Abstract

The paper evaluates the design considerations and execution results of the first commercial field trial of a novel acid fracturing technique, using encapsulated citric acid as the etching agent. The treatment was executed in the Maraca tight carbonate oil reservoir (16,000 ft, 270°F) under Lake Maracaibo, Venezuela. The main driver for this trial with an alternative acid was to obtain a significant increase in etched fracture length, over the current standard acid formulation for this reservoir of acetic and formic acid. The encapsulated citric acid is solid at temperatures up to 180°F and coated with vegetable oil to avoid dissolution in the carrier fluid. The job was designed for a target etched half-length in excess of 300 ft. The job was an operational success, placing 30,000 lbs of the solid acid in the formation. Post-treatment results, however, did not show any improvement in production from the test well. The paper also reviews several of the possibilities for the failure of the treatment and the limitations of the technology as an effective acid for fracturing carbonate formations.

Introduction

The Cogollo reservoir is a tough carbonate play under Lake Maracaibo in Venezuela. It consists of three formations: the Maraca, Lisure and Apon, which together have a total thickness of some 1,000 ft. The main producer out of the three is the porous Maraca formation. It is also the shallowest of the three (15,500–16,000 ft), having 20–30 ft of net pay and about 100 ft of gross height. The oil reservoir (28–30 API) is mainly composed of calcite (over 90%) and the reservoir temperature varies between 260 and 280 °F.

Current development strategy for the reservoir focuses on the development of the Maraca formation, targeting high porosity areas with vertical wells completed with a cased and cemented liner and stimulated during initial completion through acid fracturing. Acid fracturing has been a successful method to increase well productivity and make the development of the Maraca reservoir economical.

As a result of the low to moderate permeability of most wells in this area (2–25 mD), the main challenge in an acid fracture treatment is maximizing etched fracture length to maximize productivity. At this permeability level, fracture conductivity plays a smaller role in productivity, as acid-etched fractures in competent rocks tend to be extremely conductive. A successful campaign of twelve acid fractured wells was completed using a gelled organic acid blend of 13% acetic and 9% formic acid. Organic acids were selected as a result of their natural retardation, easier corrosion inhibition1 and better compatibility with formation fluids. Conventional HCl systems were discarded because of their high tendency to form heavy sludge upon contact with the reservoir fluid2. Even though the campaign was very successful (field production was effectively doubled in two years), computer simulations indicated effective half-lengths of only 40–50 ft, suggesting there was potential for a more retarded system to significantly enhance productivity.

Encapsulated citric acid (ECA) was envisioned as a viable alternative in the quest for longer fractures. This acid is coated in a vegetable oil that keeps it from dissolving in water-based fluids until the capsules start melting at around 180°F. The job design aimed at obtaining fracture half-lengths in excess of 300 ft by placing the citric acid in a way similar to a conventional propped fracture treatment.

Candidate Well Selection

The well selected for the trial was a typical oil well in the area with an average permeability of 8 mD completed in the Maraca interval only, with a net pay of 20 ft. Previously, the well had been successfully acid fractured with 13% acetic and 9% formic acid, but after 2 years of production the productivity index in the well had declined 33%, attributed to a significant loss in fracture conductivity.

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