Case Study of Unconventional Gas Well Fracturing in Hungary

Abstract

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. Due to lower than expected stimulation effectiveness of the first fracture, and the confirmation of the low permeability, the 2 nd fracture treatment was changed to a waterfrac design. This formation clearly falls into the category 'unconventional', and consequently was a good candidate for a waterfrac.This paper describes the pre-frac diagnostics, fracture execution and post-frac production evaluation of this unconventional gas well. Special emphasis is placed on the use of small volume injection tests (DFIT) to obtain an estimate of the in-situ kh, since it is impossible to perform pre-fracture welltests in such a formation. The result of the DFIT analysis is then used to constrain the post-fracture welltest analysis in a numerical simulation model that includes fracture filtrate cleanup modeling. Post-frac 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 cleanup effectively, and therefore showed limited stimulation effectiveness. The first ever waterfrac in a gas reservoir performed in Europe showed a more significant production improvement during the short post-frac test. The results from this well suggest that, as in North America, waterfracs appear to have better initial production than crosslinked gel fracs due to better fracture cleanup in European reservoirs with micro Darcy permeability.

Rate enhancement in unconventional gas reservoirs by wettability alteration

“…For permeability variation k ¼ 0.01e10 md, m ¼ 10 À5 e4 Â 10 À5 Pa s, reservoir pressure P ¼ 5 Â 10 6 e10 8 Pa, and length of linear flow region towards the fracture L ¼ 10e250 m, the maximum velocity where the incompressibility assumption is valid varies from 5 Â 10 À8 to 0.1 m/s. Typical velocities at fracture faces in gas reservoirs have the order of 10 À5 e10 À4 m/s (see Shaoul et al, 2012). Therefore, there are some unconventional field cases where the incompressibility assumption is valid, and where it is not.…”

Effect of Wettability Alteration on Productivity Enhancement in Unconventional Gas Reservoirs: Application of Nanotechnology

“…For permeability variation k ϭ 0.01-10.0 md, ϭ 10 -5 -4ϫ10 -5 Pa.s, reservoir pressure P ϭ 5ϫ10 6 -10 8 Pa, and length of linear flow region towards the fracture Lϭ10 -250 m, the maximum velocity where the incompressibility assumption is valid varies from 5ϫ10 -8 to 0.1 m/s. Typical velocities at fracture faces in gas reservoirs have the order of 10 -5 -10 -4 m/s (see Shaoul et al, 2012). Therefore, there are some unconventional field cases where the incompressibility assumption is valid, and where it is not.…”