World's First Combined Acid Stimulation of Horizontal Openhole Gravel-Pack Application of Coiled Tubing, Dynamically Positioned Vessel, and Floating Production, Storage, and Offloading, Offshore Brazil

Abstract

Bijupirá is the first offshore field in Brazil to produce oil for an international operator. Since reaching its plateau production in January 2004, Bijupirá has experienced significant production declines. The bulk of the production in the field mainly comes from four sub-sea wells completed with a horizontal open hole gravel pack (HZOHGP). Analysis of the causes of production impairment suggests likely fines migration and possibly BaSO4 scaling. Scale removal and matrix stimulation treatments were selected as a solution to reestablish productivity. These were performed from a dynamic positioned drillship using CT in the horizontal open hole gravelpacks. The flowbacks were done to the drillship with lift gas supplied from the FPSO. This imposed a very challenging and unique environment for successful implementation. An extensive laboratory testing program was conducted to determine any negative effects of acid on elastometers and metals used in the wells, subsea system and topsides. Reservoir cores combined with a novel geochemical simulation tool helped to select the acid system and volumes for optimum stimulation treatment whilst limiting possible damage to downhole equipment. A continuous acid mixing system was designed specifically for this operation to allow handling large volumes of fluids without the need for a stimulation vessel. Fluids were injected via coiled tubing using a special rotating jetting system to ensure mechanical clean- up and diversion. A flow back process was planned, considering the implications of injecting gas from a FPSO to a well, via gas lift valves- recovering hydrocarbons and spent acid to the drillship. This process proved to be effective in eliminating any environmental impact. This paper describes the whole process including laboratory testing, design, and simulations. Details of the execution phase are given where three scale dissolver and three mud acid treatments were performed with exceptionally outstanding results in terms of execution and production uplift. The combination of offshore, deepwater, FPSO, drillship and coiled tubing, set a milestone in the industry as first worldwide intervention of this kind. Introduction The Bijupirá field came on production in August 2003 and is both gaslifted and water flooded. It is located in 700 meter water depth and produces from 6 horizontal open hole gravel packed wells. After reaching its plateau production early 2004, the Bijupirá oil production rapidly started to decline from an initial 50,000 BOPD to close to 15,000 BOPD in less than two years in part because of increasing watercuts and mainly because of declining well rates. The bulk of this oil production came from just 3 wells BJ-Q, BJ-T and BJ-S which produced near dry oil with watercuts of less than 10%. Their oil production had declined from 40,000 BOPD to 8,000 BOPD. The declines on wells BJ-Q and BJ-T occurred gradually and constantly over time at exponential decline rates of 60% / year, whereas the decline on well BJ-S occurred suddenly during a routine scale inhibition bullhead squeeze, as illustrated in the Figure 1. The bullhead operation itself used chemicals, and procedures that were proven on other wells in the field.