Case Study of Unconventional Gas-Well Fracturing in Hungary

Abstract

Summary Well Ba-E-1 was drilled in the Tompa prospect (now the Ba-IX Mining Plot) in Hungary targeting the Miocene and Cretaceous formations between 2600 and 3500 mTVD. These are tight sandstones, and the expected permeabilities were in the range of 0.001 to 0.5 md. Two hydraulic-fracture treatments were performed. The first fracture treatment was in the lower part, and the second treatment was in the upper part of the deepest interval. With no previous propped-fracturing experience in this field, the first treatment was designed as a conventional crosslinked-gel treatment to minimize the risk of a premature screenout. Following the analysis of the data from the first zone, it became clear that the average permeability was closer to the minimum expectation of 0.001 md. Because of lower-than-expected stimulation effectiveness of the first fracture, and the confirmation of the low permeability, the second fracture treatment was changed to a water-fracture design. This formation clearly falls into the unconventional category, and consequently was a good candidate for a water fracture. This paper describes the prefracture diagnostics, fracture execution, and post-fracture production evaluation of this unconventional gas well. Special emphasis is placed on the use of small-volume injection tests, or diagnostic fracture injection tests (DFITs), to obtain an estimate of the in-situ kh because it is impossible to perform prefracture welltests in such a formation. The result of the DFIT analysis is then used to constrain the postfracture well-test analysis in a numerical-simulation model that includes fracture-filtrate-cleanup modeling. Post-fracture analysis showed that the initial proppant-pack damage is high and effective fracture length is much smaller than the created length, especially with crosslinked gel. The crosslinked-gel treatment was not able to clean up effectively, and therefore showed limited stimulation effectiveness. The first-ever water fracture in a gas reservoir performed in Europe showed a more significant production improvement during the short post-fracture test. The results from this well suggest that, as in North America, water fractures appear to have better initial production than crosslinked-gel fractures because of better fracture cleanup in European reservoirs with microdarcy permeability.