Well stimulation treatments require adequate cleaning of the tubular prior to pumping any stimulation fluids. Current known methods of designing an iron scale removal tube clean treatment employ a. "Rule of Thumb" approach for estimating a sufficient volume of acid and the pump rates.
A comprehensive study of the tubing pickle acid returns has recently been completed, testing wells throughout Central and Northern Alberta. The results were used to develop a design package, involving the relevant reaction kinetics, for optimizing pickle volumes and pump rates, and also for evaluating tubing scale removal efficiency.
Using this methodology, realistic ferric and ferrous ion content in the pumped acid, before entering the wellbore, can be correctly determined. This data could be used to specify iron loading level for competent sludge preventive acid systems. This could minimize acidizing fluid costs.
References and illustrations at end of paper
In the western sedimentary basin, many crude oils could form damaging asphaltene sludge's when contacting acidizing fluids. This sludge's are not dissolvable or dispersible by water at bottom-hole temperature. They can be dissolved only by aromatic solvents such as xylene and toluene1. It is well known that asphaltene sludge deposition could seriously plug up the wellbore and cause wettability reversal2.
There are two types of asphaltene sludge's. Those caused by H+ (from acids such as HC1) destabilization of asphaltene dispersion in crude oil usually exhibit a fine powdery texture which could be precipitated together with some high molecular weight n-paraffin (wax) and form a black viscous semi-solid mass3. Another type of sludge is caused by reaction of asphaltenes with Ferrie Ion4. The sludge is usually coal-like solid particles that could form large aggregates.
The first type of sludge can be prevented by adding into the acid fluid, a so called "Anti-sludge Agent" which is a surfactant. The second type can also be prevented or reduced if ferric ions can be removed from or transformed to ferrous ions in the acid. Complete removal of ferric ions is not reliable. Current popular acidizing fluid design uses a system of chemicals called "Iron Control Agents" that can reduce ferric ions to ferrous ions and stabilize the iron compounds in the acidizing fluids after the acid is spent5.
Based on field observations, mainly past acid tubing pickle job results, producers empirically imposed a certain level of ferric ion loading for acid fluids to be used in their wells. Associated with this loading level is a set of test criteria that should be passed before the acid fluids are accepted for acidizing the wells. The ferric ion loading level and the associated test procedure as well as the passing criteria, vary from field to field and sometimes are different for different operators even in the same field. Ferric ion loading level encountered in western Canada ranges from 200 ppm to 20,000 ppm.
The required concentration of iron control agents depends all the ferric iron loading and so does the acid fluid cost since iron control agent cost is the major part of the acidizing fluid cost.