A Case History of the Matzen Field - 16thTorton: A Story of Success! Where is the End?

Abstract

Abstract The Matzen Sand (16th TH) was discovered in 1949. To date, some 42.3 million m3 (266 million STB) oil and 3.75 billion m3(Vn) (140 billion scft) gas were produced which leads to a recovery factor of 48.5 %. Average water cut is 93.5 %. Mean porosity is 27 %. The oil has 25°API gravity. The sand is relatively homogeneous and exhibits a pay thickness between 1 m up to 70 m (3.3 ft to 230 ft). ‘Hard layers’ (tight streaks) reduce vertical permeability in certain areas. 1968 a peripheral water injection project was commenced. Discontinuous hard layers distributed throughout the reservoir affect water flooding. Sweep efficiency is related to these hard layers. Areas of unswept oil between these tight layers have been proved by reperforation of certain producing wells. In 1996 natural gas injection started into the gas cap which caused a reduction of the oil production decline by 50 %. Successful production history of the 16th TH results from proper reservoir management through 55 years of field life. Introduction The Matzen field is a giant field and the largest multi-pool oil and gas field onshore Europe. It is located some 20 km (12.4 mi) northeast of Vienna in the central part of the Vienna Basin (Austria). The field shows a typical production profile of accelerated early production followed by a long decline and an extended tail production (Fig.7). Since the discovery year 1949, 393 wells produced in the Matzen Sand. Since 1953 production follows an exponential increase of water cut (fig.6) and an almost stable decline (fig.10). Today 126 wells (tab.1) produce 425 m3/day (2,676 STB/day) with an average water cut of 93.5 % and a GOR of 220 m3(Vn)/m3. This is an average oil production rate of 4.1 m3/well/day (26 STB/well/day). In the greater Matzen area sand thickness varies between 1 m up to 70 m (3.3 ft to 230 ft). The occurrence of thin hard layers (thickness 10 cm to 1 m, 0.33 ft to 3.3 ft) is critical for production. These cemented layers represent important vertical barriers for fluid and gas flow (fig.14). Geological Description & Reservoir Characterization Matzen Field The Matzen elevation is an asymmetric elongated anticline of about 10 km (6.2 mi) length and 5 km (3.1 mi) breadth. The northeast axis of the anticline migrates from deeper to shallower horizons towards the southeast. The northwestern limb of the anticline is dissected by intersecting faults dipping to northwest and to southeast that form the pull-apart graben. The southeastern limb dips towards the southeast dipping Markgrafneusiedl fault (fig.2). The main graben of the Matzen field in the north is formed at the northwest limb of the anticline and is rotated to the north [1]. 16th TH (Matzen Sand) The 16th TH is the most important reservoir in the Matzen field and in the Vienna basin [2]. It is an up to 70 m (230 ft) blocky transgressive sand with excellent porosity and permeability. The sand was deposited on top of a regional unconformity separating the Karpatian and Ottnangian (early Miocene formations) from the younger Badenian (middle Miocene) transgression. The 16th TH can be subdivided into seven onlapping layers separated by minor flooding events. Each layer is composed of prograding sand-rich delta front deposits and transgressive shelfal sediments. Older units fill the erosional relief and each younger unit transgresses a little further to the north burying older rocks. Main sediment input derived from the northwest. The fill of the relief explains the high thickness variations over short distances. Basinward to the south the layers get thinner and shale out in minor flooding surfaces. Hence, the top of the 16th TH clearly is not a chrono-stratigraphic surface but a litho-stratigraphic top of varying age (fig.3). The mentioned flooding surfaces within the sandstone show bioturbation and higher shale content than the sands above and below. They can constitute flow and permeability heterogeneities.