Management of Sweep-Efficiency by Gas-Based IOR Methods

Abstract

Abstract High oil recoveries are being obtained in North Sea oil fields by water injection. Favourable mobility due to low viscous oils and the need for pressure maintenance have made this the preferred recovery method. However, a potential for even higher recoveries exists by complementing water injection with gas injection, in different schemes adapted to the geological structure of the reservoirs. A correct description of the heterogeneities is crucial to the success of these efforts. The higher mobility and lower density of the injection gas touches areas not exposed to the injected water. At the same time, an early gas-breakthrough will negate the possible positive effects and even reduce the oil recovery. The probability of the presence of barriers, permeability contrasts, faults and structural traps has to be quantified in order to asses the risks associated with gas injection. This may be done within the quickly developing discipline of modelling heterogeneities with stochastic methods. An overview of field tests planned and performed by Statoil will be given. Amongst these examples are (1) a study of a down-dip miscible WAG in a heterogeneous reservoir, (2) gas injection after water flooding, which has been successful in displacing attic oil in the Gullfaks field, (3) gas injection in oil producers with subsequent oil production and (4) plans for injection of gaseous water for mobility control by use of the non-newtonian rheological properties of the gaseous water. Recognising the high potential and the challenges present in gas-based IOR, Statoil has initiated a 3-years, US$ 6 million research project which will be summarily presented. Introduction In the Norwegian sector of the North Sea pressure maintenance has generally been achieved by water injection. In several fields, the low oil viscosity results in favourable mobility ratio for water flooding. However, the influence of gravitational forces causes segregation of oil and water, and viscous forces in situations with heterogeneities has caused oil bypassing. Several other situations may arise where remaining oil is the target for increased oil recovery and a mapping of the three-dimensional distribution of this oil is crucial to any decision on investments. This will be discussed in the next section. Oil production may be constrained by gas production if gas pipeline capacity is fully utilised. Injection of surplus gas may remove this constraint. In Norway, the authorities do not allow as a rule waste of resources by for example gas flaring. As a consequence, in the early period of oil production when transport systems for gas were not available, the operators had to investigate the effect of gas injection. In the case of one of the largest fields now operated by Statoil, gas injection in the Statfjord formation led to a gravity stable miscible process where the recovery is predicted to be very high, Other gas-based improved oil recovery methods will be described, with emphasis on schemes utilizing the structural/geological definition of the reservoir. There is a large literature on miscible processes, and these will not be discussed here. Performed field tests or plans for operating field pilots will be summarised, and in the last section research and developments needs will be discussed. Three dimensional distribution of remaining oil Modelling heterogeneities. One paradigm shift in reservoir management has taken place recently: the recognition that reservoirs are seldom divided in layers with constant properties: heterogeneities at different scales always exists, and in some cases they are crucial to the understanding of the flow patterns in the reservoir. This recognition has been facilitated by the increasing use of integrated teams and the advent of powerful graphical workstations, which make possible both the geological modelling in three dimensions of the heterogeneities and the simulation with high resolution of the fluid flow in the reservoir. In the following, several instances where oil recovery may be improved by some form of gas injection will be described. Fig. 1 shows schematically these methods. P. 245