Productivity of Frac Stimulations in the German Rotliegend: Theoretical Considerations and Practical Results
- Michael Koehler (Gaz de France Prod Expl Deutschland GmbH)
- Document ID
- Society of Petroleum Engineers
- SPE Europec/EAGE Annual Conference, 13-16 June, Madrid, Spain
- Publication Date
- Document Type
- Conference Paper
- 2005. Society of Petroleum Engineers
- 5.1.1 Exploration, Development, Structural Geology, 5.3.2 Multiphase Flow, 5.6.4 Drillstem/Well Testing, 1.2.3 Rock properties, 4.1.2 Separation and Treating, 1.6 Drilling Operations, 5.6.3 Pressure Transient Testing, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.6.8 Well Performance Monitoring, Inflow Performance, 2.2.2 Perforating, 2.5.2 Fracturing Materials (Fluids, Proppant), 2.5.1 Fracture design and containment, 2.5.4 Multistage Fracturing, 1.8 Formation Damage
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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.
Hydraulic fracture stimulation in the German Upper Rotliegend reservoirs started in the early 80's in deep gas wells. The reservoirs are situated in the southern Rotliegend basin, which developed between the Variscan mountain belt in the south and the Ringkøbing - Fyn High in the north. The Rotliegend basin stretches from England to Poland. It contains cyclic continental sediment sequences which formed in a rain shadow tectonic desert environment, as currently existing in North Africa, NW China or the western USA (GAST).
The potential sandstones consist of shore-parallel beach sands of the Rotliegend saline lake. The sandstone belts are stacked upon each other, sub-divided by shales - forming cycles.
The main potential sandstones are part of the Strackholt Einloh, Ebstorf and Wustrow members. The shore-parallel sandstones are mostly well sorted and fine to medium grained. In direction of the lake center, the sediments interfinger with playa flat sediments. In the landwards direction, lagoonal or alluvial fan sedimentation occurs. Lateral barriers and facies changes acting as no flow/ low transmissible boundaries are often observed.
Some typical general characteristics of the Rotliegend sands are:
Direct correlation of permeability and porosity only exists within individual facies bodies, global corre¬lations do no apply.
Even in areas with moderate porosity the rocks can have extremely low permeabilities, mainly due to clay mineral growth in the pore space.
The reservoir quality is steared by the primary depositional attributes and secondary diagenetic alterations.
Compartmentalisation can occur, which prohibits even fractured wells to produce from an economically sufficient drainage area.
Sandstone, siltstone and shale sequences frequently develop in the following main depositional environ¬ments: alluvial fans, dry aeolian dunes, dry/wet sandflats and sheetfloots sequences. In those environments significant vertical flow barriers are abundant. Those transmissible barriers prevent the communication of the entire reservoir net pay section with the well through the perforations only.
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