Comparative Study of Pressure Dependant Permeability Application for Modelling and Forecast Estimation of Well Performance in Microfractured Carbonate Formations
Authors: Gopa Konstantin , Sazonov Evgenii 
Published: October 2015

Carbonate formations of the Turney deposits in Bashkortostan contatian a significan part of the hydrocarbon reserves of the region. They become an object of a growing interest for Bashneft – oil operator in the region, as main objects of oil production such as TTNK formations are being depleted. Carbonate formations were deposited at the shelf of a paleosea during period of the early Carboniferous and are mainly presented by ramps and reef build-ups. Rocks are mostly composited by a carbonate matrix and a system of poorly interconnected natural microfractures which could be seen in the thin sections but are hardly visible in the core. Formation of the microfractures most likely occurred due to secondary processes, since there were no major tectonic movements after depostiton and during diogenesis.

Development of such reservoirs is complicated due to a high uncertainty of the formation properties, namely spatial distribution and characteristics of the natural fractures. A distinctive well production profile has high rates at the start and then a rapid decline in 3-5 times during following 4-8 months having an almost constant depression. All wells are set into production with acidizing treatment which presumably activates and connects scattered system of natural fractures in the vicinity of the well. After the system of natural fractures is drained, the well proceeds to the pdeudo-steady depleting regime of the low permeable matrix.

For modeling of the well fluid inflow in described conditions there are ussualy used models of single or dual medium. In case of a single medium, permeability in the model would be effective permeability of the combined "matrix plus fractures" medium. In that case, in order to tune model on the high starting rates, the effective permeability of the whole system would be set too high. Morever setting of negative skin-factors and positive to reproduce measured BHP dynamic would be necessary. Such an approach, as the practice shows, does not always allow reproducing production history correctly and hardly applicable for the forecast of new well.

Dual medium models in their classic version (dual porosity, dual porosity/permeabity) also do not allow to reproduce described above processes. They were designed foremost to describe fractures systems having more structured and regularized setup, in the formations where natural fractures were created mainly due to tectonic events. It is almost imposible to simulate the effect of fractures depletion and also prevent brealthrough of the injection water by high permeable fractures.

Application of the pressure dependant fracture permeability method is described in this work. In the model of a dual porosity/permeability of the real formation, we apply pressure dependence for the fracture permeability. When the well starts, after its aicid stimulation, microfractures in the vicinity of the well are being connected into the united system and work with their initial permeability providing high starting rates. After fractures depletion and following pressure decline, permeability of fractures decreases according to the pressure-permeability relation, their impact on fluid filtration reduces and main well inflow is provided by low permeable matrix. This fact is easily seen in historical data – rapid drop of the rates and pressure decline in the near wellbore zone.

This method is applied for the history matching of the one of the carbonate oilfield of Bashkortostan. Comparative analysis with other fluid flow modeling methods in carbonate microfractured formations shows an advantage of the presented method as well as for the history matching and forecast performance estimation of the new wells.