Successful Stimulation of Fordoche Field With a Retarded HF Acid

Abstract

Summary A sequential hydrofluoric acid (SHF) system yielded large-scale success in the Fordoche field in south Louisiana. The SHF was successful on 19 of 22 wells treated in various reservoirs, with an average 2.5-fold production increase. Fordoche reservoir characteristics, substantial production data on the SHF and other stimulation attempts, and some mechanical considerations that required careful engineering planning are presented. Introduction A retarded clay-dissolving system was developed for stimulating oil and gas production from deep clay-damaged sandstone formations. The SHF system uses the ion-exchange properties of clay minerals to generate in-situ hydrofluoric acid (HF) on the clay mineral by pumping alternate stages of hydrochloric acid (HCl) and ammonium fluoride into the formation.Fordoche field has been on production since 1966. Cumulative oil production through Dec. 1979 is more than 25 million bbl. The production is from deep, high-pressure, low permeability, multilayered sandstone formations. Initially, production was high (12,000 BOPD), but it rapidly declined as a result of declining bottomhole pressure (BHP) and formation damage. A gas injection program was begun in 1971 to stabilize the BHP's. To counter the formation damage, several stimulation treatments were attempted, including regular HF, HCl, sand fracture, solvent, isopropyl acetate, and acetic anhydride. These treatments were either temporarily successful or unsuccessful. In addition, frequent mechanical problems required expensive well repairs. Stimulation treatments with SHF began at Fordoche in 1977, and of 22 producing wells treated, 19 were successful. Average increase in production for the 22 wells was 2.5-fold, with a payout of 11 days. Reservoir Characteristics The Fordoche field is located in Pointe Coupee Parish, LA (Fig. 1). Initial production began in Jan. 1966. The field was developed with 55 oil completions and seven gas completions in 36 wellbores drilled to an average depth of 14,200 ft. The 3,520 surface acres contain seven pay zones in the Sparta and Wilcox Eocene formations. Fig. 2 is a typical well log showing these sands. Successful SHF stimulation was performed on both the Sparta and Wilcox formations. Basic reservoir data for the Sparta B and Wilcox 8 (W-8) sands (Table 1) indicate major differences between the two reservoirs.The Wilcox sands consist of two gas-condensate reservoirs (W-4 and W-5) and three oil reservoirs (W-8, W-12, and W-15). The five intervals are similar reservoir properties. They all are of deltaic origin and vary from slightly shaly and fine grained to silty, consolidated sandstone and contain various amounts of clay. The mineralogy of core material from the W-8 sand is shown in Table 2. Laboratory core studies indicate a high solubility of the W-8 formation with HF (Table 2).The Wilcox sands exhibit high original BHP's (9,400 to 11,000 psia) and temperatures (250 to 280 degrees F). Studies indicated that these reservoirs are very sensitive to water; oil production rates have decreased drastically when wells produced more than 2 % water. The wells' tendency to plug limits production and necessitates frequent stimulation to restore wells to commercial production. JPT P. 1485^