Acid stimulation of sour wells, including gas, oil and water injection, presents special challenges. Add high temperatures and specialty equipment and sometimes permanent production damage can result. Avoidance of damaging precipitates from unwanted reactions, which can permanently plug a producing interval while providing protection of all production equipment, is sometimes a tenuous balance of design.
Precipitation of elemental sulfur, reaction products from incompatible additives used in an acid stimulation fluid and re-precipitation of iron sulfide scale dissolved during the pumping of acid through corroded tubulars are the sources of potential permeability restrictions. Corrosion protection to the proper level of weight loss (< 0.02 to 0.05 lb/ft2) and localized surface defects (pitting and stress cracking) both in live and spent acid solutions are essential to protecting the production equipment and treating tubulars.
This paper reviews those practices, which reduce the risks associated with coiled tubing failure and acid stimulation success of H2S producing wells. Discussions of the methods of avoiding unwanted damage mechanisms, which could result from acid, H2S and/or acid additives interactions are presented. A case history from Saudi Arabia is presented that discusses damage to the outside surface of a coiled tubing string due to acid and H2S corrosion. A Canadian case history describing a mechanical failure of coiled tubing as the result of hydrogen embrittlement is presented. Guidelines are offered for coiled tubing usage to obtain successful stimulation while controlling risks of tubing failures.
Types of workover treatments vary from conventional overbalanced acid fracturing and bullhead matrix acid jobs to underbalanced washing or wellbore cleanout operations. Table 1 lists the relative importance of reactions that may occur in the treatment of sour wells. Under conditions where a well is dead or non-reactive fluids are pumped ahead of a treatment under pressure, the H2S in the wellbore will be flushed into the formation so that contamination with H2S will be avoided. However, when treatments are being performed underbalanced or when iron sulfide scales are being dissolved the coiled tubing and the metal tubulars will be exposed to contamination by H2S. Another occasion of concern is during the recovery of spent or partially spent acids, which have been depleted of inhibitors.1
Operations described above can have disastrous consequences if H2S escaped at the surface putting personnel in a life-threatening situation. In addition to the poisonous H2S potential, there are the mechanical dangers of parted pipe and the physical contact of treating fluids being released over the location.
H2S is a very poisonous gas that is produced in many areas around the world as part of the daily hydrocarbon production stream. Breathing of this gas in sufficient quantity shuts down the respiratory control center of the brain, which leads to the heart stopping due to lack of oxygen. In the petroleum industry, we normally speak of the concentration of H2S in percent. From a personal health safety viewpoint, parts per million are the units of concentration of concern (1%=10,000 ppm).
