Schlumberger

Industry Article: Continuous Improvements in Acid Fracturing at Lake Maracaibo

Publication: Journal of Petroleum Technology
Volume: July
Issue: 2006
Publication Date: 07/01/2006

The Maraca limestone formation is a part of the Cogollo carbonate group below Lake Maracaibo, Venezuela. Acid fracturing with an organic-acid system has proved to be a pivotal completion strategy for achieving higher productivities and increasing reserves. The main challenges are the relatively high hydrogen sulfide (H2S) content, 280 degF reservoir temperature, the asphaltenic nature of the crude, and the long completion tubing that limits pump rate. The Maraca limestone formation is a 20- to 30-ft upper member of the Cogollo carbonate group at 15,000 to 16,000 ft in the Urdaneta West field below Lake Maracaibo, Venezuela. Reservoir volumes and productivity are attributed mainly to matrix development, in contrast to the rest of the Cogollo, where natural fractures play a major role in well production. When stimulation and development strategy began focusing on the Maraca group, a number of matrix stimulations were performed, most using coiled tubing at low pumping rates and volumes. The first few treatments used 15% hyrodochloric acid (HCl), but the acid system was switched quickly to an organic acid blend (13% acetic and 9% formic) because of the high reactivity of the HCl and its tendency to form sludge after contacting the asphaltenic Maraca oil. These matrix treatments had modest results, and the gained productivity was lost after a few months of production.