In-situ In-situ Cross-Linking Acid Diverting Agent (ISCADA): A New Solution to Stimulate Multi-Layered Reservoirs

Abstract

Stimulation of wells completed in multiple layers has always remained a challenge with regards to stimulating all the layers uniformly and effectively. Problems are more severe if the sub-layers of the formation have different petro-physical characteristics and have varying degree of permeability contrast. Stimulation treatments without diversion system applied in these multi-layered wells mainly stimulate the high permeable streaks repeatedly. PH and temperature sensitive In-Situ Cross-linking Acid Diverting Agent (ISCADA) is developed to temporarily block the high permeable or undesired layers. This diverter chemical with maintained pH is cross-linked in-situ at reservoir temperature. Breaking time of the diverter could be tailor-made by optimizing the breaker depending upon the requirement of treating time. The diverting agent has the advantages of breaking on its own and clearing the blockage of the high permeable zone to regain its initial permeability while facilitates the acid to stimulate the desired zones. The alternating stages of diverting agent, retarded acid and main acid can be designed as per the well conditions. ISCADA can be applied in both sandstone and limestone reservoirs. Trial application of ISCADA system was done in the limestone formations of Mumbai high field and other sandstone reservoirs. Substantial improvement in productivity observed using ISCADA as diverting agent and subsequent application of compatible acid formulation. Remarkably water production has come down in some wells. This paper outlines the laboratory development of the diversion stimulation system; case histories of its application in limestone and sand stone reservoirs. Introduction Stimulation of multi-layered wells has always a remained challenge with regards to uniform and effective stimulation of all the layers. Stimulation treatments without diversion system applied in these heterogeneous multi-layered wells mainly stimulate the high permeable streaks repeatedly leaving other layers untreated. With the aim of stimulating the multi-layered wells uniformly, efficiently and cost-effectively an acid diversion system is essential. In the context of Indian reservoirs diverting acid into untreated zones has remained great challenge in the thick, and high permeability onshore sandstone formation as well as multi-layered carbonate formations in the offshore. Diversion is the process by which the treating fluid is diverted from high permeable zone and allowed to enter the lower permeable zone, thereby making the homogenous distribution of treating fluid in the zones of interest. A critical factor to the success of an acidization treatment is proper placement so that designed volumes of treating fluids contact productive intervals. Effective placement of the treating /stimulation fluids evenly across the zones of interest is important specially when treating long perforated intervals or multiple zones. If there are significant variations in reservoir permeability, the acid will tend to flow primarily into the highest- permeability zones, leaving lower- permeability zones virtually untreated. Even in relatively homogeneous formations, the damage may not be distributed uniformly without the use of techniques to improve the acid placement; much of the damage may be left untreated. Thus the distribution of the acid into the formation is an important consideration in matrix acidizing, and treatment design should include plans for uniform acid placement. The in-situ cross-linking acid diverting agent (ISCADA) is both pH and temperature sensitive. This diverter chemical with maintained pH is cross-linked at reservoir temperature. The cross-linked, high viscous diverting agent then blocks the subsequent treating fluid to enter into the high permeable layer and divert them to the zone of interest. The alternating stages of diverting agent, retarded acid and main acid can be designed as per the well condition. This paper discusses the laboratory development of the ISCADA and general guidelines for implementation.

Proper Design of Polymer-Based In-Situ Gelled Acids to Improve Carbonate Reservoir Productivity

“…The apparent viscosity of the in-situ gelled acid is a preferred mechanism in diversion because it creates a viscosity differential in treated and untreated zones. Halder et al (2004) concluded that field application of polymer-based in-situ gelled acids demonstrated an effective diversion in high permeable heterogeneous reservoirs. Based on lab results and field data, these systems can be applied as diversion agents for multi-layered wells in carbonate reservoirs up to 120°C.…”