Success in matrix acid stimulation of carbonate formations depends largely on the acid distribution and the created wormhole length. Treatment design is still as much art as science, especially where field experience is used to quantify the design parameters for candidate wells. However, each well is different, and this generic approach for treatment design can result in under- or over-treating. Another challenge for long horizontal well matrix acidizing is the distribution of acids throughout a long horizontal well. Recently, controlling the perforation density and distribution has been implemented in long horizontal well matrix acidizing and proved successful in improving well productivity. This approach basically involved pumping acid through a perforated work string inside a large open-hole area. The acid distribution with limited entry into the surrounding a horizontal well is mainly controlled by perforation distribution around the work string.
In this paper, a fluid placement simulator is used to study the key parameters in optimal acid distribution with this novel limited-entry technique in long horizontal well matrix acidizing. The algorithm is based on a 2-D radial flow system that iterates time to quantify wormhole propagation and injection rate. Wormhole velocity is calculated by a laboratory empirical model that depends on two parameters measured from core flow testing. Therefore, the algorithm captures the essential physics and chemistry of the acid reaction in a carbonate porous medium. Also, this model includes such effects as multiple formation layers with independent formation parameters and allows for modeling zonal coverage, zone skin factor, and total skin factor. The main difference in terms of the improvement of formation damage from both openhole and this limited-entry completion is investigated.
The developed algorithm is a relatively simple tool that can be used by field engineers to design and optimize an acid treatment, addressing key questions such as: What type of acid and what strength should be used? What are the optimum acid volume and pump rate? What are the best diversion method and staging design? This paper demonstrates the key issues related to the interaction between the wormhole model and zonal coverage. In addition, different scenarios and a field case example are presented and discussed.