Abstract

Conventional stimulation techniques used to treat double porosity type (low permeability / hairline fracture) carbonate formations are met with limited success where most regular acid fracture treatments yield approximately the same results as regular matrix acid treatments of a much smaller scale.

This paper outlines the details & results pertaining to some recent successful applications of a new technique whereby Coiled Tubing is used to create multiple, relatively short-length lateral holes into a carbonate reservoir using nothing but hydrochloric acid pumped through a specially designed jetting assembly. The discussed jobs were the first ever field applications of Coiled Tubing "Acid Tunneling" and the results showed one operator that this method represents a viable alternative to conventional stimulation treatments in said reservoirs.

Results indicate that this technique may be applied to all carbonate reservoirs, regardless of their inherent petrophysical properties, where a different approach to stimulation treatments is required.

Discussed are formation characteristics, acid volumes and strengths, job parameters witnessed, tools used and results obtained to date.

Field and Formation Characteristics

It is estimated that carbonate formations account for approximately 35% of the world's petroleum reservoirs yet these same reservoirs are estimated to contain about 60–70% of the world's traditional hydrocarbon reserves. Important carbonate reservoirs are found in most parts of the world with some notable ones being located in Sumatra, Borneo, China, Brazil and Venezuela. Carbonate rocks are typically low porosity/low permeability and naturally fractured, but offer two particularly useful but often-overlooked properties: mechanical integrity and high solubility in acids. Typical solubility of carbonates in hydrochloric acid is greater than 95% and often exceeds 99.5%.

The Mara field in Western Venezuela (Northwest Maracaibo Basin) is one of these petroleum producing areas that contain massive carbonate formations. This field presently has about 400 wells, of which 95 are actively being produced by means of Electric Submersible Pumps (ESP) and/or Progressive Cavity Pumps (PCP) and all of these wells are produced from the Cogollo lower Cretaceous group (95% Carbonates with a variety of micro fractures), which is comprised of the following formations:

  1. Maraca: Glauconitic and calcareous clays at the base of the formation with bioclastic massive limestone accounting for the upper portions of the zone.

  2. Lisure: Glauconitic clays, glauconitic sandy limestone with traces of shale.

  3. Apon: Black bituminous laminar limestone.

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