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Hydraulic fracturing - acidizing
The in-situ stress profiling, 3-D fracturing models, and real-time monitoring and
analysis are as important in acid fracturing as they are in proppant fracturing.
Matrix acidizing
Matrix acidizing is the process of injecting acid into the producing formation at a
pressure between the pore and the fracture pressure, to react with rock deposits or other
formation damage which reduces the productivity of the well.
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Real-time monitoring and optimization
Three methods of evaluating matrix acidizing exist. The fITSt method (McLeod,
1968) uses well tests to estimate the prCduetivi:y of the well before and af"er treatment.
The reduction in skin (any wellbore proouction flow restriction) is the e,~tion tool
used to see if the job was a success. The second method (Paccaloni, 1979) uses the fIrst
real-time tool for evaluating the matrix-acidizing job during the treatment. During the 80s
this method was widely used. It utilizes predicted damage-ratio curves for a steady-state
radial flow on a pressure versus acid flow-rate graph. As the acid etches the formation,
the damage ratio is reduced, and it can be easily determined whether the job has obtained
the maximum damage ratio of 1.0 or if more acid is needed to continue improving the
damage ratio. This tool also verifIed what people in the industry had speculated on for a
long time, that in order to optimize an acid job, the acid pump rate should be increased
towards the end to lower the damage ratio even more. The third method (provost, 1989)
integrates the transient-flow solution to the Paccaloni method. Their real-time model
predicts the skin as a function of acid volume injected and time. From the use of their
model one can see whether the skin IS continuously decreasing, flattening or increasing,
and from tl'le skin monitoring the job can be optimized in real time by stopping the
acidizing at the lowest skin value.
Quality control
During the past decade, qualitY control during acidizing has been strongly
emphasized by the oil companies. The engineer on site is responsible for following the
company check list to elimjnate any problems due to bad quality control. Emphasis has
been placed on very close collaboration between the service company and the oil
company persoonel. It is has been proven that frequent checks and monitoring have
produced better acid jobs (Ely, 1989).
Small-scale laboratory tests
To perform better acid jobs, laboratory experiments are conducted on core
samples from the pay zone. Core samples are pilt in a closed unit and reservoir
conditions are applied. A scaled-down acid job is then done in the laboratory. and the
cores are tested and checked to monitor change in damage. From the laboratory
experiments the correct volume and concentration of acid is obtained for maximum
conductivity to fluid movement. A successful test on the core samples is then scaled up
and applied on the fIeld (Gidley, 1987).