Experience in Acid Diversion in High Permeability Deep Water Formations Using Visco-Elastic-Surfactant

Abstract

Abstract It is a great challenge to divert acid into untreated zones in a thick, heterogeneous, and high permeability sandstone formation. The heterogeneity can be created by hydraulic fracturing, acidizing, or the nature of the reservoir. Common diverting agents do not work well in these situations. In high permeability porous media, foam has the tendency to segregate, gaseous phase will occupy smaller pores while the aqueous phase occupies the larger pores1. Due to the relative permeability effect, the higher permeability streaks becomes the preferable path for acid treatment fluids2. Therefore, limited effective diversion can be achieved by foam. Other diverting agents rely on particulate matter or polymer solution to plug off thief zones temporarily. However, the invasion of the undissolved particles and polymer residue can cause further formation damage. Owing to its rheological properties, and its lack of solids, Visco-Elastic-Surfactant diverting agents (VESDA) have been proven to be effective for acid diversion in carbonate formations3, where large flow channels are generated due to acid-rock reaction. This current study extends the application into diversion in high permeability, highly heterogeneous sandstone formations. Throughout this paper, the term VESDA is used to refer to the VES diverting agent. Laboratory tests have shown that VES is capable of increasing the flow resistance in the high permeability rock (simulated by a proppant pack) and will divert treatment fluid into the low permeability sandstone matrix. The process was more efficient if multiple stages of alternating VES and acid were used. Field case histories in Gulf of Mexico are also presented in this study to demonstrate the effectiveness of the VES material for acid diversion in the highly permeable and heterogeneous sandstone reservoirs. Introduction When a well penetrates through multiple zones in a heterogeneous reservoir, it is difficult to treat all of the zones evenly during matrix acidizing. Job design and fluid selection need to be made by considering the permeability contrast, saturation4 in the formation, and availability of the material. On deep water offshore platforms, the problem is further complicated by the space constraint and transportation of the chemicals. Commonly used diverting agents include polymer gels, foams, oil soluble solids materials5, and rock salt. These materials either require more complex process, such as foam, or fail to reach the full stimulation potential due to damage induced by residue precipitation, saturation alteration, or solid invasion. VESDA overcomes these difficulties by providing a easy, effective, and clean solution to the acid diversion process.