Successful Stimulation of Sandstone Formations through a Comprehensive Approach to Reservoir Characterization and Understanding in the Bachaquero Field, Lake Maracaibo
Authors: Martin Alberto , Manuel Santaella Jose , Daniel Martinez Marcos , Prieto Argenis 
Published: March 2017

The Bachaquero field is located on the east side of Lake Maracaibo, where oil exploitation has been occurring for more than 50 years. Primarily composed of sandstone, most of the producing reservoirs in this formation are from the Tertiary period and can be found in the Miocene Epoch. Nonconsolidated and poorly consolidated sands are also common in this field.

Complex mineralogy has been identified as the primary cause of production decline for wells in this field, with fines migration being the principal mechanism of formation damage. Other factors also influence the decline in production rate, including high-permeability formations, salinity, pH changes, and drag forces caused by fluid-flow velocity, multiphase flow, turbulence, and fluid viscosity.

Chemical stimulation has become a useful technique for enhancing production, and matrix acidizing with hydrofluoric-acid (HF) systems has proven to be very effective in this field. Matrix stimulation is a technique that has been used extensively since the 1930s to improve production from oil and gas wells and to improve injection into injection wells. Matrix stimulation is accomplished by injecting a fluid to dissolve and/or disperse materials that impair well production in sandstones or to create new, unimpaired flow channels between the wellbore and a carbonate formation. In matrix stimulation, fluids are injected below the fracturing pressure of the formation. Substantial production improvements can be achieved with matrix stimulation if treatments are engineered properly.

It is well known that HF reacts with clays present in the reservoir to dissolve them and restore original permeability, but some of those reactions are not always desired. Secondary and tertiary reactions of HF with aluminosilicates can promote nonsoluble fluorosilicates precipitation, which requires that fluids be tailored for compatibility with the formation's mineralogy. Variations in mineralogy determine which fluid performs better, and a high presence of feldspars requires more conservative treatments to avoid undesirable precipitations. A stronger retarded HF (RHF) has also been used to treat wells that are deeper in the formation.

Other good practices in addition to primary acid selection are also applied to help ensure treatment success. The stimulation treatment design includes pumping formation conditioning fluids before and after the main acid; using different types of organic solvents to dissolve asphaltene deposits in the well; performing NWB, HCl, and HCl/organic acid blend preflushes and post flushes to treat calcium deposits and control pH and iron precipitation in the reservoir; achieving short-term clay inhibition and long-term clay stabilization; and using other fluids, such as relative permeability modifiers (RPMs) for water-control applications and diversion of treatment in laminar reservoirs with petrophysical heterogeneities. Each of these combined practices have resulted in successful stimulation of the field.

This paper discusses a comprehensive approach that has been successfully applied in wells located in the Bachaquero field in the Maracaibo basin. The workflow includes a candidate analysis, from the reservoir description, mineralogy, and identification of the formation damage mechanism, to stimulation treatment design, laboratory fluid-systems tailoring, placement and diversion techniques, pretreatment operational task fulfillment, field execution, quality control, and post-job evaluation through analysis of records and statistics.