The Algyo Field in Hungary: Modeling Fifteen Reservoirs with Interfering Aquifers
Authors: Pach F. , Gilicz A. , Palasthy Gy. 
Published: December 2007

In petroleum practice it often happens, that several reservoir-aquifer systems are located close to each other, either vertically, or horizontally. Pressure changes of the individual reservoirs mostly differ due their different production histories, so they may interfere with each other. Observations may indicate the interference of reservoirs, but the magnitude and significance of interference can be rarely determined. Conventional material balance methods can investigate only one-reservoir-one-aquifer systems. Simulation methods are even in such "simple" systems rather time consuming, and they are even more time consuming if they are applied to several interfering reservoirs simultaneously. We have developed and applied an easy to use material balance model, which accounts for interference as well.

Introduction

The Algyo field is the largest hydrocarbon accumulation in Hungary. The oil reservoirs with gas caps and gas reservoirs are located above each other with a few meters thick shale interbeds in-between. Reservoirs contact different size and activity aquifers, their initial pressures are close to hydrostatic. These and some satellite reservoirs interact with each other through their aquifers. It was observed e.g., that pressure declined in some reservoirs prior to their production.

Production of the reservoirs started partly simultaneously, partly sequentially at the end of the sixties. Oil reservoirs were produced with close pressure maintenance by double sided water injection - water was injected along both the GOC ad WOC, the pressure drop being about p/pi 5 25%. Gas reservoirs were produced continuously, they are in the final stage currently, pressure decline is about p/pi = 50 80%.

The seasonable, intensive production of remaining gas caps starts in the near future, whereas the intensity of water injection reduces continuously. Due to the differing production histories of the different type reservoirs, production characteristics (e.g. pressures) are strongly influenced by interference. Because of the geological situation, technical possibilities and in order to reduce costs, the production of the reservoirs was realized with common (sometimes with commingled) well. Multifunctional completion raised the chances to well failure significantly. This could have been further enhanced by rock compaction due to the steady, high differences in reservoir pressures, erosion, formation break through, etc. All this rised the chances for the development of spontaneous fluid migration between reservoirs. In the model we have developed we wanted to account for the two main phenomena occurring, namely interference, and fluid migration.

Production history of the Algyo field

The Algyo hydrocarbon accumulation (Figs. 1 and 2.) consists of sandstone type reservoirs, located vertically above each other in a range of 1700-2100 m ss. The reservoir fluids are saturated light oil and hydrocarbon gas. Production of the reservoirs started along with exploration. At the end of the sixties natural energies (solution gas drive, gascap drive and natural water influx) provided the displacement energy. The well network was developed according to the requirements of production and water injection. A few years after production start the double sided water injection technology was introduced. First the water injection along the WOC, then along the GOC was started. P. 903^