Productivity Impact From Matrix Stimulating Thick Intervals Using a Nondamaging Self-Diverting Acidizing System: A Northwestern Kazakhstan Case Study

Abstract

Abstract A carbonate formation in northwestern Kazakhstan is a challenging environment for acid stimulation because of its thick intervals, heterogeneity, and natural fractures. From production log interpretation, it was observed that the layers were not being depleted uniformly because of the difference in flow properties, resulting in an uneven distribution of flow from perforations. The challenge was to achieve a more uniform flow profile. The paper will show the effect a self-diverting acidizing system has brought to field productivity when benchmarked to historical treatment methodologies. A net gain of 1900 cubic meters per day (m[3]/d) was realized as a result of the treatment campaign on 17 wells. The benefits to reservoir management from treating the heterogeneous long intervals and achieving uniform flow profiles will be discussed in detail through a comprehensive review of pre- and post-treatment production profile logs. The new treatment methodology employed has been used in freshly drilled wells. The paper will discuss the effect on productivity as well as how the methodology has been incorporated into subsequent field development planning. Introduction Acid treatments in Kazakhstan are performed in both carbonates and sandstones. Typically stimulations are executed with a small volume of hydrochloric or hydrofluoric acid, often without very good results. Diversion techniques are usually not used. General reasons for unsatisfactory treatments include the reactive fluid volume placed and the diversion of acid into producing zones. Another reason is associated with acid placement. Usually it is bullheading, which leads to spending acid inside the wellbore and placing reaction products into the formation, which damages it even further. Up to now only a few wells in a few fields in Kazakhstan have been treated with self-diverting acid, all in carbonate reservoirs: Alibekmola (more than 20), South Alibekmola (2) and Zhanazhol (1). Worldwide, viscoelastic acid was recently used on more than 700 wells, proving its success. Field Description Alibekmola Field. The Alibekmola oil-gas condensate field is in Mugodzhary region of Aktubinsk province (Fig. 1). It is in a brachyanticlinal fold of submeridional orientation that is complicated by two domes at the top of the second carbonate bed (KT-II) of Middle-Lower Carboniferous age (Fig. 2). The northern dome has been outlined by the isohypse -2950 m and measures 4.7 × 2 km with a displacement of 70 m. The southern dome is also enclosed by the isohypse -2950 m; it measures 7.8 × 3 km with a displacement of about 300 m. Productive beds KT-II and KT-I are separated by terrigenouse-carbonate rocks in the lower part of Moscovian stage (Podolsky horizon). Productive formations in KT-II consist of dark grey, organic-detrital, fissured limestones with interlayers of dolomitized limestones, dolomites, and rare interlayers of terrigenouse rocks. The reservoir is bound by tectonic faults on the western and eastern flanks. [1] The reservoir rocks have both matrix and fracture porosities. Open porosity varies from 4 to 15%. The oils in KT-II are light, with a density of 822–835 kg/m[3]. They contain sulfur (0.73–1.48%), paraffin (1.2–1.35%), and resin (2.86–11.9%). Bottomhole temperatures are 65–70°C and formation pressures range between 300 and 330 atm. Permeability ranges from 0.1 to 25 md. Alibekmola wells have as many as 30 perforated intervals over as much as 400 m length.