The Macaé formation (Cretaceous age, Campos Basin, Brazil) is predominantly an oolitic and oncolitic grainstone and packstone limestone with a bottomhole static temperature (BHST) of about 150°F. The formation permeability often exceeds one darcy. The mineralogy is composed of calcite (98 to 99%) with about 1% halite and < 1% quartz; therefore, the formation is a potential candidate for acid stimulation. This limestone is atypical because of its texture—a granular aggregation of carbonate particles, poor cementation, and a moderate-to-low rock strength. Core analysis and rock mechanics assessment revealed that much of the rock is weak and potentially prone to deconsolidation after acid stimulation. Weakening of the rock matrix often leads to borehole instability and loss of wellbore integrity at the anticipated drawdown required to meet completion objectives.

The two exploratory wells in this study were cased with a perforation density of about 5 shots per foot (spf) over relatively large perforated intervals spanning 180 ft and 269 ft, respectively. Hence, adequate diversion of acid into the desired zones was a technical challenge. In addition, the crude oil was heavy (17 to 21°API), and compatibility with the stimulation fluids was a concern. In this paper, we demonstrate the fluid selection process and its field implementation supported by integration of mechanical rock testing, core-flow, and solids migration tests using preserved core. Results indicate a high dependency of the mechanical strength on acid type. Based on the laboratory results, an optimized fluid system was selected that showed the best compromise for maintaining rock integrity, fluid efficiency, zonal coverage and fluid compatibility.

Both wells responded well to the prescribed treatment: Well A tested an 269 ft measured depth (MD) or 194 ft true vertical depth (TVD) perforated interval on natural flow at approximately 600 barrels oil per day (BOPD) with a productivity index (PI) of 2 bbl/D/psi, good reservoir quality, and a negative post-acid skin value (s~ -4).

Well B tested a 180 ft MD/TVD perforated interval with an electric submersible pump (ESP) at stabilized bottomhole pressures at rates of 3,600 and 2,500 BOPD. The well exhibited a productivity index (PI) of 2.5 to 4 bbl/D/psi, extremely good reservoir quality (average 500-md), and a negative post-acid skin value (s~ -4).

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