Use of Viscoelastic Surfactant-Based Diverting Agents for Acid Stimulation: Case Histories in GOM

Abstract

Summary Viscoelastic-surfactant (VES) -based diverting products have been used successfully in matrix acidizing in the Gulf of Mexico (GOM) in recent years. The uses of VES diverters range from remedial matrix-acid or nonacid-cleanup treatments to use before gravel- or frac-packing operations to clean up long intervals after perforating. Success or failure of these treatments is often related to the efficiency of diverting agents, especially for acid treatments on wells with long, heterogeneous intervals or multiple-zone completions. A VES diverting agent is of particular interest to remedial treatments of frac-/gravel-packed wells, because damage to the near-wellbore area and completion should be minimized for optimum production. Laboratory studies and field applications have demonstrated the nondamaging properties of a VES fluid. This paper reviews the properties of the vesicular-type VES diverting fluid, reviews the operational considerations, and presents several case histories with VES diverting agents in the GOM. The bottomhole static temperatures (BHSTs) of these cases range from 140 to 290°F for both gas and oil wells. With an internal breaker system, the diverter does not require contact with formation fluid, brine, or acid to clean up and provide optimum production. In some cases, as many as four stages of diverters were pumped in the treatment, and successful diversion was observed for each stage. The paper outlines field-operational considerations and detailed fluid properties, which were tailored to specific well conditions and formation characteristics. Details and pressure responses of the treatments are discussed. Introduction Matrix-acid stimulation has been frequently used in the GOM to remove formation damage caused by drilling-mud invasion, clay swelling and clay migration, and production impairment from inorganic scaling. When acid is injected into the reservoir below the fracturing rate and pressure, acid will typically enter the region with the highest permeability. Proper diversion is required to direct the treatment fluid into the damaged zone to achieve maximum benefit of the acid stimulation. Another area that employs acid treatment in the GOM completions is damage removal before gravel packs or frac packs. Gravel packing has been extensively used as a standard sand-control treatment in the GOM for several decades. Before or after the placement of gravel with completion fluids, low-density brines, or linear gels, an acid treatment is usually pumped to remove the near-wellbore formation damage or high skins encountered in wells perforated overbalanced (Unneland and Waage 1996; Matherne and Hall 1996). In other cases, a clay-acid package is often pumped into the formation before the gravel pack to stabilize the residual clays. The effectiveness of these treatments is often directly related to the ability of the acid treatment to remove the near-wellbore damage and connect the wellbore to the formation. In addition to determining the most effective combination of acid blends and volumes for each particular reservoir, treatment design and planning are done to ensure that the correct procedure is followed to place the acid across the entire interval. Staging of the treatment is used to force acid across the entire interval, treating the damaged portion of the completion. The successful acid placement in matrix treatments of openhole horizontal wells are even more difficult because of the length of the zone treated and potential variation of the formation properties. A successful diversion technique is critical to place the acid at the location where damage exists. A variety of diversion techniques have been practiced in acid stimulations (Rae and di Lullo 2003). Diversion sometimes can be achieved by simply increasing pumping rate during the acid treatment. Viscous diversion techniques such as hydroxyethyl cellulous (HEC) gelled brine and nitrogen foam are often used, especially when the temperature of the well is not very high. Other materials used as an acid-diverting agent include acid-insoluble particulates. A mechanic method such as a ball sealer also has been successful when it is practical. Most of these methods have some limitation to their application, especially in a completion with a sand-control method installed. VES-based diverting products have been developed and widely used in recent years, and their use has found application beyond the limitations of the previously mentioned techniques.