Aggressive concentrations of CARBOHSP reached 10-16 ppa in TSO designs in the Rotliegend reservoirs, increasing production by 4.75 fold. Extensive welltesting indicate that high conductivity fracs performed better than model predictions, doubling kh by enhancing vertical transmissibility and eliminating fines production.
The main production of German gas fields is from sandstones of Upper Rotliegend age (Permian period). The Upper Rotliegend is characterised by a sequence of sandstones, siltstones and shale beds. Most of the present gas fields produce from low permeability layers (mainly 0.1-20mD) in a high pressure and high temperature environment. Hydraulic fracture stimulation has been developed to a regular completion concept in the last 15 years. Fracture stimulations, if applicable, are planned in all new drilling projects as a standard procedure to enhance the productivity of a well.
Due to the nature of the Upper Rotliegend depositional environment, a vertical to horizontal permeability anisotropy of about 1/10 can occur within the sandstones. Hydraulic fracture stimulations, in comparison to perforations, have proven to (i) bridge vertical flow barriers within sandstone/ siltstone sequences as well as (ii) to improve the connectivity of the stimulated layers horizontally. The result is an overall permeability capacity increase, vertically as well as horizontally. In addition, the effective productivity index enhancement pre and post frac, the fold of increase (FOI), is usually higher than theoretically expected from hydraulic fracturing.
Based on an “effective wellbore radius concept” to calculate the inflow performances of a fractured well, sensitivity studies were carried out to investigate the stimulation treatment results under different reservoir conditions. The main parameters, which have an impact on the fold of increase, are discussed.
Based on transient well test analysis, 20 fracture treatments are compared with a theoretical radial inflow model with respect to their expected and observed productivity enhancement.
Author(s): SPE, SPE, M. Koehler, Gaz de France Produktion Exploration Deutschland GmbH
Paper Number: SPE 94250