Introduction of an Innovative Acid-Fracturing System to Edwards Limestone in South Texas
Authors: Veillette Christian Patrick , Cuzella Jerome , Mueller Fred A. , Loayza Michael Paul , Still John W. , Gurmen M. Nihat 
Published: May 2016

Carbonate reservoirs hold more than 60% of world's oil and 40% of world's gas reserves. Increasing demand along with decreasing reserves push the exploration and production activities to deeper and hotter zones in these prolific reservoirs creating a demanding environment for stimulation. The industry needs innovative products and associated technologies to address this challenge and meet future energy requirements. This paper presents one such technology, a solid acid product and its introduction to the Edwards Limestone in South Texas.

The Edwards Limestone wells are typically completed horizontally and often require hydraulic fracture stimulation to be commercially productive. The relatively low permeability (0.008 - 0.319 mD as Kair), high Young's Modulus (5.0E6 - 5.5E6 psi), presence of natural fractures, minimal stress barriers to control height growth, and formation temperatures up to 375 °F constitute a challenging environment for a successful stimulation treatment. Conventional stimulation approaches using acid fracturing and proppant fracturing techniques both have delivered acceptable results. However, they both have some drawbacks preventing them from being optimized solutions. The conventional fluid systems for acid fracturing increased the production, in most cases. However, the wells have suffered from a very steep decline in production. The steep decline after these treatments can be attributed to the minimal etched fracture half length developed due to rapid acid spending in the near wellbore. Moreover, the rapid spending in the near wellbore area also results in the loss of the etched fracture conductivity as closure pressures increase. The conventional fluid systems for proppant fracturing have resulted in better production results than the conventional acid treatments in this area. However, the production increase was still below expectations for horizontal wells. In addition, the proppant admittance problems required pumping very viscous treatment fluids at high rates bringing a fresh set of predicaments: high horse power requirements and water production due to excessive height growth.

As the conventional approaches failed to provide optimal results, the search ensued for an innovative technology to enhance production. One such technology is the solid acid product. This stimulation product is an inert material at surface conditions that is pumped as slurry in an operation similar to a typical proppant fracturing treatment. The product hydrolyzes only after the solid particles are in the reservoir at downhole conditions exposed to heat and water for a certain time. This enables the fracture etching process to occur during shut-in and flowback of the well thus solving the problem of not creating long effective etched fracture half lengths with acid. The dissolving capacity of the hydrolyzed product is similar to acetic acid. Because the material is inert at surface conditions, it does not result in any tubular corrosion during the pumping process. Furthermore, the health, safety, and environmental issues associated with live-acid handling on the surface are eliminated. The product is a lightweight material with a specific gravity of 1.25, so a viscous fluid is not required to suspend or carry the product; hence water production risk is reduced due to excessive height growth.

The innovative solid acid product provided both production enhancement and operational advantages in the Edwards Limestone in South Texas. These results are supported by laboratory tests and case studies from multiple-well field trials. The laboratory results include product information and its interaction with core samples whereas the case studies provide the details on design, execution, post-stimulation, and long-term results. In conclusion, the effectiveness of this innovative product demonstrated on the Edwards Limestone of South Texas could have worldwide application on acid fracturing candidates of similar lithologies and reservoir conditions.