The Development and Successful Field Use of Viscoelastic Surfactant-based Diverting Agents for Acid Stimulation

Abstract

Abstract The success of a stimulation treatment or gravel packed completion is often dependent on the ability of the diverting agent to force the acid treatment into different portions of the reservoir. Diversion of acid treatments in the Gulf of Mexico (GOM) have been predominately accomplished by using either foam, HEC, or particulate matters to "temporarily" block off the zones and force the acid elsewhere. Recently, viscoelastic surfactant based diverting agent has been successfully used in acid stimulation in GOM. The fluid generated its viscosity through the densely packed surfactant aggregations, called vesicles. Normally, densely packed conventional surfactant vesicles require high concentration of surfactant to generate adequate viscosity. A new fluid was developed by incorporating a polyelectrolyte with the surfactant to facilitate the vesicle formation, to reduce surfactant concentration and to enhance thermal stability of the fluid. The rheological properties of the fluid can be adjusted by fluid pH, surfactant concentration, and properties of polyelectrolyte and temperature. An internal breaker package was developed to break the surfactant gel and reduce the fluid viscosity to that of water at the desired time and temperature. This system does not require contact with formation fluids, brines, or acids for clean up to provide optimum production. The Core tests were performed and the results demonstrated that there is no formation damage observed in treated core. This paper describes the fluid properties at various pH's, surfactant and polyelectrolyte concentrations, temperatures, internal breaker loading, and salt concentrations along with two successful case histories. Introduction Matrix acid stimulation has been extensively used as ways to stimulate the production, remove formation damage caused by drilling mud invasion, clay swelling and clay migration. For most matrix acid treatments, acid is injected into the reservoir below fracturing rates and pressures, and the fluid will typically enter the region with the 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 reservoir. Without proper diversion, acid tends to flow to the higher permeability zone and leaving the low permeability zone untreated. High rate water packs are extensively used as standard sand control treatments, especially in the GOM. Before or after the placement of gravel with completion fluids, low-density brines, or linear gels, a large acid treatment is pumped to remove the near wellbore formation damage or high skins encountered more often in overbalanced perforation wells1,2. In other cases, a clay acid package is often pumped into the formation before the gravel pack to stabilize the residual clay. The results of these treatments are 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 insure 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 clean portion of the sands. The successful acid placements in matrix treatments of openhole horizontal wells are even more difficult due to the length of the zone treated and potential variation of the formation properties. A successful diversion technique is critical to place the acid to the location where damage exists. Without good diversion, the results of the acid treatment could lead to either incomplete damage removal and/or requirements for uneconomical volumes of treatment fluids. A well-developed diverting agent that does damage the formation after the treatment is critical to the success of any matrix acid stimulation treatment and successful sand control completion.