Without onsite blending and acid on-the-fly processes it is common to use 20 to 30 poly tanks on a single job.

To avoid preblending massive volumes of acid at well sites requiring acid treatments, Halliburton developed the “acid on the fly” (AOF) blending system. In the AOF system, acid mixtures are created when added to the carrier fluid as it is pumped downhole. Doing so can eliminate the use of 20–30 poly tanks (6,000 gal each) on a single job. Developing the solution entailed addressing a range of challenges, including: health, safety, and environmental concerns; transportation regulations; the need for extreme flexibility; and finite weight and size limitations.

Initial development of the solution was prompted by acidizing activity in the Elk Hills field, near Bakersfield, California, which saw 26 “large” acid treatments in one year of field operation. Total acid volumes per well ranged from 10,000 gal to more than 150,000 gal, based on treatment volumes of 40 to 150 gal/ft in pay intervals as thick as 2,000 ft (610 m).

AOF treatments may be monitored by a fiber-optic stimulation monitoring service to provide continuous temperature profiles over the entire length of a well during acid-stimulation treatments. The temperature data is processed in real time and converted into injection profiles that give a qualitative indication of fluid distribution across pay intervals.

As the acid treatment progress is monitored and, if necessary, re-engineered while pumping, the acid program is modified on-the-fly and the new treatment design is delivered in real time. Mixing on the fly eliminates 24 man-days per year of personnel exposure to transportation of acid to well locations. Time-on-location savings to the operating company using the AOF blending system is estimated to be 3,212 hours per year for an equal number of acid jobs performed.

Application of acid on-the-fly is prevalent in the Elk Hills field. This field, formerly a US National Strategic Petroleum Reserve, is the seventh largest in the continental United States, with cumulative production exceeding 1 billion b/d of oil and 1 trillion scf/d natural gas. There are numerous complex reservoirs in the Elk Hills field. Several require acid stimulation to be commercially viable. Acid stimulation by conventional means was a logistical challenge because of the extremely large acid volumes and variable injection rates required.

Before development of the AOF blending system, preblended acid product was moved and processed several times, originating from the vendor to the facility storage tank, to the blend tank, to the transport, to a jobsite poly tank for storage. Depending on the conditions, it was common to use 20–30 poly tanks (6,000 gal capacity each) on a single job. Further, not all job sites could accommodate the required battery of poly tanks.

At the end of each job there was typically 80 to 100 gal of acid left in each tank. This residual was vacuumed out and disposed of, in addition to the cost of mobilizing and renting the tanks.

The AOF blender system/process with its AOF delivery is inherently more efficient than the blend-and-batch acidizing method. When used with the fiber-optic monitoring system, there is no overproduction of acid. Raw materials are delivered to the well site and blended at the point of use, then consumed immediately; most unused chemicals are restocked or used on the next well. It is estimated that the AOF blender and process significantly increases operational efficiency, resulting in 20 to 40% increase in jobs completed due to greatly reduced setup time.

By reducing the transportation, handling, and storage activities required to blend acid with conventional methods, there is a potential savings of as much as 50% on chemical delivery charges, including driver/operator.