Development of the Strasshof Tief Sour-Gas Field Including Acid-Gas Injection Into Adjacent Producing Sour-Gas Reservoirs

Summary OMV operates two producing sour-gas reservoirs in lower Austria: the Reyersdorfer dolomite (shallow reservoir) and the Schoenkirchen Uebertief dolomite (deep reservoir). A new, separate reservoir called the Perchtoldsdorfer dolomite (Strasshof Tief field) has been discovered, and options for how its acid gas can be handled are being investigated. The two currently producing reservoirs deliver to a gas plant with a 30-tonne/d sulfur plant. The sulfur plant is too small to accommodate the additional production. OMV has evaluated acid-gas injection as an alternative to a new, larger sulfur plant. Acid gas could be injected into either the Reyersdorfer dolomite or the Schoenkirchen Uebertief dolomite. In either case, injection would be occurring concurrently with production. The intent of this project was to determine at a scoping level if sufficient injectivity and storativity are available in either the Reyersdorfer dolomite or the Schoenkirchen Uebertief dolomite. Compositional modeling and the prognosis of the breakthrough time at the producing wells were carried out to determine the contamination risk to existing production. The simulation work included generating compositional numerical-simulation forecasts of production-rate/composition forecasts under concurrent injection/production scenarios; modeling in-situ miscibility and gravity-separation effects of acid gas; and evaluating risk scenarios for existing production to determine the optimal solution. Introduction OMV's recent discovery of the Strasshof Tief reservoir prompted a review of whether acid-gas injection could be a viable alternative to a new or expanded sulfur plant. The issues were whether to inject into the Reyersdorfer or Schoenkirchen Uebertief reservoirs (Figs. 1a through 1c and Fig. 2), how injection would affect the existing recoveries, when breakthrough would occur, and whether there would be sufficient injectivity and storativity in both reservoirs. A complicating factor in the analysis is that the size of the Strasshof Tief is unknown at this time (testing was scheduled for 2006). The composition of the Strasshof gas is also unknown, but it was estimated on the basis of Modular Formation Dynamic Tester (MDT)* samples from the Perchtoldsdorfer dolomite and the known composition of the adjacent sour-gas reservoirs in the dolomite rock. Our review of the problem was broken into two phases. The initial phase was a brief analytical review to estimate the injectivity and storativity of each reservoir and to assess which reservoir was clearly more suitable. In the second phase, the selected reservoir was simulated to determine breakthrough times and whether there was an impact on recovery. Because of the accelerated schedule of this project, where initial simulation results were necessary to initiate discussions with regulatory agencies and obtain approvals so that 2006 development plans could proceed, it was agreed that geological models would be built for both reservoirs immediately so that the simulation could proceed when a decision was made after the initial review.