Fishbones Post Acid Injection (FPAI) Modelling the effect of continuing acid stimulation post Fishbones jetting

Fishbones Jetting technology is a well-stimulating technology that accelerates the production of hydrocarbons from oil and gas wells, and can help increase recovery rates. The method involves the generation of numerous laterals in the reservoir that effectively drain the reservoir. An analog to this method is the trees' roots, which have many branches to maximize water drainage from the soil. In a Fishbones Jetting operation, hydrochloric acid is pumped down the well to jet many small diameter tubes from the wellbore up to 12 meters into the reservoir. The underlying idea of the Fishbones Post Acid Injection (FPAI) project is to develop a Numerical Simulation Model to account for the rock dissolution caused by acid while acid is jetted through to extend the needles into a hydrocarbon reservoir and calculate consequent improvement in the well productivity, resulting from the additional flow area created by the rock dissolution by acid. This production contribution is in addition to the production created by the lateral holes. Additional channels can develop as acid is distributed further in the rock, increasing wells' productivity. A numerical simulator was developed to calculate the amount of wormhole creation and the change of productivity index based on the extended acid treatment once the Fishbones lateral is created. Extensive testing in a laboratory environment was conducted to generate PVBT curves and establish an empirical database for case-specific rock formation. The data was later utilized to verify the correct implementation of the analytic calculation method. The project's end product is simulation software that calculates the effect of post-acid stimulation after the Fishbones laterals are created regarding the change in productivity index. The case-specific calculations utilize standardized reservoir properties and laboratory test data using formation-relevant core samples (PVBT curves).

Case studies using the newly developed software confirmed the coupled effect of needles bridging near wellbore damage and the low-speed reactive flow contributing to the overall stimulation effectiveness. The case studies showed that the major contributing factor to the increased productivity index is the generation of Fishbones laterals. The post-stimulation acid treatment increased the productivity index marginal in the considered case. Hence, it can not be generalized that the post-stimulation acid treatment only gives a minor uplift. It might have a more significant uplift for other formation materials and layering. With the conducted research project, Fishbones can now evaluate the case-specific effects of post-stimulation acid treatment in a scientifically correct manner. The process allows for optimal reservoir drainage combined with efficient acid usage, ultimately leading to a more environmentally friendly solution at a lower cost.

The underlying idea is to develop a Numerical Simulation Model to account for the rock dissolution caused by acid, while acid is jetted through to extend the needles into a hydrocarbon reservoir, and calculate consequent improvement in the well productivity, often associated with a negative "skin", resulting from the additional flow area created by the rock dissolution by acid. This production contribution is in addition to the production created by the lateral holes ("needles") themselves. As the acid jets through carbonate reservoirs (e.g., chalk and dolomite), it is reasonable to assume a series of "wormholes" will be developed within the rock matrix, because of the rock dissolution by acid. The well productivity will ultimately be impacted by the length, diameter and distribution of wormholes created by the acid jetting. The wormhole geometry, path, and distribution are controlled by the heterogeneity of the pore network and the reaction kinetics between the acid and the minerals/rocks. The pore network is determined by nature. Engineers help to determine the reaction kinetics, that is, to balance the acid rock reaction rate with the acid injection rate. A dimensionless number is used to compare that, called Damkohler number (NDa = Rate of reaction / Rate of fluid flow).