In this paper we present case studies describing the approach adopted to solve scaling issues in a complex well architecture, an analysis of the scaling root causes, and the construction of a novel execution plan incorporating scale inhibitors, diverting agents with different acid systems to maximize the treatment efficiency.
Even when producing at a low water cut fraction, most of the offshore multi-fractured wells in the field experienced scale deposition phenomena because of instability of the calcium ions present in the formation water. When pressure drawdown is applied on the producing wells, a progressive and severe worsening of production performance was observed, and in certain cases this led to a complete obstruction of the well.
Previous stimulations executed on the under-performing wells were able to temporarily restore the production. Those treatments were performed using a conventional HCl acid system with coil tubing and these yielded positive results initially, but performance progressively decreased after a few months.
For this reason, it was a priority to analyze the root cause of the deposition and define an improved method to extend the effectiveness of the intervention. Scale tendency analysis of the formation water highlighted the instability and predicted calcium carbonate presence at the reservoirs’ pressure and temperature range. Based on the evaluation of Saturation Index it was determined that calcite build-up can occur at any point in the production system. This was confirmed by field evidence, with scale deposit samples recovered at the choke, surface line and along the completion tubing.
A nitrified organic acid blend was applied to invade deeply into the fracture body, together with a liquid scale inhibitor squeeze treatment that was designed to prevent further re-depositions in the short-term. A diversion technology was implemented to treat the multi-fractured horizontal wells in efficient manner by rig-less bullheading.
Furthermore, due to unavailability of a rig in place, efforts were made to solve the different challenges to operate in rig-less mode: a lack of space on the production platform deck prevented any pumping intervention, and the well restart and clean up was executed directly in a high-pressure sea line.
This alternative approach, with novel technologies for diversion and scale inhibition, yielded excellent well responses to the placement of the acid mixtures, which were designed to dissolve the carbonate scales with minimum impact on the sandstone formation, completion equipment, and production facilities. The selected solid diverting agent self-degraded by hydrolysis once in contact with water base fluids in the high temperature environment. This diverter was able to effectively distribute the acid treatment into each of the fractures: the particle size distribution was designed to efficiently bridge on the proppant pack in the fractures.
The well start-up production rates confirmed the major benefits resulting from this approach: a higher Productivity Index was estimated on all the applications when compared to past conventional stimulations. Moreover, the use of a scale inhibitor extended the post-stimulation well life from few weeks, up to several months or years and therefore reduced the frequency of future well interventions. This novel alternative approach resulted in a more cost-effective well intervention solution and addressed the challenges of an intense offshore rig-less stimulation campaign in the field.
