The Kharyaga field is located in Timan-Petchora region of Northern Russia, 60 km North of the Arctic Polar Circle. The field is producing principally from a Devonian age carbonate reservoir, this limestone formation having an average porosity of 8 to 12% and permeabilities ranging from 1 to 200 mD. The wells are completed as cased-hole, with a 7inch liner through the reservoir section. The perforated intervals range from 30 to 80 m in length and the wells were traditionally matrix-acid stimulated following perforation. The wells produced initially unassisted then later on, a work-over campaign was launched in order to equip the well with electrical submersible pumps (ESP).

Work-over operations were taken as an opportunity to re-stimulate lower-performing wells of the field. Several options were considered for this purpose from mechanically-diverted acid squeeze to propped hydraulic-fracturing or acid-fracturing this later option being ultimately selected. However, the rather adverse conditions existing both downhole and at surface rendered such an operation quite challenging. In the first place, the tectonically active nature of this region (West of Ural) led to anticipate significantly high fracturing gradients and difficulties to initiate a fracturing regime. Secondly, the weather conditions, with surface temperatures ranging from +20°C in summer to -50°C in winter and logistical issues necessitated adjustments to the fracturing fluids and job design. Finally other factors such as a high tendency of the Khariaga crude to precipitate wax, the presence of H2S or the flow-back through the ESP called for additional precautions.

In this paper we will show that despite these rather adverse conditions successful acid-fracturing treatments can be implemented in this region; we will discuss some of the key parameters that helped base the choice of treatment type and we will describe the lessons learnt during such treatments.

In addition, as the data-set available for this acid-fracturing treatment is unusually comprehensive with image logs, production profiles, pre and post-frac thermal logs, bottom-hole gauges and extended post-frac decline, an integrated evaluation of the treatment will be proposed.

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