An Efficient Chemical Treatment to Tackle Low Productivity of Challenging Tight Dolomite: Wormholing and Remediation of Scale-Based Damage

Abstract

Economical production from unconventional reservoirs including tight dolomite require some forms of stimulation techniques to increase the effective contact areas between wellbore and formation. However, productivity improvement of these formations with conventional techniques (e.g. acid stimulation) is very limited and mostly unfeasible. In this paper, an efficient chemical treatment is proposed to stimulate tight dolomite formation through wormholing mechanism and scale-based damage removal. The formation damage in tight reservoirs are much more severe due to the smaller pore/throat size. Among them, the scale-based permeability impairment or phase trapping can cause significant production lost. In this study, the proposed treatment fluid is used to remove the scale-based formation damage, mostly caused by drilling mud. To this aim, the damage removal efficiencies of dolomite cores, artificially damaged by scale precipitation, were investigated after HPHT coreflood treatment. In addition, the performance of the treatment fluid was evaluated as a mean to bypass the damaged zones around hydraulic fractures (caused by liquid phase trapping or significant net stress). To evaluate this, a series of coreflooding experiments were also performed on untreated tight dolomite cores and the feasibility of the wormholing mechanism was studied. The permeabilities of tight dolomite rocks were measured before and after the treatment. To visualize the wormhole propagation inside the cores, computed CT scanning were performed. The rock-fluid interaction was also investigated by analyzing the effluent samples by ICP. The main mechanism of this treatment technique is pore body/pore throat enlargement by slow rock dissolution. From the pore scale analysis, it is found that even at lower concentrations, the active ingredient reacts with rock minerals. A damaged dolomite core was also treated, and the results showed that the removal of Barite-based scale can be achieved even in the presence of native calcite or dolomite minerals. Also, it is found that wormholing can be only achieved at certain concentrations (>10 w%). It also depends on the injection rate and other field conditions such as temperature. Even at low concentration, the rock permeability of the damaged dolomite core can be increased by a factor of 35 (Kf/Ki=35). Finally, dolomite reservoir cores (25-30 μD) were treated at low injection rates (0.08-0.1 ml/min) imposed from the well injectivity condition. It was shown that despite an order of magnitude lower injection rate (compared to those in conventional acidizing) still an optimum injection rate is needed to extend the wormhole across the core. It is also verified that the active ingredient can be used in alcohol-based solutions for special applications such as tight gas and gas condensate reservoirs. The corrosion rate is far below the accepted corrosion level of 0.05 lb/ft2 and it is fully compatible with other additives and high salinity brines. The proposed treatment method is cost effective and experimentally proven to be efficient and long-lasting. Such treatment is recommended to tackle the low productivity of unconventional tight reservoirs. This treatment can be even applied to remove the additional formation damages usually caused during conventional stimulations such as hydraulic fracturing to boost the production.