Optimization of Limited Entry Matrix Acid Stimulations with Laboratory Testing and Treatment Pressure Matching

Abstract

In the Danish Central Graben, tight chalk reservoirs have been successfully developed through drilling, completion and stimulation of long horizontal wells. One of the most successful technologies which has opened access to the far reaches of several major fields is the Controlled Acid Jetting (CAJ) technique. CAJ treatments are performed with a treatment pressure at formation below fracturing pressure and at the maximum rate possible. Diversion is via maximum rate/pressure, combined with the limited entry technique; therefore diversion stages are not normally included in CAJ treatments. A horizontal CAJ completion may range from 5,000 ft to as long as 20,000 ft, but is most often on the order of 10,000 ft. Acid coverage is normally 1 bbl/ft of 15% HCl. Injection rates as high as 40-50 BPM are often required to ensure complete coverage of the CAJ liner. There are still areas to be investigated to maintain an effective optimization of the CAJ technology. One of these areas to be investigated is the frictional pressure drop across the perforations (ΔPperf), which is influenced by the perforation coefficient (Cp). Usually in the design and operation of the stimulation, Cp is assigned a specific value when calculating the pressure drop across the perforations. Field experience and previous studies discovered that the Cp may change due to the viscosity of the fluid injected. This paper will present the results of a study of the perforation coefficient through laboratory experiments. A set-up for measuring the pressure drop across an orifice for different types of fluids at different flow rates was constructed. The perforation pressure drops were measured and then used to calculate the fluid specific Cp in order to clarify any changes in relation to the viscosity. The results of the laboratory study are utilized in a real-time CAJ stimulation diagnostic tool which has been used in several different chalk formations. The predicted acid coverage of the long CAJ liners is shown to be very sensitive to the assumed value of Cp. Overall, the outcome of this project was considered as a further contribution to the process of achieving a better understanding of the CAJ stimulation in general and about the impact of the perforation coefficient in specific.

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