The target reservoirs are several limestone layers separated by shales and extend over a vertical height of up to 100 m. The reservoirs are developed differently in almost every well, and natural fractures exist in some, but not in all, wells. The wells are completed with regular 13 3/8-in. casing to around 900 m, 9 5/8-in. casing to around 3400 m, and a 7-in. liner through the reservoir section at around 4100 m. Surface temperatures range from +20°C in summer to -50°C and lower in winter, with bottomhole temperatures (BHTs) around 50°C.
Both for producers as well as injectors, a properly designed matrix acid treatment covering the entire reservoir proved to be the most successful production enhancement technique. Most of the wells were completed in a way that allowed the matrix acid stimulation treatments to be conducted in two stages. Both stages covered more than one perforation set. However, for the second stage the job team had to consider that the first stage covering the lower zones was already completed, and fluid would most likely be pumped into this lower zone. The challenge was to provide good diversion over the entire reservoir for one stage and, even more difficult, to divert the majority of the acid into the upper zone for the second stage. In addition, this reservoir oil has relatively high paraffin content, and paraffin deposition, hindering if not eliminating production, will occur below a certain temperature. This fact had to be considered in the matrix acid stimulation design.
For a single-stage treatment, or the treatment of the lower zones, a combination of Insitu Crosslinked Acid (ICA) and maximum pressure/maximum rate technique (MPMR) proved to give excellent diversion over these intervals. ICA is a chemical diversion technique. It features a thin, gelled acid with a viscosity of approximately 25 cP that forms a highly viscous crosslinked gel when the acid spends on the formation to a pH of approximately 2. The crosslinked gel can effectively stop any further fluid invasion, and following acid stages will be diverted to different parts of the zone. As the spending continues further, the crosslink will break again at a pH of approximately 4. The MPMR technique uses the concept of dynamic diversion, whereby the pressure remains constant just below frac pressure, and the rate is increased as the acid spends on the formation. Bottomhole (BH) gauge data are available to show the effect of diversion and eliminate any possible effects from friction and hydrostatic pressures that can affect surface data.
For the treatment of the upper zone, when the lower zone was already stimulated and could not be isolated by mechanical means, additional diversion was required to seal off the perforations of the lower zone. A combination of Biodegradable PerfPac Balls and ICA was pumped at the beginning of the treatment to divert the remaining matrix acid treatment away from the lower zone and optimize stimulation of the upper zone.
This paper shows how a properly engineered matrix acid treatment using a combination of diversion techniques can result in optimized stimulation treatments. In a carbonate formation, efficient diversion of acid fluids is even more important than in a sandstone reservoir because the acid-carbonate dissolution reaction rate is so fast.