Abstract

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.

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