Gas-Injection Pilot in the Hochleiten Field

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 89356, "Gas-Injection Pilot in the Hochleiten Field," by K. Potsch, SPE, R. Ramberger, SPE, J. Glantschnig, SPE, S. Baumgarthuber, SPE, and F. Gossnitzer, SPE, OMV A.G., prepared for the 2004 SPE/DOE Symposium on Improved Oil Recovery,Tulsa, 17-21 April. The Hochleiten field, a marginal field north of Vienna, Austria, is highly com-partmentalized and contains very viscous oil. Waterflooding worked only in a part of the reservoir. Poor communication and large pressure differences were observed across the field. A CO2 injection was attempted in an area in which no waterflood had been tried. Use of CO2 improved the inflow capacity of the injector, but oil production from affected wells increased only slightly. To match the actual response of the reservoir, reservoir-model adjustments became necessary. To support the existing reservoir description, an N2-tracer test was initiated to determine the preferred communication paths. Introduction The Hochleiten field was discovered in 1973. Production began in 1974. The main reservoir is the Tertiary Lower Sarmatian at a depth between approximately 710 and 800 m subsea. The structure map in Fig. 1 shows that the reservoir is highly compartmentalized with two major, probably sealing, faults. The reservoir is oil bearing, with small gas caps in Sectors A and C. Approximately 30 wells have been drilled. A waterflood, comprising two wells each in Sectors A and C, was initiated in 1991 with mixed success because of the high level of structural and stratigraphical heterogeneity. Sector B was not included in the waterflood program because the production history did not define fault locations of impermeable layers. In 1997, a detailed and integrated reservoir study was initiated to understand reservoir behavior. On the basis of new 3D-seismic data, a team of geoscientists and engineers revised the structural and stratigraphic framework of the field, leading to a new reservoir model for dynamic simulation. Several recompletions and infill-drilling locations were identified. It became evident that lateral-facies changes, as well as reservoir compartmentalization, led to early pressure decline or early water breakthrough. Combining a material-balance model with neural-net-work models in one sector of the reservoir showed that the connectivity between wells and layers generally is poor, and, in particular, water injection at downdip locations could not support the pressure sufficiently in the updip areas of the structure. It also was found that the dissolution of gas increased oil recovery more efficiently than waterdrive alone. Therefore, gas injection is expected to be more efficient than waterflooding to access bypassed oil.