Tuesday, December 12, 2017

The Stimulated Completion

Stimulated completions fall into two categories , acidizing and hydraulic fracturing. Occasionally, the two are combined in an acid-frac, which improves productivity by using acid to etch surfaces of hydraulically induced fractures, preventing flull closure.

Success of stimulation depends largely on how well the perforation allows delivery of treatment fluids and frac pressures into the reservoir. Because these fluids and pressure induced fractures are intended to move beyond the perforation, shot phasing, density and hole diameter are of higher priority that depth of penetration. Underbalance perforating is often used because cleaner perforation tunels give fluids more direct paths to the reservoir.  In some cases, such as TCP with high shot density guns, underbalance can be increased to where stimulation is not required to improve productivity. However, stimulated reservoirs are usually of low permeability, greatly limiting the surge available to clean the perforations. Further increases in underbalance may achieve no improvement in cleaning.

 Uniformity of perforation diameter is essential to accurately determine the cumulative area of the casing entrance holes. Knowing this are and pumping pressure allows calculation of flow rate into the formation, needed to monitor progress of the stimulation. 

 A number of studies have investigated the relationship between perforation phasing and the development of hydraulic fractures. In general, hydraulic fractures propagate normal to the minimum stress in the portion of the reservoir undisturbed by the presence of the wellbore. The general conclusion is that for an ideal fracture job, perforations are aligned with the maximum stress direction, so fractures extending from the perforations will lie in the plane that has the least resistance to opening.

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