Performance Analysis of Acid Fracturing Treatments in a Sour Gas Carbonate Reservoir

Abstract

This paper presents a new insight into the factors that affect the initial productivity of Acid Fracturing (AF) treatments in a sour gas carbonate reservoir. Acid Fracturing is a stimulation technique used to improve the productivity or injectivity of wells. It involves pumping acid into a formation at above fracture pressure, and can treat deep into the formation. Despite the benefits of acid fracturing, conditions are not always favourable for acid fracturing. This could be due to reservoir or well limitations for example, very low formation permeability, formation temperature especially in dolomite formations, and/or well completion integrity. In this application, a total of 12 wells were stimulated after initial completion by acid fracturing. The treatment design is a single stage Pad-Acid treatment which involves pumping 1) pre-acid, 2) gelled water, 3) main acid, 4) diverter, 5) repeat of steps 2 to 4 as required, then followed by 6) a post-acid flush and 7) over-flush. Optimisations made on the treatment design as the campaign progressed include use of specially formulated diversion system, ramp up of acid injection rates and volumes. The analysis of pre and post stimulation productivity performance show the following: An average productivity improvement of 9 times the pre stimulation productivity,A direct relationship between productivity improvement and increasing the rates/volumes of acid injection during the treatment,That treatment volume did not always result to productivity increase but the manner and sequence of fluid injection were of greater effect on productivity.Fluid diversion system blended with particulates show greater increase in productivity than wells treated with diversion systems with no particulates. Based on the results, it can be concluded that, an optimized method of ramping up acid injection rate/volume as well as careful selection of diversion system can lead to productivity improvement of up to 9 times the pretreatment productivity in Carbonate reservoir stimulation.