Continuous Improvements in Acid Fracturing at Lake Maracaibo

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 96531, "Acid Fracturing in Lake Maracaibo: How Continuous Improvements Kept on Raising the Expectation Bar," by B. Burgos, Shell Venezuela S.A.; M. Buijse, SPE, Shell Intl. E&P; E. Fonseca, Shell Venezuela S.A.; and A. Milne, M. Brady, SPE, and R. Olvera, Schlumberger, prepared for the 2005 SPE Annual Technical Conference and Exhibition, Dallas, 9-12 October. 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°F reservoir temperature, the asphaltenic nature of the crude, and the long completion tubing that limits pump rate. Introduction 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 with 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 (CT) 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. Acid Fracturing With Organic Acid Acid fracturing was considered as an alternative to stimulate Maraca wells. Not only would acid fracturing increase well productivity, but it also would help retain the generated conductivity for a longer time period. Acid fracturing is a stimulation technique where acid, usually HCl, is injected into the reservoir at fracturing pressures. As the acid flows along the fracture, the fracture face is dissolved in a nonuniform manner, creating conductive or etched channels that remain open when the fracture closes. The effective fracture length is a function of the type of acid used, the acid reaction rate, and the fluid loss from the fracture into the formation. Acid fracturing was preferred to hydraulic fracturing because proppant cleanout in a sour well with CT required operational and safety resources not yet in place. Additionally, the high conductivity of an acid-etched fracture made acid fracturing a more attractive technique if comparable fracture lengths could be achieved.