The Successful Use of Polymer-Free Diverting Agents for Acid Treatments in the Gulf of Mexico

Abstract

Abstract The success or failure of a gravel pack or stimulation treatment performed in the Gulf of Mexico (GOM) is often times directly related to the ability of the diverting agent to force the treatment into different portions of the reservoir. Without the success of these diverter stages, entire treatments can be channeled into small portions of these "unconsolidated" reservoirs. Since most damaged intervals are often times the last intervals to be contacted by the acid, the treatment must be chemically forced to attack these intervals early in the treatment to insure proper stimulation and damage removal. Diversion of acid treatments in the Gulf of Mexico (GOM) have been predominantly accomplished by using either foam, HEC, or particulate matters to "temporarily" block off the zones and force the acid elsewhere. Foam and HEC are the preferred method of diversion due to their "low damaging" characteristics, but are severely hampered by temperature limitations. Foam diversions can also be difficult to control due to foam stability. Although HEC is essentially non-damaging, production can be hindered due to the viscous nature of HEC. Particulate matters such as carbonates, sized salts, and benzoic flakes are not as affected by temperature but can often times be difficult to remove and in some instances impede the production results. This paper describes the development of a new "solids-free non-damaging" diverting agent based on viscoelastic surfactants. The unique fluid properties of this system allow acid diversions to be tailored to specific well conditions and formation characteristics. By incorporating an internal breaker this system does not require contact with formation fluids, brines, or acids for clean up to provide optimum production. This paper describes the fluid properties at various pH's, temperatures, and salt concentrations along with several successful case histories. Introduction Prior to the introduction of FracPacks in the GOM, standard sand control treatments would consist of a large acid treatment followed by placement of gravel in completion fluids, low-density brines, or linear gels. The success or failures of these treatments were often directly related to the ability of the acid treatment to remove near-wellbore damage and connect the wellbore to the formation. Prior to the actual sand control or stimulation treatment, expensive in-depth research was conducted on cores to determine the most effective combination of acid blends and volumes for each particular reservoir. Job design and planning was done to insure that the correct procedure was followed to insure that the acid was placed across the entire interval. Staging of the treatment was attempted to force acid across the entire interval, treating the damaged clean portion of the sands. It was often this portion of the treatment, the diverter stages, that was the most limiting or often times damaging portion of the treatment. As with most treatments injected into the reservoir below or above fracturing rates and pressures, the fluid will typically enter the region of lowest stress or highest permeability. In most cases this is the portion of the reservoir that will benefit the least from stimulation due to the apparent "cleanliness" of the sand. Fluid entering the damaged or less permeable zones will rely upon rate or the diverting agents to temporarily block the "clean" zones and force the fluid to other regions.