The Use of Acid Stimulation for Restoring to Production Shut-in OIl Fields

High operating costs and low oil prices caused oil and gas production from the St. James lease to be shut in early 1998. The St. James lease is located in the Las Cienegas field in an historic area of Los Angeles California approximately two (2) miles from the downtown Civic Center. The principal goal of this DOE project is to show how long-term shut-in oil and gas wells can be returned to profitable operation. This goal is aided by improved oil and gas prices and the use of modern stimulation technology. Low oil and gas prices in 1998-1999 led to thousands of oil wells being shut-in throughout the United States. Based upon the premise that oil and gas prices will remain relatively stable in the future, revitalizing shut-in oil and gas wells is now an attractive option for many operators. The effort and expense required to return a shut-in well to production depends, to a large extent, on its condition at the time of shut-in and the efforts taken to preserve the well’s productivity while it was shut-in. Obviously, if a well was shut-in because of an equipment failure (tubing leak, pump failure, etc.), the mechanical problem must be corrected first. However, wells with no obvious problem at the time of shut-in other than poor economics – most likely have developed problems during the shut-in period. St. James’ experience has indicted that wells in the Las Cienegas field, which have been shut-in for a long period of time, in this case more than seven (7) years and are returned to production often produce at a rate of 30-50% less than prior rates. Scale deposition caused by filtrate invasion has been found to be the major damage mechanism. Other production-impairment mechanisms are believed to be fines migration and clay dispersion and swelling. Project Description/Accomplishments Five Las Cienegas wells, all of which have a high tendency to form calcium carbonate scale were carefully selected for returning to production. A program was set up to acid stimulate these five wells after production had been restored and tested. Laboratory and field tests indicate, for this field and other surrounding fields that the mud acid systems can cause a matrix breakdown and further well bore damage. Most treatments in this field using a mud acid have an average life of less than six months. There are stimulation techniques and materials now available that often minimize these damage factors. One technique St. James used for their acid stimulation program was to modify the hydrochloric acid used for scale removal and perforation cleanout by adding phosphonic acid to the treating chemical. The phosphonic acid reacts with aluminum in clays and feldspar to form a temporary protective film, thus allowing deeper penetration and more effective reaction on the migratory silicates and other fines. Another side benefit of the phosphonic acid is its ability to inhibit the formation of calcium carbonate scale. Well work on the five wells was expected to be completed within one year of the project start. Unfortunately, new metering and sampling and pipeline equipments required by the Southern California Gas (SCG) delayed this project.