Water Shut-off Field Experience with a Relative Permeability Modification System in

Abstract

The Marmul field in Oman is exhibiting a low oil recovery caused by the high oil viscosity (80 mPa.s) and the moderate to strong edge water drive. The poor mobility ratio has resulted in early increased water production through water channeling and coning. The technology evaluated to reduce the water production made use of a so-called relative permeability modifier system which was composed of a cationic polyacrylamide and the crosslinker glyoxal. This gel system reduces the relative permeability for water, while only marginally affecting the relative permeability for oil. The aim of a relative permeability modifier system is a decreased water production and potentially an increased oil production. The major advantages for such a system are that the treatment is bullheaded down the well and detailed information on the inflow performance is not required. This was advantageous for the Marmul field in which all the wells have been gravelpacked. Selection criteria were used to establish which wells would be treated and the final candidates were randomly located over the field. The treatment design consisted of the injection of three equal sized polymer/crosslinking stages with increasing polymer concentration. Five of the initially six treated wells showed a positive response to the gel treatment, i.e. (large) water-cut reduction and increased oil production. A simulation study on one of these treatments was performed to understand the behaviour of these treatments. Initial results could not match the observed field behaviour and only after introduction of flow barriers (shale layers) in the reservoir model a match between simulation and field could be achieved showing the significance of small details in local geology around the well. However, these flow barriers could not be seen on the open hole logs. Subsequently eight treatments have been performed. However, they were disappointing and no clear explanation for the deviating behaviour could yet be given. The causes for the poor performance could be poor candidate selection and/or a poor understanding of the gel system. Poor candidate selection is considered doubtful because the wells in the first six and the last eight treatments were randomly chosen throughout the field. Therefore, the emphasis is now on the gel system to determine more accurately its chemistry (e.g. polymer/crosslinker concentrations needed) and the volumes required to obtain an optimum result with respect to decreased water production and increased oil production. P. 307