One of the critical measures of success in matrix acidizing is to get the acid distributed across all the zones of interest, thus ensuring they are all stimulated and maximum production is achieved. Because most producing zones are not homogeneous in terms of permeability, porosity, water saturation, or the degree of damage they have experienced, uniform distribution of acid during the acid job is seldom achieved. This task is further complicated when high water-saturation zones form part of the pay interval; not only do the producing zones get inadequate stimulation but the water zones get preferentially stimulated to produce large quantities of unwanted water that can kill the well or increase the lifting and disposal costs for the well.
Many acid-diversion techniques, mechanical and chemical, are available to help achieve even distribution of acid, but addressing water control and stimulation of the preferred zones in one treatment is uncommon. A new acid-diversion technique using associate polymer technology (APT) applied in the western-desert region of Egypt achieved tremendous results when compared to similar wells treated in the area. In one case, the production results from the well treated with APT were as high as 5,000 BFPD with negligible (0.02%) water cut compared to another well that produced similar fluid quantities, but with more than 50% water cut.
This paper discusses the first use of APT technology in the Al-Amein dolomite reservoir of Egypt and compares it with the earlier uses of other diverting methods in the same field. Acid-job simulation results obtained from a matrix-acid job simulator is also presented to demonstrate the ability to design such jobs before execution and to optimize the placement technique and the choice of stimulation fluids for a reservoir.
The Razak field was discovered in August 2002 in the western desert of Egypt (Fig. 1). The Razak field is in a 7-km radius area containing Razak and North Razak wells. This field is producing from the Al-amein dolomite (Figs. 2 and 3) of the Alamein carbonate member, consisting mainly of carbonates with subordinate shales. The unit ranges from 235 to 245 ft thick.
Based on cores, the Alamein dolomite of Razak field is a massive, hard to very hard dolostone. It is mostly medium-grain sized crystalline, fractured, and vuggy. Because the porosity is both intercrystalline and vuggy, porosity in clean sections ranges from 4 to 10%, and permeability ranges from 33 to 222 md. Water saturation in pay zones is around 15%, and initial estimated oil recovery is about 33% with a very active water drive.
Because the well NR-1 did not flow before an acid stimulation, a stimulation job was conducted. Because this was the first well to be acidized, the importance of diverter stages was not well recognized at that time. Post-stimulation pressure buildup analysis on Well NR-1 showed inadequate production from the well and high skin factor. Thereafter, Wells NR-2, NR-3, and NR-4 were stimulated with acid and diverter stages. The improvement in production was much better, as expected. The pressure-buildup analysis showed a negative skin factor. Also, a production logging tool (PLT) was run on Razak-28 (Fig. 4).