation of fieldwide EOR in the Mangala Field. This paper presents details of the polymer injectivity tests including bottom-hole pressure measurements and production logging conducted during the tests. Details of the single well fine scale modeling to match the injectivity test pressure behaviour are then described.

The Mangala field in the Rajasthan state of western India was the first major oil discovery in the Barmer Basin. It contains over 1 billion bbls of paraffinic oil with average viscosity of ~15cp and wax appearance temperature only ~5°C less than reservoir temperature. The initial development plan is a hot waterflood to prevent in situ wax deposition; however, chemical EOR methods are expected to play an important role in the ultimate field development strategy. EOR assessment was part of the initial field development planning process. Screening studies were conducted soon after discovery followed by comprehensive laboratory evaluations of chemical flood potential which had been identified as the most suitable process. The laboratory evaluation along with parametric simulation studies indicated significant chemical EOR potential. A closely spaced normal 5-spot pilot was designed to test chemical EOR processes in the early part of the field development. Logging observation wells are strategically placed in the pilot to periodically monitor changes in oil saturation. The initial waterflood phase in the pilot has been completed and injection of the polymer slug is in progress. Comprehensive monitoring and quality control procedures are being followed to ensure smooth operations. Extensive PLTs, time-lapse saturation and resistivity logs, pressure surveys, tracer surveys, detailed produced fluid analyses and tests for monitoring the quality of injected fluids are all performed routinely. Dynamic reservoir modeling has been a critical part of the analysis to understand polymer flood behavior. The initial polymer flood phase in the pilot will be followed by ASP (alkaline-surfactant-polymer) injection for further evaluation of chemical EOR potential. This paper discusses the operational aspects of polymer flood facilities, quality control and monitoring program followed, challenges faced, polymer injection results and plans for future ASP injection.