Acid Fracturing in Lake Maracaibo: How Continuous Improvements Kept on Raising the Expectation Bar
Authors: Burgos G. , Buijse M. , Fonseca E. , Milne A. , Brady M. , Olvera R. 
Published: December 2007

The Maraca limestone formation is a thin (20-30 ft) upper member of the Cogollo carbonate group located at a depth of 15,000-16,000 ft in the Urdaneta West Field below Lake Maracaibo, Venezuela. Reservoir volumes and productivity are mainly attributed to matrix development, in contrast with the rest of the Cogollo, where natural fractures play a major role in well production. Porosity development is found in oyster rich wackstone/packstone facies of limited aerial extent.

The current development strategy for the Cogollo focuses on the Maraca member, which has porosities in the 5-14% range and low to moderate permeabilities (1-50 mD). Acid fracturing has proven to be a pivotal completion strategy for achieving higher productivities and also increased reserves by improving the drainage efficiency through the connection of the porous oyster mounds. The main challenges in this environment are the relatively high H2S content (2-3%) and reservoir temperature (280°F), the asphaltenic nature of the crude and the long completion tubings that limit the pump rate.

The initial stimulation strategy consisted of small HCl matrix treatments pumped with coiled tubing at low rates with modest results. The combination of sludge forming tendencies of the HCl when in contact with the crude, and the high reaction rates at downhole temperature, led to a switch to organic acids. A mixture of 13% acetic and 9% formic acid was used in matrix treatments with more encouraging results.

Having found a more compatible acid system, and after several successful matrix treatments, a decision was made to go for acid fracturing with the organic acid system as a way to achieve higher productivities. A multistaged (pad/polymer gelled acid) treatment was designed including a closed fracture acidizing (CFA) stage to maximize fracture conductivity. A total of 13 treatments were pumped with this design, achieving consistently much higher productivities than those after matrix treatments.

After evaluating the results of this campaign it was determined that higher productivities could be achieved -especially in the higher permeability wells- if fracture conductivity was increased.5% HCl was incorporated in the acid mix to increase reactivity and dissolving power especially in the near wellbore area and three wells were treated with the new acid mix, two of which were re-treatments.

Further studies of the acid mix led to the removal of the acetic acid from the mixture, and lab work showed that with the right additives the HCl concentration could be increased. A new mixture of 11% formic and 7% HCl was deemed to have similar live acid penetration capabilities than the previous acids, but with much higher dissolving power. Additionally the acid system was combined with a visco-elastic surfactant to provide greatly improved acid leak-off control in the fracture.

A total of 12 treatments with the new system have been performed, resulting in a 50-100% improvement in productivity when compared to the original mixture, having a great impact in overall field production. The paper presents the details of the design changes, lab testing and post-job analysis performed as well as further possible improvements to the technique.