|Publisher||Society of Petroleum Engineers||Language||English|
|Content Type||Conference Paper|
|Title||The Role of Reservoir Lithology in Design of an Acidization Program: Kuparuk River Formation, North Slope, Alaska|
|Authors||Boyer, R.C., Wu, Chia-Hsin, ARCO Oil and Gas Co.|
SPE California Regional Meeting, 23-25 March 1983, Ventura, California
|Copyright||Copyright 1983 Society of Petroleum Engineers or AIME|
Sandstone lithologies in the Lower Cretaceous Kuparuk River formation were tested to study formation damage during acid treatments. Two lithologies were analyzed before and after acidization: 1) a glauconitic sandstone cemented by siderite, and 2) a sandstone containing clay minerals with minor carbonate cements. These samples were treated with hydrochloric acid containing iron-chelating and silt-suspending agents, with hydrofluoric acid, and with fluoroboric acid, all at reservoir pressure and temperature. Formation damage was evaluated by scanning electron microscopy (SEM), X-ray diffraction, petrographic analyses, and acid effluent analyses. petrographic analyses, and acid effluent analyses. Carbonate cement and clay mineral content were critical factors that governed the extent of formation damage.
Hydrofluoric acid is commonly used to stimulate sandstone reservoirs. However, results showed that this acid caused formation damage in both Kuparuk River lithologies. SEM examination of siderite-cemented sandstones after treatment showed abundant iron-rich and poorly crystalline fines dispersed in pores. These mobile fines included partially pores. These mobile fines included partially dissolved siderite, iron precipitates, and colloidal silica. In sandstones with clays, hydrofluoric acid partially dissolved and released clay particles which partially dissolved and released clay particles which could plug pore throats during production. Hydrochloric acid with iron-chelating and silt-suspending agents gave the best results in siderite-cemented sandstones. This acid prevented iron released from siderite and glauconite from forming precipitates and cleaned up the wellbore. Fluoroboric acid was the best treatment in sandstones containing clay minerals. SEM examination showed that this acid cemented undissolved clay particles to framework grains. Thus, pore throats remained open.
Selection of the acid treatment compatible with reservoir lithology will minimize formation damage and lead to increases in production.
The initial acidization program implemented on Kuparuk wells consisted of 7.5% to 15% hydrochloric acid preflush followed by 12% hydrochloric/3% hydrofluoric acid. Twenty-five gallons of HCl preflush and an average of 50 gallons of HCl/HF acid per foot of perforations were used. Production tests showed decreases in rates as a result of HCl/HF acid treatments. A well with initial production of 1650 BOPD declined to 1333 BOPD. The acid program was then changed to 12% hydrochloric preflush containing iron-chelating and silt-suspending agents. Twenty-five gallons of acid per foot of perforations were used. The HCl preflush was immediately produced back. This design is producing good results with a few outstanding cases. A typical well which initially produced 1260 BOPD increased to 3828 BOPD after treatment.
The objective of this study was to determine why the HCl preflush with iron-chelating and silt-suspending agents was a successful acid treatment, whereas the HCl/HF acid program showed rate declines. Also, a fluoroboric acid (HBF4) treatment was investigated as a possible addition to the HCl preflush. Fluoroboric acid is a retarded system preflush. Fluoroboric acid is a retarded system which slowly hydrolyzes to generate HF acid. This differs from the rapid reaction of conventional HCl/HF acid treatment.
Four adjacent 1-in. diameter by 2-in. long core plugs were cut in both Kuparuk C and A zones from plugs were cut in both Kuparuk C and A zones from Well 1E-5. The 1E-5 core is a preserved oil-base core. Mineral analyses were conducted on 1/4-in. slices from both ends of each core plug before acidization. The analyses included:
o X-ray diffraction (XRD) for semi-quantitative analysis of bulk mineralogy and clay-size fractions;
o Thin section petrography for evaluation of mineralogy, texture, and pore types;
o Scanning electron microscopy (SEM with an energy dispersive attachment (EDAX) to identify pore-filling materials and observe their crystal habits.
|File Size||1,169 KB||11|