Semi-analytical and numerical investigation of hydraulic fracturing and geological barrier integrity based on a case study

Abstract

In this paper, numerical and semi-analytical investigations were conducted to understand the hydraulic fracturing operation in the tight gas reservoir only identified by the code A7 in the North German Basin. Two simulators, FLAC3Dplus (numerical, full 3D model) and MFrac (semi-analytical, modified model based on the conventional pseudo-3D model), were used to model the fracturing operations including fracture propagation, proppant transport and settling. A comparison of the two simulators was carried out through simulations. Meanwhile, the function of the geological barrier integrity in A7 was also studied and confirmed. The simulations were based on the history matching of the in situ measured well head pressure. At the end of the simulation, a long fracture (length ≫ height) was modeled by both simulators. Although the results express some differences in the modeled fracture length and width (average), their results for fracture pressure, fluid leak off and proppant distribution are comparable. That means they would provide similar productivity in the later production. Investigation of the geological barrier integrity confirms that the cap rock, formed from rock salt and anhydrite, normally has higher minimum horizontal stress and lower permeability providing enough resistance to prevent the fracture from propagating in the vertical direction of the cap rocks. The case study reveals that, even when the injection volume was increased 10 times the initial volume, the integrity of the cap rocks could not be broken. Despite the presence of interbedded shale formations, in the reservoir as those of the cap rock an impediment function, if their thicknesses are too small to prevent their breakage and the injection volume too large for them to resist fracturing.