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

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. Reservoirs in the deep water of the Gulf of Mexico are normally completed by gravel pack or frac-n-pack, due to their sand producing tendency. Frequently, a fluid loss control pill is required during well completion operations after perforating. Therefore, before gravel pack or frac-n-pack, a matrix acidizing job will be performed to cleanup up the remaining drilling, perforating, and pill damage. Because of the high permeability of these formations, acid diversion is a challenging task. Foam segregation can cause the acid to preferentially enter the higher permeability layers, leaving the formation partially treated. Solid-containing diverting agents can invade into the matrix causing additional damage. This defeats the purpose of acidizing treatment. The VESDA possesses the unique characteristic of being more viscous in the aqueous phase and non-viscous in the hydrocarbon phase. When it enters the acid treated zone, which is fully saturated by an aqueous phase (acid) near the wellbore, its viscosity becomes a resistance to the following treating fluid. Thus the treating fluid has to enter the zones that are still highly saturated by hydrocarbon. The VESDA also cleans up easily without leaving any residue upon contacting liquid hydrocarbon during flow back. Hence the full benefit of the acid stimulation can be achieved.

Recent Advances in Viscoelastic Surfactants for Improved Production From Hydrocarbon Reservoirs

“…This may make it unfavorable when used with dry-gas wells. Surfactant-based acids were introduced in the petroleum industry by Chang et al (2002) and Qu et al (2002).…”

Recent Advances in Viscoelastic Surfactants for Improved Production from Hydrocarbon Reservoirs

“…Viscoelastic surfactants have also been developed and applied for acid diversion (Chang et al 2001a;2001b). As mentioned before, viscoelastic surfactant based fluids develop their viscous nature through the formation of micellar structures.…”

Potential Application of Viscosified Treatments for Improved Bullhead Scale Inhibitor Placement in Long Horizontal Wells – A Theoretical and Laboratory Examination

“…The mechanism by which even placement of acid is achieved has been described by several workers. 28,[31][32][33] At the injection flow rate, the shear stress is such that the polymer solution displays low viscosity. On entering the production zone, this fluid enters the highest permeability zone preferentially.…”