"A Multiple-Zone Acid Stimulation Treatment Of A Horizontal Well, Midale, Saskatchewan"
Authors: McLellan P.J. , Lawrence K.H. , Cormier K.W. 
Published: March 2010

Abstract
This paper describes an analysis of a multiple-zone acid stimulation treatment of a horizontal well in the naturally fractured Midale reservoir of southeastern Saskatchewan. Extensive characterization of the natural fractures with a wellbore imaging tool and core studie.s indicated that most of the natural fractures in the stimulated section were nearly perpendicular to the well trajectory and were spaced at 1.1 fractures per metre on average. A total of twenty seven, 4.5 m long, selected intervals of the open hole completed section were stimulated with retarded 15% HCI acid. Many of the pressure records also show classic fracture breakdown, propagation and shutin characteristics. The well had a modest productivity improvement after the treatment although most of the produced fluid was water. The reduction in the near wellbore skin is believed to be due to the opening and propagation of partially cemented natural fractures which were previously damaged with solid particulates from the drilling mud. This treatment demonstrates the potential for selective acid stimulation of horizontal wells completed openhole, provided that good zonal isolation is maintained.

Introduction
Despite the worldwide proliferation of horizontal wells in a variety of reservoir settings very little experience has been reported on the performance and comparison between different completion and stimulation methods. In particular, openhole completions which are a popular option for competent naturally fractured carbonate reservoirs, have not received much attention to date. Multiple hydraulic fracture stimulations of conventionally cased and cemented horizontal wells and matrix acid stimulation of openhole completions have been recently reported(1–3). It is the intention of this paper to describe a field case history of a multiple-zone acid stimulation treatment of a horizontal well and to highlight several important insights into the effectiveness of such stimulation techniques as derived from treatment pressure analysis. A complete and relatively unique dataset describing reservoir properties, in situ stress orientation and magnitudes, treatment pressures, and interpreted fracture diagnostics for the length of an acid stimulaled well is presented.

Midale Field
The Midale field is located in southeastern Saskatchewan along the northern margin of the Williston Basin as shown in Figure 1. The field was discovered in 1953 and since then has been extensively drilled with over 300 wells. The reservoir is at an average depth of 1400 m and contains nearly 82 million m3 of original oil-in-place (OOlP). The Midale field was unitized for waterflooding in 1962. Because of a recognized poor volumetric sweep efficiency attributed to high permeability anisotropy an infill drilling program was commenced in 1984. A miscible CO2 flood pilot project was also commenced in the same year. Details of the Shell Midale reservoir and its performance may be found in earlier publications by Beliveau(4–6).
Based on several favourable reservoir attributes the naturally fractured Midale reservoir was considered a prime candidate for the application of horizontal wells. In 1989 three horizontal wells were drilled in the different parts of the field to tap into unswept portions of the reservoir and to evaluate geologica1 heterogeneity.