Foamed High-Concentration/High-Volume Mud Acid Stimulations at South Belridge Field

Abstract

Abstract This paper documents the design process and production response of a successful foamed, high hydrofluoric (HF) acid concentration (10% hydrochloric (HCl) acid/5% HF), high volume (1 50 gal/ft) mud acid stimulation program performed in 1991 on thermal production wells in the Tulare formation of the South Belridge Field. The recommended program was based on the results of laboratory work on Tulare core and a lookback study of a 1989/1990 mud acid stimulation program. In comparison to the prior stimulation program, the 1991 program gave better results; both the total program success rate and the treatment cost per incremental BOPD improved by a factor of 3. Introduction An economic evaluation of the 1989/1990 South Belridge Tulare mud acid stimulation program performed on Shell Western E and P Inc. (SWEPI)-operated thermal production wells showed inferior performance in comparison to wellbore recompletion options. The 1989/1990 program used a low concentration (13–1/2% HCl/1–1/2% HF), low volume (101 10 gal/ft), diverted mud acid treatment. A mud acid optimization study was initiated to improve the performance of stimulations for South Belridge Tulare production wells. Laboratory work was requested to test various acid systems on fresh Tulare core. A lookback study was initiated to evaluate production and operational aspects of the prior mud acid stimulation program. The laboratory work identified the 10% HCl/5% HF mud acid system as a more favorable treatment for Tulare clay/shale impaired completions. The major conclusion of the lookback study identified well candidate selection as the primary reason for the poor performance of the 1989/1 990 mud acid program. Based on these results from the laboratory work and the lookback study, an 11 -well program was performed to test the recommended mud acid treatment. SOUTH BELRIDGE TULARE CHARACTERISTICS The Tulare formation at the South Belridge Field consists of multiple sands separated by significant shale zones. These unconsolidated and immature sediments form an asymmetric anticline over the South Belridge structure located on the west side of the San Joaquin Valley, Kern County, California (Refer to Figs. 1 and 2). The lower sands (E-F-G) are lacustrine deltaic, the middle sands (C-D) meandering stream deposits, and the upper sands (A-B) a lower braided stream environment. The reservoir sands are unconsolidated quartz and feldspar grains with lithic fragments (shales, carbonates, volcanics, micas) that are sometimes mixed with a clay and opaline matrix. The sands average 36% porosity and have a range of 1 – 4 darcy permeability. Table 1 is a summary of X-ray diffraction data of the Tulare sand indicating that the feldspar content averages 50%. Clays, primarily montmorillonite and illite, exist in continuous shales between the sands and as discontinuous thin shale laminae within the sands. Due to the deposition environments, the lower sands contain more shale laminae than the upper sands. The clays possess a high cation exchange capacity (15–53 meq/100gm), indicating a high sensitivity to freshwater swelling and dispersion. The produced oil is very viscous (1,875 cp @ 95F, initial reservoir temperature) and has a low API gravity ranging between 12 and 15 degrees. Heat, injected in the form of steam, is required to lower the viscosity to readily produce the oil. Typical Tulare steam drive patterns are inverted 5 acre 9-spots. Most of the South Belridge Tulare has been under steamflood operations over the last 7–13 + years. Temperature and steam identification logs indicate that the Tulare is thermally mature over a large portion of the region. There are over 3000 active producers in the South Belridge Tulare. The typical producer is completed with an open hole, gravel packed slotted liner across several or all of the sand intervals (Fig. 3). P. 185^